The Effects of a Long-Term Male-Biased Sex Ratio on Population Dynamics and Viability of Tuatara (Sphenodon punctatus)

Sex ratio is a critical parameter in the population dynamics and ecology of a species. Changes in population sex ratio can influence sexual selection and individual behaviour, which ultimately impact reproductive rates and population viability. For territorial species, sex ratio variation can influence territorial behaviour, and thus, the pattern of activity areas where animals acquire critical resources to perform biological processes. However, empirical studies addressing population consequences of sex ratio through behaviour and patterns of activity are rare. A biased sex ratio has been recognized as a major factor contributing to extinction in small populations, due to the vulnerability to environmental, demographic, and genetic stochasticity.

Species with temperature-dependent sex determination (TSD) are prone to sex ratio biases because hatchling sex ratios are influenced by thermal conditions during embryonic development. A warming climate poses an increasing concern for the population viability of species with TSD, because rapidly shifting temperatures may strongly skew offspring sex ratios. Tuatara (Sphenodon punctatus) are long-lived reptiles endemic to New Zealand. They are particularly at risk based on their rare pattern of TSD, where males hatch at higher nest temperatures.

Since the late 1990s, there has been an increasing male bias in a small, isolated tuatara population on North Brother Island. Survival and body condition of adult tuatara have declined, with a steeper rate of decline in females. Population viability analyses predicted the extinction of this population under the previous demographic parameters. However, longer term studies are needed to examine whether the sex ratio ultimately fluctuates around a more balanced sex ratio despite fluctuations based on the climate, or if the male bias becomes more extreme under climate warming.

I explored the effects of a male biased sex ratio on population dynamics and viability of tuatara on North Brother Island using updated survey data over 30 years. I estimated the current sex ratio, body condition, survival rates, and population size and temporal trends in these parameters. In addition, I quantified tuatara activity areas on North Brother Island and in two translocated populations, which can reflect resource competition and the effects of sex ratio and population density. Then, I performed behavioural observations on captive populations to investigate how dominance is associated with intra-specific interactions to understand the implications of a male bias for population viability. Lastly, I updated the population viability of North Brother tuatara population based on the recent population parameters.

There has been a continual male bias over 30 years in the North Brother Island tuatara population. Survival and body condition have declined over time, with a steeper rate of decline in females. In my recent data, I found a decrease in population size, as well as a further lengthening of the female breeding cycle. By contrast, the male bias has become less skewed over time in new cohorts of this population after 2005. Also, there has been an increase in male body condition and in survival of both sexes in new cohorts. The activity areas in males are larger than females, and activity areas became larger more recently under a smaller population size. Behavioural observations on captive tuatara showed larger males had an advantage in accessing females based on smaller distances to females, males had dominance over females through maintaining larger activity areas, and males and larger individuals had dominance in aggressive interactions. Based on the updated population parameters, the population viability analysis predicted a lower probability of extinction for this tuatara population than previous studies. However, the low female fecundity and the likely further skewed male bias under a warming climate could drive this population to extinction at a rapid rate if demographic parameters further shift.

This project revealed how a male bias can influence individual behaviour and patterns of activity areas, and impact parameters, such as body condition, survival and population size, and ultimately, influence population viability. As more new cohorts enter this population, the population could experience a less skewed sex ratio and improved survival over time. Therefore, longer-term monitoring of the North Brother Island tuatara population is needed to investigate the lagged impact of environmental temperature on sex ratio and other population parameters, as well as population viability. Changes in sex ratio can be an early indicator of decline due to the effects of climate warming for species with TSD, an issue that thus far has not been detected in other tuatara populations. This study provides implications for the conservation and management of other species with TSD, which can also be prone to skewed sex ratios in a warming climate.