Some of our happy group at Lighthouse Cave, San Salvador, Bahamas.
Life as A Loggerhead
What determines being male or female in reptiles? After looking into sex determination, I could not help but wonder about the reproduction of Loggerhead sea turtles. Observation of clutch sizes, nesting sites, environmental factors, and genetic and cultural traits have helped researchers come up with some very helpful hypotheses about how sex is determined in Loggerheads. Sex-ratio studies among reptiles have been very popular due to the skew toward females. Nesting sites, sex ratios and sex determination are all very intertwined and important to each other. In this paper, I will be focusing mainly on sex determination and sex ratios among Loggerhead sea turtles, however to discuss this, we must also look at nesting sites and a few other various areas of loggerhead development. To begin I will examine some general information on Loggerheads to get us acquainted with the turtles we will be concentrating on.
Loggerheads (Caretta caretta) can be found in temperate and tropical regions including the Atlantic, Pacific, and Indian Oceans. Loggerheads are in the Cheloniidae family. Loggerheads occupy almost all the oceans of the world. The variation of habitat for Loggerheads has not yet been extensively hypothesized over. Adult Loggerheads average about 200 to 300 lbs. and about 3 ft. in length. The carapace of a typical adult is light brownish and the plastron is yellowish. They are named after their big heads and have an amazingly strong jaw. They eat crabs, mollusks, and other marine life, however they are not considered fish eaters. Loggerheads have many different threats to them. The first and biggest would be the human threat. Humans do not only threaten the turtles themselves, but also their environment. Humans not only cause confusion with lighting (the turtles are drawn after hatching to the ocean from the reflection of the moonlight, but the coastal cities sometimes make them head in the wrong direction), but with trawling nets, long-line fishing rigs, pollution and more. Loggerheads were put in the Endangered Species Act as a threatened species in 1973. They hold this standing today. Other threats include ghost crabs, sea gulls, large fish and sharks.
When Loggerheads are laid, they are in the incubation stage for 45 to 95 days. The mean clutch size is 100 to 126. Then the hatchlings follow the reflection of the moon and stars into the ocean where they spend the next (estimated) 3 to 7 years out at sea until they are “young adults”. Sometimes there is confusion among the hatchlings because they see the lights from the costal towns and developments, and they end up going off in the opposite direction of the sea. Many people that live or work on the coastline have started lighting ordinances to stop this problem. The costal developments are convinced to use red lights or to turn out there lights at least during the nesting season. The nesting season lasts from May to August. Nesting and hatching happens primarily at night. One adult female has, on average, intervals of 14 days between hatchings. During one nesting season, one female nests between 1 to 7 times.
The 3 to 7 years (after hatchlings leave for the ocean and before the return to the same island) is referred to as the “lost years” because the hatchlings are in the ocean, and there has not been any study conducted of where the majority of the hatchlings go. However, some have been spotted drifting with gulfweed. At about 20 to 30 years, Loggerheads are at their adult form and are matured. Most females’ head back to the beach where they were hatched to nest. Freedberg and Wade propose that nest-site selection combined with ESD (environmental sex determination) “facilitates the maternal manipulation of offspring sex ratios toward females”. (Freedberg and Wade, 2000) This means that female sea turtles have “cultural traits” which enable them to go to a beach that has the right type of environment for their offspring. Many, as said before go back to the beaches where they were hatched. Freedberg and Wade created a novel model for interpreting sex ratios in sea turtles. Instead of using genetic defects, such as mitochondria and bacterium Wolbachia, they said that culturally transmitted traits are what influences behavior. Their model describes two types of adult females, one carrying the cultural trait (C) and one not (c). Female C has more female offspring because she contains the trait. This trait enables her to know the right types of environments to deposit more female offspring. Female c randomly chooses nest sites. “Although nest-site fidelity is not equivalent to nest-site inheritance, additional evidence suggest that adherence of individual females to a particular nest site reflects transmission of a nesting location from mother to daughter, a phenomenon termed philopatry (Reinhold 1998). Nest site may be imprinted on female offspring when they first hatch; upon maturation, they return to construct their own nests in the same area. This type of inheritance can be viewed as cultural because it is characterized by the imitation of a factor that influences behavior.” (Freedberg and Wade 2001) This makes sense to apply to sea turtles, however I think we cannot disregard the hypotheses made before this because it usually is a combination of attributes that produce certain actions. Dinosaurs also obtained their sex from ESD and not GSD (genetic sex determination). Turtles and crocodiles are the oldest living reptiles and take after dinosaurs in this way. One may hypothesis that dinosaurs are distinct today because of skewed sex ratios due to climate change causing males or females to die out therefore cause the extinction of all.
