Early determinants of lifetime reproductive success differ between the sexes in red deer.

In polygynous, sexually dimorphic species, sexual selection should be stronger in males than in females. Although this prediction extends to the effects of early development on fitness, few studies have documented early determinants of lifetime reproductive success in a natural mammal population. In this paper, we describe factors affecting the reproductive success of male and female red deer (Cervus elaphus) on the island of Rum, Scotland. Birthweight was a significant determinant of total lifetime reproductive success in males, with heavier-born males being more successful than lighter ones. In contrast, birthweight did not affect female reproductive success. High population density and cold spring temperatures in the year of birth decreased several components of fitness in females, but did not affect the breeding success of males. The results confirm the prediction that selection on a sexually dimorphic trait should be greater in males than in females, and explain the differential maternal expenditure between sons and daughters observed in red deer. Differences between the sexes in the effects of environmental and phenotypic variation on fitness may generate differences in the amount of heritable genetic variation underlying traits such as birthweight.     

Effects of selective harvest on antler size in white-tailed deer: A modeling approach

Selective harvesting in wild deer (Odocoileus spp.) populations is a common practice that may influence antler size. However, in free-ranging populations, response due to selection is unknown or difficult to quantify because antlers are influenced by nutrition and population demographics. We used quantitative genetic models to predict how white-tailed deer (O. virginianus) antlers would respond to selection and what variables (i.e., population size, age structure, mating ratio, and heritability) most affected antler size. We validated our quantitative genetics program by comparing model results with a population of deer used for controlled breeding experiments; modeled antler points (AP) and score increased (2.2–4.3 AP and 48.5–97.7 cm, respectively) after 8 years of selection, similar to observed increases in AP (3.2) and score (92.3 cm) from the controlled population. In modeled free-ranging populations, mating ratio, age structure, and heritability were more important in influencing antler size than size of the population. However, response to selection in free-ranging populations was lower (0.1–0.9 AP) than controlled breeding populations even after 20 years of selection. These results show that selective harvesting of free-ranging white-tailed deer may be inefficient to change population-level genetic characteristics related to antler size. Response of antlers in free-ranging deer will be less than controlled populations, and possibly modeled free-ranging simulations, because individual reproductive success of males is lower, breeding is done by a large group of males, and reproductive and survival rates are lower. These factors, and others, reduce the amount of improvement that can be made to antlers due to selection. Therefore, selective harvesting in free-ranging populations should be justified for managing population demographics and dynamics, but not for changing the genetic characteristics of populations. © 2011 The Wildlife Society.     

Assessing the intensity of sexual selection on male body mass and antler length in roe deer Capreolus capreolus: is bigger better in a weakly dimorphic species?

Little is known about traits under sexual selection in territorial mammals with low sexual size dimorphism. We examined the potential for sexual selection on male body mass and antler length in the European roe deer Capreolus capreolus, a territorial ungulate in which males are less than 10% heavier than females. Independently, both body mass and antler length (irrespective of age) had a positive effect on male yearly breeding success. However, when corrected for body mass, antler length at a given mass only had a slight effect on male breeding success. This suggests that: (1) ‘bigger is better’ and (2) sexual selection is responsible for at least part of the observed variation in body mass and antler length in roe deer. High body mass and large antlers may be advantageous to males for two reasons: (1) they enhance fighting ability and dominance, so allowing males to defend better their territory and hence access mates, and (2) they attract females because they are honest signals of male phenotypic quality. This suggests that, even in weakly dimorphic ungulate species, sexual selection may lead to a marked influence of body mass on male breeding success, as long as body mass is also strongly selected in females.     

Fluctuating asymmetry in a secondary sexual trait: no associations with individual fitness, environmental stress or inbreeding, and no heritability

It has been suggested that fluctuating asymmetry (FA) in secondary sexual traits may be a useful indicator of either individual quality or environmental stress. We tested this concept using a series of analyses of FA in male antler size in a wild red deer (Cervus elaphus) population, using four measures of size repeated across successive years on the same individuals. We found no consistent evidence of correlations between traits in levels of FA, nor of any associations between known environmental or developmental conditions. None of the four measures of FA showed a significant heritability (average h2 = 0.041), nor was there any evidence of inbreeding depression. For three of the four traits, fluctuating asymmetry did not predict either annual or lifetime breeding success. However there were significant associations between breeding success and FA in antler length. Given the series of null results in our other tests, it seems likely that this was a direct mechanistic effect rather than because measures of FA were indicative of individual quality or condition.     

Getting the timing right: antler growth phenology and sexual selection in a wild red deer population

There has been growing interest in the determinants of the annual timing of biological phenomena, or phenology, in wild populations, but research on vertebrate taxa has primarily focused on the phenology of reproduction. We present here analyses of the phenology of the annual growth of a secondary sexual characteristic, antlers in red deer (Cervus elaphus) males. The long-term individual-based data from a wild population of red deer on the Isle of Rum, Scotland allow us to consider ecological factors influencing variation in the phenology of growth of antlers, and the implications of variation in antler growth phenology with respect to the phenotype of antler grown (antler mass) and annual breeding success. The phenology of antler growth was influenced by local environmental conditions: higher population density delayed both the start date (during spring) and the relative end date (in late summer) of antler growth, and warmer temperatures in the September and April prior to growth advanced start and end dates, respectively. Furthermore, there was variation between individuals in this phenotypic plasticity of start date, although not in that of end date of growth. The phenology of antler growth impacted on the morphology of antlers grown, with individuals who started and ended growth earliest having the heaviest antlers. The timing of antler growth phenology was associated with breeding success in the following mating season, independently of the mass of antlers grown: an earlier start of antler growth was associated with siring a higher number of the calves born the following spring. Our results suggest that the phenology of traits that are not directly correlated with offspring survival may also regularly show correlations with fitness.     

