Males beetles who mate more often are more insecure about their social status than those mating less. The study, published online in Evolution last month, suggests that the social sensitivity of male behavior is linked to how often they have sex.
Dead mice are valuable breeding resources for male Nicrophorus vespilloides burying beetles, and when they find one that’s suitable, they’ll emit pheromones to attract females. But chemical signals also attract other males, who then show up to compete for access to the carcass. The winner is determined by size: The larger male burying beetle usually wins the competition. The dominant male and female pair (called resource holders) turns the carcass into a home for raising their young. Pairs of beetles who aren’t resource holders have alternative strategies to up the chances of their reproductive success: Subordinate females might lay eggs near the carcass and mate with multiple males to create uncertainty over paternity, while subordinate males sneak around and copulate with females (called a satellite behavior).
A team led by Mauricio Carter and Nick Royle from the University of Exeter worked with burying beetles who were artificially selected for either high mating rates or low mating rates. High mating rates offer paternity protection—that is, the more sex you have, the more likely you are to become a father.
They laid out freshly thawed mouse carcasses and watched for resource-holder and satellite behaviors. Compared to males with low mating rates, the beetles with high mating rates are more sensitive to how much larger (or smaller) they are than their competitors when they’re the resource-holder. Males selected for high mating rates would rapidly ramp up dominant behavior if they’re larger than their rival, and they would quickly scale down dominant behavior when they’re smaller.
Body size, the authors write, influences the response of individuals’ signaling behavior to changes in the social environment. "What is really fascinating is that this social sensitivity has evolved in response to selection on mating behavior: Males that have more sex really are more insecure about their social status,” Royle says in a news release.
Being flexible in response to changes in social context is called behavioral plasticity, and it’s pretty common among animals but poorly understood. “Plasticity of behavior is important because it allows organisms to respond rapidly to changes,” Carter adds, “increasing the persistence of populations in the face of environmental fluctuations.”