Natural selection and the nesting behavior of ridley sea turtles
Mike Salmon, Professor
Department of Biological Sciences
Florida Atlantic University at Boca Raton
In the last issue of Florida Environmental Outreach, I summarized the basics of Darwin’s ideas used to generate his theory of natural selection. The concepts are amazingly simple, so much so that anyone can comprehend what he said. The evidence Darwin presented in his book, the Origin of Species, and accumulated since then provides overwhelming support for those ideas though there still remain details that require clarification. Most important, however, is that the foundations of his theory have passed repeated testing and retain their explanatory power. When applied to history of marine turtles, Darwin’s theory provides a logical explanation for how these animals changed and diversified through time. New discoveries in the field of genetics (all of which occurred after Darwin’s death) confirm relationships between the 7 currently extant species in ways that are consistent with his theory.

For example, as Darwin predicted, their genes indicate all of them descended from a common ancestor.

Darwin’s genius was to explain the processes underlying how species changed through time. The driving force was competition between individuals that each generation produced too many offspring in a world where the essentials for life (food, shelter, mates; biologists call these “resources”) were in short supply, and where predators and pathogens posed a constant threat. Those individuals that best surmounted these challenges left behind more offspring than those that were less competent. Darwin called these individuals the most “fit” in the struggle for survival. But he soon realized that survival, per se, wasn’t really the best descriptor because organisms might outlive their competitors, but if in the process they didn’t produce offspring they made no further contribution to the evolution of their species. What really counted was how many offspring each individual contributed to the next generation, not how long they lived after reaching sexual maturity.

Put in modern terms, the most fit organisms were those that passed on their genes to the largest number of offspring.
In a second book, dealing with what Darwin called sexual selection, he focused more closely on the relationships between the sexes and, in the process, clarified the distinction between survival and fitness. He noted that the males of many birds, insects and mammals often possessed bizarre weapons (horns, spines, canines, etc.) and ornaments (think lion manes and peacock tails!); some of these features were
Figure 1. A solitary Kemp’s ridley turtle photographed as it was about nest during the day at Padre Island, Texas (Cynthia Rubio, photo).
designed for combat with other males while others appeared to function only to attract females. But most important of all, these features did nothing to enhance male survival; in fact, they often had the opposite effect and made males more vulnerable to injury (in fighting contests) and more conspicuous to their predators. Clearly, these sexually selected features didn’t evolve to promote male survival! Females, however, rarely if ever possessed those characteristics. From those comparisons, Darwin concluded that each sex differed fundamentally in how they enhanced their fitness. For males, fitness was promoted by dominating other males or by presenting females with an appearance, a song or a dance (“courtship”) that to females made that male attractive and provided him with lots of mating opportunities.
For females, however, fitness was enhanced by being inconspicuous, selecting an attractive mate
(that would pass on those traits to her sons), and obtaining enough food to produce well-provisioned, healthy offspring. Females also benefited by providing various kinds of parental care
Figure 2. Activity at an Olive ridley arribada at Ostional Beach, Costa Rica (Michael Jensen, photo).
that enhanced the chances of offspring survival. In marine turtles, that care takes the form provisioning each egg with enough energy (yolk) to promote embryonic development and to power a hatchling’s migration from the nest to oceanic nursery areas. Offspring survival is also enhanced by selecting the “best” sites for nesting – those where the nest won’t be disturbed or detected by predators during the two months it takes to complete incubation.

The marine turtles provide us with good examples of how females make those nest site choices. Let’s turn our attention to a pair of species - the Olive and Kemp’s ridley turtles – that are particularly unique and interesting. Ridley turtle populations show a polymorphism in how females select nesting sites.
Some females are solitary nesters (Figure 1), that is, each female independently decides
when and where to nest during a particular season of the year. Solitary nesters nest twice each year at 12-14 day intervals. If this timing sounds somewhat familiar, it should. Most marine turtles do exactly the same thing though species differ in how many times they nest each season. Solitary females select a different beach each time they nest. That beach can be close to where they nested previously or many miles distant, so they show little fidelity to particular beaches though they tend to nest in the same geographic region.

