Litter Gender Composition, Food Availability, and Maternal Defence of the Young in House Mice (Mus Domesticus)

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Abstract
Litter gender composition (LGC) of 33 litters of outbred Swiss albino mice was manipulated at birth to obtain 100%-male litters (MM), 50%-male litters (MF) and 100%-female litters (FF). Litter size was 10 in all cases. Litter defence by their respective dams (5-min exposure to an unfamiliar adult male) was scored on postpartum day 7. The influence of litter gender composition on the female's litter defence was tested, with identical procedure, under conditions of restricted feeding. In this case, dams were fed from postpartum days 1-7 with 80% of their normal daily requirement. Under both ad lib. and restricted feeding body weight of mothers and their litters was assessed daily. Under ad lib. feeding, dams rearing MM litters showed higher scores of Total Attacking Time than both MF and FF dams and higher frequency of Attacks and Tail Rattling episodes than MF dams. Food-restricted females showed a sharp decline in body weight and their litters a slower gain in weight than control animals. LGC did not affect mother and litter weight changes in both conditions but MM females tended to cannibalize more pups on days 6-7 than MF and FF females. Food-restricted MM and MF dams showed, respectively, lower Total Attacking Time and higher number of Attacks and Tail Rattling episodes than their own Control groups. These results show that, under ad lib. feeding, LGC affects significantly female mouse litter defence. Some possible underlying mechanisms are discussed. These results support only partly the TRIVERS-WILLARD'S hypothesis (1973) about sex-biased investment in the offspring since, even though mouse dams under ad lib. feeding defended more vigorously all-male litters, as predicted by the model, among food-restricted animals no clear shift toward a prevalent defence of female-skewed litters appeared.

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