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Sex ratio alterations related to environmental factors occur in several mammals, but no mechanism has been identified to explain the adjustment. Intrauterine position (IUP) may provide the context in which such alterations occur. Previous studies on house mice and gerbils reveal that the position of a fetus in the uterus in relation to the sex of its neighbors influences its later anatomy, physiology, and behavior. The anogenital distance (AGD) of females located between two males (2M) is longer than that of females not between two males (OM). We have found that the IUP, as determined by cesarean section and by an index of the AGD, correlates with the sex ratio of the litters produced by female mice. The sex ratio of the first Litter born to 2M females was 58% males, for iM females was 51% males and for OM females was 42% males. The effect on sex ratio continues into the second litter. The number of pups produced by mothers of different HUPs in her first two litters did not differ, suggesting that the sex ratio adjustment occurs prior to parturition. These results provide a basis for the natural variability observed in sex ratios of litter-bearing ammals, and suggest that one or more intrauterine mechanisms may be responsible for environmentally related sex ratio alterations.Parental reproductive fitness would increase if the sex ratio of their offspring could be adjusted to relate to environmental conditions. Trivers and Willard (1) hypothesized that under favorable environmental conditions males would be favored and that under adverse conditions females would be favored in polygynous species because a few strong males could sire many grandchildren, whereas females have a more sure but limited reproductive potential.A local resource competition model has been proposed (2, 3) to explain sex ratio alterations specifically for those species in which females show fidelity to their natal area and males leave the natal area to breed elsewhere. In this model, daughters of high-ranking mothers may achieve high reproductive success through inheriting the mother's high rank. However, since males breed in different groups, their reproductive success is presumably not influenced by the mother's rank. Thus, the local resource competition model predicts that, for certain species, high-ranking females would produce a female-biased sex ratio (3).Among invertebrates, breeding adults can adjust the sex ratio oftheir offspring to maximize their fitness (4). Data from mammals have been more controversial. Clutton-Brock and Iason (5) review a number of studies conforming to a high standard of evidence and conclude that significant variation in sex ratios at birth in nonhuman mammals exists and that sex ratio varies with the costs or benefits of producing male or female offspring. Investigators have also shown that manipulating food can influence sex ratios at birth. Experimentally supplementing the food supply of wild opossums (Didelphis marsupialis) results in male-biased sex ratios (6), and restricting the food available ...

No abstract

We quantified whether local populations of early juvenile blue crabs (J1-2) could be enhanced through the translocation of crabs to underutilized nursery habitats, and if enhancement success, survival, and potential impacts of stocked crabs on their benthic prey varied in a density-dependent manner. Using plankton nets, ∼143,000 blue crab megalopae were collected as they ingressed into Pamlico Sound, NC. Of these, ∼13,800 early juvenile blue crabs (J1-2 stages) were then stocked at potential nursery sites relatively far removed (32-70 km) from their initial settlement areas using a replicated before-after control impact (BACI) experimental design. On average, there was negative enhancement success (−34%) five weeks after local crab enhancement, and no evidence of density-dependent enhancement success, mortality, or impact on potential crab prey. Poor stocking success was likely due to pelagic emigration from enhancement sites relative to controls. Attempts to assess the feasibility of stocking blue crabs at local scales of small coves should (i) probably not consider J1-2 stages because of their apparent propensity to emigrate from these areas, or (ii) further assess the effects of geomorphology and wind fetch of release sites on density-dependent emigration.