Why are elephants so small?

April 13, 2022

I have long intended to write a paper entitled Why Elephants Are So Small, as a companion piece to Why Giraffes Have Short Necks (Taylor and Wedel 2013). I’ve often discussed this project with Matt, usually under the acronym WEASS, and its substance has come up in the previous post, and especially Mickey Mortimer’s comment:

I think it would be interesting to read a study on that — the order in which various factors restrict body size without transformative adaptations. Similarly, what the differences would be for an aquatic animal like a whale.

That is exactly what the WEASS project was supposed to consist of: a list of many candidate limitations on how big animals can get, some rough attempt to quantify their Big-O behaviour, some discussion of which factors seems to limit the sizes of modern terrestrial animals, and how dinosaurs (especially sauropods) worked around those limitations.

(Whales are different. I have in my mind a half-formed notion for a third paper, completing the trilogy, with a title along the lines of Why Whales Are Dirty Cheaters.)

What are those candidate limitations? Off the top of my head:

Biomechanical:

  • Bone strength
  • Cartilage strength
  • Cartilage thickness
  • Muscle strength
  • Nerve length and conduction time

Metabolic:

  • Blood pressure: column height and capillary length
  • Lung capacity
  • Tracheal dead space
  • Digestive efficiency
  • Metabolic overheating

Those are just some of the physical limits. There is anecdotal evidence that elephants are not very close to their mechanical limits in their usual behaviour: they could get bigger, and still work mechanically. (Follow the link at the start of this paragraph. You will thank me.)

There are plenty of other factors that potentially limit organism size, including:

Behavioural:

  • Feeding rate
  • Ability to navigate dense environments
  • Predator avoidance with limited athleticism
  • Difficulties in mating

Ecological:

  • Territory requirement
  • Time taken to reach reproductive maturity
  • Reproductive rate
  • Birth size
  • Lack of selection pressure: when there are no predators bigger than a lion, why would elephants need to evolve larger size?

I’m sure I am missing loads. Help me out!

I am haunted by something Matt wrote a while back when we were discussing this — talking about how alien sauropods are, and how easily we slip into assuming mammal-like paradigms.

We are badly hampered by the fact that all of the 250kg+ land animals are mammals. We only get to see one way of being big, and it’s obviously not the best way of being big. Our perceptions of how hard it is to be big are shaped by the animals that are bad at it.

So having written this blog post, I am wondering whether it’s time to breathe life back into this project, started in 2009 and repeatedly abandoned.

55 Responses to “Why are elephants so small?”

  1. Mickey Mortimer Says:

    Very cool. I look forward to any results.

  2. Leelan Lampkins Says:

    This would be very interesting. There used to be bigger elephants. But no bigger whales. Surely there is some reason for this.

  3. Mike Taylor Says:

    The simple reason there were no bigger whales earlier in evolutionary history may just be that big whales are a recent development, and they’re still in the process of getting big.

    On the the other hand, I seem to remember a recent study (anyone have the ref?) suggesting blue whales are hitting a limit based on the energetic demands of lunge-feeding.

  4. Brad Lichtenstein Says:

    I would agree that fossil elephants show better than that delightful video that modern elephants aren’t near their limit. And today at least, it’s not (just) about lions – it’s about humans directly selecting against large size, and even more recently against larger territories that might be needed to get big.

  5. dale mcinnes Says:

    Think about the question you just asked “Why are elephants so small?” This is a question based solely on Space-Time. You could just as easily ask “Why are dinosaurs so small?” All I would need to do is pluck you from the Anthropocene and set you gently down, anywhere on Earth in early Carnian times. You would be amazed at the size of the pseudosuchian crocs and wonder why dinosaurs never reached anywhere close to that size. Your study should start there. You may find that what works for dinosaurs simply doesn’t work at all for mammals. Mammals are on another trajectory different from dinosaurs and hence may need entirely different adaptations to get really big like the fossil indricotheres and fossil elephants. Adaptations that wouldn’t really suit dinosaurs. It’s like asking why no mammal ever once developed feathers or a parallel analog to them. What works for dinosaurs may never work for mammals and vice-versa. Start with the feathers. That’s how hard it will be to arrive at any useful data. There’s a reason this study really, truly, never got off the ground in the first place, Good luck.


