The Evolution Of Cooperatively Breeding Society In Birds-the Role Of Climate

Cooperative breeding in vertebrates describes social systems wherein a breeding pair plus one or more helpers rear a single brood of offspring.Helpers aid parental activities,such as constructing nests,defending territories,incubating eggs and provisioning young.Among the nearly 10000 birds in the world,about 9%are inferred as cooperatively breeding species in which more than 400 species are confirmed as regular cooperative breeders.Helping has long been regarded as a paradox against the rule of Darwin's natural selection,because helping others cannot maximize their own fitness.Many hypotheses have been published to explain the paradox,including inclusive fitness theory,ecological constraints hypothesis,life history hypothesis,monogamy hypothesis and climate hypothesis.However,none hypothesis can give a general explanation of the evolution of cooperative breeding,perhaps because of the complexity and diversity of cooperative breeding systems.Environment conditions play a key role in survival and breeding of birds.Historically,climate variation had a significant effect on animals' distribution,geographical expansion,and the evolution of animals' behavior.This study attempts to further explore the evolution of avian cooperatively breeding systems,through interspecific comparative analyses and individual species investigation with an eye to the effect of climate.The following four expects are the major results.(1)The effect of climate on the sex composition of helpers.Help in caring for offspring among cooperative birds is usually undertaken by males only or by both sexes,which is species-specific.Sex bias in helping should evolve in response to sex-specific ecological constraints on independent breeding,with mate shortage for males and breeding vacancy shortage for both sexes.Given that male-biased adult sex ratios are prevalent among birds,I predict that male-only helping mainly occurs in temperate species where fast population turnovers deriving from low adult annual survival allow all adult females to hold breeding vacancies,whereas some males overflow as helpers,and both-sex helping in tropical species where saturated habitats prevent not only males,but also females from breeding themselves.Interspecific comparative analyses based on a dataset consisting of 138 globally distributed cooperative bird species,reveal that,as expected,adult survival tends to be associated with both-sex helping.Furthermore,sex bias in helping is predicted by latitude and ambient temperature.The findings of demographic response of species to climate as a potential determinant of bias in helper sex uncover how ecological constraints operate to limit independent breeding in sex-specific ways.(2)The effect of climate on helper effect.The additional helper effort,may lighten parents' workload leading to an increased breeder survival,but it can also boost the reproductive production of parents,when parents hold their effort unchanged.Helpers may accrue indirect fitness benefits through helping kin breeders,predicted by inclusive fitness theory,therefore they invest much more in raising related offspring than non-kin offspring.The monogamy hypothesis also suggests that helpers lower their effort with increasing extra pair paternity.Environment conditions also play a role in affecting helpers' decision on levels of provisioning,whether to fully invest in the current brood or to save energy for future reproduction.Interspecific comparative analyses show that helpers' effect on reproductive success depends on extra-group paternity and environment conditions.When more extra pair mating happens,helpers' inclusive fitness benefits lose and do not work hard any more.Helpers' additive effect on reproductive success may be balanced,when breeders take a load-lightening strategy under favorable environment conditions with less nestling starvation.However,when the environment conditions are poor,breeders cannot reduce their work effort to avoid nestling starvation,then helper's additive effect may be apparent in contrast with unhelped nests.(3)The effect of climate on temporal and spatial variation of cooperative intensity.Cooperative intensity in birds varies greatly with time and space.Previous studies on the evolution of cooperative breeding in birds focused on binary classifications of cooperative systems(cooperative or non-cooperative),with the variation of cooperative intensity neglected.According to ecological constraints hypothesis,cooperative intensity should be a response of the degree of ecological press.Survival rates,sex ratios,food availability of a population fluctuating with climates,leading to a gradient of ecological press,influence the cooperative intensity of a bird species finally.This is the first study in birds using a continuous variable-cooperative intensity through phylogenetic comparative analyses,investigate the effect of climate.Results suggest that climate has a significant influence on the degree of bird cooperative intensity.This research shows that temporal variation of proportion of helped nests is associated with temporal variation of temperature,across temperate species or tropical species.The spatial variation of cooperative intensity is related to the sex composition of helpers and ambient temperature.Species with both-sex helpers have higher proportion of cooperative nests than that with single-sex helpers and the difference is significant,suggesting that the former species facing larger ecological constraints.With helper sex controlled,ambient temperature also has a positive effect on cooperative intensity,perhaps because the increasing survival in tropical leading to more redundant helpers.(4)The effect of climate on the cooperative breeding of a population of ground tits.The ground tit is a social monogamous bird,with a facultatively cooperatively breeding system.Helpers are made up by male adults only,suffering a shortage of female mates.This investigation based on a long-term study of 11 years,was designed to clarify the underlying mechanism of the evolution of cooperative breeding.Results show that last winter temperature together with precipitation in March and April before breeding seasons predict the proportion of cooperative nests well.The three climate predictors jointly explained 92%variation in proportion of cooperative nests among years.