The Tuatara’s tale

Everyone has heard of the tuatara. It’s as famous as the Coelocanth for being a living fossil (an oxymoron if ever there was one). The tuatara is the sole surviving member of an entire order of reptiles that emerged in the Triassic. It’s a cute little thing that is definitely out of its depth in the unprecedented conditions of the Anthropocene. And, as are many cute and rare things, it’s doomed because of climate change.

Rising temperatures look set to produce male-only offspring in the tuatara, condemning the ancient reptile species to extinction by 2085, computer modelling predicts.

Nature News

This story was widely covered in 2008, and originated with a study by Nicola Mitchell and colleagues in the formerly prestigious Proceedings of the Royal Society B series: “Predicting the fate of a living fossil: how will global warming affect sex determination and hatching phenology in tuatara?”

According to Mitchell et al, rising temperatures will eventually ensure that all tuatara hatch as males, which will presumably become increasingly frustrated and lonely, before the species finally dies out altogether. This might take some time: there’s a tuatara called Henry who holds court at Southland Museum, Invercargill, and must be 120 by now.^

The notion that tuatara might be doomed passes an elementary sanity check; here’s how the story goes. We all know that reptiles have temperature-dependent sex ratios. For tuatara, this means that when eggs are incubated in warmer-than-normal conditions, males predominate in the offspring. An entirely neutral, and factual, observation, unburdened by any climate-related portents of impending doom. But now let an enterprising researcher ask an innocent question: what happens to the sex ratio in tuatara under the projected conditions of anthropogenic climate change? We already know the answer: it will become male-biased. We intuitively grasp the notion that if we keep pushing the temperature higher, sooner or later there will only be male offspring. Ergo, the species is doomed because of impending climate change.

The tuatara species in question was down to a few hundred individuals on a small islet called North Brother Island. Small in this case really does mean small – about the area of a football stadium (it’s bigger than a footy pitch, but if you tried to play footy on it, you’d rapidly lose your ball off the edge of a cliff into the Cook Strait).

Let me just rewind a bit here and ask what we as a species would be worried about if we were down to 350 individuals stuck on an island the size of Wembley Stadium: would it be global warming, or maybe something else, like why are we excluded from all the rest of the land habitat?

Why might we have been driven to the precipice of extinction on a tiny rocky island? Did carbon dioxide have a role in it? Here we have a rather spooky echo of the Puffin’s Tale. Our old friend the rat is largely to blame. As with the chicks of puffins, the eggs of tuatara are rat fast food. Where there are rats, if there are tuatara, no juveniles are seen. Humans liked to eat them as well, once upon a time, another small reminder that poor people will eat anything out of necessity, while the wealthy will pop down to Waitrose and elbow one another out of the way for a free coffee. Anyway thanks to humans and their friends the rats, tuatara were soon driven off the large islands of New Zealand, persisting only here and there, and thriving only in rare places were there were no rats.

At this point I must ask the reader to consider what measures might be taken in the following circumstances: a species restricted to a single islet thanks to rat predation of eggs; a temperature-dependent sex ratio; a warming climate, potentially warming enough to swing that sex ratio to infinity males per female.

“Let’s find another island,” the reader might say, “further south, in a colder direction; let’s eliminate all the rats there, then translocate some tuatara – ooh, and let’s make it a tad larger then 4 hectares.”

Now the title of the Nature News page quoted above is “Condemned to single-sex life by climate change.” Of course unless you are a species that can reproduce parthenogenetically, or one whose individuals are immortal, then “single-sex life” means you have bought a ticket on the bus to extinctionville. Driven out of its entire range, bar a tiny nugget, by humans and rats, clinging on by its claw tips there, the extinction of the tuatara, when it occurs, will be blamed instead on human civilisation itself, via its exhalations of carbon dioxide.

A surprising taxonomic intervention

Taxonomists can be lumpers or splitters. Lumpers combine multiple species into one when they judge that the differences between them do not amount to much. Splitters take an individual species and divide it into two or more. Originally this might have been based on minor morphological features but of course it is increasingly done via DNA sequencing. A long time ago the tuatara was considered to be several species of the genus Sphenodon. Then they were all lumped together as Sphenodon punctatus. Then in 1990 S. punctatus was split once more into S. punctatus (sensu stricto) and S. guntheri. The last surviving population of S. guntheri is that on North Brother Island, hence the real fear of its imminent extinction in 2008.

However, in 2010 Hay et al did some genetic jiggery-pokery, and came to the conclusion that: “Without conducting formal species descriptions here, it now seems most appropriate to consider tuatara as a single species, S. punctatus, that contains distinctive and important geographic variation.”

So in 2010, by the stroke of a pen, S. guntheri was made extinct, or ceased to be, or rather it never was. Should the North Brother Island population of tuatara go extinct, a species will not be lost (although a significant part of the genetic diversity of a species will be).

Since the tuatara is one species instead of two, the North Brother Island population is now part of a population of c. 40,000 instead of 400. That still does not sound like very many even though it is a hundred times larger. And where do the rest live? Well, it might be surprising to discover that there are numerous small populations on islands 4 degrees further north than North Brother Island (i.e. in a warmer direction). The Hen and Chickens Islands are one such island group. In the 1990s, rats were eliminated on Whatupuke (1993), Lady Alice (1994) and Coppermine (1997) (the crossing to the first mentioned is rather rough for western stomachs*). And as you might expect, once the rats were gone, juveniles were seen again.