  6. What about allometric scaling issues with feeding?

    – Elephants chew their food with two rows of grinding teeth. Uniform scaling up the animal by N will result in no better than N^2 growth of chewing surface area.
    – However, If sauropods just cropped and swallowed, the volume of the bite would scale as N^3, tracking the growth of the mouth volume.

  7. Michael Traynor Says:

    First, thanks, Mike, for the link to the sliding elephant video. That is delightful.

    Second, of course you should do the WEASS paper. It’s your time not mine! And I like the list of factors you already have, some I’d not have considered.

  8. Alex Ruger Says:

    Dale,

    I think in critiquing the question you’ve actually articulated why it’s valuable. Distantly related animals arrive at similar solutions to gigantism, but do so in ways that are contingent on their specific evolutionary histories. Both elephantids and sauropods have graviportal adaptations, reached sizes that dwarf most of their contemporaries, and did so while largely eating plants.

    Exploring how such disparate groups arrive at these very surface level similarities will help elucidate those differences. As Mike says in the post, sauropods are alien. I would go so far as to describe most non-avian dinosaurs with that adjective, but sauropods easily take the cake.

    In principle, it’s really no different than the example you use of feathers. Comparing the simple filaments and fibers of some archosaurs with fur would actually be pretty interesting – two different integuments from two groups that (at one point or another) convergently evolved tachymetabolism. What do those differences tell us, especially in light of the whole organism’s biology and history?


  9. What about the composition of the prehistoric atmosphere? I thought for example that insects were able to grow larger because there was more oxygen at one time etc…

  10. Mike Taylor Says:

    Brad, excellent point that the modern ecosystem actively selects against large size — and for the most tragic of reasons.

    Dale, yes of course mammals and dinosaurs did things differently — that’s pretty much my point!

    Chris Green, yes, I had “Feeding rate” in the Behavioural section, and had in mind limitations like the one you propose here. It’s interesting to think about whether ingestion rate for a crop-and-swallow animal is proportional to the square or the cube.

    Michael Traynor, thank you, I now have one more reason to seriously consider de-mothballing the WEASS project!

    Alex, I agree entirely: what Dale I think saw as writing off the question, I see as restating it in a different form. What it comes down to is this: of all the various limits on growth, which ones that hold back elephants, and which ones held back sauropods, and where does each group excel than the other fails in.

    Dominic, the atmosphere changed so much through the Mesozoic — high O2, low O2; high CO2, low CO2 — that I don’t think any such explanation works as a reason for sauropods flourishing across 150 million years.

  11. llewelly Says:

    with respect to feeding, I thought sauropod heads showed negative allometry, in constrast to sauropod neck length, which shows positive allometry. If that’s correct, that would imply the volume of food reachable via the neck may be more imporant than the volume of food croped by taking a bite.

    With respect to proboscideans, for acquiring food, it seems to me the proboscidean trunk is approximately analogous to a sauropod neck; it gives access to a large volume of food at a range of heights with relatively little movement. However it may be that as trunk length increases, bringing the end of the trunk back to the mouth becomes relatively less efficient, in comparison to sauropod swallowing food down a similarly long neck.

    But here you’ll face a serious preservation problem: elephant trunks are much less likely to be preserved in the fossil record than even sauropod necks; to the best of my knowledge, there are only a handful of partially preserved trunks from woolly mammoths found frozen in permafrost, and no other fossil proboscidean trunks. Usually trunks of fossil proboscideans are assumed to be “long enough to reach the ground”, but actually, nobody knows.

