Xyleborinus saxesenii, welcome to the eusociality club!

By Jack Boyle

Ants, termites, and some bees and wasps are the classic examples of “eusociality,” an extreme form of social behavior in which a group of closely related insects form a colony so cooperative that the colony itself is in many respects a single organism, with the individual insects working together as closely as the organs within a body.

As we slowly learn more natural history, we find eusocial behavior in more and more species—mostly in insects, but also in a shrimp and even a mammal, the naked mole rat.  None of these are quite as spectacularly cooperative as ants and termites, but we still call them “eusocial” if they meet three criteria:

1) Overlapping generations within a nest: offspring stay in the nest well after maturation.

2) Reproductive division of labor: only some of the adults produce more offspring; the rest are engaged in—

3) Cooperative brood care: the adults in the nest work together to rear the young, even if those young are not their own.


Check out Naked Mole Rats, the eusocial mammal:


Our authors, Biedermann and Taborsky, studied the ambrosia beetle species Xyleborinus saxesenii, which was a good candidate for eusociality.  In this species, a single female excavates a nest in which to raise her offspring.  When the first offspring become adults, they delay their dispersal from the nest in which they were born (possibly to help raise their siblings), so we can check off Criterion One.  Like all ants, bees, and wasps, these beetles are haplodiploid, meaning that females have two copies of each chromosome, while males have only one.  Haplodiploidy is thought to predispose organisms to eusociality, due to some counterintuitive kinship-relationships.  For instance, females are more closely related to their sisters than they are to their own offspring, which is believed to promote cooperative behavior within a colony.  Furthermore, ambrosia beetles are most famous for practicing agriculture: they eat a fungus, which they cultivate in rotting wood.  Agriculture is very rare in the animal kingdom in general, but is not unusual among the eusocial ants and termites.

Xyleborus saxesenii, an ambrosia beetle. (Photos by Fdcgoeul via Wikimedia Commons)
Xyleborus saxesenii, an ambrosia beetle. (Photo by Fdcgoeul via Wikimedia Commons)

However, nobody had done the basic natural history for X. saxesenii that was required to prove Criteria Two and Three, for the simple reason that X. saxesenii lives inside tree trunks.  To get a peek into the home life of these beetles, Biedermann and Taborsky developed a sawdust-based culturing medium which they could pack into tubes in the lab, letting them take a look at what goes on inside the X. saxesenii nest.  The authors made hundreds of these artificial nests, and observed them regularly to record what each individual was doing.

Biedermann and Taborsky observed ambrosia beetles in artificial burrows, quantifying how much time they spent engaged in 11 different behaviors, 6 of which are shown here.   In this way they showed that different life stages of the beetle performed different tasks.  “Teneral” refers to adults which recently molted and therefore have soft and pale exoskeletons.  Larvae spent most of their time “digging”—their term for excavating the burrow in which the colony lived—and “balling,” or collecting excrement and other trash into compact balls.  These balls are then removed by “shuffling” them out of the nest, a task performed by larvae and adult females.  Adult females also spend their time “cropping,” or cultivating the fungus, and “blocking,” or guarding the nest entrance.  All life stages, but particularly adult males, perform “allogrooming,” which simply means grooming another individual.  In this case, grooming involves cleaning off the food fungus, which grows not just on the wood, but on the beetles as well.  If not cut back occasionally, the fungus can overgrow and kill the beetles, which is prevented by this allogrooming.
Biedermann and Taborsky observed ambrosia beetles in artificial burrows, quantifying how much time they spent engaged in 11 different behaviors, 6 of which are shown here. This showed that different life stages of the beetle performed different tasks. “Teneral” refers to adults which recently molted and therefore have soft and pale exoskeletons. Larvae spent most of their time “digging”—their term for excavating the burrow in which the colony lived—and “balling,” or collecting excrement and other trash into compact balls. These balls are then removed by “shuffling” them out of the nest, a task performed by larvae and adult females. Adult females also spend their time “cropping,” or cultivating the fungus, and “blocking,” or guarding the nest entrance. All life stages, but particularly adult males, perform “allogrooming,” which simply means grooming another individual. In this case, grooming involves cleaning off the food fungus, which grows not just on the wood, but on the beetles as well. If not cut back occasionally, the fungus can overgrow and kill the beetles, which is prevented by this allogrooming.

Their most important results are summarized in the Figure above.  Biederman and Taborsky show that the adults remaining in the nests cooperate to help raise their sisters, by harvesting fungus and disposing of waste.  The most exciting part of their observations was seeing the way that the larvae cooperated right back with their siblings: larvae were responsible for enlarging the nest and for wadding up waste for convenient disposal by their adult siblings.  While larvae are known to perform more subtle tasks in many eusocial species—often digesting what is hard-to-digest food for the adults—this is currently the most spectacular known instance of larval helping in a eusocial organism.

These direct observations allowed the authors to finally check off Criterion 3 for eusociality.  They also used the same experimental setup to show that there was indeed reproductive division of labor among the beetles (fulfilling Criterion 2), but they describe that in a different paper, so I will not go into the details here.  Xyleborinus saxesenii, welcome to the eusociality club!

Jack Boyle is a graduate student in the Organismic & Evolutionary Biology department at Harvard University.

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