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What I mean is most (all?) eusocial species are tiny (ants, social bees, termites, and even mole-rats).

I know that my father (who is a biologist) told me that the reason most massive animals (like orangutans, tigers, bears, great white sharks, basking sharks, and blue whales) are solitary is because when you are big, you need a lot of food, so you do not always want to share it with a pack.

So, is a eusocial species weighing on average 2,000 kilograms (or two metric tons, or 4,400 pounds, if you want) (2,000 kilograms is the mass of a massive adult female great white shark) possible? If so, where could it evolve?

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    $\begingroup$ tiny comment: since the adjective eusocial is pronounced with an initial consonant sound yu-so-shel, not a vowel sound oo-so-shel, the article should be a not an -- "a eusocial species". $\endgroup$ – Ross Presser Jun 21 at 13:45
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    $\begingroup$ If your father the biologist says no, why are you putting trust in random strangers on the Internet? (Your father is right about why it doesn't happen.) $\endgroup$ – RonJohn Jun 21 at 17:29
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As a counterpoint to the splendid answer by Legio, which looks at the background theory, I've taken a different tack:

This is slightly tough as the definitions of eusociality are the subject of hot debate, so I'll take the points one by one.

Cooperative brood care including offspring from other individuals:

Killer whales live in matrilines, the children remaining with their mothers for life. These matrilines collect together in groups of up to four to make pods, comprising perhaps twenty individuals.

African elephants are similarly matriarchal, they live in groups with up to three family lines, taking care of each-other's children.

Overlapping generations within a single group:

Killer whales: yes, see above.

Elephants: yes, with the exception of the males who leave the group circa age 15 and are then solitary. (Male bees (drones) are similarly driven out of the hive in harsh winters and they're considered a eusocial species)

The division of labor into reproductive and non-reproductive groups:

Killer whales: Not as such, but the matriarch continues to fulfill her role even after her reproductive days are over.

Elephants: The matriarch continues in her role as with whales, they have distinct nursing casts for the very-young, and separate "juvenile care" units for the growing children. But to be fair this doesn't stop them reproducing as-such, so it's a case of "nearly, yes".

Edit: As it turns out, a recent study in Zimbabwe has shown that female African elephants may potentially remain fertile until natural death by old age.

Individuals from one "caste" lose the ability to perform tasks in other "castes":

It's unclear from my basic researches if this is the case for either elephants or whales, but the indications are it's probably not the case, though some elephants prefer to specialise.

In conclusion:

Probably, somewhat - in the cases above, but not rigidly fitting some of the definitions. To quote Hobbamok's comment: "any world you build can very very reasonably include giant eusocial species with slim to even no deviation from our reality".

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    $\begingroup$ Nice answer, and to put it into context to OPs question about possibility: since IRL we're almost there (or already are depending on the definition), any world you build can very very reasonably include giant eusocial species with slim to even no deviation from our reality $\endgroup$ – Hobbamok Jun 21 at 8:35
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    $\begingroup$ Thanks, @Hobbamok I've added your quote to my conclusion as it's what I probably should have thought to write in the first place ;) $\endgroup$ – A Rogue Ant. Jun 21 at 9:04
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Dinosaurs were huge and plenty of them lived in herds, oh and elephants too.

Eusocieties happen when helping your cousins and brothers is as good of an option as helping yourself, which means when there are many of them.

Sacrificing yourself, your own reproduction to save a stranger is stupid, sacrificing yourself to save 2 brothers is efficient, as efficient as if you reproduced yourself.

Sacrificing yourself to save 4 first degree cousins is efficinet, or 8 second degree cousins or 16...and so on..

Eventually when you have 100'000 sisters like an ant, it's more efficient to die and save 100'000 sisters instead of being selfish and maybe produce a few offsprings, selfish offsprings which wont help you out when they grow up.

There's a reason many animals adopt close relatives whos parents died, theres a reason gays and lesbians did not go extinct and they are found in many species, because even if they don't reproduce,they help their family survive better. There's even evidence of gay animals protecting sisters and cousins from being raped, instead of leaving and building their own family, they stick with the old one and help out.

Big animals are already eusocial, just to a lesser extent...to make them more eusocial make them have more brothers and sisters.

You can even see it in human children, I don't remember which experiment, old memory....they found out that male children will often display self sacrificing acts in oder to defend their brothers if they are many, like stepping in front of the danger and covering the other siblings behind their back, but will cower and hide if they are only two...unless its a female....male kids will almost always self sacrifice for a sister.

Iyou have to think like an animal, animals don't care about evolution, they don't have plans for the next millenia, but selfish animals have selfish genes which produce selfish offsprings...which means that selfsih parents will have to compete with their selfish offpsrings.

Cooperating animals help one another, selfish animals eat their own children to eliminate the competition...literally.

When animals start having hundreds of offsprings at once or live long enough to produce multiple generations, then selfish genes always fail against cooperative ones.

