Year

2005

Degree Name

Doctor of Philosophy (PhD)

Department

School of Biological Sciences - Faculty of Science

Abstract

To investigate the evolutionary rationale for the seemingly altruistic behaviours commonly seen in cooperatively breeding Australian passerines, I examined alloparental behaviour in the White-browed Babbler Pomatostomus superciliosus (WBBA). Toward this end, I analysed behavioural, hormonal, and genetic factors in both free-living and captive WBBAs. Studies of free-living birds examined social and reproductive behaviours and hormonal correlates to reproduction. With captive birds, I performed both observations and manipulative experiments focusing on intragroup social structure, social behaviours, and the endocrine correlates to such structure and behaviours. The WBBA was selected as a study species as they live in sedentary, year-round social groups that engage in cooperative breeding. Field work was conducted in Back Yamma State Forest, in the central west region of New South Wales, Australia. In this population of WBBAs, groups included many close genetic relatives, and neighboring groups also shared several related individuals. There were multiple breeding pairs within most groups, and reproductive behaviours between breeding pairs were similar to those of many biparentally breeding songbirds. However, nest defense and postfledgling care were undertaken by large cooperative groups. In free-living WBBAs, plasma levels of testosterone (T), estradiol (E2), progesterone (P), prolactin (Prl), and corticosterone (B) were measured, and laparotomies were performed to ascertain gonadal condition. Endocrine profiles in WBBAs were similar to those reported for a number of passerines and likely reflected physiological changes necessary for breeding, such as spermatogenesis and ovulation. Males� T profiles resembled those of some polygynous passerines, in that plasma T levels rose after the completion of the female partners� clutch. This may reflect the possibility for extra-pair copulations in WBBA groups with multiple breeding females. There was some indication that WBBAs� endocrine system may have been fine-tuned to support alloparental behaviour. In adult males that chaperoned fledglings and juveniles, elevated plasma Prl titres may have facilitated alloparental care. Furthermore, elevated plasma P levels in some adult females may have been related to non-breeding behaviour and perhaps also to care of post-fledging young. Unlike many temperate zone species, many WBBAs maintained recrudesced gonads for much of the year, reflecting their extended (if not perpetual) breeding season. Studies on captive WBBAs were conducted in aviaries at the University of Wollongong. Extensive observations were undertaken to investigate intragroup social structure and associated allofeeding behaviour. Despite an absence of aggression, intragroup social structure appeared stratified and was maintained by ritualised behaviours and vocalisations. In particular, allofeeding behaviour appeared to act as an important social signal within groups, indicating high social standing of the feeder and low social status of the receiver. Plasma levels of B and Prl were measured and compared to social factors, but I found no hormonal correlates to WBBA social status or behaviour in groups with stable social structures. To further examine the relationship between the endocrine system and social behaviours and structure, manipulative experiments were carried out on captive WBBAs. Removal of group members from socially stable groups elicited no overt aggression, and exchange of members between groups elicited little aggressive behaviour; however, both experiments resulted in significant social restructuring. Nevertheless, I found no significant hormonal correlates (T, E2, and B were measured) to social instability caused by these perturbations. Another social behaviour, roost nest building, was correlated with elevated plasma T and E2 levels, in some months of the year but not others. Field and captive studies of the WBBA supported hypotheses suggesting that (1) alloparental behaviours evolved via kin selection mechanisms and (2) alloparental behaviours are important signals of quality used to help select mates and/or attract collaborators. In WBBAs alloparental behaviours seem to be either directed toward kin or co-opted as a means of advertising social status.

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