Finally a recent paper in Oryx by Price et al reported on tuatara translocated to sites 2-4°C warmer than the source site (Stephens Island, near North Brother Island). Their conclusion:

“The fact that several tuatara populations have so far demonstrated high survival rates, growth, and some evidence of reproduction at sites that can exceed mean temperatures on Stephens Island by 2–4°C suggests that these warmer climates have not negatively affected the survival of translocated individuals, that possible local adaptations to the Stephens Island climate have not impeded their ability to establish at new sites, and that tuatara may be capable of tolerating the warmer air temperatures predicted for the 2100s.”

So although it’s early days for a naïve species that has been around for >200 million years, the signs so far are good.


Mitchell et al 2008

The fateful Nature News report, 2008

Hay et al 2010

Price et al 2020

^ Henry the tuatara is still alive, unless the website needs updating. Mind you he is not yet listed as 120 years old despite having been 110 years old a decade ago: he’s “over 110 years old.”

Southland Museum’s breeding program has been so successful that they have supplied  the other New Zealand zoos with sufficient tuatara for their needs and populated an outdoor enclosure to supply individuals for recolonisation efforts elsewhere.

* With apologies to our New Zealand readers.


  1. Earlier this week I watched a programme where green turtle eggs were being dug up from sands in Australia that are too hot (naturally because of climate change). The turtles were at risk because hotter sands produce an overwhelming preponderance of females, so endangering the species. So the eggs were being transferred to cooler sites where some males might hatch.

    So why do higher temperatures produce females in turtles, but males in tuatara? Why should warmer conditions cause changes in the sex ratios at all? What could be the evolutionary advantage?

    Whenever I hear stories like these involving species being put at risk of extinction because of human activities resulting in climatic heating, I wonder how the species escaped the Holocene Optimum or even the last Interglacial when temperatures were markedly higher.

    BTW this must be the only discussion of tuatara that does not mention their third eyes.

    Liked by 1 person

  2. My ignorance of many things never ceases to disappoint me. Suitably encouraged by this article I went to our old friend Wikipedia and looked tuatara up. I was pleasantly surprised to see that Wikipedia doesn’t seem to be blaming climate change, and does go into quite a lot of detail about rats.

    Thank you for a fascinating insight.


  3. The momma tuatara probably does like the turtles, alligators, and other reptiles, lays her eggs in a nest that is somewhat temperature controlled. Aint Mother Nature grand? She don’t need no sinkin’ models to insure the survival of her critters!


  4. “Why should warmer conditions cause changes in the sex ratios at all? What could be the evolutionary advantage?”

    This is the right question. With zero knowledge my scientific wild-ass guess is that constant varying around an average produces more competition of males for females in some years, and vice-versa for others, which sharpens the adaptation rate wrt all the long-term environmental changes the species may face (for instance, only those who can cope better with a long-term alteration of food-supply are going to get mates). So temperature was just a usefully varying engine to hang the system upon. In humans it’s war. Males are overproduced, so in years of peace there’s too many males, and in years of war there’s too many females. As we always have intermittent wars, it’s as reliable as varying temperature. Given that the longer-term variation of temperature over decades (el nino etc), and indeed much higher scope still within say millions of years, will be enormously higher than the trivial changes observed for a few seasons by human investigators, it think it pretty likely that evolution will not have been caught out by this. The system probably tracks the average in some way and moves the switch point accordingly. As indeed attested by the fact that the same species lives happily in a much higher temperature on different islands to the north, as JIT reports. And indeed if transplanted groups are happy too, then digital resets to the average can be accommodated too (and this also makes sense, because over many millions of years there would likely have been countless sudden climate shifts spanning only a few seasons). Given I’m on a swag here already, I’ll leap into the swag squared that meta-genetics could provide a mechanism for passing down the switch threshold changes.


  5. @ Andy, Alan the main theory as explained in wiki is that the fitness of the two sexes is not fixed at a common temperature. So the fitness of males might be higher at higher temperatures in this case. That would select for (or not select against) temperature dependent sex ratios. However going back one step, it’s harder to see why the fitnesses of the two species might be different at different developmental temperatures in the first place. There are potential answers in the size differences in the adults. But I dunno. This is somewhat newish – my 1963 Time Life Reptiles has no mention of the phenomenon. Wiki says it was reported first in 1966.

    @ Pamela the ostensible reason for the panic was that on such a small island there would be limited scope for female tuatara to use behavioural means to pick burrows with the right temperature “in a warming world,” i.e. even the coolest burrows would end up too hot.

    @ Alan the third eye: I dunno what it is for. It seems to be redundant, but, if so, it isn’t clear why it is still there even if only in vestigial degree. Clearly an ancestor must have had some use for it, but I don’t know how useful an eye in the top of your head would be if you lived in a burrow. Which invites a lot more questions, like how long have tuatara lived in burrows (an immediate corollary: what did they do for burrows before they shared them with sooty shearwaters (and others) who actually did the digging?)


