On the skewed sex ratio of the Kakapo Strigops habroptilus: sexual and natural selection in opposition?

Abstract

The Kakapo Strigops habroptilus is unique amongst parrots in many respects, not least its use of a lek breeding system. Since Merton et al. first described the phenomenon, little new information has come to light, chiefly as a result of the extreme rarity of Kakapo. Observation of an extreme sex ratio skew in favour of males has received little attention and has been considered spurious. I used subfossil material to examine the nature of the sex ratio prior to the arrival in New Zealand of humans and demonstrated that a sex bias in favour of males in the order of 2:1 existed at that time. Although apparently untenable on traditional theoretical grounds, this extreme skew has been persistent and has resulted from the interaction of sexual and natural selection on dimorphic male and female Kakapo. Relatively small, sole caring females may suffer the consequences of parental investment as a result of demands made by offspring over an extended rearing period. In particular, larger male chicks may exert a greater cost on the parent bird than do female siblings. Additionally, brood reduction in certain circumstances may favour the survival of dominant male siblings.