The mating system and stability of pairs in kiwi Apteryx spp.

Different populations of kiwi Apteryx spp. live in very different habitats and climatic conditions, and they vary greatly with regard to population de nsities and sex ratios. Populations also differ remarkably in their social and mating patterns. By comparing four populations, we asked whether kiwi m ating systems are primarily shaped by the availability of mates as caused b y different operational sex ratios ("environmental polygamy potential"), or whether they depend on the costs and benefits of desertion by either sex f rom parental care, especially on the limitations to desertion due to high p recopulatory investment ("parental limitation"). Most kiwi have long-term partnerships and very high partner fidelity, but i n one population half of all pairs split each year. This is a relatively de nse population with a strongly female-biased sex ratio and a complete lack of territorial behaviour. We argue that pair stability and territoriality a re related in kiwi, and that the loss of territoriality and the high divorc e rate in this divergent population result From the female-biased sex ratio . Data analyses did not reveal any reproductive advantage from divorce and re-mating with a different partner in kiwi. We suggest that divorce results from the interaction of surplus females with paired males and is hence "fo rced" upon the pair. Our analyses of social systems of the four populations of kiwi suggest that both territory defence and the degree of polygamy depend primarily on pare ntal demands and not on the distribution of resources and mates. Hence, we regard the "parental limitation hypothesis" as being the more adequate one to explain mating patterns in kiwi.