Since they’ve finally filmed a live adult giant squid, I’ve been reminded of a question: What is the upper limit to animal size? I’m interested in physical rather than environmental limits (e.g., don’t worry about finding enough to eat). And mobile instead of sessile animals. This is a rather complicated question.
The maximum size of land animals is limited by scaling laws. The cross-sectional strength of (exo)skeletons increases as the square of length, while the weight increases as the cube. So weight increases faster than strength and at some size the animal can’t support itself. Thus elephants have massive bones compared to mice. The largest dinosaurs are presumably near the limit (is this true?).
Animals using powered flight will obviously face a different upper limit. Are the pteradons at the maximum size, or could something bigger exist?
I think I have a good understanding of those cases (comments are surely welcome). My question really centers on animals where weight is not a factor. There are two cases, neutrally buoyant aquatic animals and neutrally buoyant airborne animals. Both are supported by the surrounding fluid medium. Mechanical strength is needed only to hold the animal together and provide some amount of propulsion.
Blue whales are the largest aquatic animal, and I don’t know what the largest airborne floater is. How much bigger could an animal get?
A neutrally buoyant airbore animal would (by definition) have an average density equal to that of air. I can’t think of an example and I question whether this is possible. What components could an animal have whose density would offset that of the components (e.g. water) that are substantially more dense than air?
It’s true that certain spiders can “soar” (sometimes considerable distances) by extending strands of silk. But they are not neutrally buoyant - they make use of rising air, with respect to which they are descending (albeit slower than the air is rising).
I think it’s possibly a bit misleading to talk about ‘limits’, because that implies there’s some definite cutoff point, up to which everything is possible, but beyond which it is abruptly impossible. In reality, it’s a curve where bigger body size is possible, but pays ever decreasing dividends in one or more crucial areas.
A big bag full of hydrogen. I can easily imagine something akin to an airborne jellyfish, with a modified mantle that it could inflate with the gas, which would be obtained from carbohydrates or possibly water. In the same manner as a bird, it would most likely have body parts designed to be lighter. Mobility would be the biggest problem with such a creature. It would probably not be able to escape predators easily, and would either have to rely on poisons or venom to defend itself or some sort of camouflage, or both.
I can’t think of an example animal either. But any of a number of gases could be used for buoyancy (CH[sub]4[/sub], N[sub]2[/sub], Ne, He, H[sub]2[/sub]).
Mangetout, I think there are limits, determined by the properties of biologically-created materials. We might have look at clades differently. For example, the theoretical maximum size for a land mammal will be different than an arthropod. But it seems that “what is the largest size of terrestrial mammal species we can reasonably expect to survive?” is answerable. And testable, to some extent, by comparison to the fossil record.
There certainly is a maximum upper size based on local gravity, I should think. At a certain point no body design can sustain life even minimally, as it will collapse under its own weight.
I suppose this is possible - we could imagine a larval stage where the hydrogen is ‘brewed’ from readily available ingredients, and the imago inflating its bladder and ascending for everlasting flight.
The fact that no prototype exists argues that this is not a big evolutionary winner. Such a creature would certainly be mobile, but would have little to no control over that mobility. It would inhabit an environment where the food resources available to a drifting organism are marginal.
