Among polygynous mammals, males differ markedly in their reproductive success, and a great deal of effort has been made to understand how selective forces have shaped traits which enhance male competitiveness. Male reproductive success in red deer is believed to be related to body size and antler size which, in turn, determine fighting ability. In red deer populations from northern Europe, males defend harems but, in the Mediterranean (where the rut takes place when food resources are scarce), males may also defend territories that contain food resources that attract females (Carranza et al. 1996).

Male reproductive success is not only determined by the ability of males to gain sexual access to females, but also by their ability to fertilise ova. However, the possibility that males may also differ in their fertility has been ignored, under the assumption that male infertility is rare in natural populations because selection against it is likely to be strong. We have found that, in natural populations of Iberian red deer, there are major differences between males in fertility rates, and that such differences are mainly determined by sperm swimming velocity and by the proportion of morphologically normal spermatozoa (Malo et al. 2005a). The size and the complexity of male antlers is associated with relative testes size and sperm velocity, suggesting that antlers honestly advertise male fertility (Malo et al. 2005b). These findings reveal a new function for male red deer antlers which were regarded exclusively as weapons.

We have examined how sperm design influences sperm swimming velocity and have found that spermatozoa with elongated heads, short midpieces, and those in which the relative length of the rest of the flagellum is longer, swim faster (Malo et al. 2006). We have discovered a large degree of inter-male variation in sperm design which underlies differences in sperm swimming speed which, in turn, determine differences in male fertility rates.

Many populations of Iberian red deer are fragmented by the use of fences. We have analysed whether inbreeding influences parasite load using heterozygosity (based on microsatellite analyses) as an indirect measure of inbreeding. We have found a significant association between genetic variation and parasite burden which is mediated by the strong influence of genetic variation upon body condition (Gomendio et al. 2006).

Collaborators:

• Julián Garde, Instituto de Investigación en Recursos Cinegéticos (CSIC-UCLM-JCCM),    Albacete (Spain).

• Christian Gortázar, Instituto de Investigación en Recursos Cinegéticos (CSIC-UCLM-JCCM),   Ciudad Real (Spain).

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