Ovarian growth and folliculogenesis in breeding and nonbreeding females of a social rodent, the Zambian common mole-rat, Cryptomys sp.

Abstract

Zambian common mole-rats are subterranean rodents that live in families with only one female breeding. Her offspring remain in the parental nest and do not reproduce. Behavioral experiments (Burda, '95) demonstrated that their apparent "sterility" is based on incest avoidance and individual recognition of family members. To elucidate whether some kind of morphologically apparent ovarian suppression still takes place in daughters, ovaries of females of known age, weight, and reproductive histories were examined histologically and morphometrically. The body mass of old females (more than 3 years of age) begins to decrease, and the ovaries seem to begin to atrophy at the age of about 3-6 years. Ovaries in neonates exhibited primordial and primary follicles, sometimes clustered in nests. Ovaries of adult nonbreeding females expressed all stages of the follicular development up to tertiary follicles. Many unruptured luteinized follicles were present, but true corpora lutea as a morphological sign of ovulation were missing. Unruptured luteinized follicles also could be found (additionally to true corpora lutea) in ovaries of breeding females. The number of primordial follicles dropped rapidly during the first 2 years of age; the number of primary, secondary, and tertiary follicles was subject to individual variation; and there was no clear correlation with age or reproductive status. While a tendency to form accessory unruptured luteinized follicles may just reflect taxonomic affinities of bathyergids to hystricomorphs, the otherwise complete folliculogenesis in "sterile" daughters and the presence of unruptured luteinized follicles even in breeding females are further evidence that there is no hormonal suppression of the ovarial cycle. We suggest that ovulation in nonbreeding females is not actively suppressed by the breeding female, but instead that it is not released because the triggering mechanisms, most probably repeated copulation, are missing.