In the lower Rio Grande Valley of south Texas, more than half the nesting groups of Harris's hawks (Parabuteo unicinctus) include at least one auxiliary group member in addition to a breeding pair. To provide further insight into cooperatively breeding raptors, I evaluated sociality in Harris's hawks through the dual benefits framework. I explored the formation, structure, and stability of cooperative group formation across a spatially variable study area, which includes high levels of urbanization and development as well as remote, undisturbed native habitats with low anthropogenic impact. I used color banding, regular censuses of active territories, and a microsatellite relatedness analysis to examine patterns of sociality, including delayed dispersal, the effect of auxiliary group members on reproductive output, parentage of broods, and the relatedness of auxiliaries compared to the nestlings in their territories. I confirmed cooperative polygamy with genetic techniques for the first time in Harris's hawks and found 58% of juvenile hawks delayed dispersal for at least 6 mo. Using the dual benefits framework, I found social associations that formed through delayed dispersal followed predictions for resource-defense benefits, but sociality among mature non-related hawks more closely followed predictions associated with collective action benefits, specifically reproductive output was significantly reduced in undeveloped habitats, presumably due to a less predictable prey-base.

Although group-living is widespread in animals, the degree of social complexity varies markedly within and among taxa. One important precondition for the evolution of higher forms of social complexity is increasing group size. However, this imposes a challenge: finding sufficient food for growing numbers of individuals. One hypothesis is that the (in)ability to avoid resource competition as group size increases, could partly explain variation in social complexity among vertebrates. Increasingly, evidence suggests that resource competition can be reduced via three forms of individual specialisation. These are foraging niche specialisation, specialisation to a role under division of labour (DoL), and as a mediator of these two, personality variation. Yet few studies have directly investigated the role of these specialisations in mediating the costs of increasing group size in social vertebrates. In this thesis, I first review the evidence to date that specialising to a foraging niche, and/or to a task under DoL, is (1) mediated via personality variation and (2) can be a means of reducing competition, generated by increasing group size, in social species (Chapter 2). Then, using the cooperative breeding chestnut-crowned babbler (Pomatostomus ruficeps) as my model system, I empirically test some of the hypotheses posed in this review, regarding foraging niche specialisation and associations with personality variation. In Chapter 3, I show that babblers do show personality variation in traits likely to facilitate niche segregation, and in Chapter 4 that variation among individuals within groups is sufficient to lead to intragroup niche specialisation. However, I find that the level of variation within groups is not associated with group size. Then in Chapter 5, I show that in a direct measure of foraging niche, there is only limited evidence for intragroup specialisation, and again that any specialisation is not associated with larger group sizes. I therefore find no evidence that niche specialisation is a means through which babblers can overcome the costs of increasing group size. I discuss the implications of these results for the rise of social complexity in this system, and social vertebrates generally.

No description available.

Thesis (Ph. D.)--University of Chicago, Committee on Evolutionary Biology, August 1999. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--Oregon State University, 2006. / Printout. Includes bibliographical references (leaves 173-195). Also available on the World Wide Web.

Thesis (Ph. D.)--University of Chicago, Committee on Evolutionary Biology, March 2001. / Includes bibliographical references. Also available on the Internet.

Large individual differences in cooperative contributions are common within animal societies such as cooperative breeders, where helpers care for offspring which are not their own. Understanding this variation has been a major focus in behavioural ecology and while evidence has shown that individuals are capable to adaptively adjust their cooperative behaviours, the physiological mechanisms underlying such adjustments remain poorly understood. Steroid hormones are prominent candidates to regulate cooperative behaviours due to their ability to integrate internal physiological state and environmental stimuli to produce an adaptive behavioural response. In this thesis, I investigate the effects of two steroid hormones, Cortisol (CORT) and Testosterone (T), in the regulation of cooperative behaviours in the Damaraland mole-rat (Fukomys damarensis). Because these hormones are susceptible to both modulate and be modulated by cooperative contributions, I experimentally tested both sides of this relationship. I show that, despite the absence of correlation between CORT and T and cooperative contributions, experimental increases of cooperative contributions elevate CORT levels, but not T (Chapter 3). Additionally, experimental increases of CORT levels in female helpers raised their cooperative contributions by more than one half demonstrating the regulatory effect of CORT on cooperative behaviours (Chapter 4). As breeding opportunities are likely to affect cooperative contributions and because T is a likely candidate to mediate a trade-off between future reproduction and current cooperation, I tested the effects of experimental increases of T levels in female helpers. I show that such elevations have no measurable effect of aggression, dispersal tendencies (both important to attain a breeding position) or cooperative contributions (Chapter 5). Taken together, the results of this thesis demonstrate that CORT can both respond to and regulate cooperative behaviours and suggest that this hormone may play a major role in the adaptive regulation of cooperative behaviour.

This study addressed two aspects of ecological theory developed primarily in North America and examined these theories using an Australian passerine as a model species. The first theory concerns the mechanisms by which habitat fragmentation affects avian populations. I investigated the mechanisms causing the decline of the Brown Treecreeper (Climacteris picumnus) in fragmented habitat, and specifically considered the effects of isolation and habitat degradation, which are potentially important in Australian woodlands, and edge (patch size), which are important in North America. Brown Treecreeper groups were as productive in isolated patches as in connected patches of habitat regardless of patch size, yet unpaired males were common in isolated fragments of habitat. I conducted a field experiment that confirmed that female dispersal was disrupted among isolated fragments. Thus, my results suggested Brown Treecreepers were declining due to disruption of dispersal by habitat fragmentation rather than degradation or edge effects. I compared the results of an individual-based, spatially explicit simulation model to field observations and concluded that territory spatial arrangement and matrix composition altered dispersal success, recruitment, and subsequent population growth. With the aid of a geographic information system, I determined that both landscape factors (fragmentation patterns within 4.5-km) and habitat characteristics (cavity density) explained Brown Treecreeper presence and absence from random locations in woodland habitat. The birds appear to be absent from suitable habitat in unsuitable landscapes. The second theory I addressed concerns the maintenance of avian cooperative breeding. The most widely accepted models to explain cooperative breeding suggest that individuals that delay dispersal obtain a payoff under conditions in which the quality of breeding positions varies greatly. These models arose chiefly from a few long-term studies in North American. This is an unfortunate bias because the occurrence of cooperative breeding among birds of Gondwanan origin is 22%, whereas the worldwide incidence is only 3%. I used demographic and habitat data to examine the influence of habitat and cooperative breeding on Brown Treecreeper fitness. Group size affected one component of fitness and habitat variables affected another. High cavity density may be favorable due to intense inter-specific competition for suitable cavities, which Brown Treecreepers require for roosting and nesting. Low tree density may be advantageous by favoring ground foraging, in which Brown Treecreepers frequently engage. Experimental manipulations of important habitat variables are needed to determine whether variability in these ecological factors is critical in maintaining group formation in this species. / Ph. D.

Thesis (Ph.D.) -- McMaster University, 2007. / Supervisor: Sigal Balshine Includes bibliographical references.

Thesis (M.S.)--State University of New York at Binghamton, Department of Biological Sciences, 2008. / Includes bibliographical references.