Ecological factors have been recognized as important in the evolution of mating systems, and thereby in the evolution of sexually selected characters, since the 1970s (Emlen & Oring 1977
; Hamilton 1979
). Much of the research in the field since then has concentrated on the role of OSR, which is now indeed recognized as being one of the fundamental determinants of mating system diversity (Emlen & Oring 1977
; Kvarnemo & Ahnesjö 1996
; Reynolds 1996
; Kvarnemo & Ahnesjö 2002
), although Shuster & Wade (2003)
have recently put forward an alternative view. Our data support a role for both OSR and crowding in determining horn diversity in this community of beetles: hornless beetles tend to experience high levels of crowding and to have female-biased OSRs.
Negative relationships between the size, presence or use of male weaponry and crowding (or measures related to crowding such as population density) have been found in three other studies that concentrated on patterns within individual species. Firstly, male mites of the species Sancassania berlesei
that are reared at low densities will develop into ‘fighter’ morphs, with armoured and pointed legs that they use to kill rivals, whereas males reared at high densities develop into ‘scrambler’ morphs that are unarmed and do not fight (Timms et al. 1980
; Radwan 1993a
). Secondly, Moczek (2003)
found that there was a lower proportion of horned major morphs of males of the dung beetle Onthophagus taurus
in higher-density populations. Finally, Connor (1989)
compared male mating success in the forked fungus beetle, Bolitotherus cornutus
between high- and low-density trees, and found that horn length was positively correlated with mating success on two out of three low-density trees but not on any high-density trees. Our data indicate that these intraspecific patterns are reflected in interspecific patterns of weapon diversity.
If male dung beetles in crowded species are competing via scramble competition for matings, then high fitness will be related to mobility, the ability to find females and success in sperm competition. Of these, the last is likely to be of considerable importance: in the frenetic world of the Southern African dung pat, females of these species are likely to encounter, and mate with, a large number of males, leading to high levels of sperm competition. There is strong theoretical and empirical evidence that sperm competition selects for males that invest heavily in sperm production (Parker 1998
; Simmons 2001
), and importantly two recent studies have indicated a link between investment in horns and investment in testes in onthophagine beetles. Firstly, in the species Onthophagus nigriventris
, males that were prevented from growing horns grew relatively larger testes (Simmons & Emlen 2006
), and secondly a comparison of testis size across 16 species of Onthophagus
with dimorphic males found that horned major morphs tended to invest less in testis mass than hornless minor morphs (Simmons et al. 2007
). One plausible explanation for the loss of horns in more crowded species is therefore that males of these species experience high levels of sperm competition and trade off weapon size against testis size: this is a clear direction for future research into this system.
By contrast with these studies that have shown the evolution of male weaponry to be favoured in less-crowded conditions, both Zeh (1987)
and Tomkins & Brown (2004)
have reported positive relationships between population density and weapon size, in pseudoscorpions and the earwig Forficula auricularia
, respectively. In these cases the benefits of aggression and investment in weaponry may outweigh the costs even at the highest densities: whether this occurs in a specific system will depend on details of the intrasexual contests such as how costly the weapon is to produce; how long contests take to resolve; how serious the risk of injury is; and on details of the reproductive system such as patterns of sperm precedence and whether reproduction is iteroparous or semelparous.
Operational sex ratio has been shown to be an important force influencing the intensity of sexual selection in a wide variety of species (Kvarnemo & Ahnesjö 1996
; Reynolds 1996
; Kvarnemo & Ahnesjö 2002
), and it is perhaps not particularly surprising that it is related to horn presence in these animals: hornless species show more female-biased OSRs than horned ones, most probably because intrasexual competition between males is reduced in these species. Nonetheless, the correspondence between possession of a horn and OSR is striking: all of the hornless species had female-biased sex ratios, and two of them had OSRs of less than 40%, whereas two-thirds of the horned species had slightly male-biased or even sex ratios. We do not currently know the reasons behind these biased sex ratios, and this is another direction for future research.
As with any field study, our conclusions rest on a number of assumptions, and as a consequence some caution must be exercised in interpreting these data. Most obviously, we are assuming that sampling these animals by using baited pitfall traps gives a good estimate of the density and aggregation experienced by these animals in the wild. This assumption is, we feel, a reasonable one: these beetles locate dung pats by odour, and the mechanism by which they arrive in a pitfall trap is therefore the same as the mechanism by which they would arrive at a dung pat under natural conditions. We are also assuming that the degree of crowding that we measured reflects the most important crowding that these species have experienced during their evolution, and clearly we cannot rule out the possibility that the crowding measured during the two field seasons was exceptional: nonetheless, we have no particular reason to believe that this is the case either.