Genetic patterns of paternity and testes size in mammals

Abstract

Background: Testes size is used as a proxy of male intrasexual competition, with larger testes indicative of greater competition. It has been shown that in some taxa, social mating systems reflect variance in testes size, but results are not consistent, and instead it has been suggested that genetic patterns of mating may reflect testes size. However, there are different measures of genetic patterns of mating. Multiple paternity rates are the most widely used measure but are limited to species that produce multi-offspring litters, so, at least for group living species, other measures such as loss of paternity to males outside the social group (extra group paternity) or the proportion of offspring sired by the dominant male (alpha paternity) might be appropriate. This study examines the relationship between testes size and three genetic patterns of mating: multiple paternity, extragroup paternity and alpha paternity.

Methodology/principal findings: Using data from mammals, phylogenetically corrected general linear models demonstrate that both multiple paternity and alpha paternity, but not extra group paternity, relate to testes size. Testes size is greater in species with high multiple paternity rates, whereas the converse is found for alpha paternity. Additionally, length of mating season, ovulation mode and litter size significantly influenced testes size in one model.

Conclusions/significance: These results demonstrate that patterns of mating (multiple paternity and alpha paternity rates) determined by genetic analysis can provide reliable indicators of male postcopulatory intrasexual competition (testes size), and that other variables (length of mating season, ovulation mode, litter size) may also be important.

Figure 1. Relative testes size and multiple paternity rates.

Regression line (y = 0.0311×- 1.2339) shown through the phylogenetically corrected residual testes size. Multiple paternity rate is significant in the full model (t = −2.90, P = 0.006; see Table 1).

Figure 2. Relative testes size and length of mating season.

Short mating season (<6 months); Long mating season (≥6 months). Length of mating season is significant in the full model (t = −2.90, P = 0.006; see Table 1).

Figure 3. Relative testes size and ovulation mode.

Ovulation mode is significant in the full model (t = 4.16, P<0.001; see Table 1).

Figure 4. Relative testes size and litter size.

Regression line (y = 0.8173×- 1.7911) shown through the phylogenetically corrected residual testes size. Litter size is significant in the full model (t = 3.32, P = 0.001; see Table 1).

Figure 5. Multiple paternity rates and social mating system.

SM: socially monogamous; SP: socially polygynous; MM: multi-male. Multiple paternity rates differed significantly between social mating systems (F _2,61 = 4.58 P = 0.014); SM was significantly different from MM species (Tukey HSD = 0.036), but not SP species (Tukey HSD = 0.839). MM and SP species did not differ significantly (Tukey HSD = 0.079).

Figure 6. Relative testes size and alpha paternity.

The regression line (y = 0.511×-0.492) shown through the phylogenetically corrected residual testes size. Alpha paternity is significant in the full model (t = −2.32, P = 0.032; see Table 1).