Table 1 Number of litters born to juveniles, yearlings, and adults available for each year hunting season and for each age class Year hunting season. Free-ranging populations of wild boar, red deer Cervus elaphus , and roe deer Capreolus capreolus are present in the forest. In this example, the relative cost of producing a male M compared with a female F is 1. High-quality mothers should provide greater investment to one sex than the other. Advanced Search Abstract For species in which reproductive success is more variable in one sex than the other, the Trivers and Willard model TWM predicts that females are able to adjust their offspring sex ratio. Each year between October and February, wild boars are intensively hunted during weekends using drives. Age was assessed by tooth eruption and replacement patterns Matschke ; Baubet et al. According to the WM, we expected to find a positive relationship between the sequence of increasing production costs and relative maternal condition. A national forest of ha constitutes the core of the forest, whereas the periphery ha is private or communal.
Figure 1 View large Download slide Relationship between maternal body condition and the specific sequence of increasing production costs expected under the WM. Under the WM, we expected a positive relationship between a specific sequence of increasing production costs based on size and sex composition of the litters and maternal body condition. In our population, the generation time was around 2 years due to high hunting pressure Gaillard et al. Williams generalized the TWM to polytocous species by including changes in litter size in the model. According to the extrinsic modification hypothesis, we expected to find an increasing proportion of males with increasing food resources. Whereas our knowledge of the physiological mechanisms involved in sex ratio variation has recently improved Krackow ; James ; Cameron , most evolutionary models are intended to assess the pattern rather than the process of variation in sex ratio for a recent review, see Sheldon and West In this example, the relative cost of producing a male M compared with a female F is 1. We were able to sex fetuses visually without ambiguity from a length of 40—45 mm i. According to the WM, we expected to find a positive relationship between the sequence of increasing production costs and relative maternal condition. For instance, given a maximal litter size of 3 and given that producing a male M costs 1. The fetal sex ratio was negatively related to increasing litter size, providing some support for the TWM. In , roe deer, red deer, and wild boar were hunted in the national part of the forest. Sex ratio was male biased for litter sizes up to 6 and then became female biased in larger litters. In our study site, mast production was assessed indirectly by the analysis of stomach contents for a similar approach, see Bieber and Ruf Previous tests of the TWM have been inconsistent, and whether the TWM applies to species with several offspring per litter is unclear due to possible trade-offs between size, number, and sex of the offspring. High-quality mothers should provide greater investment to one sex than the other. Advanced Search Abstract For species in which reproductive success is more variable in one sex than the other, the Trivers and Willard model TWM predicts that females are able to adjust their offspring sex ratio. Hence, there was no bias in the identification of sex composition within a litter in relation to the size of the litter. According to the TWM, high-quality mothers should provide a greater investment in sons than in daughters when males have a greater variance in individual fitness than females, providing that 1 sons benefit more than daughters from extra investment by mothers, 2 offspring quality is a good proxy of adult quality, and 3 offspring quality is related to maternal quality. Skewed sex ratios have often been reported, but whether variation of offspring sex ratio is adaptive in vertebrate populations is still controversial for reviews, Clutton-Brock and Iason ; Frank ; Hewison and Gaillard ; West and Sheldon Wild boars were classified into 3 age classes Pedone et al. Likewise, changes in resource availability did not influence the fetal sex ratio, so that the extrinsic modification hypothesis was not supported. Finally, we also tested whether changes in litter size influenced sex ratio variation. Polygynous and sexually dimorphic species are good candidates for the TWM, and among them, ungulates have been especially well studied for reviews, see Hewison and Gaillard ; Sheldon and West However, results are often inconsistent because of 1 the strict conditions that must be fulfilled for the TWM to apply for recent case studies, see Leimar ; Blanchard et al. Using data collected on the highly polytocous wild boar Sus scrofa scrofa; mean litter size of 4.
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