Other ridley females nest in aggregations known as arribadas (Figure 2). These turtles also nest twice each year but unlike the solitary females, arribada females show strong fidelity to a single beach and only nest there. They arrive at the site a few days in advance and can be seen swimming in groups just a few meters from the beach. Then, suddenly, large numbers of turtles begin crawling up the beach at the same time, as if they received a signal that starts the process. At different arribada beaches, turtles vary in what time of day they nest.
Figure 3. Nesting regions exploited by Olive and Kemp’s ridley turtles in the eastern Pacific, Gulf of Mexico, and off the northeastern coast of South America. Orange circles are the arribada sites; dark lines tracing the coastline show the regions exploited by solitary nesting turtles. See the text for details. (from Bernardo and Plotkin, 2007, in: Biology and Conservation of Ridley Sea Turtles, Johns Hopkins University Press, Baltimore, Maryland 21218)
Some nest only during the day, others only at night, and still others throughout both the day and night. But regardless of what time of day nesting occurs, the turtles are present at the beach for only a few (2-7) days and then, as suddenly as nesting started, it stops. About a month later, the turtles return and mass nesting occurs again.

A map (Figure 3) shows where both species of ridley turtles nest. As far as we know, Kemp’s ridley turtles have a single arribada beach located on the east coast of Mexico,
Figure 4. Hypothesis to account for the balance between solitary and arribada nesting in ridley turtles. Solitary nesting is favored when few turtles nest on any beach, making it difficult for predators to find the nests. However, when more females nest at those sites predators are attracted, destroy the nests, reduce female fitness and make solitary nesting a less favorable strategy. That promotes an increase in arribada nesting. Initially, those nest have high survival probabilities because few predators are aware of places where synchronized nesting activity and hatchling emergence occurs over such a short time period (a week or less). As a result, more females are recruited to the the arribada site, increasing the density of nests. Females returning to the site a month later begin to dig up and destroy the nests of the females that nested earlier, reducing female fitness. The pendulum then swings back in the opposite direction to favor solitary nesting. The result is a balance between the females that opt for each strategy (modified from Bernardo and Plotkin, 2007, op. cit.).
a place called Rancho Nuevo. Olive ridleys have several arribada beaches on the west coast of Mexico, Nicaragua, Costa Rica and Panama, and another arribada in the Caribbean located close to the border between Suriname and French Guiana. Ridley turtles also nest as solitary individuals on the northeastern coast of Mexico, southwestern Texas, and in Brazil. Solitary nesters on occasion nest at sites in close proximity to arribada sites but never at arribada sites. The arribada females nest only at those sites, even when another beach that appears suitable (at least to a human) is close by – in fact, next door!
Why do these two female nesting strategies co-exist? No one knows! Hypotheses center on the relationships between three variables: the density of nests on the beach, the probability of nest detection by predators, and nest loss because females nesting later can destroy the nests that other females placed on the beach the month before. Solitary nesting is favored when there are so few nests on any given beach that they escape detection by predators (Figure 4).

That results in lots of surviving offspring that once mature are recruited to those sites, increasing nest density over many generations. But as the number of solitary nests on the beach increases, more predators are attracted so nest survival plummets.

At this point, arribada nesting becomes the optimal female strategy because predators don’t detect either the nesting activity of those females or the emergence of their hatchlings when these events are confined to short periods of only a few days.

Eventually, however, so many females join the arribada that when they crawl ashore to nest, they dig up the nests (and kill the eggs) of the females that nested previously. The site is then exposed to predators that are attracted to all the broken eggs on the sand surface.

Nest survival and female fitness once again plummets, resulting in a shift that returns to favor the solitary nesting females.

What does all this tell us? It suggests that sometimes there is more than one solution to the problem of how to promote fitness, especially for females whose alternatives for enhancing offspring survival can be quite diverse.

Different individuals may then opt for different strategies and if both behavior patterns result in a favorable outcome (the production of more young), both may very well persist as a “balanced polymorphism”.

Many examples of these polymorphisms are known to biologists, and each of them is unique and interesting in their details. But certainly for those of us that study marine turtles, this example is the best!
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