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?
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
I haven't read the paper yet, but I won't let that stop me from firing a speculative suggestion off from the hip: perhaps the larger, heavier females were more likely to become mired around the edges of the waterhole, while the smaller, more nimble males tip-toed across the mud without getting stuck?
Now off to read the paper and discover why my "explanation" sucks......
I don't understand how the first scenario is supposed to work - even if the males die younger, all individuals died sooner or later, so there should be equal numbers of dead moa of each gender around (assuming the birth ratio was approximately 50:50), even if there, at any given time, where a substantial surplus of living females around.
I agree with Andreas and this is what crossed my mind. The male Moa still died so therefore they would still have created fossils. Which makes me think they died in areas where they were not able to be preserved, which fits with the second hypothesis.
Not a scientist, but on comment no 2, I thought the point about males dying young might have been that smaller fossils are more fragile and thus more likely to disappear or disintegrate through natural processes (rain, wind, even earth movements). Not having read the paper I don't know whether that possibility would match up with the environment described.
Those are some very interesting observations and ideas.
It sounds like there is a basic assumption of a 50:50 sex ratio at birth. Is there any reason to question this assumption?
I don't know very much about the topic, but why couldn't the females be producing more female chicks. My understanding is that the Kakapo can alter the sex ratio of their chicks according to how much protein based foods they eat. Why not something similar with the Moa?
@4: The suggestion is that they died as "subadults or adults". by which time they'd be large enough one wouldn't expect a massive discrepancy in perservation potential based on size alone. They're smaller than the females, but still pretty big animals.
(That said, I'm no taphonomist, and it's possible my expectations of the size dependence of perservation potential are wrong.)
Maybe they had a social structure similar to that of African elephants or sperm whales: groups consisting of a large number of females with a single or a few males, and the rest of the males either solitary or in all-male groups that frequented different waterholes to the mostly-female groups?
Like Shiva, I think it could be behavioural. Females that hung out in groups would be easier to round up, and being larger they would be the preferred target.
Also they may have had some sort of spatial sexual segregation, like the kakapo where males live high up in the hills whereas females live lower down in the forest.
Or, explanation number three, moa had a harem-based social system, like some animals like elephant seals and baboons have today. Few males would be present in a general population due to this system, and the majority of the males would instead be loners, surviving and searching for mates on their own.
Isn't it usual for males to be larger than females in harem breeding systems?
Perhaps the Haast's Eagle preyed on the smaller of the sexes, aka the males. Since predatory birds are rarely seen hunting in packs, why would an eagle take on the much larger female when a smaller male would fully satisfy its appetite? Perhaps what we are exhibiting is a sexual altruism. What would precipitate is fewer males in the population. Since female numbers determine population size and not male numbers, the population would not experience a decline if mainly males were preyed upon. This could help to explain how a population of such giants could persist on a small island under top-down regulation.