We present estimates of the selection on and the heritability of a male secondary sexual weapon in a wild population: antler size in red deer. Male red deer with large antlers had increased lifetime breeding success, both before and after correcting for body size, generating a standardized selection gradient of 0.44 (+/- 0.18 SE). Despite substantial age- and environment-related variation, antler size was also heritable (heritability of antler mass = 0.33 +/- 0.12). However the observed selection did not generate an evolutionary response in antler size over the study period of nearly 30 years, and there was no evidence of a positive genetic correlation between antler size and fitness nor of a positive association between breeding values for antler size and fitness. Our results are consistent with the hypothesis that a heritable trait under directional selection will not evolve if associations between the measured trait and fitness are determined by environmental covariances: In red deer males, for example, both antler size and success in the fights for mates may be heavily dependent on an individual's nutritional state.
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"Omitted confounders have been addressed in the selection (but not performance) literature (Price et al. 1988; Rausher 1992; Queller 1992; Scheiner et al. 2002; Kruuk et al. 2002; Hadfield 2008; Morrissey et al. 2010; Shaw and Geyer 2010; Stinchcombe et al. 2013). What is missing from this literature is any analysis of the sensitivity of effect estimates to the presence of random, omitted confounders. "
[Show abstract][Hide abstract] ABSTRACT: Multiple regression of observational data is frequently used to infer causal effects. Partial regression coefficients are biased estimates of causal effects if unmeasured confounders are not in the regression model. The sensitivity of partial regression coefficients to omitted confounders is investigated with a Monte-Carlo simulation. A subset of causal traits is "measured" and their effects are estimated using ordinary least squares regression and compared to their expected values. Three major results are: 1) the error due to confounding is much larger than that due to sampling, especially with large samples, 2) confounding error shrinks trivially with sample size, and 3) small true effects are frequently estimated as large effects. Consequently, confidence intervals from regression are poor guides to the true intervals, especially with large sample sizes. The addition of a confounder to the model improves estimates only 55% of the time. Results are improved with complete knowledge of the rank order of causal effects but even with this omniscience, measured intervals are poor proxies for true intervals if there are many unmeasured confounders. The results suggest that only under very limited conditions can we have much confidence in the magnitude of partial regression coefficients as estimates of causal effects. This article is protected by copyright. All rights reserved.
"Pradhan and van Schaik proposed that differences in male strength could arise in species with high predation , but mountain gorillas currently have no natural predators. Variations in male strength could also depend on other environmental factors, genetics, and age –. See Section S7 in File S1 for potential explanations about how such variations could lead to stronger dominant males in one-male groups than in multimale groups. "
[Show abstract][Hide abstract] ABSTRACT: Infanticide can be a major influence upon the social structure of species in which females maintain long-term associations with males. Previous studies have suggested that female mountain gorillas benefit from residing in multimale groups because infanticide occurs when one-male groups disintegrate after the dominant male dies. Here we measure the impact of infanticide on the reproductive success of female mountain gorillas, and we examine whether their dispersal patterns reflect a strategy to avoid infanticide. Using more than 40 years of data from up to 70% of the entire population, we found that only 1.7% of the infants that were born in the study had died from infanticide during group disintegrations. The rarity of such infanticide mainly reflects a low mortality rate of dominant males in one-male groups, and it does not dispel previous observations that infanticide occurs during group disintegrations. After including infanticide from causes other than group disintegrations, infanticide victims represented up to 5.5% of the offspring born during the study, and they accounted for up to 21% of infant mortality. The overall rates of infanticide were 2-3 times higher in one-male groups than multimale groups, but those differences were not statistically significant. Infant mortality, the length of interbirth intervals, and the age of first reproduction were not significantly different between one-male versus multimale groups, so we found no significant fitness benefits for females to prefer multimale groups. In addition, we found limited evidence that female dispersal patterns reflect a preference for multimale groups. If the strength of selection is modest for females to avoid group disintegrations, than any preference for multimale groups may be slow to evolve. Alternatively, variability in male strength might give some one-male groups a lower infanticide risk than some multimale groups, which could explain why both types of groups remain common.
"These findings reveal a new function for male red deer antlers and suggest that among mammals the degree of elaboration of male secondary sexual characters may signal important aspects of male reproductive quality to females and males. Previous studies demonstrated that antler size is related to the number of calves fathered by males, and it has been assumed that this is exclusively the result of males with large antlers being able to win more fights with other males (Kruuk et al. 2002). Our findings suggest that males with large antlers could also achieve higher reproductive success through their enhanced ability to win fertilizations both in competitive and non-competitive contexts and the possible preferences shown by females to mate with them. "