  12. Mike Taylor Says:

    Negatively allometric heads is news to me. I’m not saying it isn’t true, just that I’ve not heard of it before. Do you have a reference?

    I think that for elephants, the problem is not the trunk but the chewing. But I suppose I have no strong basis for that belief.

  13. LeeB. Says:

    There are probably lots of different size constraints for different groups of animals; those for ruminants which are fore gut processors seem to involve the digestive system.
    Very large extinct camels seem to avoid this, some seem to reach several tonnes in weight.
    Those for other mammals may involve the digestive system or possibly the gestation time for unborn young.
    Male proboscideans seem to continue growing through their life which suggests that if their teeth lasted longer they would have got even bigger; yet the teeth of large mammoths seem more complex than those of even larger strait tusked elephants.
    As well as sauropods hadrosaurs, proboscideans, indricotheres, megatheriid ground sloths and diprotodonts seem to reach multi ton sizes; the sauropods obviously are the largest terrestrial group but a comparison to all these other groups might bring up interesting insights.

  14. Mike Taylor Says:

    LeeB, yes on the multi-group comparison. That’s what we did in the summary talble for Why Giraffes Have Short Necks: see https://peerj.com/articles/36/#table-3

  15. llewelly Says:

    On negative head allometry: I’m thinking I misremembered; I’m looking for a reference and what I’m seeing is several papers (such as Sander et al 2013 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0078573 ) showing no positive head allometry – which isn’t the same as negative head allometry. My apologies.

  16. Mike Taylor Says:

    No apology necessary, thanks for doing my research for me :-)

  17. Mike Taylor Says:

    (BTW., that paper covers a lot of the same ground than WEASS was intended to address.)

  18. Mark Witton Says:

    I’d really like to see this paper come together, so consider this another vote for dusting the project off. I guess one question I’d add, which is probably already on the cards anyway, is why _everything_ is so small compared to sauropods. Elephants are an obvious comparison (and ecologically apt) but why do herbivorous theropods, ornithischians and mammals only graze the greatness of sauropod size with a few big outliers? What was the special ingredient that made even modestly-sized sauropods bigger than everything else to ever walk on land?

  19. Mike Taylor Says:

    Sheer awesomeness, Mark. Sheer awesomeness.

  20. Mike Taylor Says:

    (But seriously, thank you for the vote.)

  21. Bruce Ediger Says:

    Can you talk some more about “tracheal dead space”? Is this the volume of the trachea? Wouldn’t sauropods have a relatively large tracheal dead space? What happens if an animal’s exhalation volume (if that’s the phrase) is smaller than tracheal dead space?

  22. Mike Taylor Says:

    Yes, exactly, Bruce. If sauropods had in inefficient lung arrangements that we mammals make do with, then the volume of the trachea would be a big problem indeed. Happily, it’s one of the problems that they neatly sidestepped, by instead having an avian-style flow-through respiratory system with voluminous air-sacs.

  23. LeeB. Says:

    That works as long as the air sac volume is greater than the volume of air in the trachea so that on inhalation it inhales both the air in the trachea plus some oxygenated air from outside.
    Another way to arrange this would be to have an elastic trachea that was squeezed on exhalation to remove all the deoxygenated air and then expanded on inhalation to draw in fresh oxygenated air..


  24. Great thing! I have thought about this question since several years. At physiological level the great differences of the respiratory systems are surely one point. That elephants chew is another point. Additional to this, my thought is that the overall proportions of both groups has massiv implications of anatomy and size limits. Very likely the fact of long necks and very long tails play a role for the sauropod gigantism. Terrestrial mammals never achieved these features. Giraffes have the longest necks in great mammals, but don’t come up to very big sauropods. Mammal necks a limited in length by the weight of the chewing apparatus. All big mammals have very short tails which contribute nothing to size and mass or to locomotion. The very long and muscular tails of sauropods contribute their own to size and body mass. And they have a important function in locomation (attachement area for muscles which move the hindlegs etc.). These points leads to the fact, that the complete systems of muscles, tendons and joints of the vertebral column from sauropods (dinosaurs) and elephants (mammals) are very different in several points because inherited features from different origins (more lateral motions during locomotion in the sauropsid lineage to sauropods and more dorso-ventral motions during locomotion in the derived synapsid lineages to mammals including elephants). It’s only a thought, but perhaps it may be help (and perhaps the paper will disprove my thoughts, but this is okay :) ).

