Behavioral Ecology Blog

Just in case you have always wondered, “old” is not an insult in the bird world… Actually, multiple studies have not shown that old birds have a competitive advantage over younger birds when it comes to extrapair access to fertile females. The latest (and one of the best in my opinion) is a a study published in Behavioral Ecology by Schmoll, et.al. entitled “Male age predicts extrapair and total fertalization success in the socially monogamous coal tit” 1st, this is a coal tit:

And here is the Abstract:

An important step to a better understanding of the adaptive significance of extrapair mating behavior in socially monogamous species is to uncover the identity of extrapair sires (EPS). Here, we used a combination of multilocus DNA fingerprinting and microsatellite analysis to identify EPS in the socially monogamous coal tit (Periparus ater), a passerine bird with high rates of extrapair paternity. We then analyzed how fertilization success was related to male age in 2 consecutive first brood periods based on knowledge of the exact age of the majority of territorial males. EPS were significantly older compared with the males they cuckolded. Furthermore, extrapair and, as a consequence, also total fertilization success were positively related to male age, while within-pair success was not. Interestingly, fertilization success did not increase linearly with male age but leveled off for older age classes and was most parsimoniously described by the inverse term of male age. Results were consistent over the 2 years, while the demography of the study population differed with respect to the age distribution of territorial males. Furthermore, we also show that individual males increased their extrapair fertilization (EPF) success with age indicating that cross-sectional analyses were not confounded by cohort effects. Together with the results from other species, these findings suggest that male age (or a strong correlate thereof) is a major determinant of EPF success in several socially monogamous bird species.

So basically the authors take their knowledge of the demography of the population, use genetic techniques to assign parentage, then compare age and fertilization success…. Not so surprising, older birds do better…. The graph to the left illustrates this point(A and B represent different years).. Points above the 1:1 isoline represent cases when the extrapair sire was older then the social mate. For me anyway, then graphs were not that convincing, but in table 3, they show that there were statistically significant differences in the predicted direction.

Now what I like about this paper is that the authors present correlational data, and treat is as such. They do a good job of stating that the cause of the relationship could be age, or something correlated with age. They talk a lot about the fact that age might be used by animals as a proxy for good genes- a hypothesis that I am a fan of. Animals that live long by definition have genes that are favorable under the current selection regime. They have withstood the test of time..

Of coarse this ignores stochasticity and the issues with the heritability of fitness is generally thought to be lowish in these situations..

Anyway, so while relatively speaking, the older birds do better, there comes a point when older doesn’t help you anymore (The relationship is asymptotic). The authors speculate that this might be as a result of senescence or reduced fertility in old age, but there is no real data to support either of these hypotheses.

For me, I would like to know more about some of the potential correlates of age. I guess that while I think the longevity/good genes hypothesis is a good one, I think that before a hypothesis of indirect benefits is accepted, direct benefits type things need to be ruled out. Here are my questions:

1. Do the extrapair sires provide parental care?
2. Is age correlated with territory quality in this population?
3. Do females solicit extrapair copulations, or is it rape (or whatever you’d like to call it)?
4. Do ECP’s have an effect on female reproductive success?