The strongest correlate o f male mating success was the distance from a territory to the lek centre: males closer to the centre mated with more females. With increasing distance from the lek there was a growing but modest benefit o f defending a territory next to a drainage line. Momentary mating success was found likely to reflect relative lifetime reproductive success, thus it indeed seems that males gathered on leks due to mating benefits. However, lek males incurred several costs as well: (a) they were in
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worse body condition than resource defenders, (b) hyenas, their presumed main predator, were found in higher densities on lek, and (c) lek males engaged in agonistic encounters more frequently than resource defenders. Severe fighting was more frequent on the central lek and this could explain w hy central lek males were larger than other males. Furthermore, lek males were older than resource defenders as judged from horn wear, and lek males in more central positions had darker
facemasks.
Do these findings tell us anything about why females gather on leks? With respect to the direct comparison model, none o f the morphological or behavioural traits expected to correlate independently with male mating success in the multivariate analysis were found to do so. However, the overriding influence o f centrality is consistent with the position model according to which male-male competition elicited by female preference for centrality separates the superior mates from the rest. The increased fighting rate o f lek males compared to resource defenders also agrees with this model, as does the larger body size o f central males, since fighting ability is likely to depend on body size (e.g., Davies and Halliday, 1977; Jarman, 1983; Miller, 1975; Owen-Smith, 1993). That female ungulates may use the outcome o f male combat in their mate choice has also been reported from a resource- defending ungulate, the pronghorn antelope (Antilocapm americand), where oestrous females incited fights by moving between males after which they mate with the winner (Byers et al., 1994).
The fact that lek males might be relatively old on average could be explained by fighting skills increasing with experience through life. I f survival is heritable, females may gain indirect benefits from the association between centrality and longevity, not only because their sons are more likely to obtain a central position as they get older, but also because their offspring would be likely to enjoy a longer reproductive lifespan (Halliday, 1983; Kokko, 1998; however, see Brooks and Kemp, 2001). For the importance o f male age in determining mating success in topi see also secs. 7.4.4 & 7.5.3.
Apart from viability and body size, the results suggest that the blackness o f the facemask may be indirectly favoured by sexual selection due to its association with centrality. In the lekking lechwe or kob, no correlations between black colouration and mating success were found (Balmford, 1990; Nefdt, 1992). Traits, that could be indirectly selected but were not investigated in this study, include an efficient immune response (Moller, 1997; Pomiankowski, 1989). I f only healthy
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individuals can succeed in the intense inter-male competition, centrality could be a reliable signal o f health and efficient immune response. In Soay sheep {Ovis aries),
parasite resistance has been found to be heritable (Smith et ah, 1999). W hile females could gain indirect benefits in this way, direct benefits due to reduced risk o f disease transmission appear less likely: the most likely diseases to be acquired through brief mating encounters are STDs (Daly, 1978) and the high number o f mating partners and poor body condition o f lek males suggest that they may be more, rather than less, likely to acquire and transmit such diseases.
The overriding importance o f central location on the lek seems to be a general phenomenon among lekking taxa. A meta-analysis o f traits associated with male mating success in lekking species found that next to territorial attendance the strongest correlate was distance from the lek centre; body size demonstrated only a weak correlation (Fiske et ah, 1998). Higher mating success on central territories has been found in other lekking ungulates as w ell (kob (Balmford et ah, 1992a), fallow deer (Clutton-Brock et ah, 1988), blackbuck (Isvaran and Jhala, 2000), lechwe (Nefdt, 1992)) and in birds (e.g., Gratson et ah, 1991; Hôglund and Lundberg, 1987; Trail and Adams, 1989; van Rhijn, 1991; W iley, 1973).
