Brett Smith for redOrbit.com – Your Universe Online
Sexual dimorphism, in which male and females of the same species differ significantly in appearance, is fairly common among birds. Typically, the male of the species either towers over the female or is equipped with elaborate plumage, as in the case of the peacock. However, for New Zealand´s extinct, flightless giant moa, the roles were reversed, with the female often weighing three times as much as her male suitors.
According to a new report in Proceedings of the Royal Society B, the moa´s environment and lack of competitive large herbivorous mammals probably allowed for the evolution of larger females within the species.
“A lack of large land mammals — such as elephants, bison and antelope — allowed New Zealand’s birds to grow in size and fill these empty large herbivore niches,” explained study co-author Samuel Turvey, a research fellow at the Zoological Society of London (ZSL).
“Moa evolved to become truly huge, and this accentuated the existing size differences between males and females as the whole animal scaled up in size over time.”
To investigate this unusual phenomenon, Turvey and his ZSL colleague ValÃ©rie A. Olson examined the physiological remains of moas and their closest relatives, including other ratites — large, flightless bird species such as emus and cassowaries. Using the known lengths of femur specimens from these birds, the researchers were able to calculate an approximate body mass of the birds.
The team was looking to determine whether the pronounced sexual dimorphism was unique to the giant moa or if its nearest relatives also exhibit this species feature, but the dimorphism is less obvious due to their smaller scale.
They found that evolutionary pressures allowed the moa to get larger over the course of generations, with the females unusually being disproportionately more affected. Meanwhile, the moa´s closest relatives experienced sexual dimorphism much closer to those expected of their evolutionary group.
“We compared patterns of body mass within an evolutionary framework for both extinct and living ratites,” Turvey said. “Females becoming much larger was an odd side-effect of the scaling up of overall body size in moa.”
In their conclusion, the authors noted that the male moa of New Zealand evolved on a path much closer to other members of their phylum.
“In contrast to male“¯Dinornis, female Dinornis body masses, therefore, evolved at least slightly independently of phylogeny, indicating that there has been some adaptation of female body size to ecological conditions,” they wrote.
The researchers also noted the moa´s slow growth rate as a clue to why the females became larger than the males.
“Intraspecific competition was, therefore, probably a major factor in moa evolution, with selection for increased investment to produce competitively fit offspring probably more intense among females than males,” the report said.
Because the moas´ experienced such a relatively long transition into adulthood, they most likely need a mother that was capable of protecting them and providing nourishment over a longer period of time. Hence, the larger female body size, the research team said.
Several different moa species inhabited New Zealand until about 700 years ago when they were hunted to extinction by the first Polynesian settlers. According to archeological records, the“¯indigenous MÄori“¯peoples still hunted them at the beginning of the 1400s, often driving them into pit traps and pillaging their nests.