Advanced Search Abstract Both population density and sex ratio shape competition for mates, resources and mating costs. Thus they may critically affect the intensity of sexual selection in the populations. Susceptibility to inter- and intrasexual competition, which changes with age in a large number of species, may additionally influence population response to these demographic factors.
In this study, we monitored 16 seminatural populations of common lizards Lacerta vivipara to determine whether the reproductive output varied with male and female densities as a function of the individual sex and age. Our results suggest that the intensity of sexual selection was weaker in male-biased populations, supporting new theoretical models. In populations with a male-biased sex ratio, reproductive success was more equally distributed between males and, unlike female-biased populations, the choosiest females middle-aged did not obtain sires of higher quality than low-performance females.
Our results also suggest that age may influence the intensity of sexual conflict. Middle-aged females the class with the best performance produced offspring with a lower body condition in male-biased populations, suggesting that they may be the preferred target of male harassment. By contrast, a male-biased sex ratio appeared to be beneficial for low-quality females, allowing these females to obtain higher quality sires and to produce offspring with a better body condition.
These age- and sex-dependent responses to population density and sex ratio have important implications for population ecology and sexual selection. Population density and sex ratio are important parameters shaping inter- and intrasexual competition Emlen and Oring ; Kokko and Rankin because they establish the rate at which individuals encounter competitors or potential mates.
Increased density of one or both sexes is expected to increase intraspecific competition for resources and to modify other fitness-related factors such as parasitic prevalence or predation Begon et al. Temporal and spatial fluctuations in demographic parameters that drive changes in competition for access to mates and resources are thus expected to affect the fitness of both males and females Kasumovic et al. The consequences of demographic parameters on sexual selection are, however, expected to vary with population-specific mating system Emlen and Oring ; Kokko and Rankin Contrasting results indeed show that demographic effects on competition and the ensuing reproductive success are complex.
On one hand, male biased sex ratio and high density may lead to increase male—male competition and choosiness of females Gwynne ; Grant et al. This pattern would be due to an increased variance in male quality when the density of males is higher, giving females more opportunities of mate choice at lower mate searching cost Owens and Thompson Moreover, successful males would control the access to mates of competitors by defending or monopolizing females.
Conversely, when sex ratio is female-biased or density is lower, females would be less choosy because of a low male encounter rate. On the other hand, new theoretical works have proposed opposite predictions for demographic effects on competition for mates Kokko and Rankin If females suffer from increased harassment for copulation in male-biased or high male density populations Clutton-Brock and Parker ; Stockley , the cost of choosiness for females may increase.
Females may become more likely to accept copulations indiscriminately to avoid harassment. Some studies have indeed confirmed that females become less choosy with increased density and male-biased sex ratio Rowe ; Lauer et al. Additionally, one can expect variation in the effects of sex ratio and density on competition when there are changes in the relative competitive abilities of the individuals in the populations.
In particular, age-dependent sensitivity to demographic parameters may affect population dynamics, thereby influencing the response of populations to natural and sexual selection Pfister In a large number of animal species, individual performance is known to follow age-dependent patterns Martin ; Gaillard et al.
Typically, survival and reproductive success increase with age in younger individuals, subsequently decreasing as individuals get older a phenomenon called senescence; Comfort Long-term studies have shown, for instance, that the survival of females of the prime age group shows little or no dependence on population density in ungulates, whereas old and young females are more sensitive to density pressures review in Gaillard et al.
Additionally, in years of food scarcity, only young Tengmalm's owls Aegolius funereus fail to breed, whereas no such differences between age classes are observed when food is abundant Laaksonen et al.
The costs associated with mate choice, including the costs of searching for a mate, mate sampling, and resistance to sexual harassment, are also expected to vary with individual age. Here, we aimed to investigate the effects of population density and sex ratio on the intensity of competition within and between sexes and age classes. We analyzed observational data from 16 seminatural populations of common lizards, Lacerta vivipara, with varying male and female densities.
This data set provided us with a rare opportunity to study the influence of both population density and sex ratio on the reproductive output of males and females Rankin and Kokko Moreover, as individual performance is age-dependent in this species Ronce et al. The common lizard is a promiscuous species Laloi et al. Males compete aggressively for females and may harass females to force them to copulate Heulin We recorded individual mating and reproductive success, together with offspring quality.