Revisiting the allometry of antlers among deer species: male–male sexual competition as a driver

The positive allometry between antlers and shoulder height reported for cervids has previously been interpreted as resulting from a high male‐male competition in large species that form large breeding groups. We aim at revisiting relative antler size variation among deer species by including more species (n = 31) and by testing both direct and indirect influences of different sexual selection proxies on the relative antler length using path analysis. The absence of direct effect of mating tactic on relative antler allometry indicates that the strength of fights does not differ among mating tactics. On the other hand, the main effect of breeding group size is revealed by polygyny. Highly polygynous species have relatively longer antlers than less polygynous ones but the difference in the relative effect of breeding group size on relative antler length is weak. Strong direct effect of breeding group size and indirect effect of mating tactic shaped the observed variation in the relative antler size among deer, suggesting that the amount of male‐male competition is the main evolutionary force of antler allometry in cervids.     

Repeatability of size and fluctuating asymmetry of antler characteristics in red deer (Cervus elaphus) during ontogeny

Fluctuating asymmetry (FA) has been suggested as a measure of the sensitivity of development to a wide array of genetic and environmental stresses. It has been also suggested that antlers in red deer could be important during social and rutting displays. We used antler measurements of 51 males that were measured over subsequent seasons, from 3–8 years of age, and analysed three antler traits: antler weight, length, and the number of antler tines (antler size). We calculated absolute and relative FA. All three size traits were highly significantly intercorrelated. By contrast to this, the FA of the three traits, did not show such relationships. With increasing age, antler size and FA also increased. When testing the repeatability of FA and antler size, there was a principal difference in the pattern between FA and antler size, with the latter being much more consistent than the former. This suggests that antler size, not FA, may be a good predictor of the bearer's quality in mate selection. This fits well with the good-genes hypothesis that the development of extravagant secondary sexual characters can be an honest advertisement of heritable male quality.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 215–226.     

Does fluctuating asymmetry of antlers in white-tailed deer (Odocoileus virginianus) follow patterns predicted for sexually selected traits?

Secondary sexual characters have been hypothesized to signal male quality and should demonstrate a negative relationship between the size of the trait and degree of fluctuating asymmetry because they are costly to produce. We collected morphometric and antler data from 439 white-tailed deer (Odocoileus virginianus) in Oklahoma, USA, in order to determine whether measures of antler asymmetry follow the patterns predicted for sexually selected characters. Relative fluctuating asymmetry was negatively related to antler size for all deer and within age groups up to five and a half years of age. We did not detect an association between asymmetry and antler size among deer that were six and a half years or older. When categorizing deer by antler size, we found that deer with small antlers (< or = 33rd percentile) had greater levels of relative asymmetry than deer with large antlers (< or = 67th percentile). The relative asymmetry of antlers was negatively related to age and was greatest in deer that were one and a half years old. Relative asymmetry was also negatively related to carcass mass, inside spread, skull length and body length. These data suggest that asymmetry in the antlers of white-tailed deer may be a reliable signal of quality and, as such, may be important in maintaining honesty in intrasexual advertisements during the breeding season.     

Intralocus sexual conflict over human height

Intralocus sexual conflict (IASC) occurs when a trait under selection in one sex constrains the other sex from achieving its sex-specific fitness optimum. Selection pressures on body size often differ between the sexes across many species, including humans: among men individuals of average height enjoy the highest reproductive success, while shorter women have the highest reproductive success. Given its high heritability, IASC over human height is likely. Using data from sibling pairs from the Wisconsin Longitudinal Study, we present evidence for IASC over height: in shorter sibling pairs (relatively) more reproductive success (number of children) was obtained through the sister than through the brother of the sibling pair. By contrast, in average height sibling pairs most reproductive success was obtained through the brother relative to the sister. In conclusion, we show that IASC over a heritable, sexually dimorphic physical trait (human height) affects Darwinian fitness in a contemporary human population.     

Antler size and fluctuating asymmetry in red deer (Cervus elaphus) stags and probability of becoming a harem holder in rut

Red deer hinds play a significant role in selecting stags for mating. We tested the hypotheses that in red deer ( Cervus elaphus ) the probability of becoming a harem holder (and hence achieving reproductive success) occurs in stags bearing larger, branched antlers and showing low fluctuating asymmetry (FA). Eleven antler characteristics were measured; absolute and relative FA were calculated on 51 cast antler sets from 19 individually recognized stags. Probability of becoming a harem holder (PBHH) was originally analysed separately, i.e. for antler size and FA of each antler characteristic and calculated factors for both antler size and FA. If analysed separately, large antler size and low relative but high absolute FA increased PBHH. When we combined antler size and FA of antler characteristics in one model using antler size and FA factors, however, PBHH and achieving reproductive success were mainly dependent on increasing antler size and enhanced antler branching rather than on FA. We conclude that in contrast to antler size, FA is unlikely to play any significant role in sexual selection as an indicator of individual quality.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 59–68.