  6. Echoing Mark “ My ignorance of many things never ceases to disappoint me”.

    I seemed to recall the tuatara’s third eye was indeed light-sensitive, so I consulted my friend Wiki. There I found much I didn’t know or remember.

    Did you know for instance that lampreys have TWO additional eyes, a parietal eye like our beloved Tuatara and linked to the pineal gland and another linked with a parapineal organ. There is even speculation that “old four eyes” might be the original condition of all vertebrates, as well as be-speckled schoolboys of the rotund variety.


  7. JIT: “So the fitness of males might be higher at higher temperatures in this case…” Well indeed I’m well out of domain, but that sounds very like someone trying to back-fit a marginal species-specific effect as an explanation instead of returning to fundamental evolutionary theory to figure out the real advantage. And as you note it doesn’t gel well with other known facts, not least per Alan occurring upside-down for other animals, and 1966 was also before many evolutionary aspects were properly appreciated too. There’s no point in improved fitness for only males or only females of the same species regarding some universal environmental variable they’re both subject to, because both halves are needed. Playing games that leverage competition for advancing the whole species, and which may involve environmental variables, is a different matter. Competition can be sharpened down either both lines or weighted to one (frequently males fighting for females). Fitness is a species related thing, or rather a populational thing of which species are the result, not a male-half-species related thing.


  8. Andy what I find fascinating is that the mechanism for temperature-determined sex ratios would appear able to turn off the usual mechanism that controls sex, namely the nature of the sex chromosome that was within in the fertilising sperm (excuse my French). Daddy Tuataras must feel absolutely superfluous.

    I also seem to recall that there was a fad sometime ago that suggested it was possible to influence the sex of human offspring, I believe this had something to do with how warm or cool the mother keeps herself. Most unlikely.

    Liked by 1 person

  9. Andy, Alan, I resort to wiki for evidence of a partial proof that the theory may hold for at least one species:

    In support of the Charnov and Bull hypothesis, Warner and Shine (2008) showed confidently that incubation temperature influences males’ reproductive success differently than females in Jacky Dragon lizards (Amphibolurus muricatus) by treating the eggs with chemicals that interfere with steroid hormone biosynthesis. These chemicals block the conversion of testosterone to estradiol during development so each sex offspring can be produced at all temperatures. They found that hatching temperatures that naturally produce each sex maximized fitness of each sex, which provides the substantial empirical evidence in support of the Charnov & Bull model for reptiles.

    (The Charnov & Bull hypothesis was that different developmental temperatures maximised the fitness of the two sexes.)

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  10. JIT: Reece et al, the first paper I found looking into the C&B theory (for Loggerhead turtles as it happens) gives a C&B conformance example of Gamarus dubeni, which reveals that it isn’t male fitness per se that is the underlying reason. Really, it is an extension of the fitness competition between males, for females (biasing the early born to males during short photoperiod, giving them longer to get beefy as size wins out in this competition). This benefits the whole species due to the constant improvement via said competition. Pretty much where I was going. The process is blind so it leverages anything going, but the leverage would stop (eventually) if the process went so far as to burden females, say, or anything else to the net detriment of the whole species. Nor would there be selective value if it made males fitter yet this didn’t pass on as a benefit to the whole species somehow (say in communal species where males don’t compete, maybe fitter males might get more food for everyone, but these species are not so, I think).

    Reece et al says the C&B theory would explain Loggerhead characteristics ‘if larger size at hatching leads to a greater increase in lifetime fitness for females than males’. But they don’t know it does, and what seems critical to me is that unless the greater fitness for females also (net) benefits the species via some route, at best it would be neutral regarding selection and likely wouldn’t happen. I guess if there’s no detriment to males, fitter females may simply bear more eggs for longer. But in that case, why have temperature involved at all and what happens regarding cold seasons? Up to the point where it interferes with breeding because males can’t mount, why not just have bigger females hatchlings anyhow? Unlike photoperiod, which always does the same thing, temperature change is random, and we also know for tuataras that the system works fine in islands with much higher absolute temperature, also with (males in this case) no bigger anyhow, one presumes. Surely it is the changing nature of temperature that is being leveraged for some reason? .


  11. @ Andy I have an ingrained allergy to anything that sounds a bit like group selection, probably because it was widely reviled when I was studying my ecology degree. I will admit it is possible. But it was inculcated in us to search for adaptive significance at the level of the gene – Dawkins’ The Selfish Gene was required reading and was hot science at the time. So I always instinctively search for a way that an individual “mutant” could buck the system, invade the existing status quo with a new (any new) strategy that has a higher fitness. If a life history or reproductive system can be invaded by an alternative then either nature is wrong or you haven’t completely grokked the system itself. (Always the latter.)

    Another explanation given at wiki is phylogenetic inertia, where the temperature-dependent sex ratio persists because it does not have any fitness cost. As you imply above, this seems counterintuitive. There is a reason that “standard” mating systems end up close to 50:50 male:female. That is because if one sex is rare, there is a selective advantage that accrues to an individual which produces more progeny of that sex. (Most easily seen in extreme cases, say start at 100:1, but if you allow selection to act long enough, it zeroes in on 50:50.)

