Conditional allogrooming in the herb-field mouse

Among members of the family Muridae, the herb-field mouse, Apodemus microps, is unique in that aggression is almost entirely lacking. This species, therefore, is a model organism for experimental studies of social behavior without the confounding influence of aggression. We used video surveillance cameras to assess the importance of self-grooming and allogrooming in the social life of this species. Detailed analysis of individual behavioral sequences using Markov chain methods revealed that selfgrooming is a relatively stereotypic, sex-independent activity usually lasting about 8 s. Allogrooming is conditional in the herbfield mouse, because it takes the form of a reciprocal strategy, with the differences between nonmatching bouts varying according to whether the initiator of allogrooming is male or female and whether both interactants are of the same or opposite sex. Our analysis revealed that the exchange of allogrooming bouts between individuals of the same sex is reciprocal, but that males allow females to ‘‘defect’’ more often than vice versa, and males groomed females for longer than predicted by the distribution of individual self-grooming bouts. In those species where the demand for mating by males is far greater than that offered by females, in other words where females may select mates, asymmetry of allogrooming may provide a mechanism for females to test the ‘‘suitability’’ of males for mating. It may also provide the means for males to stimulate females before mating. Furthermore, allogrooming was the only sex-dependent behavior of the several tested in our experiment. As such, we suggest that allogrooming is the predominant premating mechanism in this species.