Résumé
Cooperative actions are beneficial to the group, but presumably costly to
the individual co-operators. In cooperatively breeding species, helping to
raise young is thought to involve important energetic costs, which could
lead to elevated exposure to reactive oxygen species, resulting in
oxidative stress. However, identifying such costs can be difficult if
individuals adjust their investment in helping in relation to
environmental conditions or their own physiological condition.
Experimental approaches are therefore required to quantify the costs of
helping but, to date, these have been infrequent. Here, we combined
correlational and experimental data to investigate the oxidative cost of
helping-at-the-nest and how this affects helping decisions in wild
sociable weavers Philetairus socius, a colonial cooperatively breeding
bird. At the correlational level, we found that the probability of helping
was influenced by the interaction of an individual’s oxidative state and
age: compared to younger birds, older individuals were more likely to help
when they had higher oxidative damage, and the opposite trend was found
for younger individuals. After experimentally increasing the energetic
cost of flight, manipulated helpers in breeding colonies decreased
nestling feeding rates and incurred an increase in oxidative damage, which
was not present in manipulated helpers in non-breeding colonies. This
indicates that individuals decreased their helping behaviour to minimise
the associated costs. These results suggest that oxidative stress can
influence helping decisions and underlie a trade-off between cooperation
and self-maintenance, which is central to understanding when helping might
take place in this and other species.