    The problem with the phylogenetic inertia theory is that it just pushes back the required explanation. So: there is no advantage to temperature-dependent sex ratios now, but there was when it evolved, and we still have to explain that. For the Gammarus duebeni situation (we’ve now moved to Amphipoda) we have to ask whether a genetic sex determining mutant could invade this setup. Would 50:50 M:F always beat the early majority for males programmed by photoperiod? I suppose this depends on the mating system at least in part, because earlier females would also end up being more fecund at reproduction time, just as early males would be larger. But if the mating system is contest competition, winner takes all, then it may pay to produce a male as early as possible. But then you get into a bit of a spiral: why not lay half as many eggs, so doubling the mass a male has at birth, thereby guaranteeing victory in contest competition over the weedier “normal” males…? Wherever we look, something else usually cuts in, pushing everything back to where it is now – in this case, probably survival rates.

    I’ll read the pdf tomorrow and see if it makes things clearer…….

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  12. JIT: “But it was inculcated in us to search for adaptive significance at the level of the gene – Dawkins’ The Selfish Gene was required reading and was hot science at the time…”

    Yeah, it was partly the inculcation at the time that bothered me, much as cultural inculcation / group-think still does 😉 I thought TSG was a good insight but way OTT in terms of emphasis of only one level (gene) in a multi-level system. And since I wasn’t studying formerly or was not within an org where my salary depended on my views, I resisted the view. The big needle is moving the right way (imho) now, towards group selection in a multi-level context. The fact that the early maths for group selection turned out to be rubbish, didn’t help its cause.

    “why not lay half as many eggs, so doubling the mass a male has at birth, thereby guaranteeing victory in contest competition over the weedier “normal” males…?”

    Likely because predation rates for the young are high, so halving their chances would ‘cut in’ too much as you put it. A birthing system that’s geared for double the size would likely be hooked to about that size throughout, and there’s further advantage in not putting extra resource into those who only get predated anyhow. By using photoperiod all eggs can remain small, so total numbers much higher, and the extra beefiness is said to come from more feeding time experienced by the surviving progeny, not from more resources gifted by mummy to start with. But this is actually a much more straightforward situation to think through than the size / temperature thing for the Loggerheads or Tuataras, and indeed there’s probably way too many variables to figure it out. Just for a start, temperature changes will have a strong random element (albeit average seasonality as well if far enough from the equator) while photoperiod is a much more fixed seasonal equation. However, this complexity makes me think it’s much less likely rather than more likely to be a simple case of purely extra male fitness for the Tuataras 0:


  13. P.S. Group selection explains altruism. I once saw Dawkins challenged about the issue of altruism in the context of a gene-only selection theory, I think on a Channel 4 TV program quite some years back. He said “it’s a special case”, and literally threw his hands into the air while pronouncing this. Game over, I thought.


  14. While I accept that group selection is possible I don’t think it generally happens. There are patterns of selection that are well described by both group and individual but where it seems rational to me to adopt the latter. Kin selection is one such branch of theory where individuals appear to be making a sacrifice for the benefit of the group but under closer examination, they (via relatedness) benefit.

    The reason I think gene selection made such an impression on us as ecology undergrads was not just because it was the new thing and because group selection was treated as a quack theory with minor if any applicability to real-world situations. It was because we spent a long time in animal behaviour seminars trying to understand whether certain strategies were evolutionarily stable (ESSs). If the entire population was doing one thing, could an individual mutant arise that would benefit from adopting an alternative strategy? Of course this was hard to do quantitatively – we had the elementary models of the prisoner’s dilemma and the hawk, dove and the bourgeois, but the fitness levels assigned were arbitrary.

    Nevertheless for me for group selection to be an explanation, the behaviour or life history must be vulnerable to invasion by a mutant with alternative behaviour/life history strategy. If altruism collapses when a mutant individual raises its own fitness at the expense of the group, then you can invoke group selection for the stabilisation. The introduction to my old copy of Krebs & Davies Introduction to Behavioural Ecology uses the example of a bird species that produces 2 eggs per clutch and are not overexploiting their food resource. A selfish mutant with a clutch of 6 eggs would spread in this population. The clutch size that we end up seeing ought to be the one then that maximises the number of surviving young. The clutch size that benefits the population by sparing the habitat could only evolve by group selection, but it would not be stable against selfish mutants.

    I wonder if Dawkins was talking about human altruism? There is certainly a case to be made for group selection in humans, because of our culture, but as ecologists we were sternly warned never to use humans as an example in discussions, essays, or anything else!


  15. Jit: we’ll have to agree to disagree. Kin selection is a sub-category of group selection, and group selection will occur wherever there is ‘correlated interaction’ (see Darwinian populations and natural selection by Peter Godfrey Smith, for instance). As far as I recall the above Dawkins interview was talking about altrusim generically, but I can’t be sure as it’s too long ago. In any case cultural evolution is only the extension of the same evolutionary principles into the new domain made available by advanced brain power, and indeed starts early as some animals have cultural behaviours. Humans and their cultural behaviour are not exceptions to any of the rule systems. The selfish always win *within* groups, mutant or systemic, obviously. But in the bigger picture *the least selfish groups* always win out. And because of systemic / periodic group mingling, the genes from the winning (less selfish) groups, end up being spread throughout all groups and displace the selfish ones. Which is ultimately what causes the mechanisms of selfishness suppression to arise within groups.

