The Mystery of “Enough”: What proportion of males to females is necessary to maintain a healthy sea turtle population?
By Jeanette Wyneken and Selina Heppell
Loggerhead sea turtles mating, Florida Keys, U.S.A. © 2005 DAVID DOUBILET
Populations are groups of animals that share genes and other characteristics such as nesting or feeding locations. The “health” of a sea turtle population—its likelihood of persistence over time—may be evaluated with information on how fast the turtles within the population grow, how long they live, how productive they are as adults, and the numbers and proportions of juveniles and adult males and adult females in the population.
A healthy population is a resilient one that can withstand natural variability, such as hurricanes, disease, large-scale nesting beach erosion, and some human impacts. Predicting how populations will decline or recover in response to changes (both positive and negative ones) is fundamentally important as we try to identify the most effective conservation methods. The best conservation mechanisms depend on sound understanding of the turtle populations and the threats that cause declines. There are many black holes in our knowledge of the structure of sea turtle populations, and in examining any individual population, one of these unsolved mysteries is: Are there enough males present for the population to survive?
One egg and one sperm: a simple recipe to make a turtle, but it is a recipe with a lot of critical biology behind it. To produce viable hatchlings, there must be enough mature males and females in the right place at the right time and enough good nesting beaches to incubate the eggs. Because higher temperature nests tend to produce more females than males, the sex ratios of these commonly tropical and subtropical turtle nests—and, thus, the sea turtle populations—are often skewed.
For a healthy population to have “enough” male and female breeders under skewed conditions implies at least three things: first, that females are able to find viable males; second, that one male can mate with many females; and finally, that females can store sperm and then allocate these gametes over the season to successfully fertilize several clutches of eggs. Currently, clutches of infertile eggs are rare, and some clutches have more than one father.
Simply having enough males to fertilize the eggs may not be enough. The consequence of small proportions of males in populations is unclear. Does a low number of viable males reduce the number of healthy, fertile eggs that each female lays? Does a low number of males increase the number of years between nesting seasons? Our understanding of these aspects of sea turtle biology is truly limited because we do not know the resilience of populations with low numbers of males. In some animal species, when animal populations are seriously depleted, there may be insufficient numbers of males to mate with the females or too few breeders to ensure adequate genetic mixing, thereby exacerbating the population decline and complicating recovery efforts.
One critical need for assessing sea turtle population health is to understand how fast turtles grow, how long it takes them to grow up to adulthood, and if males and females reach reproductive maturity at the same age or even the same size. New methods for counting “growth rings” in the bones of dead turtles can help us to better define how long sea turtles live, as well as how old they are when they migrate into new habitats or reproduce for the first time.
It is essential that populations have a critical mass that can cope with change. In most instances, we do not know what that number should be for sea turtles, but we do know that global sea turtle numbers are a small fraction of what they were a few centuries ago. While we are working to restore their numbers, we must simultaneously understand how individuals and their environments vary and what factors are most important in determining how their populations will respond to change. Ultimately we must understand the family of “hows”: how many, how big, how long, and how old is enough to ensure that populations will be healthy “enough” for sea turtles to thrive.
This article originally appeared in SWOT Report, vol. 2 (2007). Click here to download the entire article as a PDF.