Highly skewed sex ratios and biased fossil deposition of moa: ancient DNA provides new insight on New Zealand’s extinct megafauna

Abstract

Ancient DNA was isolated from the bones of 267 individuals of the extinct New Zealand moa (Aves: Dinornithiformes) from two late Holocene deposits [Pyramid Valley (PV) and Bell Hill Vineyard (BHV)]located 5.7 km apart in North Canterbury, South Island. The two sites’ combined fossil record cover the last 3000 years of pre-human New Zealand and mitochondrial DNA confirmed that four species (Dinornis robustus, Euryapteryx curtus, Emeus crassus, and Pachyornis elephantopus) were sympatric in the region. However, the relative species compositions in the two deposits differed significantly with D. robustus and E. crassus being most abundant at PV while E. curtus outnumbered the other three moa taxa combined at BHV. A subsample of 227 individuals had sufficient nuclear DNA preservation to warrant the use of molecular sexing techniques, and the analyses uncovered a remarkable excess of females in both deposits with an overall male to female ratio of 1:5.1. Among juveniles of E. curtus, the only species which was represented by a substantial fraction of juveniles, the sex ratio was not skewed (10 male, 10 female), suggesting that the observed imbalance arose as a result of differential mortality during maturation. Surprisingly, sex ratios proved significantly different between sites with a 1:2.2 ratio at BHV (n = 90) and 1:14.2 at PV (n = 137). Given the mobility of large ratites, and the proximity of the two fossil assemblages in space and time, these differences in taxonomic and gender composition indicate that moa biology and the local environment have affected the fossil representation dramatically and several possible explanations are offered. Apart from adding to our understanding of moa biology, these discoveries reinforce the need for caution when basing interpretation of the fossil record on material from a single site.