    This is always going to work where there are a number of groups. In your example the group with no strategy to limit clutch sizes within a permanently food-poor environment, would lose membership size and may eventually even collapse within their piece of geography, because it’s unsustainable practice. This leaves behind the successful groups who would take-over that geography when it recovered enough. In practice it rarely comes to collapse, because via systemic means or periodic major upset events groups always tend to be isolated for periods, yet mingled intermittently in-between. Meaning that the more successful groups with higher memberships due to same, swamp the less successful ones in the ultimately shared gene pool. So a strategy to limit clutch sizes will spread throughout the whole species and give less and less opportunity to cheaters. There are very many specific detailed ways in which this actually occurs in the wild (and likewise models thereof), but I guess the simplest to grasp is the ‘haystack model’, which I presume would be in Wiki. I guess K&D 1st edition back in 1978 and only 2 years after The Selfish Gene, is not going to feature any argument against the fashionable wave; but it would be nice to at least include the counter theory in later editions. Maybe they did, I see they’re up to the 4th, and fairly recent date (2012).


  16. Andy, my Krebs & Davies is the ’87 one. I looked into getting the 4th edition just now as an update, but baulked at the £30+ for second hand. (A skinflint, I usually baulk when a second hand book goes over a tenner. These days new hardbacks even seem to be creeping up towards £20. But I did happily shell out £109 for a beetle book last year.) The blurb for the 2012 one says they brought a microbiologist on board. Now that is an intriguing addition to a behaviour book.

    Looks like wiki is as down on group selection as I am! I had to go elsewhere to read up on the haystack model. I wasn’t convinced. The model seemed to specify a situation that group selection might work, but it didn’t seem to me to have a real-world application. The version I read about had a haploid mouse with dominant/recessive alleles for aggressive, i.e. not collegiate, multiplication. The 0.75 of the starting populations that expressed the aggressive reproductive strategy bore fewer offspring than the 0.25 of the starting populations that were more measured. That was because the populations that multiplied faster used up the food resource (the haystack).

    The taxon group I’m most familiar with is aphids. What you generally see in these critters is: a population that multiplies at immense speed. Aphids are born pregnant. Left unchecked, their numbers have amazing compound growth. Now, they quickly start to damage the quality of their food, the plant. At this stage they produce winged forms, which disperse and become the founders of new populations. The best analogy I can think of is a forest fire, with smuts starting new fires wherever they land.

    The point is, throttling back might make a population more successful, but not if faster reproducers are constantly raining in. So with the haystack model, it requires that the mice do not begin to scatter when the food resource declines. In practice, the winning strategy is to multiply as fast as possible, and move to new patches to exploit as soon as the rewards at the present one begin to decline. But you can tweak the numbers to get the group selection model to work. The question is: is the real world like that?


  17. JIT: “Looks like wiki is as down on group selection as I am!”

    Absolutely, although I haven’t looked for years they were always and inappropriately (i.e. very biased) dead against it. No different than the climate-change case. But it’s still handy to see simple things like haystack. The winning strategy is the one that wins for each situation, and clearly as life covers every breeding rate from faster than fire to glacially slow, and a massive range of every other characteristic one can think of too, then there’s a vast number of situations and matching strategies. Your K&D example *defined* that large clutches were ‘over exploitation’. The simple haystack model is similarly defined within a specific circumstance, but there’d be nowhere to scatter to if this was their whole environment, due to geography or population pressure of peers or whatever other boundaries. Group selection is by no means tied to a specific example strategy, nor will it occur if there aren’t groups. But wherever there are groups, evolution will take this path, and in some cases ensuring the groups becomes built into the system, so to speak. As it happens populational groups are extremely common in higher species at least, and indeed altruism is pretty common too. This virtually ubiquitous real-world evidence continues after decades to be a blind-spot for theories promoting purely gene-level selection (rather than multi-level). But fortunately the rigid consensus on same does seem to be thawing somewhat. The multi-level model requires that you first find out which level the most important evolution of interest is occurring on within particular circumstances, and sometimes this will *not* be the group level. Group level features are common however, in humans not least, and we are no special case.


  18. P.S. I didn’t mean you were as biased as wiki, being open to argument. But from the typical edit wars years ago, wiki was not.


  19. No, I’m open to argument. I’m sure there are modelled situations where group selection works perfectly well. The advantage of the haystack model over the real world is that it doesn’t need groups! It just has a single founder with either RR Rr Rr or rr genes, which multiplies. Since the rr conserve their haystack resource better, then the frequency of r increases in the next generation.

    But as soon as we add mortality from external sources or immigration/emigration, not to mention density-independent mortality e.g. weather, the advantage of rr-founded groups starts to fade.

    The model seemed to have been selected to circumvent an objection to group selection, which is that the moderate has to evolve within a population of aggressives and come to exclude them. That seems difficult to me. Of course altruism can evolve within small groups and familial groups, but rather than the “isnt’ nature wonderful?” that we used to get, we now have ways to prove that altruism benefits the individual altruist (e.g., a classic example, regurgitated blood meals in vampire bats).

