How burying beetles spread their seed: The Coolidge effect in real life

The Coolidge effect is a well-known phenomenon in the behavioral sciences. It was first observed in several different mammals and refers to the decline in the sexual interest of males over the course of repeated encounters with the same female, coupled with renewed interest in a novel female introduced when the male no longer shows any desire to mate with the original female. Among a handful of other invertebrates, this effect has also been described for burying beetles (Nicrophorus vespilloides Herbst) based on lab observations of males and females in small containers without access to carrion. In the field, the only repeat encounters between males and females occur on carcasses, which can be utilized as food or buried for reproduction. In the present study, we placed dead mice in the field to investigate how often natural breeding groups include males and several females. We found that many breeding groups in the field satisfy this condition necessary for a Coolidge effect. In addition, we used direct observations of undisturbed breeding assemblages in the lab to assess whether males really exhibit the Coolidge effect in a more natural context, when they are engaged in burying and preparing a carcass. Since males encounter dominant and subordinate females at different rates, we compared how often successive encounters and matings occurred with the same female, which revealed that males in these undisturbed breeding groups actively avoid mating with the same female twice in succession. This shows that the Coolidge effect is not a laboratory artefact, but is part of the natural repertoire of behaviors in male burying beetles.