The Evolution Of Cooperatively Breeding Systems Of Tibetan Ground Tits Under High-altitude Environments

Cooperative breeding is a reproductive system composed of two or more breeding individuals,plus one or more "helpers" which exhibit parent-like behavior towards young that are not their own offspring.The origin and evolution of this peculiar system has engendered great interest from behavioral ecologists for decades.Although series of long-term demographic studies with the application of new research techniques have been coming to fruition,we are still far from reaching a point to have a general understanding of this phenomenon for the existence of great diversity on different species.Cooperative breeding was found to occur most frequently in tropical and subtropical areas.It is comparatively rare in cold and harsh environment.Exploration into the operating of this system at different or special circumstances remains as an effective approach for detection of the underlying mechanism for this unique social system.The Tibetan ground tit,Parus humilis,is an avian species endemic to the open alpine steppe in the Tibetan Plateau.The existence of facultative cooperative breeding system of this species was confirmed based on years of field research at a variety of locations around the Tibetan Plateau.The proportions of cooperative breeding units varied between 10-50%within populations,based on different locations and breeding seasons.The fickle and inclement climate conditions and anoxic environment set a unique background that can hardly be found anywhere else in the world.The study of evolution of life-history traits and social systems on this species in such special ecological context can provide new clues to our understanding of avian cooperative breeding system.Multiple social and ecological factors may get involved for the occurrence of cooperative breeding behavior.To fully understand the origin and evolution of this system requires for pervasive exploration on the structure and dynamic of the social groups of the concerned populations,as well as the fate and fitness gain of group members.My doctoral studies focus on the data from 10 year's fieldwork at Dangxiong(2005-2014)and 4 years data at Tianjun(2009-2014)in some chapters.The database was based on the collections and conclusions from a series of graduate students.So far a more precise understanding on the structure and dynamics of social groups and individuals had been achieved.(1)Cooperative breeding system of Tibetan ground titFor both study plots(Dang-Xiong and Tian-Jun),the majority of cooperative breeding units of were established by grown male offspring forgo their own reproduction and help their parents to raise subsequent broods on their natal territory.This breeding system of Tibetan ground tit can be assumed to be facultative'helper-at-the-nest' system commonly found among avian cooperative breeders.The proportions of the cooperative breeding units were 27%(Dang-Xiong)and 41%(Tian-Jun),respectively for each population.Most cooperative breeding groups we found only one helper at the nest(Dang-Xiong:85%,101 of 119;Tian-Jun:84%,57 of 68).The vast majority of the helpers are males,for only two helpers were identified as females for each research site(Dang-Xiong:1%,2 of 135;Tian-Jun:1%,1 of 87).The majority of the male helpers were yearlings(Dang-Xiong:83%;75 of 90;Tian-Jun:90%,35 of 39).A considerable proportion of the events the helpers were related to at least one of the breeders(Dang-Xiong:63%,81 of 129;TJ:40%,29 of 73).'Parents-Son' is the most general pattern in our known-related cooperative breeding units(Dang-Xiong:67%,54 of 81;Tian-Jun:55%,16 of 29).Ecological research of other species at Tibetan Plateau indicate the phylogenetic history as a critical factor to the occurance of cooperative breeding.The survival of ground tit individuals also largely depends on the family group living.Thus,being a help may perform as an alternate route to stay in a certain social group.There are great diversity among the features of cooperative breeding system we found at different field sites of Tibetan ground tit.Understanding the origin and evolution of this system at certain species requires for integrated analysis base on ecology,inheritance and life history trait.(2)The structure and seasonal dynamics of winter groupsDuring the winter at Dang-Xiong,individuals of Tibetan ground tit residents commonly in social groups that defended territories resemble to those used during the breeding season.The structure of the groups are relatively stable during our period of survey(November to December).Group size varied between 2-9 individuals,which includes breeders,helpers,offspring and immigrants.The population sex ratio were significantly biased towards males(0.57),and highly significant in hatch year birds(0.67),as we found more hatch-year males rather than hatch-years females remain with their parents and natal territory.Most of the immigrants are hatch year birds(banded:69%,non-banded:93%).Although we found more female immigrants,the deletion of female would not be fully compensated due to the total number of the immigrants were small.As the next spring arrived,cooperative groups were established as part of hatch year males remain with their parents and act as helpers.(3)Annual dynamics and LongevityLife history hypothesis predict that the cooperative breeding are more likely to occur in population with low annual turnover rates and mortality.In our research,however,fluctuations were detected at a large proportion of breeding groups.As only one third(Dang-Xiong:36%,Tian-Jun:33%)of the breeding pairs remain to be intact next year,about 29%of them we only found one of the breeders(Dang-Xiong:Only Males:17%,Only Females:12%;Tian-Jun:Only Males:18%,Only Females:15%),for another one third of the breeding pairs,neither of the breeders were found in side of our research areas(Dang-Xiong:35%,Tian-Jun:33%).High annual adult vanish rates were detected in both relevant populations(Males:Dang-Xiong:49%,Tian-Jun:53%;Females:Dang-Xiong:51%,Tian-Jun:53%).We found greater vanish rates at fledglings(Males:Dang-Xiong:80%,Tian-Jun:83%;Females:Dang-Xiong:89%,Tian-Jun:97%).A large proportion of male fledglings appeared as helpers nest year(Dang-Xiong:46%,Tian-Jun:97%).Vanish rates of female fledglings were significantly greater than male fledglings.The average longevity of males are(1.68±0.15 years)significantly greater than females(1.37±0.12).It is turned out the life history trait of Tibetan ground tit were not consistent with the prediction of life history hypothesis,which indicated the limitation of this theory.Habitat saturation did not appear as an critical constraint for breeding independent,it could not explain the extremely male-biased helping behaviour at two populations.High turnover rates increase the probability of acquisition of breeders' status at the next breeding season for survived helpers.(4)The natal and breeding dispersal of Tibetan ground titAs the dispersal is either delayed or limited,it would lead to spatial clustering of close relatives.Under this scenario,individuals are more likely to interact with kin.The sex-biased helping behavior maybe closed related to pattern of dispersal between two sexes.Both the breeding and natal dispersal of Tibetan ground tit in the two populations were found to be limited.Although both sexes present natal philopatry tendencies,female-biased natal dispersal can still be detected in both proportions of "further dispersers" and dispersal distances at Dang-Xiong population.But the breeding dispersal,which would only take place when breeders get widowed,were not sex biased,The female-biased natal dispersal of Tibetan ground tit were attribute to the discrepancy of intrasexual competition between young bird of two sexes.As the chance of acquiring breeders' status at territories far away from home are slim for males,most of them would breed inside or close to the natal territories,if their failed to fill a reproductive vacancy at those areas,they were force to help and wait.(5)Lifetime reproductive success:the helping effect and fitnessThe accession of helpers did not increase the numbers of fledglings for the receipients.Individuals who had ever appeared as helpers did not own a greater longevity,either,than those who appears as breeders in every breeding season of their entire life span.The reproductive success of helpers who acquired breeding status were not significantly greater than early breeders.The helpers may conduct other alternative ways to increase their inclusive fitness,or,the helping behavior per se,were just an affiliated behavior for delay dispersal.