Stud males and dud males: intra-uterine position effects on the reproductive success of male gerbils

Sexually mature male Mongolian gerbils, Meriones unguiculatus, from known intra-uterine positions were each paired with a succession of nulliparous females. Those males that had matured in intra-uterine positions adjacent to two males (2M males) sired an average of 28% more young than did those males that had matured in intra-uterine positions between two females (2F males). In a second experiment, female gerbils chose between pairs consisting of one adult 2F male gerbil and one adult 2M male gerbil. When in induced oestrus (but not when in other hormonal states), the females spent significantly more time near 2M than near 2F males. Female gerbils in oestrus thus tended to affiliate with the more reproductively capable male in their vicinity.