    Humans seem to do some pretty unwise altruistic things, but if you trace that behaviour back to an origin in a small band of related individuals, it seems to make more sense. In small groups where individuals can be recognised, it pays to be nice, because the likelihood of re-encountering an individual is high. You’re more likely to get robbed in a city than a village.


  20. Group selection (which is also in a multi-level context, i.e. gene-level and cell-level etc selection will all be happening simultaneously), is much misunderstood, and for instance ‘groups’ is any kind of correlated association. The haystacks represent the group association phases. To say there are ‘not groups’ is to completely miss what’s happening. This association occurs within the real world constantly, and creates results that simply cannot be explained by gene-level only theories. Indeed the whole group thing came from trying to explain these real-world results, for which Darwin in 1870 likewise proposed groups; it is not only inherent in models.

    “The model seemed to have been selected to circumvent an objection to group selection, which is that the moderate has to evolve within a population of aggressives and come to exclude them.”

    I don’t really understand this sentence. The model is an attempt to portray real-world behaviour in an easily graspable form, and was originally created to show the difference between kin selection and group selection. Re the latter part of the sentence, the moderate manifestly *do* evolve, we are surrounded by them in a plethora of species including ourselves. It is not an attempt to address an objection, it is an attempt to explain this inescapable fact of real-life.

    “Of course altruism can evolve within small groups and familial groups, but rather than the “isnt’ nature wonderful?” that we used to get, we now have ways to prove that altruism benefits the individual altruist (e.g., a classic example, regurgitated blood meals in vampire bats).”

    Vampire bats do not yet qualify in this behaviour for true reciprocal altruism, as it has not yet been shown they have a mechanism to explicitly punish cheaters. This is a problem for other proposed examples too, most typically warning calls. However, where RA is formally shown to be fulfilled this is usually within small groups, hence generically this is typically explained as far as I’ve seen via notions of ‘kin selection with timing delays’. But kin selection can be mathematically shown to be (a subset of) group selection. And GS needs no concession to timing delays, or indeed explicit on-the-spot-fines for cheaters, because the behaviour is instilled at a deeper level. And also warning calls would be hard in many cases to bound to only kin. I’m not sure where you’re coming from with the “isn’t nature wonderful thing”, this hasn’t been a serious argument for 150 years.

    Part of the problem here is that the selfish gene has become so much of an embedded orthodoxy, many folks simple can’t see past it, and literally work backwards from it as though this must be shiningly obvious. Consider below I found on a blog article about altruism [emphasis original]:

    “…we discussed birds who give alarm calls to warn of predators, at some risk to themselves. However, this act of altruism may ultimately be an act of selfishness—in fact, considering the selfish gene theory, it *must* be. By the simple fact of natural selection, we can infer that giving that alarm call is more beneficial to the individual’s genes than not giving it would be. There are any number of reasons this could be the case. If a single bird simply flew away upon spotting a predator, it would lose the advantages of living in a flock. If it froze and hid, but the rest of the flock kept moving around and making noise, that would draw the predator closer anyway, so it would be best to call a quick warning so the entire flock can hide. Finally, there’s the simple likelihood that by taking a small risk to itself, the individual giving the call can protect many of its relatives.”

    This is not an explanation, it’s an act of faith, with some floundering to fulfil it and of which only an appeal to kin-selection has any basis in theory. Co-operation explains as much as competition at all levels, and indeed I’ve not seen a good argument based on selfish gene theory why we don’t all die of cancer before we are even born. Which is not to say I’ve seen arguments, whose details I now don’t recall but seemed highly convoluted. MLS requires no convolution for same. Ultimately, ‘selfishness’ for units only wins out always if populations are always homogeneous wrt those units. But extremely frequently they are not, at all levels they are instead ‘lumpy’. Wherever intra-populational flows exceeds (even for a period of time, not necessarily always) extra-populational flows, are associations that support the spread of co-operative behaviours. Hence indeed emigration / immigration that systemically exceeds intra-populational flow will turn off the basis which supports co-operative behaviours – but the point is that it’s exceedingly common in nature that this does *not* occur. And for groups that have existed long enough, mechanisms become incorporated to actually stop it occurring, such as shunning out-groupers. Such groups are better able to face any environmental threat, including weather, by means of their co-operative support.


  21. Andy, what I meant by the haystack being chosen to circumvent the necessity for an invasion by moderates was this: because the haystack populations are seeded by a single individual, each population only has a single kind of behaviour. If the starting population for each haystack was 100, there would be a mix of behaviours, and the aggressives would outcompete the moderates. Thus the model makes sense as set up, but does not seem to make sense except within these precisely-set parameters. That makes me suspicious.

    The haystacks aren’t groups when they start each cycle: they’re individuals.

    Moderates evolve, yes of course, but in my example above where the haystacks are colonised by a mix of individuals, they lose. For this to work they have to win out within the population. (I dunno, maybe I’m missing something…)

    Re: vampire bats, I am not up to date with experiment and observation, but the original work was compelling enough. Are you saying it has been superceded? Maybe I ought to check for myself when I have a mo.

