Good sperm producers are more likely to participate in incubation in the Asian barn swallow Hirundo rustica gutturalis.

Sperm competition favors increased investment in post-copulatory sexual traits (e.g., long sperm) while reducing parental investment. The relationship between the two investments, however, remains unclear, although it affects the direction and speed of the evolution of each trait. Here, using the Asian barn swallow Hirundo rustica gutturalis, we examined the relationship between total sperm length (i.e., a post-copulatory sexual trait in birds) and male incubation participation (i.e., a paternal investment). This study system provides a unique opportunity to test the relationship, because male incubation has evolved as a derived trait and has not yet been fixed in the populations. After controlling for potential confounding factors (i.e., body condition and pre-copulatory sexual traits), we found that the probability of male participation in incubation increased with the total sperm length. Given that long sperm would secure within-pair paternity, incubation investment by males with long sperm would be adaptive in these sparse populations, rather than pursuing unlikely opportunities for extrapair mating at the expense of participation in incubation. The observed pattern was contrary to the negatively correlated evolution between the total sperm size and male participation in incubation in the family Hirundinidae, indicating that the direction of the relationship between post-copulatory sexual traits and paternal investment can be different from the general pattern of the clade depending on the ecological and evolutionary settings.