Cause having possibly 2 or 3 generations of offsprings helping you is better than having to kill as many of your offsprings as possible to not be outcompeted.

This is not a choice made by the animal trying to figure out the most efficient way, it just happens that cooperative animals are more successfull and starve selfish animals or at least out reproduce them. And selfish animals are often more successfull in harsh environments like the deep dark sea or the desert or except when they find a good niche which allows them to prosper regardless.

Good rule of thumb is, if you have more 2 siblings then sacrificing yourself for their good will always be more efficient than building your own family, so it will slowly...really slowly develop your genes, passed by your siblings into an eusocial society. To speed up the process, just have more sibblings.

The only problem is, big animals have long pregnancies, so evolving eusocieties is not impossible but it is incredily slow and tedious, our planet is really young and our universe is also young...it might take so much more time to see fully eusocial gigantic fauna.

By the time an elephant has had 1 new generation, a cat produces two generations, cats live less but evolve faster.

Heck...for most of the time earth existed there was only micro fauna or animals as small as a lizard...big animals have just appeared on the universal clock, dinosaurs died half a second ago on the universal clock and life on earth was born yesterday...

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    $\begingroup$ Note that for many eusocial insects, their haplo-diploid sex-determination means that sisters generally share more than 50% of DNA, and can be as high as 75%. $\endgroup$ – eyeballfrog Jun 21 at 13:31
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    $\begingroup$ Sharks seem like the obvious counter example to the idea that this is inevitable. Many species have multiple births and long lifespans (including great whites) but they are about as far as you can get from being eusocial. And not like they haven't had long enough to develop that level of behavior, they just don't need it. $\endgroup$ – user3067860 Jun 21 at 18:34
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    $\begingroup$ "There's even evidence of gay animals protecting sisters and cousins from being raped" Do you have a citation for this? I'm not questioning it so much as looking for details since I've never heard anything like that before.. $\endgroup$ – TimothyAWiseman Jun 22 at 19:21
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The most common reason for a species becoming eusocial is that the males only have one of each chromosome. That means that all their sperm is identical. If the queen mates for life with one male her female offspring will inherit 100 % of the male's genes and 50 % of the queen's genes. It means that the sisters will share 75 % of their genes.

That's the magic thing. For the females it makes more sense to help their sisters who they share 75 % genes with rather than having their own offspring who they share only 50 % of genes with.

Eusociality has evolved at least 8 times in hymenoptera, order that contains wasps, bees, and ants. The males have one of each chromosome and the females have a pair of each chromosome. Eusociality has evolved only a handful of times in all other orders. It can happen but it's a lot less likely.

If the shark males were haploid, meaning they only have one of each chromosome instead of a pairs of chromosomes like the females and they mated for life, they could evolve eusociality.

https://en.wikipedia.org/wiki/Evolution_of_eusociality

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On the one hand, not all eusocial insects contribute much to the colony's central food stores. Deborah Gordon (a computational biologist specializing in red harvester ants at Stanford) recounts an incident in her book Ant Encounters where a fellow biologist was trying to find a bullet ant's nest; he set out a dead cricket as bait in a bullet ant's path, hoping that it would lead him back to its nest. Unfortunately for him, the ant simply ate the cricket and kept going. She suggests the possibility that ants that come from small colonies may only feed themselves and the brood (rather than keeping large central food stores).

On the other hand, there is the issue of why an animal that large can't simply get food and reproduce on their own. Most ants (and eusocial insect species in general) have at least some degree of division of labor, with some ants (typically the older ants) being foragers, some queens producing eggs (most ants are sterile females), and some taking care of larva. On the other hand, most large animals are quite capable of reproduction and food-gathering on their own (or in small "packs" like lions and wolves). You don't really see as much division of labor among, for example, Great White Sharks because they simply don't need it - they can do everything they need to themselves or in relatively small groups anyway, so there's less of a value add to a colony-type arrangement.

Also, some other comments and answers have pointed this out, but ants in the same colony can be more than 50% related, which means that they have a strong incentive to be "altruistic" and cooperative towards others in their colony.

Incidentally, this is a little off-topic, but the fact that foragers tend to be older may be in part due to the fact that eusocial insect foragers tend to "smell like" foragers, which may help eusocial insects know what kind of food to forage for; also, there is some evidence that ants in particular perform a form of social distancing with foragers to prevent the spread of disease. Also, foragers are at higher risk from accidents, disease, and predators, so if older ants are performing more dangerous jobs it would effectively raise the average lifespan of the colony.

In short, in order for something like that to "work," you would need some kind of extensive division of labor, closely related individuals within a group, and a large foraging range.

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Unless you consider it cheating, laying down whatever laws say it is possible on your world, is your privilege.

If at least partial adherence to your father's knowledge and wisdom is required then move back a level and create a food source abundant enough for your eusocials but quite unattractive to other critters.

Throughout, please remember from the point of view of the ant and, the mole-rat isn't "tiny" at all.

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