    The alarm call: I agree, if this can’t be explained by individual fitness, it is evidence for group selection. Does a bird that keeps feeding have an advantage over one that calls the alarm? To cheat means to feed for the proportion of time you should be scanning for predators. And the proportion of time spent scanning ought to depend, in an optimal situation, on the size of the flock. If that assertion is true, it means the cost of vigilance (= energetic benefit of cheating) declines exponentially as group size increases. But the second factor is the risk of getting attacked. This seems to me to be inversely related to the overall vigilance of the group, which would be (as a first approximation) proportional to the fraction of the time spent checking summed over all individuals i…n. If in the first case, all check at a proportion of time p, then the group vigilance is p*n. If we now add a cheater, it becomes p*(n-1).

    So if n is large, a single cheater has no effect on the overall vigilance. If n is small, it makes a big difference. So I would expect no cheats in small groups, purely for the effect on the cheater’s own fitness. Going further it would be possible to find (purely with individual selection) a balance point in larger groups where the advantage of cheating = the cost of cheating. The key thing is, if a cheater gene can successfully invade this setup, but hasn’t, then group selection must be holding it together.

    Liked by 1 person

  22. JIT:

    “…what I meant by the haystack being chosen…”

    The maths works for the generic case, so any number in the populations and any potential starting behaviours. I haven’t looked at the Wiki for many years but I assume it is trying to explain it in the simplest manner possible, so used numbers people could actually ‘see’ without calculation. It has been proven that the generic maths for Trait or Group/Multi-level Selection is exactly equivalent to Kin Selection, and vice versa, and I think you are happy with KS. Supporters of Selfish Gene sometimes say therefore: “ah ha! you G/MLS folks have just discovered a different way of stating KS, hence nothing is really different and so SG is still the only show in town!” However, G/MLS folks point out that yes, they are not challenging gene level anyhow and that KS is indeed fine to explain a bunch of co-operative behaviours within a gene-level only context. But it’s NOT the only show in town because selection can ALSO occur on other levels simultaneously. AND you only need correlated association for this to happen at the ‘group’ (a better word might be ‘association’) level, plus because G/MLS isn’t specifically tied to kin, this means indeed there *can* be non-kin co-operation when such associations occur. The reason G/MLS is equivalent to KS, is that kin form a correlated association of exactly the type G/MLS predicts. But it manages to predict this *without* needing kin gene-content figures, only *the association*. Hence if there are indeed similar associations *without* kin, or maybe with ‘diluted’ kin’ where KS is breaking down, G/MLS says you will still get the same co-operative behaviours.

    “Moderates evolve, yes of course, but in my example above where the haystacks are colonised by a mix of individuals, they lose.”

    Of course they lose *inside* any group association. Hopefully wiki says exactly that. But there will always be variance, and the group associations that succeed more (so will support higher numbers) are the ones with less cheaters (cheaters bring down the fitness of their team, so ultimately team numbers). Longer-term, genes are mingled throughout the population, yet with a flow not rising above the threshold to erase the ‘lumpiness’ of the associations. Hence the stock of moderates always increases and the stock of cheaters always falls. If the extra-populational flow between associations systemically exceeds the intra-populational flow, the lumpiness will become too smooth and the generation of co-operative behaviours will stop; cheaters will be back on the rise. But nature is replete with situations where this doesn’t occur. Not only that, in systems where groups have evolved for a long time, behaviours are established to maintain the lumpiness, via shunning out-groupers for instance. One can also deduce intuitively that there’ll frequently be much lumpiness, for instance… though some kind of environmental event can raise a barrier that splits a population, which eventually becomes 2 species, many species are generated by populations under their own steam, as it were. I.e. they get so ‘lumpy’ that at least two of lumps aren’t just associations within a species anymore, the extra-populational flow drops to zero for a long time and eventually they can’t even breed if they met again.

    “For this to work they have to win out within the population.”

    Exactly. They do this via the above.

    “Re: vampire bats, I am not up to date with experiment and observation, but the original work was compelling enough. Are you saying it has been superceded?”

    I may be out of date myself, but I recalled several *candidates* for Reciprocal Altruism were not clinched because this is hard to do, one reason being that to make the grade there has to be explicitly shown punishment for cheaters, plus a full round whereby those that gift are repaid by the recipient. You need a lot of observation which for practical purposes have to be of modest populations in a bounded area, and this introduces another big problem (below). Anyhow I thought Vampires may be one such and looked on wiki, which said indeed there wasn’t yet proof of explicit cheater punishment. However, that was a 10 second glance at a wiki page, so IOW could be nonsense! Probably best you read something proper, it may well be confirmed for all I know by now. The introduced problem above is that exactly the best sort of population to observe has lots of close kin within it, and unless you watch for generations you don’t know who is related to who. So how do you know that you are observing Reciprocal Altruism, and not just altruism from KS?? Quite a snag. This is the reason why cheater policing and true reciprocity must be observed within definite non-kin, these aren’t necessary for KS altruism but *must* occur for RA. Meanwhile in a mathematical vein, some folks have found that RA won’t latch in on its own (albeit that might be different in nature), and there’s various proposals along the lines of RA and KS working to reinforce each other within a population. This makes the potential distinguishing of them worse if they’re somehow leaning on each other, but anyhow I’ve noted the two are often talked about together now.

