Lloyd (1967) proposed that a measure he called mean crowding should be used as an estimate of the number of conspecifics that an individual animal is likely to encounter within a particular patch of habitat. We measure population density as the number of conspecific male competitors that an individual male is likely to encounter per patch of resource, which can be estimated using a quantity called ‘mean crowding’. Here, we focus on the effects of operational sex ratio (OSR) and population density in determining patterns of presence or absence of weaponry in a community of Southern African dung beetles. Most research to date, however, has focused on individual species: despite some interspecific studies, notably those on agamid lizards ( Stuart-Fox & Ord 2004 Ord & Stuart-Fox 2006), the factors that lead some species to gain or lose sexually selected traits, or to grow larger or smaller ones, are generally poorly understood ( Wiens 2001). More recently, there has been substantial research interest in how ecological factors such as the operational sex ratio can influence mating systems and sexual ornamentation ( Emlen & Oring 1977 Hamilton 1979 Kvarnemo & Ahnesjö 1996, 2002 Reynolds 1996 Shuster & Wade 2003). The first attempt to understand these patterns was by Darwin (1871), who realized that interspecific differences in mating systems can explain some of the diversity of sexual ornamentation in birds: plumage is more exaggerated in polygynous birds, and monogamous birds tend to be monomorphic. The distribution of these traits is often unrelated to phylogeny: in some taxa sexually selected traits are expressed in some species but not in other closely related species ( Andersson 1994). One of the most striking features of sexually selected traits is the tremendous variation that exists in the size and shape of such traits between species. The influence of mean crowding on horn diversity between species probably reflects the difficulty of guarding and monopolizing females when many competitors are present, meaning that males who adopt ‘scramble’ tactics tend to be favoured. Analysis of data from 14 species using a generalized least-squares model incorporating phylogenetic influences found that both OSR and mean crowding were significant predictors of horn presence, with hornless species tending to show female-biased sex ratios and high levels of crowding.
We investigated the roles of sex ratio (measured as operational sex ratio, OSR) and population density (measured as mean male crowding, a measure indicating the average number of conspecific males that an individual male animal will encounter) in determining horn presence in a community of South African dung beetles. It has long been recognized that some of this diversity can be explained by differences in mating systems between species, but there remains substantial variation between species with similar mating systems.
Sexually selected ornaments and weapons are exceptionally variable, even between closely related species. The influence of mean crowding on horn diversity between species probably reflects the difficulty of guarding and monopolizing females when many competitors are present, meaning that males who adopt 'scramble' tactics tend to be favoured.
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