Why do older males sire more offspring despite less active mating?
In short: In the world of socially monogamous birds, older males consistently sire more extra-pair offspring. However, recent research reveals a surprising twist: these older males are not necessarily more aggressive or better at initiating matings. Instead, the data suggests that female choice and post-copulatory factors may drive this reproductive advantage.
While it is a robust finding that older males sire more extra-pair offspring in species like the house sparrow (Passer domesticus), the mechanism is not what many assumed. Two primary theories—male manipulation (where older males use experience or coercion to force matings) and female choice (where females actively seek out older males)—have been largely unsupported by recent behavioral data.
A study of captive house sparrows found that while middle-aged males (considered 'old' in the wild) gained the most extra-pair paternity, their actual mating behavior did not differ significantly from younger males. Furthermore, copulations were more likely to be initiated by females than by males. This suggests that the 'old-over-young' advantage may not be about who initiates the act, but rather about what happens after the copulation occurs.
Researchers are now looking toward post-copulatory mechanisms, such as superior sperm competition, to explain why older males succeed. Additionally, the benefits for females remain a subject of intense debate. While some argue females seek 'good genes' from older, proven males, other studies show that extra-pair offspring are not always more likely to survive, and genetic benefits may only become visible under unfavorable environmental conditions, such as in experimentally enlarged broods.
This complexity highlights a major gap in behavioral ecology: the costs of extra-pair behavior are often understudied compared to the benefits. Because costs can be indirect—affecting long-term fitness or offspring quality rather than immediate survival—the true evolutionary equation of extra-pair mating remains one of nature's most intricate puzzles.
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Sources
- Summary by Intellect · source: Male age is associated with extra-pair paternity, but not with extra-pair mating behaviour | Scientific Reports
- Also: Benefits of extra-pair mating may depend on environmental conditions—an experimental study in the blue tit (Cyanistes caeruleus) - PMC
- Also: Extra-pair copulation
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