The evolution of expendability: Why some ants traded armor for numbers

Ants with lots of workers tend to put less energy into making them armored.

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[Lexus Lunar Lorry]

obligatory Pluribus mention

We're social, but not biologically eusocial. It's not unique to insects though, there's naked mole rats. And shrimp!

I wonder what the dividing line is between kin selection and eusociality - or is there even a clear boundary? For example, if we pick on hominids, there's the hypothesis that homosexuality/asexuality/childfree behavior massively reduces the reproductive fitness of the individual, they still have utility for the tribe by providing members that aren't directly occupied by childrearing roles (plus bisexuality can be used to help cement social bonds if you're a bonobo).

The obvious line that people point at would be differences in body morphology, but the eusocial shrimp that you link don't have huge differences between fertile and infertile females - only the fact that the former can be seen carrying eggs around.