INFANTICIDE IN THE BANK VOLE (CLETHRIONOMYS-GLAREOLUS) - OCCURRENCE AND THE EFFECT OF FAMILIARITY ON FEMALE INFANTICIDE

We studied infanticide in the bank vole (Clethrionomys glareolus), in a species whose social organization is characterized by strict female territoriality during the breeding season. One possible origin of fema le territoriality could be to protect the nest site and pups from pote ntially infanticidal conspecifics. However, direct evidence of the occ urrence of infanticide, and thus of its possible role in territorial b ehaviour of the bank vole females, is totally lacking. Observations in the laboratory, but also in the field and small enclosures yielded a proportion of 30% or more of infanticidal cases of both females and ma les intruding a strange nest. If an individual of either sex was infan ticidal it killed all the pups in the nest. In 36 trials in eight 25-m (2) enclosures we tested the occurrence of infanticidal behaviour in v oles from three female categories: mutually familiar females with neig hbouring ''territories'' and mutually unfamiliar ''stranger'' females either breeding or non-breeding. In ''stranger'' females the proportio n of infanticidal individuals was 25% in non-breeding and 42% in breed ing ones. Mutual familiarity between neighbours decreased the frequenc y of infanticide significantly to only 6%. In a control group of adult non-familiar males the proportion of infanticide individuals was 67%. Thus, both sexual selection (males) and resource competitions (female s) infanticide was observed for the first time in the bank vole or in general in Clethrionomys spp. Territoriality in the bank vole may func tion to deter potentially infanticide intruders from entering the nest ing area and killing pups. It might be that in a breeding system of ph ilopatric females, mutual familiarity between neighbouring females wou ld decrease the frequency of infanticide. This would be reflected in h igher survival of young in a breeding population of philopatric female s with a high degree of kinship and/or familiarity.