Female sea turtles will often mate with more than one male during the breeding season. This type of mating system is termed polyandry. In addition, a female can store sperm in her oviducts for long periods of time and use the sperm to fertilize her eggs later. An interesting consequence of this is that within a single turtle nest, it is possible to find hatchlings that were sired by different fathers. So in the same turtle nest you might find not only siblings, but also half-siblings. But what are the potential benefits of polyandry and multiple paternity for sea turtles?
The suggested advantages of polyandry include fertilization assurance and genetic benefits. In other words, having more than one mate can decrease the chance of having one “bad” (for example infertile) mate while increasing the chance of having at least one “good” (for example exceptionally fit) mate. However, a study published in 2004 showed that multiple paternity did not correlate with any estimator of reproductive success in green turtles. Comparing single-fathered clutches to those with multiple fathers, no evidence for genetic benefits was detected with fitness indicators such as clutch size, hatching success or offspring quality. This research therefore suggested that there are no direct genetic advantages to polyandry for female sea turtles. So is polyandry simply a consequence of the incidence of male-female encounters?
“Initially, there appeared to be a simple correlation between population size and the frequency of multiple paternity in sea turtle populations,” comments Dr Patricia Lee, a lead author of the 2004 study. “However, exceptions kept cropping up. For example, multiple mating by female leatherback turtles was relatively infrequent even for moderately sized populations, whereas for a similar sized loggerhead sea turtle population in Greece, over 90% of the females was found to be polyandrous.”
In a new study published recently in Advances in Marine Biology, Dr Lee and her colleagues explored the idea that frequency of multiple paternity was linked to the local density of a nesting population. By examining data from rookeries around the world they found a tight relationship between how densely populated a rookery was and the occurrence of multiple mating within that rookery. For example, individuals that congregate in small areas and do not move very far are more likely to encounter other individuals more frequently.
In summary, multiple paternity occurs more often at densely populated rookeries. While the benefits of polyandry are still unclear, it appears that female turtles are only opportunistically polyandrous. Dr Lee concludes: “Although there may be many reasons as to why females would choose to mate more than once, they would first have to have the opportunity to meet more than one male before they are able to have this choice.”
“A review of patterns of multiple paternity across sea turtle rookeries” was published by Advances in Marine Biology (2018). Authors: Patricia L.M. Lee, Gail Schofield, Rebecca I. Haughey, Antonios D. Mazaris and Graeme C. Hays.