Ontogeny of agonistic behaviour in dispersing blind mole rats (spalax ehrenbergi)

Abstract

The blind mole rat (Spalax ehrenbergi) is a solitary and aggressive fossorial rodent that inhabits its own individual tunnel system. Mole rats breed mainly in the winter, giving birth to a litter of three to four young on average [Nevo (1961) Mammalia 25:127–143]. Intraspecific interactions between adults occur mostly during the winter, which is the rainy season in Israel; after parturition, interactions occur between the mother and her offspring and between the siblings themselves until dispersal. Until now, the subterranean habitat of mole rats has prevented direct observation in the wild. Thus, there has been no direct evidence for the proximal factors that drive juvenile mole rats out of the maternal tunnel system as they shift from the social life-style of the young to the solitary life-style of the adult. In the present study, female mole rats were caught in the wild with their offspring and brought to the laboratory. The dispersal process of the young from the maternal nest was followed under two housing conditions: (1) restricted housing, simulating the physical conditions under which dispersal is delayed past the usual age, as often occurs during late winter floods in the field in areas of nonporous clay soils, and (2) nonrestricted housing, simulating the physical conditions that allow the young to disperse from their maternal tunnels at any time. In each housing condition, the mothers raised either one or three pups. During pup development, we monitored both maternal and juvenile agonistic variables during intraspecific interactions. Five major behavioural phases were identified during the ontogeny of the mole rat pups. Maternal aggression toward pups increased mainly in the first three phases of pup development, and sibling aggression increased from the third phase until dispersal. Under the restricted housing conditions, there was a delay in both the appearance of aggression in the “family group” as well as in dispersal age compared with the nonrestricted conditions. In the nonrestricted conditions, increased litter size resulted in delay in dispersal age. We suggest that the main proximate factor initiating natal dispersal is increased intolerance and aggression among siblings rather than maternal aggression. Only after the young began dispersing from the maternal tunnels did maternal aggression dramatically increase, thus preventing the offspring from returning. However, following floods, inhibition of aggression may enable the mole rat pups to stay with the family group and to disperse later when the physical conditions permit it. Aggr. Behav. 24:455–470, 1998. © 1998 Wiley-Liss, Inc.