Social life histories: jackdaw dominance increases with age, terminally declines and shortens lifespan

Abstract

Behaviour may contribute to changes in fitness prospects with age, for example through effects of age-dependent social dominance on resource access. Older individuals often have higher dominance rank, which may reflect a longer lifespan of dominants and/or an increase in social dominance with age. In the latter case, increasing dominance could mitigate physiological senescence. We studied the social careers of free-living jackdaws over a 12 year period, and found that: (i) larger males attained higher ranks, (ii) social rank increased with age within individuals, and (iii) high-ranked individuals had shorter lifespan suggesting that maintaining or achieving high rank and associated benefits comes at a cost. Lastly, (iv) social rank declined substantially in the last year an individual was observed in the colony, and through its effect on resource access this may accelerate senescence. We suggest that behaviour affecting the ability to secure resources is integral to the senescence process via resource effects on somatic state, where behaviour may include not only social dominance, but also learning, memory, perception and (sexual) signalling. Studying behavioural effects on senescence via somatic state may be most effective in the wild, where there is competition for resources, which is usually avoided in laboratory conditions.

Keywords: ageing; birds; corvids; dominance; senescence.

Figure 1.

Social dominance in relation to age and size (tarsus length). Statistical analysis was carried out using raw data, but for graphical purposes, individuals were grouped by tarsus size (tarsus ≤ 45.8; or >45.8) and age. Lines show the predicted values of the logistic regression in table 1 for the average tarsus length of each group (44.8 and 46.8). Low rank indicates high dominance.

Figure 2.

Social dominance in relation to age and lifespan. Statistical analysis was carried out using raw data, but for graphical purposes, individuals were grouped by median average age and age. Lines show the predicted values of the logistic regression (equation (2.3)) for the mean values of average age (2.6 and 6.8). Low rank indicates high dominance.

Figure 3.

Terminal decline in social dominance. To show the terminal decline, social dominance is plotted in relation to years prior to disappearance (death), but note that the terminal effect was tested in a model based on age rather than age until death (table 1) and hence the line was drawn by eye rather than being a model fit. Low rank (0) indicates high dominance.

Figure 4.

Reinforcement effects of behaviour up and down the social ladder. (a) Positive and (b) negative reinforcement leading to increasing and decreasing social dominance with age at different life stages. Each pillar (phenotypic state, dominance and the ability to secure resources) will positively affect the next pillar, causing an upward spiral with age (a). Physiological state deteriorates with age owing to senescence, ultimately turning the direction of the effects (b). (Online version in colour.)