Another factor in reptiles with ESD is the temperature of their nest.“…high temperatures produce female hatchlings and low temperatures produce male hatchlings…”(Janzen and Paukstis, 1991) According to Freedberg and Wade this would mean that the females would know that the nesting site in which they were hatched was warm enough for creating more females. Then the sex is determined after being laid.
“The gonadal sex of the embryo is determined not so much by temperature during a short phase of incubation, but rather by a cumulative effect of temperature during this phase.” (Janzen and Paukstis, 1991) This shows that it is not only one time in the 45 to 95 day incubation phase, but all of it together. Smith and Daniel studied the development of Loggerhead turtles, embryo, inside egg, and right after hatching. The earliest differentiation shown in them is in the 18th to 21st day inside the shell. They start to slowly move away from C-movements (tale to head sways) to variation between them all. In the 26th through the 32nd day, “several activities closely associated with behavior in the later normal sea environment.” (Smith and Daniel, 1946) These movements include snapping, nystagmie head responses to rotation, crawling and tropistic orientation toward the ocean. A few hours after hatching the loggerhead has almost all the abilities needed to survive in the ocean. “It would appear that in the loggerhead response repetition in embryo may be ruled out as a factor in the formation of integrated movements of the front flippers especially. These observations agree with the principal finding of this study, that the initial development of behavior in the loggerhead proceeds in terms of specialization of local responses within an evolving matrix of mass movements, which constitute basic behavioral patterns for the maintenance of integration of developing local movements.” (Smith and Daniel, 1946) Freedberg and Wade would relate this to their cultural traits among sea turtles, saying behavioral patterns are just that, culturally inherited traits.
Alho, Danni and Padua found similar results in Podocnemis expansa, (a type of sea turtle found in the Amazon). “Sexual differentiation was dependent on whether eggs were incubated in covered nests or uncovered nests. The temperature difference between two sets of treatments was less than 1degree C, but this difference stayed constant throughout thee incubation period…the covered nests produced more males… Uncovered nests produced more females.” (Alho, Danni, Padua, 1985) Many researchers have studied sex-determination in reptiles, especially sea turtles because their eggs are bigger and easier to gather for testing.
There are many other areas of study to be looked at in loggerhead turtles, along with all reptiles. Even the areas that I have researched need more time spent on them, maybe one day more questions will be asked as more questions are answered and then we will have more of an understanding of how and why turtles behave the way they do when laying eggs. Nest sites, embryo development, sex determination and sex ratio have started us out, now we have to look further into the species, into their lives out at sea and determine more about their sex ratio and sex determination motives.
Cultural Inheritance as a Mechanism for Population Sex-Ratio Bias in Reptiles. Steven Freedberg; Michael J. Wade. Evolution, Vol. 55, No. 5 (May, 2001), 1049-1055.
Environmental Sex Determination in Reptiles: Ecology, Evolution, and Experimental Design Fredric J. Janzen; Gary L. Paukstis. The Quarterly Review of Biology, Vol. 66, No. 2 (Jun., 1991), 149-179.
Observations of Behavioral Development in the Loggerhead Turtle (Caretta caretta). Karl U. Smith; Robert S. Daniel. Science, New Series, Vol. 104, No. 2694 (Aug. 16, 1946), 154-156.
Population Structure of Loggerhead Turtles (Caretta caretta) in the Northwestern Atlantic Ocean and Mediterranean Sea. Brian Bowen; John C. Avise; James I. Richardson; Anne B. Meylan; Dimitris Margaritoulis; Sally R. Hopkins-Murphy. Conservation Biology, Vol. 7, No. 4 (Dec., 1993), 834-844.
Reproductive Investment and Optimum Clutch Size of Loggerhead Sea Turtles (Caretta Caretta). Graeme C. Hays; John R. Speakman. The Journal of Animal Ecology, Vol. 60, No. 2 (Jun., 1991), 455-462.
SeaWorld/Busch Gardens Animal Information Database. 2002 Seaworld Inc.
Temperature-Dependent Sex Determination in Podocnemis expansa (Testudinata: Pelomedusidae). Cleber J. R. Alho; Tania M. S. Danni; Luiz F.M. Padua. Biotropica, Vol. 17, No. 1 (Mar., 1985), 75-78.
US Fish and Wildlife Service, North Florida Field Office
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