There are lots of males out there available for females to mate with – enough for every female in our sample to have mated with a different male, and no indications of any males monopolising females.
Research published in the journal Molecular Ecology
has revealed information about the mating habits of the critically endangered hawksbill turtle, Eretmochelys imbricata
. As well as highlighting the effects of conservation measures to date, it is hoped that the findings of the study, which was carried out by a team from the University of East Anglia (UEA)
and Cousine Island
in the Republic of Seychelles, will contribute to the success of future efforts on the part of conservationists.
Hawksbill turtle numbers suffered a dramatic decline due to the international trade in tortoiseshell, causing the species to be listed as endangered by the International Union for Conservation of Nature (IUCN)
in 1996. The turtles are still threatened by egg poaching on the beaches where they come to nest, habitat loss, pollution, and being hunted for their meat.
The Seychelles are one of the remaining strongholds of the hawksbill, and Cousine Island is an important nesting site. Karl Phillips, a PhD student in UEA’s School of Biological Sciences, shared with ScienceOmega.com
details of the challenges and rewards of investigating the sex life of these creatures. As the turtles spend almost all of their time out at sea, direct studies of their behaviour – including mating habits – are logistically challenging.
"Even if one put the time and effort into an observational study of mating, one would still not know the paternity outcomes; this is a problem inherent to mating system studies on any species," Phillips explained. "DNA-based methods allow for quick, comprehensive, and detailed assessments of parentage patterns, from which more general inferences about the mating system can be made."
The researchers obtained small tissue samples from adult female turtles and hatchlings, using one of several standard procedures known to cause no significant long-term harm. DNA analysis of these samples enabled them to elucidate the number of males present and how many offspring each male had paternity of.
"Within a nesting season, we sampled every female observed on our study beach, and twenty offspring from every nest lain," Phillips related. "We then obtained DNA profiles of all females and all hatchlings, and used these to reconstruct DNA profiles of the males we never saw or directly sampled."
The results revealed that more than 90 per cent of the female hawksbills sampled had mated with only a single male, storing that male’s sperm for long periods of time in order to fertilise multiple clutches of eggs. A female might lay five clutches in a nesting season, each consisting of approximately 160 eggs, with approximately two weeks between clutches.
"There are lots of males out there available for females to mate with – enough for every female in our sample to have mated with a different male, and no indications of any males monopolising females," said Phillips. "This is a key point – only by molecular techniques can we accurately count how many males are actively contributing to the next generation of a population – and good news for the population and its conservation."
A very small number of females – less than 10 per cent – had mated twice, utilising the sperm of two males to fertilise their eggs. The researchers did not find any evidence to suggest that female turtles were actively choosing males of ‘better’ genetic quality.
The hawksbill population has declined substantially in a very short amount of time, increasing vulnerability to loss of genetic variation to inbreeding and random processes. According to Dr Gallagher, the level of genetic variation among the Cousine Island turtles was higher than expected and came as a pleasant surprise.
"Given that levels of genetic variation are high and that population declines in the Seychelles are now reversing – at least on protected islands such as Cousine – this is extremely good news from a conservation perspective, as it seems the population has come through this crisis with its genetic diversity in good condition," he continued.
While the large number of males is extremely good news, the observation that every female mated with a different male has implications beyond male numbers. It suggests that males are highly mobile, which may promote genetic mixing between groups of females breeding in different areas.
"This would increase the overall genetic population size, and thereby decrease the population’s vulnerability to the negative effects of inbreeding and loss of genetic variation," Phillips explained.
From a more general perspective, the study demonstrates the feasibility of using genetic methods to indirectly study breeding behaviour in species that are difficult to observe. It also serves to highlight the extent of the inferences that can be made using evidence from DNA profiles about a species’ previously unknown life history.
"We have work in progress to compare genetic variation between females breeding on separate islands to better resolve these implications," finished Phillips. "We are also looking at samples collected in later years to assess how often, if at all, we see any of the males again."