The skeletons of female (larger, background) and male (smaller, foreground) Dinornis robustus, with a pigeon skeleton for comparison. From Allentoft et al 2010.
A little more than 700 years ago, multiple species of the gigantic, flightless birds called moas were still running around New Zealand. They ranged over almost the entirety of the North and South Islands, from the coast to the mountain forests, but when the Maori people arrived in the late 13th century the birds were quickly driven to extinction. Within a few hundred years they were entirely wiped out (along with the immense Haast’s eagle, which fed on the moas), but fortunately for scientists these birds left behind vast accumulations of bones.
Two such moa graveyards are the Pyramid Valley and Bell Hill Vineyard sites on South Island. Together they record the presence of four moa species (Dinornis robustus, Emeus crassus, Euryapteryx curtus, Pachyornis elephantopus) over the course of the 3,000 years prior to the arrival of the Maori, and these sites presented scientists with the opportunity to recover ancient DNA from a large sample of bones to investigate the population genetics of the birds, including the sex of each individual. As they collected and analyzed the genetic data, however, they found something they were not expecting. In each species and across both deposits, females, which are considerably larger and heavier than males, were significantly more common, with an average of five females for every one male out of a sample of 227. What could could account for this disparity?
Breakdown of moa population dynamics at the two South Island sites. The top chart shows the occurrence of each species of moa and the bottom chart shows the number of males and females at each site. From Allentoft et al 2010.
The authors of the new study consider two scenarios. The first is that many males died as subadults or adults, hence creating an excess of females. Among large flightless birds it is common for males to care for the young, something which requires a large energetic investment, and it may be that many males were quickly “burnt out” by their parental responsibilities. If this was the case, the deposits at each site could be taken as an accurate reflection of the moa populations which lived in the area.
The alternative hypothesis is that females were somehow more likely to become preserved at the Pyramid Valley and Bell Hill Vineyard sites. Even though females outnumbered males at both sites, the ratios significantly differed between them despite representing the same time period and being only about six kilometers apart. The authors take this as a sign that there was some kind of bias which skewed the sex ratios in the sample, perhaps being the behaviors of the moas themselves. Since the area was near a waterhole, a valuable resource, large female moas may have driven off many of the males, or males may have been incubating eggs elsewhere at the time the deposits were made.
These hypotheses are not necessarily mutually exclusive. Male moas may very well have suffered higher mortality as a result of rearing offspring and also been less numerous in the area (either because of parenting duties or being driven off by females) during the time periods when bones were deposited at the sites. Further studies of bone accumulations from different kinds of depositional environments will be required to better understand the population dynamics of moas. Even so, it is amazing that such a level of resolution can be achieved through a combination of genetic and fossil information, and such interdisciplinary efforts will undoubtedly help us to better understand the lives of these unique, extinct birds.
Allentoft, M., Bunce, M., Scofield, R., Hale, M., & Holdaway, R. (2010). Highly skewed sex ratios and biased fossil deposition of moa: ancient DNA provides new insight on New Zealand’s extinct megafauna Quaternary Science Reviews, 29 (5-6), 753-762 DOI: 10.1016/j.quascirev.2009.11.022