The hotshot model also predicts that matings are concentrated in the centre o f the lek, but in addition it demands that one male per lek should achieve a markedly higher mating success than others, which was not the case: for none o f the leks could I demonstrate any significant difference between the two most successful males. Moreover, the idea that satellite males create a lek by clustering around a particularly attractive male is refuted by the observation that leks persist longer than the lifespan o f individual males. It is known that the western Dutwa lek (SNP) existed 40 years ago (pers. comm, from H. V. B ell to P. Duncan), and the three leks in the Mara have been in their present location since at least the early 1980s. In comparison, the mean longevity o f 36 topi bulls in captivity was 8 years with a maximum lifespan o f 17 years, suggesting that defence o f a territory by an individual for longer than the maximum estimated reproductive lifespan o f 14 years is extremely unlikely.
Just as in the preceding chapter, no support was found for those o f the hotspot models which rely on improved feeding conditions on lek: males on lek were in worse body condition than resource defenders. The food shortage on lek exacerbates the demands on male quality required to hold especially the central lek territories. Male body condition was negatively affected by low rainfall and rutting; the latter
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relationship may be explained by the markedly higher territorial attendance during the rut (sec. 3.4.3) with concomitant reduction in supplementary feeding in other areas.
While the preceding chapter showed that females might suffer increased risk o f hyena predation on the lek, the results o f this chapter suggest that the same may apply to males: the density o f hyenas was markedly higher on lek territories (see also Gosling and Petrie, 1990; Rainy and Rainy, 1989). The lower vigilance o f lek males could suggest that they benefit from sharing vigilance, which may reduce the risk o f lion predation. Alternatively, lek males could be too exhausted to be vigilant. Yet another interpretation o f the result is that males scan primarily for conspecifics rather than lions. Whatever the explanation for the lower vigilance o f lek males, lion predation is likely to be o f less concern than hyena predation (cf. sec. 4.2), and altogether antipredator benefits seem unlikely as an explanation for lek evolution in topi. A study on kob also failed to find any antipredator advantages o f lekking (Balmford and Turyaho, 1992).
Thus, the results in this chapter support the position model and speak against the direct comparison model, the simple and nutrient hotspot models as w ell as the safe-site model. Any model, such as the position model, which hypothesises that females gain indirect benefits by choice, rests on the assumption o f heritability o f sexually selected traits, e.g., viability or competitive ability. Due to the expectation that strong selection on traits leads to a reduction in the underlying additive genetic variance, pronounced mating preferences in females have been seen as a paradox, the 'lek paradox': w hy choose i f traits have lost heritability (Borgia, 1979)? Or rather, upon finding that the additive genetic variance in fact tends to be higher in sexually selected traits than in non-sexually selected traits (Pomiankowski and Moller, 1995): w hy is heritability not lost i f females choose? Because sexual selection is often directional, phenotypic variation p e r se might be favoured resulting in sexually selected traits being coded by a relatively large number o f loci and thus subjected to high mutational loads (Pomiankowski and Moller, 1995). Furthermore, genetic variance in condition could prevent fixation o f a favoured trait, which is condition- dependent (Kotiaho et al., 2001; Rowe and Houle, 1996). Genetic variance in selected traits can also be maintained if the choosy sex makes occasional mistakes in mate assessment (Neff, 2000; Randerson et al., 2000).
In white-tailed deer {Odocoileus virginianus) and red deer, heritability has been found in antler size and body size (Lukefahr and Jacobson, 1998; van den Berg and Garrick, 1997; Williams et al., 1994). In the Ram mountain bighorn sheep {Ovis
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canadensis), body mass was heritable (Reale and Festa-Bianchet, 2000). In red deer,
Kruuk et al. (2000) demonstrated heritability o f two male morphological traits, birth weight and jaw length as w ell as high coefficients o f additive genetic variance in male breeding success. Total fitness, however, was not found to be heritable, possibly because o f large residual variance as the population was characterised by a strong environmentally induced variation in longevity (see also Coulson et al., 1998; Pemberton et al., 1991). These studies could indicate that competitive ability may demonstrate higher heritability than survival. If this is the case, the main benefit to females o f mating on the central lek may be that central males provide genes which give offspring a competitive advantage in intra-sexual fights rather than good genes associated with viability.