Costs and benefits of solitary living in the bush Karoo rat (Otomys unisulcatus)

Abstract

Solitary living has traditionally been regarded as the ancestral as well as most common and most primitive form of social organisation in mammals. However, recent comparative studies indicate that solitary living is not ancestral for all mammalian orders, and that solitary living is an adaptation to the environment. I show that solitary living occurs in only 22% of studied mammalian species and argue that the main benefit of solitary living is to avoid the costs of group-living. Group-living has been explained using socio-ecological models, and I used the same factors of resource distribution and predation risk to develop a socio-ecological model explaining solitary living. To reach a better understating of solitary living, I studied the social system of free-living bush Karoo rats (Otomys unisulcatus) in the Succulent Karoo semi-desert of South Africa. I used trapping and marking, focal animal observations and mini-GPS dataloggers that I fitted simultaneously to neighbouring females. I found that 96% of female bush Karoo rats were solitary living, with social groups of two or three individuals occurring occasionally. Groups always consisted of close kin, typically females. The home ranges of kin neighbours overlapped more than those of non-kin. Neighbours were, however, attracted to the same foraging grounds, irrespective of relatedness. Females tolerated one another at shared foraging grounds, which could be interpreted as byproduct mutualism, a simple form of cooperation. I recorded interactions between neighbouring bush Karoo rats both in a neutral test arena and in the field to investigate whether solitary living was due to aggression and social intolerance. Social interactions between neighbours were rare and aggression was rare in neutral arena tests. However, female bush Karoo rats were more aggressive towards non-kin intruders in the field tests. Finally, the relationship between mother and offspring remained amicable even after the offspring had dispersed from the lodges, indicating that maternal aggression was not the mechanism that led to offspring dispersal and solitary living. In conclusion, I showed that solitary living is not always characterised by aggression and avoidance, but rather that solitary species can have non-random and individualised social interactions that are influenced by kinship.