Male fertility in natural populations of red deer is determined by sperm velocity and the proportion of normal spermatozoa

Malo AF, Garde JJ, Soler AJ, Garcia AJ, Gomendio M, Roldan ER

Reproductive Ecology and Biology Group, Museo Nacional de Ciencias Naturales (CSIC), 28006-Madrid, Spain

Male reproductive success is determined by the ability of males to gain sexual access to females, and by their ability to fertilize ova. 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 before and after copulation (i.e. sperm competition). 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. In this study we examined which semen traits correlate with male fertility in natural populations of Iberian red deer. We found no trade-offs between semen traits. Our analyses revealed strong associations between (a) sperm production and sperm swimming velocity, (b) sperm motility and the proportion of morphologically normal spermatozoa, and (c) sperm viability and acrosome integrity. These last two variables had the lowest coefficients of variation suggesting that these traits have stabilized at high values and are unlikely to be related to fitness. In a fertility trial, our results show that there was a large degree of variation in male fertility, and that differences in fertility were mainly determined by sperm swimming velocity and also by the proportion of morphologically normal sperm. We conclude that male fertility does vary substantially in natural populations of Iberian red deer and that, when sperm numbers are equal, it is mainly determined by sperm swimming velocity and sperm morphology.

Biol Reprod 2004, Dec 1.