Sexually selected traits are limited by selection against those traits in other fitness components, such as survival. Thus, sexual selection favouring large size in males should be balanced by higher mortality of larger males. However, evidence from red-winged blackbirds (Agelaius phoeniceus) indicates that large males survive better than small males. A survival advantage to large size could result from males migrating north in early spring, when harsh weather favours large size for energetic reasons. From this hypothesis we predicted that, among species, sex differences in body size should be correlated with sex differences in timing of spring migration. The earlier males migrate relative to females, the larger they should be relative to females. We tested this prediction using a comparative analysis of data collected from 30 species of passerine birds captured on migration. After controlling for social mating system, we found that sexual size dimorphism and difference in arrival dates of males and females were significantly positively correlated. This result is consistent with the hypothesis that selection for survival ability promotes sexual size dimorphism (SSD), rather than opposes SSD as is the conventional view. If both natural selection and sexual selection favour large adult males, then limits to male size must be imposed before males become adults.

Summary 1.Understanding the trade-off females make between offspring size and number requires knowing how neonatal size, and traits associated with size, affect survival. 2. We studied neonatal survival in the northern watersnake Nerodia sipedon in outdoor enclosures with artificial hibernation sites. 3. From a total of 950 neonates from 77 litters collected over 3 years, we found a survival rate of 65% between birth and hibernation and 47% during hibernation. Estimated survival from birth to the end of hibernation was 31%, comparable with indirect estimates for free-living watersnakes. 4. Consistent with the 'bigger is better' hypothesis, larger neonates and neonates heavier relative to their body length were more likely to survive both the pre-hibernation and hibernation periods. 5. Survival in the pre-hibernation period also decreased with the duration of that period and varied among years. 6. Survival during hibernation was higher in warmer winters. Mass change prior to hibernation did not affect survival during hibernation. 7. These results suggest that an optimal reproductive strategy should exist for female watersnakes, producing a 'consensus' among females on the optimal size for offspring. This expectation stands in stark contrast to the pronounced variation in offspring size observed both within and among litters.

No abstract

No abstract

We used data from 88 litters of northern water snakes (Nerodia sipedon) to test predictions about how mothers would adaptively vary the sex ratios of their offspring. Larger mothers produced significantly more daughters (r 2 = 0.04, P = 0.05), and mothers producing larger offspring produced significantly more daughters (r 2 = 0.06, P = 0.02). Because neonate size did not vary with maternal size, these sex-ratio patterns were independent of each other. These patterns were more pronounced for wild females than for females maintained in captivity while gravid, but rearing conditions did not have a significant effect on sex ratio. Also, because sex ratios were similar between captive and free-living females despite captive females giving birth 16 days earlier, on average, and because sex ratios did not vary with birth date within the two groups of females, gestation appeared not to affect sex ratio. If females vary sex ratios adaptively, only the relationship between sex ratio and neonate size was consistent with our predictions. Limited evidence from other snake species also indicates variation in neonatal sex ratios that is nonrandom but not necessarily adaptive. A better understanding of these patterns will require information on the factors that affect the fitness of male and female neonates differently. An unexpected sex-ratio pattern that we found was that 14 of 19 stillborn young were male. We speculate that this pattern could be a result of male embryonic sensitivity to temperature. Thus, the need for gravid females to maintain a high body temperature so that their young are born with enough time to find hibernation sites may conflict with the need for embryos to develop at a safe temperature.Résumé : Nous avons utilisé des données sur 88 portées de Couleuvres d'eau (Nerodia sipedon) pour vérifier les prédictions sur la façon dont les mères doivent contrôler le rapport mâles:femelles de leur progéniture de façon adaptative. Les mères de plus grande taille produisent significativement plus de femelles (r 2 = 0,04, P = 0,05) et les mères qui produisent le plus de rejetons donnent naissance à significativement plus de femelles (r 2 = 0,06, P = 0,02). Comme la taille des couleuvres néonates ne dépend pas de la taille de la mère, les patterns de ces rapports mâles : femelles sont indépendants les uns des autres. Ces patterns sont plus accentués chez les femelles sauvages que chez les femelles gardées en captivité pendant leur grossesse, mais les conditions d'élevage n'affectent pas significativement le rapport mâles : femelles. De plus, comme les rapports sont similaires chez les femelles gardées en captivité et chez les femelles libres, même si les femelles en captivité mettent bas 16 jours plus tôt en moyenne, et comme les rapports ne varient pas en fonction de la date de naissance chez l'un ou l'autre de groupe de femelles, la gestation ne semble pas affecter le rapport mâles:femelles. Si les femelles contrôlent les rapports mâles:femelles de leur progéniture de façon adaptative, seule la relation entre le rapp...