Beetles that battle make better moms than ones that never fight

For burying beetles, it’s not whether a female wins or loses, it’s whether she plays the game.

Female Nicrophorus vespilloides beetles lay their eggs in dead bodies — specifically, the bodies of small mammals, like mice. But dead mammals are hard to find, and burying beetles don’t share. So females will fight over them, with the winner getting the icky dead prize.

The mom-to-be then prepares the carcass for her eggs, in which she invests a lot of nutrients. Once the eggs have hatched, she’ll supply them with food and antimicrobial chemicals and defend them from predators — providing what amounts to pretty good kid care for an insect.  

Natalie Pilokouta of the University of Edinburgh and colleagues wondered if being a winner or loser in the fight for a carcass nest mattered when it came to being a good beetle mom. So they brought virgin females into the lab and set up contests for dead mice, eventually producing three groups of beetles: winners, losers and a control group that had never seen a fight. All were then paired with a male beetle and allowed to mate.

Winning and losing beetles interacted more with their larval offspring and more often provided them with food than did beetles that never entered a battle, the researchers report June 21 in the American Naturalist. And when the scientists counted the offspring when they dispersed from their maternal parent, they found that moms that had fought had larger broods.

“Experience with a prior contest, regardless of its outcome, influenced the subsequent reproductive decisions of females, thus altering their reproductive output,” the team writes.

The beetles may be using encounters with other females as an indicator of how much competition they can expect as they go forward in life, the researchers propose. The carcass battles can result in lost antennae or limbs, so it might make sense for a beetle that battled to put more investment in the brood she knows she can produce — there might not be another. Beetles that don’t have to fight might see that as a cue that there’s not much competition where she lives, and she can hold off and save some of her energy and investment for the next brood.