We further assessed the sensitivity of the different age and sex classes to a range of population densities and sex ratios.
Lastly, we investigated in both sexes whether 1 reproductive success varied with density and sex ratio in different ways as a function of individual age and whether 2 the intersexual variance of reproductive success changed with density and sex ratio. According to our knowledge of the mating patterns in this species and the age susceptibility to interindividual competition, we can make several predictions.
First, female reproductive success has already been shown to decrease in natural populations, where density has been increased experimentally Massot et al. Here, we investigated whether in high-density populations the variance in female reproductive success was higher, due to a smaller number of females being able to reproduce successfully, the others being adversely affected by the interindividual competition.
In high-density populations, we predict that the intense competition between individuals should most strongly affect the reproductive success of lower quality individuals the youngest and oldest.
Second, male-biased sex ratio is likely to increase male—male competition for mates and sexual harassment of females. This decrease in female reproductive success should depend on female susceptibility to harassment pressure and male mating preference. If the males harass all females, the youngest and oldest females would again be the most strongly affected by an increase in the proportion of males. Conversely, if males preferentially harass middle-aged females, which are the most fertile Richard et al.
Finally, it has been suggested that females of different ages benefit differently from multiple mating and therefore have different optimal mating strategies. Specifically, middle-aged females tend to be less polyandrous than younger or older females of lower quality. This suggests that they may either be more choosy and able to obtain high-quality mates or better able to resist to male harassment Richard et al. By contrast, lower quality females the youngest and oldest are usually impregnated by a larger number of males than middle-aged females, and their reproductive success increases with the number of sires Richard et al.
Low-performance females may benefit from a larger number of mating events because 1 it increases their chances of having their eggs fertilized by at least one high-quality mate Promislow et al. We therefore predict that higher male density should favor low-quality females as these females would potentially increase their number of mates. Individuals start hibernating in late September. In our study area, individuals can reproduce once per year from the age of one year.
Maximum female and male life span in natural populations are 11 and 7 years, respectively. Clutch size ranges from 1 to 12 eggs, depending partly on body size Boudjemadi et al. Hatchlings are independent at birth with no parental care after birth Massot et al. No nuptial gifts are provided Heulin , and sperm has little effect on the nutrition of the young Depeiges et al.
Age affects individual performance by influencing current reproductive value and survival Ronce et al. The fecundity of both sexes and the probability of survival for their offspring also increase until the age of 4 years and decrease thereafter. In females, annual fecundity is highest in 3- and 4-year-old individuals, whereas the 2- and 3-year-old individuals have the highest survival rates Ronce et al. Middle-aged individuals between the ages of 2 and 4 years thus have the highest level of performance, whereas yearlings and old individuals 5 years and older perform less well.
As these individuals were part of a long-term study, all were individually marked by toe clipping and the year of birth was known for most individuals. Previous studies have shown that confined populations have similar life-history traits and mating patterns to natural populations in terms of age at first reproduction, clutch size, and proportion of multiply sired clutches Boudjemadi et al.
In June , we captured individuals in holding enclosures. These individuals were then released to create 16 seminatural populations containing a mean of As home ranges overlap to a great extent in this species, 30 adult individuals can share an area of similar size in natural conditions Massot et al.
Thus, the densities of adults created in our experiment were similar to those observed in natural populations Lecomte and Clobert The sex ratio of the created populations adult sex ratio [ASR], the proportion of adults that were male was biased toward females mean: This is generally the case in natural populations, although substantial spatial and temporal variations are observed from 0. In early June , all the surviving lizards were captured. Due to demographic stochasticity, survival rates from release to capture differed between populations, creating a continuous distribution of population density and sex ratio Table 1.
This distribution provided us with an opportunity to analyze the correlations between both population density and sex ratio with reproductive output. Population density and sex ratio in May were thus used as independent continuous covariates in the analyses. During the breeding period studied, in May , populations contained females and males.
At the first capture of June , we recorded body length SVL and body mass for all individuals. An index of body condition was calculated for males and females as the residual of the regression between body mass and body length. Table 1 Characteristics of the populations studied in May Enclosure Density total number of individuals ASR proportion of males among the adults 1.