Takahe (Porphyrio hochstetteri) are one of the world’s most endangered birds. There are only around 120 still surviving in Fiordland, although a few more now live on predator-free islands off the New Zealand coast. (If you go to Tiritiri Matangi Island in the Hauraki Gulf you’ll be bound to see them.) But while the birds are intensively managed, there hasn’t been a lot of research into their breeding systems.
A takahe on Tiri
And this is important – in small populations genetic drift and inbreeding can have profound effects on the gene pool, reducing the amount of genetic variation, & so breeding in these populations is often managed quite intensively. And while many people tend to think of birds as faithful monogamists, in fact there’s often a lot of hanky-panky going on. For example, Massey University scientist Isabel Castro studied mating systems in stitchbird & found all sorts of goings-on: polygyny, polyandry, rape…
So in 2002 Marieke Lettink and her colleagues used DNA technology to study the breeding system – and the genetic variation – of takahe. They comment that [knowledge] of a species’ mating system along with other life history parameters, is valuable when calculating the effective population size and its effects on the rate of inbreeding and loss of genetic variation. Scientists have always assumed that takahe are monogamous, but there’s always the chance of what are called ‘extra-pair’ copulations, particularly for the island populations because breeding territories there are close together. However, it’s not easy to catch takahe in the act, and lack of accurate knowledge of who begat whom can cloud our understanding of the level of inbreeding in these birds. Thus one aim of the team’s study was to determine whether the apparent, ‘social’ monogamy of takahe translated into actual genetic monogamy, using DNA profiling – a technique which would also give an estimate of the amount of genetic diversity in the population.
They focused on takahe from the offshore islands, because records indicated that breeding in these island populations was slower and less successful than in the Fiordland population. 52 birds were sampled, ranging in age from 5-month-old juveniles to fully mature individuals, and where possible samples were taken from all members of a family. The team assumed that the mature female in a territory was the mother of any chicks present, because there are no records of nest parasitism in takehe.
The DNA profiling data allowed the researchers to determine paternity for 73% of the offspring studied – & in all these cases the father was the resident male on the territory. They concluded that the likelihood of extra-pair copulations was low, despite the relatively high density of breeding pairs on the islands, and that this was consistent with the assumption that island takahe are genetically monogamous. This is consistent with results for other large, long-lived birds species that form long-term pair bonds & have only a few offspring that are dependent on their parents for long periods of time.
The data also showed a low level of genetic variation in both Fiordland and island takahe, similar to the results for other bird species that have passed through a genetic bottleneck (the Black Robin is an extreme example). This low level of variation is to be expected, since takahe numbers declined rapidly following Maori arrival, & the birds were extremely rare by the time Europeans got here. (They were in fact believed to be extinct, & were rediscovered only in 1948.) It may also explain the poor breeding success of the island populations, where fertility, hatching & fledging rates are all lower than in the Fiordland group.
One possible explanation put forward by Mettinke & her co-workers is what’s called ‘environment dependent inbreeding depression’: while they may be well adapted to the Fiordland environment, they have reduced fitness in the differing environments of the islands. And yet the highly inbred black robins don’t seem to have this problem – clearly, fertile ground for further research 🙂
M. Lettink, I.G. Jamieson, C.D. Millar & D.M. Lambert (2002) Mating system and genetic variation in the endangered New Zealand takahe. Conservation Genetics 3: 427-434