    I might be underselling their arguments here but I’m thinking along the lines of maybe you don’t always get paid back, but your kin do. A problem has been that sometimes the timescale is very long between a gift and a receipt, how does the giver know he’ll ever be paid back? On the up-side there *have* been confirmed cases of RA in non-kin afaik, but 50 years after the theory was introduced by Trivers these are quite limited and the difficulties still significant, it seems to me. Not what you’d think, considering the virtually ubiquitous presence of altruism. Anyhow, I’m probably out of date and my quick forays didn’t resolve anything. And although I don’t hold anything against these lines of inquiry if they work out, from my perspective such complications are unnecessary because if there’s sufficient correlated association there’ll be co-operative behaviours anyhow without having to worry about who is kin or not, or how long the loans are extended. Maybe they’ll find another mathematical equivalence! (This is all what I meant above by ‘KS with time-delays’, which may have seemed confusing, and indeed as noted, being definitely not an advocate my bias might be misrepresenting the situation somewhat).

    “The alarm call: I agree, if this can’t be explained by individual fitness, it is evidence for group selection. Does a bird that keeps feeding have an advantage over one that calls the alarm? … the second factor is the risk of getting attacked…”

    I think the maths is too complex for me to work out! But from a generic standpoint, sticking with SG theory says it can only be RA or KS (ignoring hybrids above, which I don’t really understand anyhow). In some cases at least KS doesn’t fit well because non-kin is involved. If it’s RA, how the hell are cheaters detected or punished? If a supposed sentinel doesn’t call out, in order to slink into a hole and save his skin, how will anyone know? “Didn’t see ‘im, gov, honest to God!” Unlike giving / receiving of support, this is impossible to know. I also saw reported somewhere that calls were more frequent nearer kin, which may be back to the hybrid thing although this still doesn’t address the detection issue, I dunno. But if it’s just that association is stronger with kin, then G/MLS already explains it 🙂


  23. Andy, I couldn’t find mention of the haystack on wiki, but I found it here: ****:// [just adding a few *** instead of http so that it doesn’t embed.] That’s where I got the idea from that there is not a group as such, at least at the beginning of each cycle. I’m afraid I skipped the maths, because I didn’t have time to work through it.

    Re the alarm call, I was more thinking that the fitness cost is the need to constantly scan the surroundings for potential predators. I don’t think the call/flight has a fitness cost. If it did, then when the bird called the alarm, the other birds would be inclined to watch it fly off and get snaffled by the hawk.

    You do need to explain punishing cheaters in the circumstance that the net cost of scanning the surroundings exceeds the benefit. But there is also a slight tension in that real-world strategies might not be optimal for a variety of reasons. An example given by Krebs & Davies is that when a gannet is given a second egg (they only lay one), it is easily able to fledge two chicks. Thus the strategy of laying a single egg is not optimal. There seems to be a lot of “inertia” in behaviour. [It would be interesting to know whether the gannet story is generally true. I suspect even fledging the single chick is hit or miss, depending on prey availability.]

    Regarding the vampire bats, the original work was mostly with kin I think, but there were unrelated individuals in the roost, albeit individuals that had been present for a long time. A further experiment was of small roosts all of unrelated individuals who were kept together long enough to bond. A bat, whether part of the roost, or a stranger, was starved for a night and introduced. The familiar bat was much more likely to be fed. Not conclusive proof that a cheat would be punished, but a necessary condition.


  24. JIT:

    “I’m afraid I skipped the maths, because I didn’t have time to work through it.”

    Well if you want to show that G/ML Selection doesn’t work, you’ll have to engage 0: The problem being that as it’s mathematically equivalent to KS for a kin context, then if you succeed you’ll bring KS down too!

    “I don’t think the call/flight has a fitness cost.”

    I think this aspect of cheating very much has a relative fitness cost. Calling draws the predator’s attention, plus proper callers ensure everyone else is warned (rotating while calling), meaning personal escape is delayed while everyone else has already started running, so increasing personal risk. And cheating will mean *not* becoming the target, so in addition to *not* calling, quietly edging through the herd in direction away from the predator, or slipping into a burrow instead of calling (e.g. meercat), or hopping ‘innocently’ into the brush while the rest of the flock are still pecking on exposed ground. All these tactics increase personal fitness at the expense of the group; it’s highly likely someone else will get snaffled.

    “It would be interesting to know whether the gannet story is generally true. I suspect even fledging the single chick is hit or miss, depending on prey availability.”

    Quite possibly, and ‘optimal’ anyhow is the variable in question, in the sense of ‘optimal for whom’? I.e. the individual parents or the group of gannets.

    “Regarding the vampire bats… …Not conclusive proof that a cheat would be punished, but a necessary condition.”

    Indeed not conclusive proof. And the problem is that after 50 years there are very limited conclusively proved cases. Strongly suggesting the mechanism is valid but is nevertheless very short of being a full explanation, even in conjunction with KS. G/ML Selection presents an explanation that does not have to have explicit cheater punishment (though is also compatible with same), and is universal for kin or non-kin.


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