There isn’t much on Earth like an ant colony or a beehive. It’s a bizarre lifestyle, creating multi-generational groups where most members don’t reproduce but protect the offspring of others. This episode, we discuss the questions surrounding the details and evolution of Eusociality.
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Sociality describes how social a species is, and as you might imagine, this varies widely. Some animals are mostly solitary; others care for their young but avoid each other as adults; still others congregate for hunting, nesting, or hibernating; and many species spend most of their lives in tribes, packs, or prides. But eusocial species are different.
Eusocial (“true” social) species tend to share certain features: individuals typically spend their lives in one particular home (a hive, a colony, etc.), where multiple generations live and work together, and the task of caring for young – providing food and protection – is assisted by individuals other than the parents of the young, including many members who don’t reproduce at all. Think of a bee hive: lots of bees living together, most of them not reproducing but helping make sure the queen’s offspring survive.
That last part is key. It’s called reproductive altruism, when individuals give up their own chance to reproduce and instead focus on making sure someone else reproduces successfully. Some eusocial species go as far as to develop a built-in caste system, with different members of the group being physically distinct for specific tasks, commonly soldiers, workers, and reproducers. This functional specialization is called polyethism.
The most famous eusocial animals are insects, especially bees, wasps, ants, and termites (not all bees and wasps are eusocial, but the many thousands of species of ants and termites are basically all eusocial), and there are also some lesser-known examples in thrips, aphids, and weevils. Outside of insects there are synalpheus shrimps, tangle web spiders (arguably, maybe), and of course the bizarre Damarland and naked mole rats, the only known example of eusociality outside of arthropods.
Since eusociality is mostly about behavior, it can be tough to study in the fossil record, although there is some rare fossil evidence. Examples of ants and termites in Cretaceous amber indicate that eusociality in these groups goes back at least 100 million years, with similar evidence in fossils for bees and wasps, and at least one study of bees in Baltic amber has shown a surprising diversity of eusocial species around 45 million years ago.
Genetic studies have helped scientists estimate the history of eusocial evolution. For example, DNA data indicates eusociality evolved only once each in ants and termites, while it has apparently evolved multiple times among wasps, bees, and mole rats. Bees are especially helpful in studying eusocial evolution, since halictid bees have seemingly evolved and lost eusociality several times.
There has been plenty of scientific discussion about how eusociality might evolve, and there are probably many possible evolutionary paths to eusociality. These paths probably start with the evolution of various key foundational features, such as a social lifestyle, nest-building, and young that need parental care, before natural selection favors the division of reproducing and non-reproducing members.
When it comes to eusocial evolution, one of the most complex questions is how evolution could produce swarms of individuals ready to sacrifice themselves for offspring that aren’t their own. So much of evolutionary theory is built on the drive to reproduce. This leads into discussions of kin selection – the hypothesis that protecting one’s family could be selectively beneficial – and even the idea that eusocial colonies can evolve as a unit, a superorganism. This is a very active field of evolutionary study.
Origins of eusociality in halictid bees (technical)
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