  25. Allen Hazen Says:

    (Addendum to Lee B.s reference to teeth.)
    Another suggestion:
    Mammals evolved diphyodonty somewhere around the Morganucodon level, and tend to have very strict, genetically determined, limits on the number of teeth they have over their lives. (Very rare exceptions: some toothed whales have a lot more teeth than the Eutherian standard, and I think either dugongs or manatees go on serially replacing molars beyond the total number of molars a standard Eutherian ever has.) Elephants serially replace their molariform teeth, but only a limited number of times (lifetime total of ?6? or ?7? in each quadrant, and the last one is often wearing out by the time the animal reaches old age. (I think there may sometimes be a supernumerary extra molar after the “normal” last one, but my recollection of wherever I read or heard this is that it is a very rare occurrence.) So elephant lifespans — and so growth spans — are limited by the “planned obsolescence” of their masticatory apparatus.

    Are they really all that small, though? The largest extinct Proboscidean species seem to have gotten up to twice the body mass of extant African elephants. (Last estimate I saw was that they were bigger than Paraceratherium.) Which is still a lot less than the biggest Sauropods, but beginning to get respectable.

  26. Mike Taylor Says:

    Well, the title “Why elephants are so small” is meant somewhat facetiously.

  27. Katherine Honish Says:

    As far as selection pressure goes, there’s the cost-benefit factor for mammals with reproduction. I don’t know how long pregnancy was for a wooly mammoth or paraceratherium, but it likely wasn’t shorter than the African elephant’s 22 month gestation period. That’s a lot of time to devote to creating only one offspring, and if elephants got bigger that time would probably lengthen and eventually impact population growth for no significant benefit as the only remaining species of elephant don’t compete with each other.
    Does lead to a variation of the question – why have modern rhinos given up the size contest to pachyderms when we know their relatives can get just as big or bigger?

  28. Mike Taylor Says:

    Yes, Katherine, these are the issues!

    On rhinos: it seems that the species we have now have opted for athleticism over size. Why? No idea. Maybe just the contingencies of how their environments have turned out.

  29. LeeB. Says:

    Elasmotheres were bigger than living rhinos and athletic; if people hadn’t come along they might have got bigger.
    Indricotheres were browsers with reasonably long necks; perhaps they would have got larger if proboscideans hadn’t spread to Asia.
    Browsing Megatheres hadn’t been getting up to 4-5 tonne size until the last few million years, perhaps with more time they might have got larger.
    The grazing mylodont sloths only seem to reach 2-3 tonnes (e.g. Lestodon) so as with rhinos perhaps it is easier to be a really large browser than a grazer.
    Elephants may to some extent avoid this constraint by using trunks to gather food.

  30. Andreas Johansson Says:

    A thought that’s occured to me is that the largest theropods, hadrosaurs, ceratopians, ankylosaurs, and stegosaurs all seem to have been roughly the size of the largest proboschideans, in the 5-20 tonne range. Proboscideans themselves and indricotheres also fall there. Is there something magical about ca 10 tonnes that makes it not worthwhile to exceed unless aquatic or sauropodal?

  31. Mike Taylor Says:

    I think that’s exactly right, Andreas. For most of these, the secret sauce would seem to be the the need to chew keeps you small, but of course that doesn’t apply to the predators. But maybe they had an independent soft limit that happened to fall in the same region.


