Polyandry and multiple paternity

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?


A pair of loggerhead sea turtles mating in the Mediterranean
(photo courtesy of Kostas Papafitsoros)

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.


The case of the black turtle

How many species of sea turtle are there? Depending on who you ask, you might get a different answer. Some might say eight, others will say seven. The debated eighth species of sea turtle is the black turtle.

To trace back the history of the black turtle, one important event stands out: sometime between 3 and 20 million years ago Central America rose up out of the sea. The creation of the Isthmus of Panama was an important paleozoogeographic event that allowed for the migration of terrestrial species between the two American continents. On the other hand, this created a land barrier between the Atlantic and the Pacific, isolating two populations of green turtles from each other. One population that is found in the area from southern California to Chile and around the Galapagos islands looks different from all other green turtles: individuals are smaller in size, their carapace is oval shaped and tapered towards the tail, and they are darker in colour. Because of this last trait, they were given the common name ‘black turtle’, but it is also known by some as the Eastern Pacific green or the Galapagos green turtle.

A juvenile black sea turtle (right) next to a juvenile green sea turtle (photo taken by Javier Rodríguez-Zuluaga, courtesy of PLoS One)

A juvenile black sea turtle (right) next to a juvenile green sea turtle (left)
(photo taken by Javier Rodríguez-Zuluaga, courtesy of PLoS One)

But is the black turtle a unique species? Studies of skull anatomy suggest that black turtles are indeed different to green turtle. On the other hand, DNA analyses reveal no genetic distinction between the two turtle populations. There is still ongoing scientific debate as to whether or not the black turtle should be considered a stand-alone species, or possibly a subspecies. Officially, neither the species Chelonia agassizii nor the subspecies Chelonia mydas agassizii are recognized but both continue to be widely used by scientists.

I contacted Dr Jeffrey Seminoff from NOAA who wrote his dissertation on black turtles in 2002. “Taxonomy is not a static thing and there will always be disagreement among scientific colleagues on the topic,” he tells me. “Today it is not an argument that I prefer to spend energy on. Instead, I am focusing on the fantastic population recovery of black (i.e. green) turtles in the eastern Pacific. These dark green turtles are doing very well currently, but we need to keep our focus on efforts to preserve this recovery!”

Regardless of their taxonomic status, it is great to hear that devoted scientists are successfully preserving these mysterious turtles!

Identifying sea turtle species

There are seven species of sea turtles and to the untrained eye they look somewhat similar. In fact, turtles are often misidentified in the press: it is not rare to see the picture of a green turtle next to an article about loggerhead turtles. So how can you correctly identify the species of a turtle?

Leatherback turtles are easy to recognize because of their unique soft leathery back. For the other six species it is a bit trickier. The key to correctly identifying a turtle species is by examining its shell and the pattern of its scales (also known as scutes).

Picture 1

The name of the different scutes on a sea turtle carapace

By counting the costal scutes on a turtle’s carapace and the prefrontal scales on its head, you can tell each species apart. For example, hawksbill turtles have four costal scutes on each side of their carapace and two pairs of prefrontal scales on their head unlike green turtles that have four costal scutes but only one pair of prefrontal scales. By using the following turtle identification key you can easily identify the seven species of sea turtles. Alternatively, check out this interactive species identification key from the very informative Sea Turtle Conservancy.

Sea turtle identification key (courtesy of seaturtle.org)

Sea turtle identification key
(courtesy of seaturtle.org)

Of course there are other ways to quickly identifying turtle species. For example, loggerheads have a typically large head and a beautiful red-orange colour. With experience you learn how to tell apart one species from the other without resorting to counting scutes, but examining the carapace is always a sure way to confirm your identification!


Can you identify the species of this juvenile sea turtle?