BEHAVIOURAL STRATEGIES, TESTIS SIZE, AND REPRODUCTIVE SUCCESS IN TWO CAVIOMORPH RODENTS WITH DIFFERENT MATING SYSTEMS

We summarize our studies on the social and mating systems of Cavia aperea and Galea musteloides, two closely related South-American rodents. In Cavia an extremely high incompatibility exists among adult males. As a consequence, only a single male can be kept together with several females even in richly structured enclosures of 20 m^2. From this, a polygynous mating system emerges. In contrast, under similar housing conditions male Galea are much more tolerant and large groups can be established consisting of several adult males and several adult females. The mating system of Galea is promiscuous because of the femaleʹs soliciting behaviour when receptive that makes it impossible for a single male to monopolize her. The diverging mating systems correspond well with functional variations in testis size and sexual dimorphism: the polygynous Cavia show low testis masses (weight of both testes = 0.58% of body weight) and body weights are 11% higher in males than in nonpregnant females. The promiscuous Galea have extremely high relative testis masses (1.86% of body weight) and non-pregnant females are 15% heavier than males. In the latter species promiscuous mating results in a high percentage of multiple paternities (> 80% in groups of 4 males and 6-7 females) as revealed by multi-locus DNA fingerprinting. Nevertheless dominant males achieve a significantly higher reproductive success than subordinates. The high frequency of overt aggression directed from dominant to subordinate males, therefore, may be a mechanism to lower the fertilizing capacity of the lower ranking males. Concerning than females which were paired with a single male. Thus, for the first time a reproductive benefit from promiscuous mating is shown for a female mammal. Field studies in the natural habitats of Cavia aperea and Galea musteloides are now performed to elucidate whether the differences in social and mating systems can be related to differences in ecological conditions.