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Although burying beetle young (beige) rely on their moms (black) for help, they can evolve to fend for themselves.

When this beetle mom disappears, her children become stronger and nicer

MONTPELLIER, FRANCE—Burying beetles are the supermoms of the insect world. Unlike most other bugs, which abandon their eggs almost immediately after laying them, these 15-millimeter-long, black-and-orange beetles protect and feed their young until they are big enough to fend for themselves. Yet the progeny can evolve to do just fine on their own, scientists reported here last week at the Second Joint Congress on Evolutionary Biology. When researchers took burying beetle mothers away from offspring for dozens of generations in a row, the fledgling beetles evolved tougher jaws and became more cooperative with their siblings—allowing them to tackle a challenging world without mom’s help.

The new work “is a lovely example of how social interactions within families can evolve differently,” depending on the context, says Allen Moore, an evolutionary biologist at the University of Georgia in Athens who was not involved with the work.

Burying beetles (Nicrophorus vespilloides) live in European forests and in North American bogs, where they scour the landscape for dead mice and birds. After stripping the carcasses of feathers or fur, adult beetles roll them into neat balls that they bury along with a female’s eggs, which she lays next to the corpse. Normally, the mom holes up with the eggs and tends the young larvae.

But over the past 5 years, Rebecca Kilner, an evolutionary biologist at the University of Cambridge in the United Kingdom, and her colleagues have been altering that family scenario in some lab-reared beetles. Half the time, they remove the female after her eggs are laid, and half the time they leave the family intact. After those larvae grow up, the team repeats the experiment over and over, and monitors each generation for physical and behavioral changes.

After 30 generations, something surprising happened: The motherless beetle larvae evolved larger, most likely stronger, jaws. Mom usually helps the newly hatched larvae gain access to the nearby animal corpse by biting through the soil surrounding it and making a hole in the carcass for them. When larvae had to do this one their own, only the ones with the biggest jaws reached their meal, so they survived, and their young tend to have ever bigger jaws, Kilner reported at the meeting.

More important, after all these generations on their own, “the [motherless] larvae are nicer to one another,” Kilner said. She and her colleagues tested how well larvae with different maternal experiences worked together to survive by putting some newly hatched larvae from the two groups in different proportions on a carcass, and comparing their survival. Larvae where all the young came from the motherless group did great, surviving just fine. “We don’t know how they help each other,” Kilner said, but she suspects the larvae work together to bite their way into the carcass—something that mothers had done for them.

In contrast, when all the larvae came from the group whose ancestors still had mothers present, they did quite poorly with a carcass without maternal help. It seems mom’s presence on an evolutionary timescale leads to ever more competitive siblings, says Joel Meunier, a behavioral ecologist at the University of Tours in France who was not involved with the work. These young likely were too busy jostling each other for access to the carcass and failed to break through, Kilner suggests.

In the past, most studies of families have focused on that kind of sibling rivalry, Meunier says. But this work and his studies of cooperating siblings in insects called earwigs are showing that siblings can evolve to work together not against one another.

Another sign of “cooperation” is that larvae from the motherless group hatch earlier and all at the same time. “There’s a critical number of larvae needed to bite their way into the carcass,” Kilner explains, so those whose timing matches the group’s do better.

The work points out that the evolution of cooperation depends not only on how an individual changes through time, says Moore, but also how the individuals interact with change. Who you grow up with matters a lot.

What matters for these six-legged families may be important for two-legged ones as well, Moore adds. “We are social and so group composition matters for our evolution as well,” he says. “The parents’ behavior depend on what the babies are doing, not just what the parents are doing.” But there are likely big differences, Kilner points out: “We don’t give birth to 25 offspring at once.”