Breeding pair

Abstract: Evolutionary researchers have recently suggested that pre-modern human societies habitually practised cooperative breeding and that this feature helps explain human prosocial tendencies. Despite circumstantial evidence that post-reproductive females and extra-pair males both provide resources required for successful reproduction by mated pairs, no study has yet provided details about the flow of food resources by different age and sex categories to breeders and offspring, nor documented the ratio of helpers to breeders. Here, we show in two hunter–gatherer societies of South America that each breeding pair with dependent offspring on average obtained help from approximately 1.3 non-reproductive adults. Young married males and unmarried males of all ages were the main food providers, accounting for 93–100% of all excess food production available to breeding pairs and their offspring. Thus, each breeding pair with dependants was provisioned on average by 0.8 adult male helpers. The data provide no support for the hypothesis that post-reproductive females are the main provisioners of younger reproductive-aged kin in hunter–gatherer societies. Demographic and food acquisition data show that most breeding pairs can expect food deficits owing to foraging luck, health disabilities and accumulating dependency ratio of offspring in middle age, and that extra-pair provisioning may be essential to the evolved human life history.

Abstract: We have begun an intensive series of investigations into the environmental and endocrine mechanisms regulating cooperative breeding in a semi-arid tropical environment. A colour-marked population of white-browed sparrow weavers (Plocepasser mahali) was studied at a field site near Nyamaluma Camp, Luangwa Valley, Zambia (13dG S). This species breeds from October to April, although some clutches may be found as early as August and young may be fed until May. Non-disruptive techniques were employed to monitor annual cycles of gonadal development, body mass and fat reserves, and small blood samples were collected to measure circulating levels of reproductive hormones and corticosterone. All individuals were released after sampling and the social and reproductive activities recorded by systematic behavioural observations. Each individual could thus be identified as: (1) a breeding adult; (2) a related helper (i.e. offspring of the breeding male and female within the group); or (3) a non-related (=invader) helper (these birds helped defend territory but did not help feed young; they were not related to the breeding pair and usually originated from outside the group). Breeding males had larger testes and higher levels of testosterone than either related helpers or invader helpers. In females, all status groups had follicles up to 2 mm in diameter, but only breeding females ovulated. There were no differences in plasma levels of reproductive hormones among different status females, except for higher circulating luteinizing hormone (LH) in invader female helpers during the first part of the breeding season. Although plasma levels of testosterone were highest in breeding males, the maximum titres were two orders of magnitude less than in males of passerine species from temperate regions. Furthermore, injections of a peptide hormone GnRH (gonadotropin-releasing hormone) increased plasma LH levels in both males and females, and were followed by rises in testosterone levels only in males, but not over the normal maximum seen in February. GnRH-induced high levels of LH did not affect testosterone concentrations in the blood of females. Results obtained thus far indicate that the hormonal control of aggression in white-browed sparrow weavers is different from that predicted by investigations on north temperate species. Whether these phenomena are typical of tropical species, particularly cooperative breeders, remains to be determined. Since most of the world's species of birds live in the tropics, and a substantial fraction of them breed cooperatively, more investigations are needed to establish the hormonal basis of reproductive behaviour and cooperative breeding. Given the baseline data already collected, the white-browed sparrow weaver will be a useful model.

Abstract: Although several different hypotheses have been proposed to explain the evolution of helping behavior, most are based on the assumption that helping enhances the reproductive success of recipient breeders. I tested this assumption by removal experiments in the cooperatively breeding Florida scrub jay (Aphelocoma c. coerulescens). This species lives in permanently territorial social units containing a single breeding pair and none to six nonbreeders, which are usually offspring of the breeding pair and which usually act as helpers by feeding the nestlings and fledglings produced by the breeding pair. Although experimental removals of non-breeders in 1987–1988 had no significant effect on breeder survival, egg production, or hatching success, experimental groups suffered higher rates of predation on nestlings (1987) and lower rates of fledgling survival (both years) than did unmanipulated controls. As a result, experimental groups produced an average of only 0.56 independent juveniles, compared to 1.62 young for controls. Analysis of the factors contributing to nestling and fledgling mortality indicates that helping behavior per se (i.e., the aid that nonbreeders provide to dependent young), not the mere presence of nonbreeders, was responsible for the greater reproductive success observed in control groups. Because survival rates of allofeeders (i.e., those nonbreeders that provisioned dependent young) were virtually identical to those of non-allofeeders, the costs of helping behavior in this species appear to be small. Furthermore, nonbreeders are more likely to provision dependent young within their social unit when those young are closely related. I therefore conclude that nonbreeders increase their indirect fitness by serving as helpers and that helping behavior in the Florida scrub jay is a trait that has current selective utility. It remains debatable, however, whether helping in this species is an adaptation that has been shaped by the process of natural selection.