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

Abstract

the other three moa taxa combined at BHV. A subsample of 227 individuals had sufficient nuclear 28 DNA preservation to warrant the use of molecular sexing techniques, and the analyses uncovered a 29 remarkable excess of females in both deposits with an overall male to female ratio of 1 : 5.1. Among 30 juveniles of E. curtus, the only species which was represented by a substantial fraction of juveniles, 31 the sex ratio was not skewed (10 ♂, 10 ♀), suggesting that the observed imbalance arose as a result of 32 differential mortality during maturation. Surprisingly, sex ratios proved significantly different 33 between sites with a 1 : 2.2 ratio at BHV (n = 90) and an entire 1 : 14.2 at PV (n = 137). Given the 34 mobility of large ratites, and the proximity of the two fossil assemblages in space and time, these 35 differences in taxonomic and gender composition indicate that moa biology and the local environment 36 have affected the fossil representation dramatically and several possible explanations are offered. 37Apart from adding to our understanding of moa biology, these discoveries reinforce the need for 38 caution when basing interpretation of the fossil record on material from a single site. 39 40

Ancient DNA microsatellite analyses of the extinct New Zealand giant moa (Dinornis robustus) identify relatives within a single fossil site

“…If adult female moa from each cohort were particularly attracted to the habitat around Pyramid Valley, whether foraging in groups of females or alone, it could explain the nonrandom relatedness distribution in our sample. It has previously been suggested that moa females maintained home ranges around the lakes, whereas males may have been more mobile (Allentoft et al, 2010).…”

“…Although 60% of the samples were from females from Pyramid Valley, the naive probability of all four related dyads belonging to this class of individuals is only 1.5%. Also, Allentoft et al (2010) showed a significant difference in the distribution of sexes and juveniles from the proximate fossil sites at Pyramid Valley and Bell Hill Vineyard (Figure 1). At Pyramid Valley, only two of 49 D. robustus individuals sampled were identified genetically as males and this extreme skew is likely a combination of higher mortality among males combined with behavioural differences or differential habitat preferences between the sexes (Allentoft et al, 2010).…”

“…Also, Allentoft et al (2010) showed a significant difference in the distribution of sexes and juveniles from the proximate fossil sites at Pyramid Valley and Bell Hill Vineyard (Figure 1). At Pyramid Valley, only two of 49 D. robustus individuals sampled were identified genetically as males and this extreme skew is likely a combination of higher mortality among males combined with behavioural differences or differential habitat preferences between the sexes (Allentoft et al, 2010). The social behaviour in extant ratites varies greatly between taxa but in southern cassowaries (Casuarius casuarius), for example, males and females have separate territories (Moore, 2007), except for a short period when breeding.…”

“…It has also been demonstrated that moa displayed reverse sexual dimorphism with the females being up to 280% heavier than the males (Bunce et al, 2003;Huynen et al, 2003). Moreover, Allentoft et al (2010) demonstrated by a molecular sexing technique that there was an overall 5:1 excess of females among 267 individuals sampled in North Canterbury. In particular, moa fossils from the shallow lake at Pyramid Valley, excavated periodically since 1939, displayed an extreme excess of 49 females to 2 males of South Island giant moa (Dinornis robustus).…”

“…Hypotheses involving female territoriality and seasonal egg incubation by males (both observed among extant ratites) have been invoked to explain the absence of males from some deposits. From the observation of an even sex ratio among juvenile moa of the taxon Euryapteryx curtus (Allentoft et al, 2010), but not among the adults, a higher mortality among males has also been suggested. Incubation by males has been suggested by Huynen et al (2010) who identified putative moa male DNA on the surface of their eggshell.…”