Figure 2: Two green turtles mating in-water. Image taken by Guinjata Dive Center.
Males, though, typically stay in the ocean where they can’t be seen. One might assess population health and estimate adult sex ratio by taking a boat into open water to count all the turtles you encounter. However, this census method is very expensive and fails to detect all the individuals in a population. Also, even if these problems could be overcome, one couldn’t be sure that every male is competitive and is able to contribute his genes to the next generation. Much depends on whether a female finds him attractive.
Thus, in reality, in marine turtle populations with skewed sex ratios, I seek to answer two questions: (i) how many males are “out there”, and (ii) how many of those males routinely mate with each female? If, from the genetics of the hatchlings produced in each nest, I can identify the males as individuals, I can answer both questions simultaneously.
Our lab is developing new techniques to understand the process of environmental sex determination, and to estimate current hatchling sex ratios on our beaches. However, because marine turtles can take 20 – 30 years to reach sexual maturity, these new techniques provide us with little understanding of the sex ratios in the adult population. In-water counts indicate that there are more sexually mature females than males; we might expect this outcome if hatchling sex ratios 20 – 30 years ago were also biased toward females. However these numbers are obtained, they are always a proportion, rather than the entirety, of the breeding population. Females, for example, do not breed every year; they require a few years of feeding to accumulate the energy used to produce the hundreds of eggs they place in nests during a single breeding year.
Mature males, however, may breed more frequently since the energetic demands involved in searching for mates (and competing with other males for mating opportunities) may not be as great. Thus, what’s really important to determine is the operational sex ratio, or the ratio of breeding males to breeding females during any one breeding season. My research project is designed to address this question by identifying parentage.
I can identify how many males a female has mated with by using genetic markers, or “fingerprints,” from small samples of blood or skin. I use tissue samples from females to identify maternal genes, and tissue samples from hatchlings to identify their genes. Half of each hatchling’s genes are maternal; half are paternal. For each hatchling, I identify maternal genes by matching their characteristics (how they migrate on the surface of a charged gel) with those of the mother; any genes that don’t match are paternal (Figure 3). The final step in this molecular detective work is to compare the characteristics of the paternal genes in tissue samples taken from hatchlings in a single female’s nest. The number of different paternal genes in a nest equals the number of males mating with the female laying the eggs placed in that nest.
Figure 3: Illustrative gel showing how paternity is analyzed from the genetics of the female (“Mom”) and three of her hatchling offspring (H1, H2 and H3). The female has two copies of the pink and orange gene. The hatchlings receive one copy of each gene from Mom. Any other genes that the hatchlings’ possess (green,blue) must have come from “Dad”. Thus, in this example,the female mated with only one male. My data indicate that in sea turtles, that’s rare, as most of my females mated with more than one male.