Showing papers by "Jean Clobert published in 2005"

Sex ratio bias, male aggression, and population collapse in lizards

Abstract: The adult sex ratio (ASR) is a key parameter of the demography of human and other animal populations, yet the causes of variation in ASR, how individuals respond to this variation, and how their response feeds back into population dynamics remain poorly understood. A prevalent hypothesis is that ASR is regulated by intrasexual competition, which would cause more mortality or emigration in the sex of increasing frequency. Our experimental manipulation of populations of the common lizard (Lacerta vivipara) shows the opposite effect. Male mortality and emigration are not higher under male-biased ASR. Rather, an excess of adult males begets aggression toward adult females, whose survival and fecundity drop, along with their emigration rate. The ensuing prediction that adult male skew should be amplified and total population size should decline is supported by long-term data. Numerical projections show that this amplifying effect causes a major risk of population extinction. In general, such an "evolutionary trap" toward extinction threatens populations in which there is a substantial mating cost for females, and environmental changes or management practices skew the ASR toward males.

Age‐specific mating strategies and reproductive senescence

Abstract: While males gain obvious direct advantages from multiple mating, the reproductive capacity of females is more constrained. The reason why polyandry evolved in females is therefore open to many conjectures. One hypothesis postulates that females gain indirect benefits by increasing the probability of siring young from high quality males. To explore this hypothesis, we used the natural variation of the reproductive value that males and females undergo through age. The age-related variation of phenotypic performance might then induce variations in mating strategies in males and females. Using the common lizard (Lacerta vivipara) as our model system, we showed that reproductive immaturity and senescence created variability in both male and female reproductive success (including survival of offspring). Consistent with theory, males at their best-performing phenotype adopted a polygynous strategy. These males were of an intermediate age and they produced offspring of higher viability than younger and older males. In contrast, females at their best performing phenotype, also of an intermediate age, were less polyandrous than other less- performing females. Middle-aged females tended to mate with males of an intermediate age and produced litters with higher viability independently from their reproductive strategy. Males of an intermediate age enhanced their fitness by additional matings with young or old females. Young and old females increased their fitness by being more polyandrous. Polyandry therefore appears as means to seek for good males. A positive correlation between males and their partners’ fitness disagree with the idea that polyandry is the result of a sexual conflict in this species.

Plastic changes in seed dispersal along ecological succession: theoretical predictions from an evolutionary model

Abstract: Summary 1 We use a deterministic model to explore theoretically the ecological and evolutionary relevance of plastic changes in seed dispersal along ecological succession. Our model describes the effect of changing disturbance regime, age structure, density and interspecific competition as the habitat matures, enabling us to seek the evolutionarily stable reaction norm for seed dispersal rate as a function of time elapsed since population foundation. 2 Our model predicts that, in the context of ecological succession, selection should generally favour plastic strategies allowing plants to increase their dispersal rate with population age, contrary to previous predictions of models that have assumed genetically fixed dispersal strategies. 3 More complex patterns can evolve showing periods with high production of dispersing seeds separated by periods of intense local recruitment. These patterns are due to the interaction of individual senescence with change in ecological conditions within sites. 4 Evolution of plastic dispersal strategies affects the patterns of density variation with time since foundation and accelerates successional replacement. An interesting parallel can be drawn between the evolution of age-specific dispersal rates in successional systems and the evolution of senescence in age-structured populations. 5 Seed dispersal plasticity could be a potential mechanism for habitat selection in plants and have implications for range expansion in invasive species because recently founded populations at the advancing front may show different patterns to those in the established range.

Juvenile growth and survival under dietary restriction: are males and females equal?

Abstract: The effects of food availability on life-history traits may be direct or delayed and may vary between the sexes. We evaluated the effects of dietary restriction early in life on growth and survival of male and female juveniles in the common lizard (Lacerta viripara) and surveyed the literature on sex-specific sensitivity to the environment in vertebrates. Juvenile lizards were reared in the laboratory during one month following birth under full feeding or under dietary restriction. They were then released in two outdoor enclosures, where we compared growth and survival between treatments during one year. Low food availability early in life led to lower body growth in a directs but not delayed, manner. The absence of compensatory growth in juveniles that experienced dietary restriction might be explained by their reduced competitiveness. Dietary restriction had a strongly negative, delayed effect on survival up to the age of one year that was mediated by selection against smaller individuals. Effects of dietary restriction were not sex-specific, as expected from the similar energetic requirements of male and female juveniles. Hence, food availability has long-lasting consequences on life-history traits that might influence population dynamics in this species.

Female common lizards (Lacerta vivipara) do not adjust their sex-biased investment in relation to the adult sex ratio.

Abstract: Sex allocation theory predicts that facultative maternal investment in the rare sex should be favoured by natural selection when breeders experience predictable variation in adult sex ratios (ASRs). We found significant spatial and predictable interannual changes in local ASRs within a natural population of the common lizard where the mean ASR is female-biased, thus validating the key assumptions of adaptive sex ratio models. We tested for facultative maternal investment in the rare sex during and after an experimental perturbation of the ASR by creating populations with female-biased or male-biased ASR. Mothers did not adjust their clutch sex ratio during or after the ASR perturbation, but produced sons with a higher body condition in male-biased populations. However, this differential sex allocation did not result in growth or survival differences in offspring. Our results thus contradict the predictions of adaptive models and challenge the idea that facultative investment in the rare sex might be a mechanism regulating the population sex ratio.

Is sexual dimorphism affected by the combined action of prenatal stress and sex ratio

Abstract: During embryonic development, offspring are exposed to hormones of both maternal and sibling origin. Maternal stress increases offspring exposure to corticosterone, and, in polytocous animals, the sex ratio or intrauterine position can influence the levels of androgens and estrogens experienced by the offspring. Such hormone exposure has the potential to influence many important morphological and behavioural aspects of offspring, in particular sexually dimorphic traits. Although well known in rodents, the impact of prenatal hormone exposure in other vertebrates is poorly documented. We experimentally investigated the relationship between maternal stress, population density, sex ratio (a surrogate for the degree of exposure to steroids produced by siblings), and sexual dimorphism in a viviparous lizard, Lacerta vivipara. Our results show that prenatal sex ratios have consequences for sexually dimorphic morphology (ventral scale count) in both sexes, but with no effect of maternal stress or any interaction between the two. Embryonic steroid exposure can potentially be an important factor in generating individual variation in natural populations of viviparous animals.