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 a nd Galea musteloides, two closely related South-American rodents. In Cavia an extremely high incompatibility exists among adult males. As a consequenc e, only a single male can be kept together with several females even in ric hly 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 ad ult males and several adult females. The mating system of Galea is promiscu ous because of the female's soliciting behaviour when receptive that makes it impossible for a single male to monopolize her. The diverging mating sys tems correspond well with functional variations in testis size and sexual d imorphism: the polygynous Cavia show low testis masses (weight of both test es = 0.58% of body weight) and body weights are 11% higher in males than in nonpregnant females. The promiscuous Galea have extremely high relative te stis 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 per centage 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 m ales, therefore, may be a mechanism to lower the fertilizing capacity of th e lower ranking males. Concerning the females' reproductive success we demo nstrated in a mating experiment that Galea which were paired with four male s and became pregnant, weaned significantly more offspring than females whi ch 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 perf ormed to elucidate whether the differences in social and mating systems can be related to differences in ecological conditions.