A pioneering study has delivered the most detailed estimate so far of infertility rates in an endangered wild animal species.
Over a decade of research by teams from the University of Sheffield, the Zoological Society of London, and the University of Auckland, New Zealand, has shed light on the reproductive issues confronting the endangered hihi, a unique songbird native to New Zealand.
This study is the first to connect small population sizes and biased sex ratios with lower fertilization rates in wild animals. It emphasizes the serious reproductive hurdles that endangered species face when they have reduced populations and skewed sex ratios.
The researchers examined more than 4,000 eggs and looked into the fertility of nearly 1,500 eggs that did not hatch. Their findings indicated that an average of 17 percent of hatching failures in the hihi can be attributed to infertility, while most failures arise from early embryo loss.
The study found that embryos are most at risk during the initial two days of their development. There was no notable difference in survival rates between male and female embryos, nor was there any significant effect from inbreeding. Furthermore, infertility rates were notably higher in years when the population was smaller, particularly when there were more males than females. This suggests that increased stress from male harassment of females may contribute to these results.
Known for high levels of male harassment and frequent extra-pair mating, the hihi exemplifies the reproductive difficulties faced by species with imbalanced sex ratios. In severe scenarios, females may endure as many as 16 forced mating attempts every hour, a behavior that can be both exhausting and stressful, likely leading to decreased fertility.
By understanding how population size and sex ratios influence fertility, conservationists can more effectively manage animal populations. This knowledge can contribute to enhanced fertility rates in these species.
Fay Morland, a PhD student at the University of Sheffield and the lead author of the study, remarked: “One of our major discoveries is that early-stage embryo mortality is the leading cause behind hihi egg failures to hatch. However, the specific reasons for failures at this stage are still unclear. This underscores the urgent need for further research into the reproductive challenges faced by at-risk species so that we can better address factors contributing to their extinction risk.”
Dr. Nicola Hemmings, from the University of Sheffield’s School of Biosciences and head of the research team, stated: “Our findings underscore the necessity of understanding the elements that influence fertility in endangered species. The correlation between male-biased sex ratios and reduced fertility suggests that managing the composition of populations could be essential for enhancing reproductive success in conservation efforts.”
