Resource Partitioning in Cooperatively Breeding Burying Beetles, Nicrophorus nepalensis (Hope, 1831)

Reproductive partitioning is how number of offspring is apportioned among the same sex group members in a cooperatively breeding group. It indicates how reproductive benefit is allocated within a group and is one of the key aspects in social evolution studies. Ecological and social factors and individual intrinsic condition could influence individual behaviors and further affect reproductive partitioning. We want to know how within group differences of individual intrinsic condition influence behaviors of group members and their reproductive partitioning. The individual intrinsic condition could show in cost of reproduction, fighting ability or cost of copulation, and, moreover, in behaviors of reproductive investment, aggression or copulation. These behaviors could further influence individual fecundity. We did our experiments with burying beetles, Nicrophorus nepalensis. Burying beetles take carcasses of small vertebrate as only breeding resource, and form cooperatively breeding groups on the carcass. The group size is varied in different environment. We did our experiments alone the elevation gradient in Hehuanshan and recorded the air temperature. We randomly grouped burying beetles of different body size and weight, and recorded the behavior videos. Burying beetles freely formed the cooperatively breeding group on the carcass we provided. In order to identify individual fecundity, female beetles were fed dyed beef to lay colored eggs. We found that female copulation is the key factor of fecundity and reproductive partitioning. Less the variance of copulation number, more even the reproductive partitioning. Larger females were more likely to copulate more, and could lay more eggs. We also found that females with more copulation showed more aggressiveness and invested more time on the carcass. Furthermore, air temperature was correlated to male group size. Male group size was positively correlated to total number and variance of female copulations. Our results show that within group differences of copulation cost influences reproductive partitioning, and environmental gradient changes social factors and then influence copulations. We believe when studying cooperative breeding of biparental care species, we should focus on not only the differences within the same sex but also the sexual conflict and possibly mutual sexual selection. If we know better about how reproductive partitioning is influenced by both sexes, the whole picture of the evolution of social group will be clearer.