Single male hihis contribute to population conservation and mitigate the effects of inbreeding by mating randomly with females.
The hihi birds are a rare species, found only in New Zealand, and it is believed that only about 2,000 are left in the wild. They have been closely monitored since being reintroduced to New Zealand, providing an opportunity for their mating behaviours to be observed and recorded.
Hihi males display two mating behaviours. They can either be paired territorial or unpaired floaters, with the hihi male being able to switch between behaviours across their lifetime. While both types of males are reproductively mature and can engage in extra-pair copulations (EPC), the floater hihi male “specializes” in this sort of mating. In EPC, the floater male seeks already paired females to mate with—using either solicited or forceful means to do so—without the intention raising the brood. In effect, the floater male hihi maintains to be an eternal bachelor; that is, they are able to pass on their genes to the next generation without pairing with any female hihis.
Researchers from the Zoological Society of London (ZSL) studied the hihi birds (Notiomystis cincta) and recently published their findings in Evolutionary Applications. Of interest to the researchers was identifying factors that promoted floater behaviour and whether such behaviour resulted in successful fertilization of the female hihis. They used a closed population sample of ~150 hihi birds (40% female, 60% male), monitored every individual in the population, and recorded a detailed pedigree.
Age was found to be the strongest determinant of floating behaviour and occurred either when the floater male hihi was young (one year) and inexperienced in defending its territory, or old (over five years) and lacking energy to defend its territory. More interestingly, however, was the finding that mating behaviour did not have a significant heritability factor for floater male hihis. Instead, mating behaviour had a strong environmental component. This essentially means that researchers were not able to find a significant link between the genetic makeup of the male hihi and their consequent mating status; however, the permanent environment, such as population density, was a greater predictor of the male hihi’s mating behaviour.
This is the first study linking floater behaviour to species conservation, as it was previously assumed that floater males did not mate and produce offspring. Floating, as a breeding strategy, can be fruitful for males that invest energy into mating pursuits instead of protecting their brood—a risky tradeoff given the territorial nature of other males.
The researchers note that the reproductive success of floater males was lower than that of territorial males, an expected finding provided that territorial males are morphologically superior. Nevertheless, floater males are able to engage in EPC and contribute to the genetic diversity of future generations. For the hihi bird and vulnerable species alike, floater males serve an important role in population reproduction and its perseverance.
From an evolutionary perspective, the outcome of floater males mating with multiple females is that a greater gene pool is established while also reducing inbreeding and its effects amongst the population. This is especially important for the hihi population because of their susceptibility to diseases and possible extinction. Interestingly, the promiscuity of the floater male hihis helps mitigate these threats when they mate with multiple females by introducing new genes to the species’s gene pool and thus allowing natural selection to run its course.
Previously overlooked, floater hihi males are important players in propagating their own species.