  32. Nitpick: aren’t Galapagos tortoises technically non-mammals larger than 250 kg?

    Also, wondering if there isn’t a population-level selective pressure towards *more* individuals (for, say, disease resistance and genetic diversity) rather than fewer but larger individuals. That’d be above and beyond the benefits to the population of having more “granular” food/range needs such that a smaller fraction of the population can die in times of resource scarcity.

  33. Matt Wedel Says:

    Also, wondering if there isn’t a population-level selective pressure towards *more* individuals (for, say, disease resistance and genetic diversity) rather than fewer but larger individuals.

    That’s a pretty darned interesting idea. I’ve sometimes wondered if really big, really long-lived sauropods might have been a sort of brake on the microevolution of their populations. Imagine a super-giant individual that lives for a couple of centuries, insulated from all but the most jarring environmental perturbations by its size, spamming the environment with a thousand eggs a year. At the other end of the scale, reproductively-active subadults might have gone through 20 generations in the same time span. Left to themselves, their genotype might evolve substantially in response to changing environmental conditions that the super-giant individual could just cruise right through (up to a point), but their offspring would be interbreeding with the “old model” offspring of the super-giant — and some, maybe many, of the subadults would simply be offspring of the super-giant. It seems plausible, and possibly inevitable, in that situation that the regular injection of “old model” genes from the long-lived super-giant would delay or even prevent a very close fine-tuning of genotype to environment at the population level.

    I also suspect that being really big pulls in different directions under different conditions. Under most perturbations, being big is probably a benefit: in times of drought or flood, big animals can travel farther, subsist on lower-quality fare, and simply survive longer on their reserves (mice starve faster than elephants). BUT there is threshold of perturbation intensity where being big becomes a detriment — like during an impact winter or other global catastrophe that collapses the whole ecosystem.

    From the Late Triassic to the Late Cretaceous, being big was a successful strategy for sauropods, in a way that it wasn’t, so continually, for any other clade of dinosaurs. If the K-Pg event hadn’t happened, I strongly suspect that sauropods would still be around, still looking much like they did during the Mesozoic.

  34. Mike Taylor Says:

    Now that is fascinating!

  35. LeeB. Says:

    This happens with large brain corals and probably other organisms which reproduce sexually and clonally.
    When the sea levels rose at the end of the last ice age they opened up new habitat for corals to colonise.
    The first brain corals to colonise the habitat just kept getting bigger and bigger and producing more gametes each breeding season as they did so.
    This can go on for hundreds or possibly thousands of years; and result in brain corals 10-15m in diameter which must produce huge amounts of eggs and sperm which can interbreed (they are self fertile); far more than any smaller colonies that came along later; and furthermore those smaller colonies are likely to end up producing offspring by crossing with the large colonies.
    So there is likely to be stasis in evolution of these colonies along a reef.
    Also the initial colonization of each reef may have been from other colonies on the deeper reef which then died out as the sea level rose.
    The outcome is that each reef has its own genetically different population of brain corals which is genetically stable over very long periods of time.
    This also applies to branching corals which break up in major storms and the fragments continue growing.
    So in any one area the branching corals can be genetically very similar.
    In effect it isn’t the survival of the fittest but the survival of the first to get there.

  36. Mike Taylor Says:

    LeeB, this is exactly the kind of background information we need! Do you have a reference?

  37. LeeB Says:

    Ill have a look and see what I can get.

  38. Francelino de Azevedo Says:

    Viviparity surely must play a part too! It’s hard to become bigger when that means carrying a bigger baby inside, sauropod eggs are much smaller proportionally compared to an elephant baby

  39. Mike Taylor Says:

    Yes, we think viviparity was a big part of the equation!

