Behavioral and adrenocortical responses to mate separation and reunion in the zebra finch

    The article discusses the behavioral and hormonal effects that pair mate separation has on avian species, as well as the impact of possible mitigating factors in reducing the bird’s stress level. Research with mammals has brought about a comprehensive understanding of the role of peptide and steroid hormones in behavior as well as their relationship with varying levels of circulating adrenal and pituitary hormones during pair formation and separation (Remage-Healey et al, 2003). However, research in birds has not been as extensive, and by studying the zebra finches, the experimenters aimed to understand how separation elevates baseline corticosterone on birds.
The Australian zebra finch was chosen as a model of bird mate separation because laboratory conditions do not disrupt significantly their sexual and social behavior and thus, it is a good model of natural interactions. These species are particularly relevant to studying the effects of mate separation since they sustain lifelong monogamous bonds and are highly sociable. Furthermore, previous studies contribute extensive insight about the selective pressures that lead to the zebra finches to have life-long pairs, as well as about the behavioral changes that arise after separation (e.g. increased activity and song rates).

The four experiments carried out as part of this study were designed to understand how acoustic and visual separation and reunion with a mate affects the bird’s behavior and levels of stress hormones, as well as the factors that mitigate this effect. Among the factors studied, social interaction in a group of same-sex individuals as well as a group of potential new mates was studied. Both physiological measures as well as behavioral observations were recorded to asses the effect of a stressful event, namely pair mate separation. In terms of physiology, the main measure taken was concentration of CORT, the primary avian glucocorticoid, since increased levels of stress increase the presence of this hormone. In addition, special attention was taken to rule out factors unrelated to mate separation but that could nevertheless increase levels of CORT. For instance, baseline measures were taken long after birds have been moved between cages, and consecutive blood sampling was limited to 24-hour periods to avoid physiological stressors. Furthermore, in each experiment there was a control group which underwent the same physical manipulations to ensure that these were not the cause of increased CORT. In terms of behavior, birds were observed for increased levels of locomotion (e.g. more perch hops), but also for behaviors associated with courtship (e.g. increased song rates), mating and pair bond maintenance.

    In the first experiment, researchers tested whether separation increased stress hormones, and if this effect could be attenuated by placing the bird in the company of same-sex individuals as opposed to complete isolation. As expected from studies with mammals, finches that were visually separated from their mates and placed in isolation had elevated levels of CORT. However, from the small sample of birds that were placed in the company of same-sex individuals, it preliminarily appears as if there had not been a significant increase of the stress hormone, suggesting that social interaction buffered the stress response.
    The main purpose of the second experiment was to asses how hormonal levels changed after reuniting an isolated bird with either the old partner or with a group of potential new mates. It was found that after the pair was reunited, levels of CORT returned to baseline, indicating an end to the period of physiological stress. In contrast, levels of CORT in birds placed with potential mates did not return to baseline.

During the first experiment, birds had been isolated visually, but they could still hear their mates. This led the experimenters to consider the frustration hypothesis, where stress was induced not because the birds where separated but because they still knew they were alive and yet could not interact with them. To rule this possibility, in experiment 3 birds were completely isolated from their mates. However, the findings did not differ significantly from those in experiment 1, which suggests that the frustration hypothesis was incorrect, and that the stress was entirely produced by mate separation.

Next, experimenters considered that housing a bird alone in a cage could be enough to increase stress levels, which gave rise to the isolation hypothesis to explain the increase in levels of CORT. From experiment 1, it appeared that social interactions attenuated stress levels, since individuals placed in a group upon separation did not show increased stress. However, due to the small sampling size, these results were inconclusive and gave rise to a fourth and final experiment. Contrary to the findings from experiment 1, placing the birds in a social group did not prevent increased stress hormonal levels upon separation, which shows that the isolation hypothesis is not appropriate. From this, researchers suggested that in species with strong pair bonds, such as the zebra finches, and the marmosets, which is one of the two primates species that is genetically monogamous (Banerjee, 2006), social interaction is not enough to mitigate stress.

Marked behavioral changes resulted upon mate separation and reunion. In experiment 4, reunion with the mate increased the frequency of affiliative behaviors, which suggests that the purpose of these behaviors is to reestablish and strengthen the bond that was temporarily lost. Finally, in all experiments it was found that zebra finches increased the frequency of locomotion and song rates when separated from their partner. This means that certain behavioral changes took place which could promote locating the lost partner, or finding a new one.

I believe that the main finding of this study was to note physiological variations between long-term and short term pair bond and to corroborate mammal pair bonding studies. On one hand, the experiment showed that that social interaction does not attenuate the level of stress in long-term bonds, even though it does in short-term bonds. This could explain why which explains how life-long monogamy developed evolutionarily and how it was sustain through physiological changes. On the other hand, the study corroborates findings from mammals which suggested that elevated levels of stress hormones delay re-pairing in monogamous affiliative bonds (Devries, 1995 as cited in Remage-Healey et al, 2003).

This study is highly relevant to the material presented in class and complemented my understanding of the relationship between hormones and behavior, with a particular focus to pair bond maintenance, as well as the physiological effects of stress. In order to fully understand what the researchers concluded regarding behavioral changes to promote finding the mate, it was essential to know about the differences between long and short-term stress. Even though mate separation in the zebra finch is a traumatic event, in order for the stress to have value for locating a partner it must be short-term only, since otherwise, memory impairments and extensive immune suppression might compromise the survival of the stressed individual. Finally, understanding the relationship between pair bond formation and “conditioned reward learning” helped me understand why affiliative behaviors should be reinforced upon being reunited.