Why some male Mongolian gerbils may help at the nest: testosterone, asexuality and alloparenting

In previous studies we have shown that those male Mongolian gerbils, Merion es unguiculatus, that as fetuses resided in intrauterine positions (IUPs) l ocated between two female fetuses (2F males) have lower circulating levels of testosterone, less well-developed genital musculature, and lower reprodu ctive success than males gestated in IUPs between two male fetuses (2M male s). We have also found that such 2F males spend more time caring for conspe cific young than do 2M males, and that presence of a 2F male, but not of a 2M male, with a lactating female and her litter decreases the cost to a dam of suckling one litter while gestating a second litter conceived in postpa rtum oestrus. Here we show that some 2F males, those with circulating level s of testosterone similar to those seen in females of their species, show n o interest in females in oestrus and fail to impregnate females with which they are paired. Such 'asexual' 2F males spend 30-50% more time caring for nestlings than do sexually active 2F males. We suggest that such asexual, h ighly parental 2F males are incapable of direct reproduction and are obliga te helpers at the nest that can contribute to their own fitness only by ass isting to rear collateral kin. (C) 2000 The Association for the Study of An imal Behaviour.