I think you can break down limitations on bulk size into four general categories:
[ul]
[li]Structural limitations: the constituents of the skeletal structure have limits on their strength and toughness (ability to withstand fracture). You can increase the bulk size and weight limit (which we’ll assume are closely correlated) by adding more legs, albeit at the expense of much greater complexity, but for a standard configuration (say, a quadruped) you can only make the leg bones so thick (with corresponding area moment of inertia) before their required strength exceeds the structural properties of the material, not to mention the strength and leverage of joints and connective tissue.[/li][li]Dynamic limitations: larger, more massive animals have more inertia; it takes more energy for them to evade predators (or predate on other animals) than smaller animals. Hence, virtually all large land animals have been herbivores, and even the largest predators (saber-tooth cats, short-faced bears, T. Rex) have been fairly specialized at attacking other large animals for food. While being larger confers some relative benefit (predators are more likely to attack smaller or weaker members of a pack) there are both tradeoffs in speed and food requirements.[/li][li]Energy/food requirements: Large animals require more food. Virtually all massive animals are either grazers or “scoopers”, like the baleen whales. OTOH, large animals are also more efficient from a heat-retainment perspective–less surface area per unit mass, all things being equal. This is one possible reason that (some) dinosaurs grew so large, if they did indeed lack autonomous temperature regulation, though that has been brought into question in a number of different ways.[/li][li]Neurological limitations: larger bodies and longer nerve paths require more complex nervous systems and stimulation/reaction times. Eventually a body becomes simply too large or complex to control from a central source. However, many creatures have ways to compensate for this; the members of class Cephalopoda have nervous systems that are controlled by a central brain but are arranged in a hierarchal, partially autonomous fashion. It would be impossible, for instance, for an octopus to process all the sensory inputs (which are not just tactition and proprioception, but also olfaction and taste. (Curiously, they don’t seem to have a sense of thermoception (heat, cold) and only very limited nociception (pain), and have been seen on land slithering through fires to get back to water, oblivious to the danger of heat.) Instead, local nerve clusters process both sensation and response impulses, transmitting only limited information to the central nervous system and up to the brain. This clearly serves them well and allows some species of squid and octopus to grow to enormous size yet maintain dexterity and nervous control needed to be general predators.[/li][/ul]
Obviously, all of these categories are relative limitations (with respect to food source, type of locomotion, environment and thermoregulation abilities, et cetera). Trying to set an ultimate limit on physical size, even for one particular class or order, involves so many interrelated variables and factors that a single number or scale is likely to be grossly oversimplified. We can look back at the fossil record and see how large certain types of animals (which we have retroactively assigned to specific taxons) have been in the past, but that doesn’t mean that, for instance, mammals can’t be any larger than the mammoth or the whale, given an appropriate set of evolutionary influences. We could establish an order of magnitude estimate on the maximum structural size and presumed mass of a quadruped, and I’d guess it to be something slightly larger than an Imperial Mammoth. But I wouldn’t bet the mortgage on it; there’s nothing so unlikely as a giraffe.
And having said that, a brief perusal of “largest land mammal” turns up the Baluchitherium/Indricotherium, a “gigantic hornless rhinoceros” about 5.5m tall at the shoulder and estimated to weight 20 tons.
So…somewhat bigger than that. (I’m hedging my bets against 50 foot tall love-addled gorillas discovered on uncharted South Pacific islands.)
The largest pteroaurs known up until now had wingspans of around 40ft (in the genus Quetzalcoatlus), and were probably capable of sustained powered flight (rather than just gliding/soaring).
The general thought has always been that his is close to the upper limit for a flying animal (I can’t find the studies that suggest this, but I think one or two have been done). However, new yet-unpublished fossils suggest there were pterosaurs TWICE the size of the largest previously known, though whether these could achieve powered flight might be up for debate.
I am not a biologist, but I would break the answer to this question into 2 categories: maximal size for a genetic “freak” and maximum size for individuals of a reproducing population. The first, a genetic freak, is hard to argue, since an animal could conceivably have specialized mechanisms to deal with gigantic size, and it’s kind of hand-wavy to guess at how big something could “conceivably” get.
The second, maximum size for a species, is a practical problem, and we are guided by extant animals and the fossil record. The largest land quadrupeds were likely ~110 ton dinosaurs (Argentinosaurus hinculensis ) and the largest aquatic animals are blue whales at ~174 tons (linky ).
It seems to me that the problem is not so much structural, as it is how to ensure the greatest mass of your offspring – in the limit at time approaches infinity, what is the expected value of the ratio of the mass of your descendents to your own mass. The blue whale has presumably reached the mass, given its habitat, competitors, and feeding mechanisms, where further increase is counter-productive.