Insights from Keewaydin Island, II.
How Are Reproductively Superior Female Loggerheads Made?

There is no question that the way large creatures go about the business of reproduction is complicated and diverse, so much so that there seems to be no end to how the process is described. Biologists have tried over the years to come up with overarching “principles” based upon characteristic of the two main participants, specifically, males and females.

Darwin was the first to do so with competence. His distinctions were based upon both morphology and behavior. He characterized males as the sex most likely to possess specialized weaponry (e.g. horns, claws, teeth) used to compete aggressively with other males for access to females. A consequence was the increased likelihood of injuries and a shortened life span. But for males, “success” was measured by the number of matings, not by how long they lived! Left relatively unstudied until recently, however, was an understanding of the qualities serving as indicators of success among females.

When Hoover examined the data, she found that clutch size (the number of eggs/nest) averaged 102 eggs per nest, but was highly variable among females (Figure 2). In addition, so was hatchling production (Figure 3). The 55 most successful females (the upper 17 % of the population) on average placed more eggs (118) in each nest than the 60 least successful (bottom 17 % of) females, that placed 90 eggs in each nest. More hatchlings (mean of 93 turtles) emerged from the nests of the successful than the unsuccessful females (mean of 42 hatchlings; Figure 3). The two female groups, however, did not differ in remigration intervals (on average, 2 years), in where on the beach they placed their nests, or in their age and breeding experience. The most obvious difference was that the successful females were about 1.3 x larger than the unsuccessful females. That difference was important because larger females at Keewadin (and in loggerheads nesting elsewhere) produce larger clutches (1).

Isotope Ratios and Female R.S.: Yakushima Island and the Refuge

Two additional studies took advantage of a relatively new and important tool for identifying differences among females in r.s.: ratios between carbon and nitrogen isotopes. Those ratios reflect the nutritional value and abundance of prey where females feed during the remigration interval. The basic rule in isotope research is: you are what you eat! That means that the isotope ratios in small tissue samples (skin and blood) from females reflect the ratios present in the food they consumed so those ratios can be used to explore relationships between where females forage, what they eat and their r.s.

Smaller turtles fed on planktonic prey found in less productive oceanic sites whereas larger turtles fed on bottom-dwelling, nutritionally richer prey found in shallow coastal waters (Figure 4). However, and unlike the Keewaydin turtles, a similar proportion of the eggs placed in the nests of both groups developed into hatchlings that left the nest. There were also no differences between those hatchlings in either their growth rates or vigor, nor was there any evidence that the two groups of females differed genetically; they differed only in size and in choice of foraging location that, once made, didn’t change (3).

Isotope analyses revealed that females nesting at the Refuge on Florida’s East Coast foraged at one of 4 locations: sites to the north of Cape Hatteras, in the Bahamas, in the Florida Keys, and in the shallows of the southeastern Gulf of Mexico (4). The most productive sites were northern and, as expected, the largest females nesting at the Refuge fed there. But contrary to expectations, those females produced clutches similar in size to the females feeding in the Bahamas and Florida Keys. The smallest clutches were produced by females foraging in the SE Gulf. That outcome may have resulted from the interaction costs associated with food availability and migration distance. Females feeding at northern sites and in the SE Gulf experienced high migratory costs as they swam longer distances than others to reach the refuge. But those costs for the northern females were probably tempered by their access to a rich food supply. The Gulf of Mexico females, however, fed where food quality was low and so experienced a combined negative migratory as well as foraging cost. And so while larger adult females have the potential to express greater r.s., whether that happens depends not only upon where she feeds, but also upon where she nests! (4)