Reproductive Cycles of Males and Females
145
Little is known about potential factors that may influence when a turtle enters
vitellogenesis (or spermatogenesis). Similar to males, female sea turtles are capital
breeders, and in at least some populations of C. mydas breeding rates are linked to
climatic conditions at the foraging area (Limpus and Nicholls, 1988; 2000; Chaloupka, 2001). These climatic alterations may influence nutritional pathways (Limpus and Nicholls, 1988; 2000) by altering factors such as the abundance, quality,
and distribution of food. In addition, climatic conditions may improve feeding rates
or digestive efficiency among individual turtles. Presumably, each year a turtle (male
or female) must make a choice whether to enter vitellogenesis (spermatogenesis) or
to remain quiescent. The factors that influence this decision could be environmental
cues such as temperature or ultimate cues such as a genetically determined energy
threshold. If the conditions are favorable, the turtle will enter vitellogenesis (spermatogenesis) and breed in the following season; if not, then the individual will
remain quiescent, at least until the following year.
Once an individual enters vitellogenesis, a series of physiological mechanisms
are initiated that promote follicular growth. The first visible signs (increased follicle
size) occur around 8–10 months prior to the breeding season (Wibbels et al., 1990;
Rostal et al., 1997). In migratory birds, hyperphagia and increased lipolysis combine
to ensure that adequate energy is accumulated and stored prior to breeding (Berthold,
1993; Guillemette, 2001). Although similar associations have not been investigated
in sea turtles, vitellogenic females showed increases in plasma hormones (corticosterone, testosterone, estrogen, and epinephrine), triglyceride, and adipose tissue
lipids. Moreover, turtles at the end of vitellogenesis (during courtship or in the early
nesting season) showed decreased plasma VTG and estrogen, elevated plasma testosterone, corticosterone, epinephrine, triglyceride levels, and maximal follicle size
(see Owens, 1997; Rostal et al., 1996; 1997; 1998; Hamann, 2002; Hamann et al.,
2002a; Hamann et al., 2002b). In addition, total lipid in yolk follicles collected from
courting females was similar to levels found in egg yolks during the early, middle,
and late nesting season (Hamann et al., 2002b). These data suggest that lipid deposition and follicular development is completed prior to the nesting season.
There are significant gaps in our understanding of vitellogenesis and its regulating factors. Specifically, investigations could target ovarian synthesis of steroids,
seasonal changes in VTG production, and exogenous and endogenous factors that
may influence the timing of vitellogenesis and the regulation of body condition. It
would be interesting to determine whether VTG production could be detected in
females prior to the visual distinction of a developing follicle.
Another interesting area of research would be to investigate whether the hormone
leptin, or an analogous hormone, is found in sea turtle adipose tissue. Leptin in
mammals appears to signal nutritional status to several other physiological systems
and modulates their function (Friedman and Halaas, 1998). More specifically, hyperleptinemia has been induced in vivo using hydrocortisone infusion (Askari et al.,
2000), and has a profound effect on appetite and energy balance in humans (Maffei
et al., 1995; Ahima and Flier, 2000). Recent experimental data have shown that
exogenous leptin induced decreased feeding rates and weight loss in lizards
(Niewiarowski et al., 2000). Indeed, Paolucci et al. (2001) found a seasonal pattern
of leptin production in an oviparous, seasonally breeding lizard. These data suggest
1123 book.book Page 145 Thursday, November 14, 2002 11:50 AM
145
Little is known about potential factors that may influence when a turtle enters
vitellogenesis (or spermatogenesis). Similar to males, female sea turtles are capital
breeders, and in at least some populations of C. mydas breeding rates are linked to
climatic conditions at the foraging area (Limpus and Nicholls, 1988; 2000; Chaloupka, 2001). These climatic alterations may influence nutritional pathways (Limpus and Nicholls, 1988; 2000) by altering factors such as the abundance, quality,
and distribution of food. In addition, climatic conditions may improve feeding rates
or digestive efficiency among individual turtles. Presumably, each year a turtle (male
or female) must make a choice whether to enter vitellogenesis (spermatogenesis) or
to remain quiescent. The factors that influence this decision could be environmental
cues such as temperature or ultimate cues such as a genetically determined energy
threshold. If the conditions are favorable, the turtle will enter vitellogenesis (spermatogenesis) and breed in the following season; if not, then the individual will
remain quiescent, at least until the following year.
Once an individual enters vitellogenesis, a series of physiological mechanisms
are initiated that promote follicular growth. The first visible signs (increased follicle
size) occur around 8–10 months prior to the breeding season (Wibbels et al., 1990;
Rostal et al., 1997). In migratory birds, hyperphagia and increased lipolysis combine
to ensure that adequate energy is accumulated and stored prior to breeding (Berthold,
1993; Guillemette, 2001). Although similar associations have not been investigated
in sea turtles, vitellogenic females showed increases in plasma hormones (corticosterone, testosterone, estrogen, and epinephrine), triglyceride, and adipose tissue
lipids. Moreover, turtles at the end of vitellogenesis (during courtship or in the early
nesting season) showed decreased plasma VTG and estrogen, elevated plasma testosterone, corticosterone, epinephrine, triglyceride levels, and maximal follicle size
(see Owens, 1997; Rostal et al., 1996; 1997; 1998; Hamann, 2002; Hamann et al.,
2002a; Hamann et al., 2002b). In addition, total lipid in yolk follicles collected from
courting females was similar to levels found in egg yolks during the early, middle,
and late nesting season (Hamann et al., 2002b). These data suggest that lipid deposition and follicular development is completed prior to the nesting season.
There are significant gaps in our understanding of vitellogenesis and its regulating factors. Specifically, investigations could target ovarian synthesis of steroids,
seasonal changes in VTG production, and exogenous and endogenous factors that
may influence the timing of vitellogenesis and the regulation of body condition. It
would be interesting to determine whether VTG production could be detected in
females prior to the visual distinction of a developing follicle.
Another interesting area of research would be to investigate whether the hormone
leptin, or an analogous hormone, is found in sea turtle adipose tissue. Leptin in
mammals appears to signal nutritional status to several other physiological systems
and modulates their function (Friedman and Halaas, 1998). More specifically, hyperleptinemia has been induced in vivo using hydrocortisone infusion (Askari et al.,
2000), and has a profound effect on appetite and energy balance in humans (Maffei
et al., 1995; Ahima and Flier, 2000). Recent experimental data have shown that
exogenous leptin induced decreased feeding rates and weight loss in lizards
(Niewiarowski et al., 2000). Indeed, Paolucci et al. (2001) found a seasonal pattern
of leptin production in an oviparous, seasonally breeding lizard. These data suggest
1123 book.book Page 145 Thursday, November 14, 2002 11:50 AM