The Cause Of Extra-pair Copulations In Varied Tit (Parus Varius)

Birds may form a mating system when they are in breeding season,while the mating system varies in different avian species.Monogamy is the most common mating system in birds.92%avian have been regarded as monogamy.However,with the application of molecular tools,extra-pair copulations(EPCs)have been found in many monogamous birds especially in passerine.It is now commonly accepted thatgenetic mating systems can not be predicted by simply observing the pattern of social behaviors.Varied Tit in the genus Parus and have been described as monogamy from the field observations.It is apparent that the monogamy of Varied Tit is not certainty in genetically.We have taken bloods from Varied Tit to perform paternity identification arid found that the mating system of Varied Tits was not the strict monogamy in genetically.The level of EPC is relatively high in passerine.There are generally four hypotheses to explain the reason of EPC in monogamy birds and they are fertility assurance hypothesis,genetic diversity hypothesis,genetic compatibility hypothesis and good genes hypothesis.The main purpose of our study is preliminary to test and verify the applicability of "good genes" hypothesis in explaining the reason of EPC in Varied Tit.We performed the sex and paternity identification for 251 offspring from 40 broods during the breeding season from March to July in 2009,2010 and 2011.We also calculated the sex ratio of offspring,physical indicators of the parental birds and offspring,the different survival competitiveness between within-pair offspring(WPO)and extra-pair offspring(EPO)and other breeding parameters.Results show that:(1)Totally,30.68%of offspring were not the genetic offspring of their social fathers and at least one EPO occurred in 55%(22/40)of the broods.Among the 22 EPP broods,53.85%offspring were EPO.(2)The sex ration of all 251 offspring was 1.09:1(131:120)and there was no significant deviation from 1:1(p>0.05).The sex ratio of 174 WPO was 0.93:1(84:90)and there was no significant deviation from 1:1(p>0.05).The sex ratio of 77 EPO was 1.57:1(47:30)and there is a certain degree deviation to male,but did not reach a significant level(p>0.05).The sex ratio of EPO in 2011 was 2.08:1(25:12)which reached a significant level(p<0.05).(3)There was a significant variation(p<0.05)between social father and extra-pair male in the length of middle toe,while there were no significant variations in other physical indicators(body length,wing length,tail length,mouth length,mouth wide,tarsus length,hallux length and weight).There were no significant variation between social mother and extra-pair female in all the physical indicators.(4)Comparisons of physical indicators between WPO and EPO were performed when they were 12-days old and the results showed that only in the mouth wide occurred a significant difference(p>0.05).(5)There was an extremely significant variation in the long diameter(p<0.01)and a significant variation in weight of the egg(p<0.05)between the within-pair broods and extra-pair broods,while there was no differences in the short diameter,hatching rate and flying rate(p>0.05).In summary,the proportion of the extra-pair broods of Varied Tit reaches up to 55%and the proportion of EPO is 30.68%.The level of EPP in Varied Tit is relatively higher than other passerine.The length of middle toe of male in extra-pair brood is remarkably longer than male in no EPP brood,while there are no significant variations in other physical indicators.We can infer that female might be the sponsor of EPC and they were in the dominant position.The mouth wide of EPO is significant wider than WPO.The mean size and weight of egg in extra-pair broods are greater than others.The sex ratio of offspring in extra-pair broods is a certain extent preferred male.That is to say,the competitiveness of EPO is stronger than WPO.Therefore,we could support the "good gene" hypothesis from the perspective of offspring,while the evidence from parents is insufficiency.Reasons are followed:(1)the sample size is relatively small,so we can not fully prove the preference of offspring in extra-pair broods;(2)there is no clear report about the function of middle toe and there is also no sufficient evidence to prove the reason of EPC was because the poor quality of social father.