Different types of oestrous cycle in two closely related South American rodents (Cavia aperea and Galea musteloides) with different social and matingsystems

A comparative approach was used to investigate two closely related South Am erican rodent species: the wild cavy (Cavia aperca) and the yellow-toothed cavy (Galea mosteloides). These species of wild guinea-pig inhabit differen t habitats and show divergent social and mating systems. Cavia have a polyg ynous mating system, whereas in Calea promiscuous mating occurs. These obse rvations correspond to functional variations in sperm characteristics, whic h might be interpreted as adaptations to different reproductive patterns in the females of both species. Twenty-two female C. aperea and G. musteloide s (11 of each species) were investigated to elucidate differences in the ch aracteristics of the oestrous cycles. The experimental design consisted of three stages, in which the degree of contact between the sexes was successi vely changed. During these stages, the reproductive cycle of the females wa s monitored by examining the condition of the vaginal closure membrane, ana lysing vaginal smears and determining serum concentrations of oestrogens an d progesterone. The results revealed different types of oestrous cycle in t he two species. Female C. aperea showed periodical cycles with spontaneousl y occurring oestrous periods, ovulation and corpus luteum activity. In cont rast, in female G. musteloides, oestrus was exclusively induced by the pres ence of a male. However, after the induction of oestrus, ovulation and corp us luteum activity occurred spontaneously without stimuli from copulation. To date this type of oestrous cycle has not been described for any other ma mmalian species. Thus, these data indicate that differences in the reproduc tive patterns of the females may have shaped the evolution of different spe rm characteristics in the males.