Social Organization And Mating Choice Of The Cooperatively Breeding Tibetan Ground Tit Pseudopodoces Humilis

Mate choice is an important component in avian mating system, there are many research about the significance of mate choice for mating behavior, and heated debate is focus on the factor effect on the process of mating choice. Tibetan ground tit is a typical passerine inhabiting the Tibetan plateau, which is a facultative cooperatively breeding bird and the level of extrapair parentage in population is relatively high. Then we consider that there may have complex mechanisms of female extrapair mate choice in this species. Results showed that extrapair mates do not have higher genetic heterozygosity than the same-sexed social mate, and heterozygosity of extrapair offspring is not significantly higher than their within-pair nestmates, no matter in cooperative or bi-parental brood. All of results fail to support the genetic diversity hypothesis or genetic compatibility explain to the mating choice in Tibetan ground tits. As regards the influence of breeding synchrony on extrapair parentage, we found synchrony index cuckolding and cuckolded broods are not significantly higher than that between broods without extrapair offspring. Furthermore, there is a significantly negative relationship between distance among cuckolding and cuckolded nests and the number of extrapair offspring, apparently, the frequency of extrapair copulation decreases by the nest density increases.In many cooperatively breeding species, the society typically consists of nuclear families that form through the delayed dispersal of grown offspring who help their parents rear subsequent broods. Following Hamilton's rule, high relatedness serves to increase fitness by facilitating kin selection and the evolution of cooperative behaviors. However, on the other hand, restricted dispersal of offspring can lead to clustering of closely related individuals, high level of relatedness among the group members will increase the likelihood of inbred matings, which may result in a decline in individual genetic diversity and fitness known as inbreeding depression, such as the decreased reproductive fitness. Relatedness analysis indicated inbreeding occurred in approximately 47.8% of matings during four continuous breeding period, which suggests inbreeding occurred but, neither social nor extrapair mate choice in cooperative or bi-parental brood was used to avoid inbreeding facultatively. Moreover, not only we did not found the heterozygosity of extrapair mate is higher than that of the corresponding social mate, but also heterozygosity of extra-group extrapair offspring were found lower than their within-pair brood mates, which is suggest that inbreeding does not decrease genetic diversity of offspring.Four-year data of dispersal in Gahai population of Tibetan ground tit, on the northeastern Tibetan plateau, the breeding ground tit population varies annually in composition, therefore, breeding pairs do not inherit their territory in last year. Totally, the pattern of dispersal shows a special small-group dispersal, when breeding season is over, female fledglings disperse to other territories firstly, leaving the small group consist of male offspring later disperse step by step. During the process of dispersal exotic female individual will take part and the social mate choice will be made in these groups, then the groups break up and every pair take independent reproduction in the next breeding period. Besides, the higher level of relatedness among male members also suggests a female-biased natal dispersal of offspring, which means that there may be some other mechanism of inbreeding avoidance in this Tibetan ground tit population. Anyway, more work is required to investigate the relationship between dispersal and social structure in this species.