  40. LeeB. Says:

    Potts D.C. Bull.Marine. Sci. 33:619-632; Chronic evolutionary disturbance by high frequency sea level fluctuations.
    It points out that the zone of active coral growth less than 20m deep has only been stable at one place on average for 3200 years during the Pleistocene; this has maximized intraspecific variation but largely inhibited the process of population differentiation.
    Potts et. al. 1985; Marine Ecology Progress Series 23: 79-84; Dominance of a coral community by the genus Porites.
    Large Porites colonies dominate communities; these can be very old.
    Brown et. al. 2009; Coral Reefs 28; 735.
    Soong et. al. 1999; Coral Reefs 18;42.
    Takeuchi et. al. 2017; Coral Reefs 36; 1317.
    Smith et. al. 2021. Sci Reports 2021; 11:15334.
    The above papers discuss very large colonies of Porites more than 10m in diameter which are very old; possibly more than 1-2000years.
    Devlin-Durante et. al. 2016; Molecular Ecology 25 no. 22; 5628-5646.
    How old are you? Genet age estimates in a clonal animal.
    This gives possible genet age estimates up to 6500 years in branching corals that break up in storms where the broken off pieces continue to grow.
    Very old ages are also recorded in clonal plants including seagrasses and trees.
    Also corals do strange things such as form syngameons where several coral species are obviously separable at some parts of their range and completely interbreed at other parts of their range.
    Presumably ecological conditions at some locations favors the survival of hybrids over the parental species; but the old age of some coral colonies slows down the hybridization process because the parental colonies persist.

  41. Mike Taylor Says:

    Lots of great stuff here, LeeB, many thanks!

  42. ncmncm Says:

    I had a lengthy rumination that WordPress took the opportunity to eat, and that I will not try to reproduce.

    Maybe our question should be, why are sauropods so small?

    There would have been a whole series of barriers to increased size that each lineage overcame independently, at different times. (IMHO these necessarily included secondary cervical two-chambered hearts,, e.g. one per vert.) Those that failed to cross one or other chasm topped out in size somewhere short of what we are schooled to consider awesome.

    It is common in scaling for a solution that worked at size x to become wholly inadequate at size 1.5x, with need to substitute another structure different in kind, a sort of phase transition. Life is not commonly good at that. ‘Pods lucked out with their pulmonary inheritance, but digestive architecture is hard to modify incrementally into the multiple, parallel tubes called for.

    I have speculated elsewhere that the mature individuals subsisted on coprophagy on the incompletely digested output of smaller ‘pods, maybe their own offsprings’.

    Whatever they achieved, they all either fetched up against a limit too hard to beat, or lost selection pressure to get even bigger. But we don’t even know what benefit they got from being as big as they got. It must have been decisive for so many species to carry it so far.

  43. LeeB. Says:

    Perhaps theropods reached a size limit and then sauropods that exceeded the size that they could prey on were away laughing.

  44. ncmncm Says:

    We may imagine the largest ‘pods at the center of a fleet of smaller offspring responsible for absorbing the onslaught of allosaurs and tyrannosaurs, and ranging out to gather vegetation, delivering the … pre-processed result to the sedentary (not to say sessile) egg-laying mothership.

    Sauropods as hyper-distended bees.

  45. dale mcinnes Says:

    Why would you suggest that the parental pods would be in the centre of a family group sacrificing their offspring at the periphery ?? I don’t know of any terrestrial vertebrates that would do that. Even crocs would defend these offspring.

  46. ncmncm Says:

    Bees, termites, and ants don’t. There is ample precedent. And, as with bees, a herd (pod?) of ‘pods sociobiologically motivated to attack a single predator might lead the latter to prefer less touchy prey.

  47. dale mcinnes Says:

    The largest, strongest termites and ants are usually on the periphery of a column protecting the interior mass. Bees ? A little different.

  48. llewelly Says:

    Queens of eusocial insects benefit from the protection of the hive because they are threatened by larger predators – sometimes many orders of magnitude larger.

    A big reproductive sauropod is in the opposite situation; probably no predators threaten it. A juvenile sauropod that acted to protect an adult would just become theropod food for no benefit to anyone but the theropod.

