Ageing and personality in a wild bird
​Blue tit are aggressive small passerines which can live for a number of years in the wild. Our oldest individuals were 7 years old. As they age, their aggression during handling decreases

​Senescence in animal personality

Historically, senescence was deemed largely irrelevant in the wild, because it was assumed that organisms in the wild really did not survive to such high ages that a senescent decline would become observable. During the last decade or so, biologists have studied individuals more intensively, and perhaps also for long enough time, to demonstrate that senescence indeed occurs in the wild. Nevertheless, most studies show a senescent decline in reproductive traits or survival.
The term "animal personality"  refers to behaviors which are repeatable. That is, when the same individual is handled it will, on average, show a similar behavioral response. In our case, we focus on behavioral responses to our handling of the birds. Clearly, being handled is a stressful event for a wild bird. Blue tits respond to handling in different ways. What we term "handling aggression" refers to the tendency of individuals to either resist handling during the entire measurment protocol (highly aggressive bird, score 5), or not resist at all (play dead, score 0). A typical blue tit will initially resists handling and measuring, but will rather soon quiet down (score 3). Other birds are intermediate to the typical score of 3 and either the extreme low or high score such that we recognise in total 5 levels of handling aggression. Handling aggression is a repeatable behavior, and we can thus - in a sense - classify individuals into aggressive personality types and less aggressive ones. We have shown earlier that males with a higher handling aggression are able to produce more offspring. Thus, the personality type of individuals is related to their performance in the wild.

Based on 10 years of data on handling aggression, we find that older birds display a lower level of aggression when handling. In principle, this can be caused either by a high probability of mortality of individuals with high handling aggression score (such that only low-aggression individuals survive to old age), or it can be so that all individuals show this decrease in handling aggression as they age. We found that the latter explanation is the driver of a decrease in handling aggression in our population.
The phenomenon of senescence is puzzling biologists, because how can natural selection allow individuals to decrease in performance in age? There are two theories outlining how senescence can evolve in nature. Thus, theoretically, evolved senescence is a possibility. Empirically, these theories have been tested, although largely under laboratory conditions. In the last decade, studies have also documented what can be considered as the fingerprint of evolved senescence. We also looked for this "fingerprint" in the senescence of hanlding aggression in our blue tits, but we did not find it. Because this is an analysis which demands very much of the data, we cannot be fully certain that this "fingerprint" truly is absent, or has merely remained undetected with the information we currently have.

Jon Brommer
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