Female burrowing beetles think small is beautiful when choosing mates, a new study has found. Although the reasons can’t yet be determined with certainty, the authors of a paper in the Journal of Experimental Biology suspect the attraction is that smaller males are less likely to get into fights.
The animal kingdom is filled with examples of males doing battle for mates. From kickboxing kangaroo contests to lion fights to the death, males of many species invest huge resources on their capacity to fend off rivals. Size is usually such an advantage that noticeably smaller males will often not even enter a contest. The only check on endless male growth is assumed to be the price that has to be paid in additional food requirements.
Yet the more we study the natural world the more we discover how diverse behavior can be. Dr. Paul Hopwood of the University of Exeter has made a study of burying beetles, Nicrophorus vespilloides, and has observed that females show a preference for smaller males.
As so often occurs in science, the unexpected discovery was made while looking for something else. Hopwood was seeking to explore the parental behavior of male beetles.
“Male parents face a choice: should they invest more in caring for offspring or in attempting to mate with other females?” The paper notes. “The most profitable course depends on the intensity of competition for mates, which is likely to vary with the population sex ratio.”
Hopwood tried seeing if, when there were more male beetles than female, some would choose to prioritize being a good father, rather than fighting for mates. In such an environment, he reasoned, it might be better to ensure the young you have survive, and keep existing mates satisfied, rather than fighting battles for access to new mates.
In a previous study, Hopwood found that an increase in competition had just this effect. This time around he tried inducing the same behavior by raising the males with differing sex ratios around them. They were then released into the wild with a dead mouse, the perfect place for this species to mate and raise a family. The study group then tried to attract female company, but some instead had to fight off male intruders who wished to get the mouse for themselves.
Hopwood’s results suggest that, for N. vespilloides at least, the juvenile environment's male to female ratio made no difference to parental behavior. In the process, however, he noted, “Smaller males attracted a higher proportion of females than did larger males, securing significantly more monogamous breeding associations as a result.”
If the females were choosing smaller males because they might make better parents, however, their hopes were dashed. “We found no evidence that males of any size, or from any social background, were more committed parents," Hopwood said in a statement. Instead he thinks, “That by being choosy about their males, female burying beetles might avoid complicated relationships involving male fights and extra female competitors."