A groundbreaking study conducted by researchers from the University of Sheffield, the Zoological Society of London, and the University of Auckland, New Zealand, has provided new insights into the reproductive challenges faced by the endangered hihi, a rare songbird native to New Zealand. By analysing 10 years of data, the team has established a link between small population size, sex ratio bias, and reduced fertilisation rates in wild animals, shedding light on the significant reproductive challenges faced by threatened species with limited numbers and imbalanced sex ratios.
Over 4,000 eggs were analysed and the fertility of nearly 1,500 eggs that failed to hatch was assessed, revealing that infertility accounts for an average of 17 per cent of hatching failures in the hihi. The majority of hatching failures were found to be caused by early embryo death, with embryos being most vulnerable within the first two days of development. Interestingly, no significant difference in survival rates between male and female embryos or any impact from inbreeding was observed. The study also showed that infertility rates were higher during years when the population was smaller and male numbers exceeded female numbers, suggesting that increased male harassment of females may play a role in reducing fertility.
The hihi, known for its high levels of female harassment by males and frequent extra-pair paternity, exemplifies the reproductive challenges faced by species with skewed sex ratios. In extreme cases, females may face up to 16 forced copulations per hour, which can be energetically costly and stressful, potentially contributing to reduced fertility. By understanding the impacts of population size and sex ratio on fertility, conservationists can better manage the numbers and composition of animal populations, ultimately improving fertility rates and increasing reproductive success in conservation programs.
Fay Morland, lead author of the study and a PhD student at the University of Sheffield, highlighted the importance of understanding the causes of embryo mortality at the early stages of development, which is the most common reason for hihi eggs failing to hatch. Morland emphasized the urgent need for more research into the reproductive challenges faced by threatened species to better understand and mitigate the factors driving their risk of extinction. Dr Nicola Hemmings, leader of the research group, emphasized the crucial role of managing population composition in improving fertility rates in endangered species, particularly in cases where a male-biassed sex ratio is present.
The study’s findings provide critical insights into the factors affecting fertility in wild animals, particularly endangered species like the hihi. By uncovering the link between small population size, sex ratio bias, and reduced fertilisation rates, the research demonstrates the importance of considering these factors in conservation efforts. Understanding the impact of male-biassed sex ratios on fertility rates can help conservationists develop strategies to manage and improve reproductive success in threatened species, ultimately increasing their chances of survival. Further research into the reproductive challenges faced by endangered species is urgently needed to identify and address the specific factors that may be driving their risk of extinction.