胎仔数与父本缺失对棕色田鼠幼仔体重发育的影响

室内饲养条件下,测量了出生后不同发育阶段棕色田鼠Microtus mandarinus的体重,并探讨了胎仔数(分别为1只、2只、3只和4只)及父本缺失(母鼠单独抚育)对幼仔体重发育的影响.结果表明,棕色田鼠出生后第一周体重的瞬时生长率最高,此后逐步下降;出生后28～35 d,体重增益最大(P<0.05).出生后7 d,不同胎仔数的体重没有明显差异,但至断乳时(出生后21 d),胎仔数为2只的幼仔平均体重明显高于其它胎仔数的平均体重(P<0.05).与双亲抚育相比,母鼠单独抚育的幼仔在断乳时的体重明显下降(P<0.05).这些结果说明,哺乳期内棕色田鼠幼仔的体重发育与胎仔数有关.由于棕色田鼠是单配制,双亲育幼,父本雄鼠的缺失减少了亲本投资,影响了幼仔的身体发育.     

Nest size affects clutch size and the start of incubation in magpies: an experimental study

Nest size has been suggested to be a sexually selected traitindicating parental ability of both males and females. To testwhether a female's reproductive decisions (e.g., clutch sizeand starting incubation) change in relation to experimentalmanipulation of nest size, as would be predicted if nest sizeis a sexually selected signal reflecting the male's parental quality, we manipulated nest size in a population of monogamousmagpies before laying by adding or removing about 20 cm oflarge sticks in the roof of magpie nests. On the one hand,we found that clutch size of reduced nests was smaller thanthat of control or enlarged nests. Moreover, clutch size was significantly related to nest size after manipulation, whichindicates that females adjust clutch size to the final sizeof the nest, nest size thereby being a good candidate for asexually selected trait. On the other hand, number of eggshatched during the first day is hypothesized to be related to the expected available resources during nestling growth, andsubsequent nestlings hatched are likely to die due to broodreduction if resources are not sufficient to raise well-developednestlings. Nest size is hypothesized to inform females abouta male's willingness to invest in reproduction, and we foundthat in broods of experimentally reduced nests, females startedto incubate earlier in the laying sequence than they did inbroods of control or enlarged nests. Moreover, in experimentallyreduced nests, fewer nestlings hatched during the first day,and the difference in body mass between the first and the fourthnestling hatched increased. This result is in accordance withthe hypothesis that the female's decision of when to start incubationin the laying sequence is mediated by nest size, a sexuallyselected trait signaling parental quality. We discuss alternativeexplanations for the results such as the possibility that nestsof different treatments may differ in their thermoregulationproperties or in their protection against predators.     

The 'big spenders' of the steppe: sex-specific maternal allocation and twinning in the saiga antelope

In polygynous mammals, males generally benefit more from extra allocation of maternal resources than females. However, limitations to sex-specific allocation are usually ignored. We propose the 'allocation constraint' hypothesis, whereby maternal resource allocation is more likely to follow life-history predictions in single sex litters than in mixed sex litters, due to limitations in prenatal resource targeting. Consequently, for polygynous species, males in mixed litters are likely to receive suboptimal maternal investment, which may have a negative effect on lifetime reproductive success. We test this hypothesis for the saiga antelope (Saiga tatarica), a highly polygynous species with the highest level of maternal allocation reported among ungulates. At such high reproductive output levels, the limitations on additional investment in males are likely to be particularly acute. However, we demonstrate high levels of sexual dimorphism in both late-stage foetuses and newborn calves, including within the same litter. Male twins with a brother tended to be heavier than those with a sister. This may be due to allocation constraints or differences in maternal quality. We conclude that an explicit focus on potential constraints can enhance the progress in the field of sex-specific maternal allocation in polytocous species.