The evolution of sexual dimorphism in New Zealand giant moa (Dinornis) and other ratites

Abstract

The extinct giant moa Dinornis is one of the most remarkable known examples of reversed sexual size dimorphism (RSD), with males weighing 34-85 kg, but females weighing up to 240 kg. However, there has been little consideration of the evolutionary mechanism that produced this level of dimorphism, and most living palaeognaths also exhibit varying levels of RSD. Using male and female body mass data for extant ratites and tinamous and four extinct moa genera, and tests of phylogenetic dependence (l) of body size evolution among these species, we investigated whether Dinornis was truly unusual with respect to RSD relative to other palaeognaths, which sex was under greater pressure to change in size over evolutionary time, and which candidate hypotheses explaining the presence and variability of RSD in the genus are most plausible. We demonstrate that the extreme level of RSD exhibited by Dinornis represents a straightforward consequence of positive allometric scaling of body size. However, Dinornis females have undergone more evolutionary change than males, and larger females from high-productivity environments are associated with greater differentiation, possibly driven by intraspecific competition and female-biased selection for increased offspring investment.

A refined model of body mass and population density in flightless birds reconciles extreme bimodal population estimates for extinct moa

“… Graphical comparison of the published estimates of mean moa population density (all species combined) across New Zealand. The studies are as follows: (1, Caughley ); (2, Batcheler ); (3, Anderson ); (4, Holdaway and Jacomb ); (5, Gemmell et al 2004); (6, Forsyth et al ); (7, Lee et al ); (8, Tanentzap et al ); (9, Perry et al ); and (10, this study). For Perry et al () and this study, values shown are the 95% quantile range (bars) and mean (point).…”

“…Where species are sympatric and have overlapping ecological niches, one species may be competitively dominant and depress population densities of subordinate species (Dayan and Simberloff ). Four or more species of moa occurred in sympatry at some South Island locations (Worthy 1990, Worthy and Holdaway , Wood et al , Fig. A).…”

“…However, knowledge about their biology and functional roles is crucial not only for understanding island ecosystems and evolutionary processes, but it could also help guide the selection of appropriate surrogate species for ecological restoration (Hansen ), or even determine appropriate de‐extinction candidates (Wood et al 2017b). Diet is one aspect of these species' biology that has been well studied, having been assessed using preserved gizzard contents (Burrows et al ), molecular and/or microscopic analysis of coprolites (James and Burney , Wood et al ), dietary stable isotopes (Clarke et al ) and morphology of the bills and skulls (Mourer‐Chauviré and Balouet , Attard et al ). Yet many other fundamental aspects of their biology, including population abundance and density, remain poorly understood.…”

“…In addition to furthering our understanding of past island ecosystems (Caughley , Forsyth et al , Lee et al , Wood et al ), population density data for large flightless birds can also help us to understand extinction processes, for example by quantifying the levels of harvesting required to bring about extinction within an observed time‐frame (Holdaway and Jacomb , Perry et al ). Therefore, we also assess and discuss the implications of our new density estimates for the speed of moa extinction using the harvest model developed by Perry et al ().…”

The Gastornis (Aves, Gastornithidae) from the Late Paleocene of Louvois (Marne, France)

“…An extreme reversed sexual size dimorphism was found in the Moa (Dinornithidae), extinct giant ratites endemic to New Zealand (Bunce et al 2003;Olson and Turvey 2013). In these New Zealand Moa, the largest females reached about 150 % the height and 280 % the weight of the largest males.…”