Experimental mate-removal increases the stress response of female house sparrows: the effects of offspring value?

Vertebrates secrete elevated levels of glucocorticoids in response to various stressors, which mobilize energetic reserves but concurrently interfere with reproduction. In accordance with life-history theory, recent evidence suggests that the corticosterone response to stress is modulated according to the value of the brood. Since brood value is positively related to parental care, the stress response modulation may be either the consequence of offspring value (e.g. large broods have high fitness potential - the brood value hypothesis) or the consequence of parental workload (e.g. large broods are energetically demanding for the parents - the workload hypothesis). In this experiment, we aimed at experimentally separating the effects of brood value and workload and to confront the latter two hypotheses. To do so, we captured the male parents from breeding pairs of house sparrows (Passer domesticus) and took them in captivity for 48 h. During the absence of males, mate-removed females made more food deliveries than controls (increased workload) but were unable to fully compensate the lack of their mate, thus their chicks were in worse condition (reduced brood value) than control chicks. After the experimental period, mate-removed females responded more strongly to the standardized stressor than controls. In both groups, the corticosterone response to stress was negatively related to the nestlings' mass gain. These results provide experimental support for the brood value hypothesis, i.e. that individuals may actively modulate their stress response (either down- or upwards) with respect to the value of their current reproduction.