Mongolian gerbils are territorial and live in family groups with established, stable male-female pairs. The objectives of this study were to investigate whether locomotor activity indicates female receptivity, and to evaluate the impact of a male on the stability of the female sexual cycle. To prevent gravidity, males were sterilised by vasectomy. Nevertheless, they behaved like intact males, displayed copulatory behaviour, and had normal mean serum testosterone levels of 1.1±0.2 ng/ml. Oestrus caused modulations in the activity pattern and hence influenced locomotor activity; female receptivity was reflected by an advance in the onset of activity. Male presence led to prolonged and irregular oestrus cycles in females. Three pairs were stable and nearly no attacks occurred for nine months. However, in four pairs, females suddenly attacked males. Males displayed flight and appeasement behaviours, but two died within 24 hours, and two were severely injured and had to be separated. Two of these females were paired again to other vasectomised males, but killed their cagemates after 3.5 and 12.5 weeks, respectively. In total, six of nine pairs were disrupted by female aggression, four males were killed and two males were separated but presumably would have otherwise died. Females presumably detected male infertility by repeated unsuccessful matings without sperm transmission and/or by physical impairments due to repeated pseudopregnancy. As a result they no longer invested in pair bonds that would eventually diminish their reproductive fitness.
Mongolian gerbils (Meriones unguiculatus) live territorially in families consisting of a reproducing founder pair and their non-reproducing young. Intra-family aggression occurs and is reported to be mainly caused by reproductive competition between females and the loss of reproducing founder animals. The current study investigated the impact of family traits (size, density and sex ratio) and aggressive inter-individual interactions on litters. Characteristics like pup mortality, litter size, sex ratio, and weekly body mass gain were tested. Across litters, significant correlations were found between litter size and family size (r = - 0.507, df = 25, p = 0.008) and between litter size and family density (r = - 0.404, df = 25, p = 0.01). Pup mortality was influenced by family size (r = 0.556, df = 25, p = 0.003) and by family density (r = 0.328, df = 25, p = 0.04). Unexpectedly, the influencing factor “occurrence of aggression” between adult family members or “expulsion of the mother” during lactation of the young had no influence on litters’ features. Family size and family density could be shown to be the most dominant parameters affecting the fate of the offspring and regulating the reproduction of the family.