104
The Biology of Sea Turtles, Vol. II
4.1 INTRODUCTION
Temperature-dependent sex determination (TSD) was first reported in 1966 in a lizard. 1
Since that time it has been shown to occur in a wide variety of reptiles, including all
crocodilians, most turtles, some lizards, and the tuatara. 2–4 The occurrence of TSD in
primitive groups of reptiles such as turtles, crocodilians, and tuatara has led some
researchers to hypothesize that it may represent an ancestral form of sex determination
from which avian and mammalian sex determination systems have evolved. 5 The
adaptive advantage of TSD is not clear, but a number of proposed hypotheses might
explain why many reptiles, including sea turtles, have retained TSD. 6–8
In sea turtles, TSD was first documented in the loggerhead, Caretta caretta . 9
Since that time it has been shown to occur in all extant species: the green turtle,
Chelonia mydas ; 10–12 the olive ridley, Lepidochelys olivacea ; 13–15 the leatherback,
Dermochelys coriacea ; 12,16,17 the hawksbill, Eretmochelys imbricata ; 18–21 Kemp’s
ridley, Lepidochelys kempi ; 22–24 the black turtle, Chelonia agassizi ; 25–27 and the
flatback, Natator depressa . 28 The occurrence of TSD in sea turtles generates a wide
variety of questions regarding the physiological, ecological, and conservational
implications of this form of sex determination. The purpose of this review is to
summarize what is known about TSD in sea turtles and to use that information to
address basic questions regarding the biology and conservation of marine turtles.
4.2 CHARACTERISTICS OF TSD IN SEA TURTLES
Although a wide variety of reptiles possess TSD, the effect of a particular temperature may vary depending on the species. 2,29 Several patterns of sex determination
have been described regarding the effects of temperature on sex determination in
reptiles. 2,29 All sea turtles examined to date appear to have a male–female (MF)
pattern in which cooler incubation temperatures produce males and warmer incubation temperatures produce females (Figure 4.1). Several terms have been created to
describe TSD in reptiles. 30 The transitional range of temperatures (TRT) is the range
of temperatures in which sex ratios shift from 100% male to 100% female (Figure
4.1). In the case of the MF pattern in sea turtles, temperatures above the TRT will
produce all females and temperatures below the TRT will produce all males. Within
the TRT, there is temperature referred to as the pivotal temperature, which is the
constant incubation temperature that will produce a 1:1 sex ratio (Figure 4.1). The
pivotal temperature can vary between and even within a species. 31 It has also been
reported that the TRT may vary among sea turtle populations. 32 Therefore, if one is
interested in studying sex determination or estimating hatchling sex ratios in a given
sea turtle population, it is optimal to determine these parameters for that particular
population, rather than extending data from one population to another.
4.2.1 A CCURACY OF T EMPERATURE E STIMATES
Before delving into the specific temperature estimates from previous studies of TSD
in sea turtles, it is necessary to address the potential problems associated with
accuracy. The goal of many TSD studies is to estimate parameters that describe TSD
1123 book.book Page 104 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
4.1 INTRODUCTION
Temperature-dependent sex determination (TSD) was first reported in 1966 in a lizard. 1
Since that time it has been shown to occur in a wide variety of reptiles, including all
crocodilians, most turtles, some lizards, and the tuatara. 2–4 The occurrence of TSD in
primitive groups of reptiles such as turtles, crocodilians, and tuatara has led some
researchers to hypothesize that it may represent an ancestral form of sex determination
from which avian and mammalian sex determination systems have evolved. 5 The
adaptive advantage of TSD is not clear, but a number of proposed hypotheses might
explain why many reptiles, including sea turtles, have retained TSD. 6–8
In sea turtles, TSD was first documented in the loggerhead, Caretta caretta . 9
Since that time it has been shown to occur in all extant species: the green turtle,
Chelonia mydas ; 10–12 the olive ridley, Lepidochelys olivacea ; 13–15 the leatherback,
Dermochelys coriacea ; 12,16,17 the hawksbill, Eretmochelys imbricata ; 18–21 Kemp’s
ridley, Lepidochelys kempi ; 22–24 the black turtle, Chelonia agassizi ; 25–27 and the
flatback, Natator depressa . 28 The occurrence of TSD in sea turtles generates a wide
variety of questions regarding the physiological, ecological, and conservational
implications of this form of sex determination. The purpose of this review is to
summarize what is known about TSD in sea turtles and to use that information to
address basic questions regarding the biology and conservation of marine turtles.
4.2 CHARACTERISTICS OF TSD IN SEA TURTLES
Although a wide variety of reptiles possess TSD, the effect of a particular temperature may vary depending on the species. 2,29 Several patterns of sex determination
have been described regarding the effects of temperature on sex determination in
reptiles. 2,29 All sea turtles examined to date appear to have a male–female (MF)
pattern in which cooler incubation temperatures produce males and warmer incubation temperatures produce females (Figure 4.1). Several terms have been created to
describe TSD in reptiles. 30 The transitional range of temperatures (TRT) is the range
of temperatures in which sex ratios shift from 100% male to 100% female (Figure
4.1). In the case of the MF pattern in sea turtles, temperatures above the TRT will
produce all females and temperatures below the TRT will produce all males. Within
the TRT, there is temperature referred to as the pivotal temperature, which is the
constant incubation temperature that will produce a 1:1 sex ratio (Figure 4.1). The
pivotal temperature can vary between and even within a species. 31 It has also been
reported that the TRT may vary among sea turtle populations. 32 Therefore, if one is
interested in studying sex determination or estimating hatchling sex ratios in a given
sea turtle population, it is optimal to determine these parameters for that particular
population, rather than extending data from one population to another.
4.2.1 A CCURACY OF T EMPERATURE E STIMATES
Before delving into the specific temperature estimates from previous studies of TSD
in sea turtles, it is necessary to address the potential problems associated with
accuracy. The goal of many TSD studies is to estimate parameters that describe TSD
1123 book.book Page 104 Monday, November 11, 2002 11:11 AM