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 (λ) 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.

Figure 1.

Male and female body sizes and SSD in ratites. Minimum, median and maximum body size estimates shown for Dinornis in bold. Dashed line represents slope of 1.0. Ap, Apteryx; Ca, Casuarius; Em, Emeus; Eud, Eudromia; Eury, Euryapteryx; Di, Dinornis; Dr, Dromaius; Pa, Pachyornis; Rh, Rhea; St, Struthio; Ti, Tinamus. (Online version in colour.)

Figure 2.

(a–f) Hypotheses of phylogenetic relationships between extinct and extant palaeognath genera, and showing ratios of male : female phylogenetically independent body mass contrasts among ratites exhibiting female-biased SSD. Pies plotted at nodes of candidate phylogenies indicate value of male contrast (dark) relative to that of female contrast (pale); pies that are 50% dark and 50% pale indicate equal contrast values for both sexes. When testing Rensch's Rule in taxa with female-biased SSD we expect male contrasts to be larger than female contrasts (i.e. pies should have larger dark than pale segments), whereas in taxa with male-biased SSD the reverse should be true. (Online version in colour.)

Figure 3.

Estimates of (a) λ for male and female ratite body masses, (b) root body mass for male and female ratites, and (c) root SSD of body size, across six phylogenies, three estimates of Dinornis body mass, and including all genera. Male trends shown as solid lines and female trends shown as dashed lines in first two plots.

Figure 4.

Estimates of λ plotted against root estimate of palaeognath male body mass (a) and female body mass (b), using median estimates of Dinornis body mass and averaged across all six phylogenetic hypotheses, showing how excluding each genus from analyses changes the estimates. Dashed lines intersect at the values of λ and estimated root body mass when all genera are included in analyses. Lines radiating from plotted genera are standard error bars. If a genus is small-bodied compared with the root estimate when all taxa are included, it will appear in one of the upper quadrants, because excluding it increases the root estimate; similarly, large-bodied genera appear in one of the lower quadrants. Genera with phylogenetically atypical body masses appear to the right of the vertical dashed line. ‘Tin’ denotes change in λ and root estimate when both tinamou genera are excluded; abbreviations otherwise as for figure 1. (Online version in colour.)

Figure 5.

Estimates of λ plotted against root estimate of SSD averaged across all six phylogenetic hypotheses, showing how excluding each genus from analyses changes the estimates. Dashed lines intersect at the values of λ and estimated root body size when all genera are included in analyses. Lines radiating from plotted genera are standard error bars. Genera exhibiting strongly female-biased SSD appear above the black dashed line, whereas those with less female-biased or male-biased SSD appear below the line. Abbreviations as for figure 1. (Online version in colour.)