    Queens of eusocial insects also benefit from food provisioning by the hive, and this is anotther way in which a big reproductive sauropod is in the opposite situation; a big sauropod’s long neck, modest non-chewing head, and huge digestive system mean it already has what may have been the best ever food provisioning evolved for large terrestrial herbivores. A hive of smaller sauropods would actually probably less efficient at provisioning food.

    So I would think sauropods would more likely to be the *opposite* of eusocial insects, with at most maybe some croc-like hatchling protection, but little or no other social behavior.

    (I do wonder if maybe Nate is joking, but I decided to treat the suggestion seriously. )

  49. ncmncm Says:

    Did you miss that scaling up a digestive tract runs into severe area/volume problems? Queen Argentinosaurus *might* have had little to fear from apex predators, but her gullet and intestine still would be hard-pressed to process enough fresh cycad leaves, on their own, to support her even without the extra drain on calories levering her ponderous bulk around the countryside.

    If you can’t split your own digestive system into a dozen parallel tracks, the next best thing is to bring a dozen others’ into play. Having those others, still only (say) elephant-sized, range out in a dozen different directions and bring it back half-digested enables gathering sustenance from a much wider area than you can stagger back and forth across by yourself.

  50. llewelly Says:

    Digestive efficiency, in calories extracted per unit of food, of back-fermenting herbivores increases as gut retention time increases. So a sauropod with a longer gut retention time gets more energy from the same amount of food. This means a single, long winding tube is much better than multiple parallel tubes, because the single tube has longer gut retention time. Parallel tubes would be an optimization for a *fast* digestive system, which might be nice for a bird, but is the opposite of what a sauropod would benefit from.

    Because the efficiency of both the digestive system and the food-acquisition system (the long neck and the non-chewing head) increase as size increases, recruiting a bunch of smaller sauropods to help the big adult actually doesn’t help the big adult.

    I think your 1950s-era assumption that huge sauropods could barely move is outdated and bit ridiculous. While I don’t think sauropods could run (except perhaps small juveniles less than 2 tons?), a walk of modest velocity wasn’t unduly difficult, and was probably reasonably energy-efficient, in terms of calories spent per kilogram-meter moved.

  51. LeeB. Says:

    Gut retention times eventually have a problem with methaneogenesis, which large mammals like elephants get around by shortening their guts; as a result they don’t fully digest the food they intake.

  52. LeeB Says:

    If you want to understand some of the limits on size imposed by digestive systems a good paper is here: http://www.rhinoresourcecenter.com/pdf_files/133/1332029763.pdf

    though bear in mind that giant camels like Gigantocamelus and Titanotylopus may have been heavier than Aepycamelus major; and Bison latifrons was also very large as indeed are the living chianini breed of cattle ( they are so big that I have seen them included in a parade on TV).

  53. llewelly Says:

    Thank you for the link to the Clauss et al 2002 paper, Lee B. I’m still reading it.

  54. ncmncm Says:

    @llewelly: Nobody said “huge sauropods could barely move”. Certainly they moved. But, on topic, the energy cost of motion goes up as x^3, where calorie absorption is only x^2, so the energy budget is crippling. Energy even just for existing without moving goes as x^3. Without an adaptation to cut energy demand, we have an animal, in effect, flying without wings.

    An objection that eusociality isn’t seen in extant reptiles does not seem to be meant seriously (although I stop short of calling it “ridiculous”, in deference to our host); we know ‘pods have plenty about them unlike other reptiles. We even have other tetrapodal eusocial species, so we know it can arise. The question is whether it solves essential problems.

  55. Mike Taylor Says:

    @mcmncm, the energy cost of motion is less than x^3, as is that of just existing. BMR tends to be around x^2.25, though this varies in different groups of animals and different studies have disagreed on the exact numbers.

    Also: if we didn’t know about naked mole rats, we would probably describe the notion of eusociality in mammals as ridiculous … but it turns out not to be. I agree there is no particular reason to think sauropods lived that way, but you are right not to rule it out a priori.


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