136
The Biology of Sea Turtles, Vol. II
5.1 INTRODUCTION
Reproductive biology and some aspects of endocrinology in sea turtles have been
widely investigated and reviewed over the last two decades (Owens, 1980; 1982;
Ehrhart, 1982; Owens and Morris, 1985; Miller, 1997; Owens, 1997; also see
Kuchling, (1999) for a review on turtle reproduction). Similar to most ectotherms,
sea turtles are seasonal breeders, although in some populations nesting occurs year
round (Witzell, 1983; Marquez, 1994; Hirth, 1997). Most populations have reproductive cycles constrained by proximal environmental conditions, aiding both
survival of the parents and offspring while allowing maximal reproductive effort
(Miller, 1997). A percentage of males from at least some populations can breed
annually in the wild (Limpus, 1993; Wibbels et al. , 1990; FitzSimmons, 1997).
This is not usually the case for most females, with the exception of both ridley
species ( Lepidochelys olivacea and L. kempii ) (Miller, 1997) and captive Chelonia
mydas (Wood and Wood, 1980). Female C. mydas appear to be incapable of
breeding on annual cycles in nature (see reviews by Ehrhart, 1982; Miller, 1997),
but a small percentage of female Caretta caretta and Natator depressus breed in
consecutive years (Hughes, 1974; Limpus et al., 1984a; Parmenter and Limpus,
1995; Broderick and Godley, 1996). In at least one species ( C. mydas ) breeding
rates are regulated to some extent by regional climatic events driven by El Niño
southern oscillation (ENSO) (Limpus and Nichols, 1988; 2000), and it appears
that levels of endogenous energy reserves may play a vital role in both intra- and
interannual reproductive effort in both sexes.
Although significant breakthroughs in these areas have been and continue to
be made, less attention has been given to developing an understanding of the
mechanisms involved in gametogenesis, ovulation and egg production, and factors
regulating the timing of reproductive cycles. These shortfalls in our understanding
of sea turtle biology most probably reflect logistic difficulties in (1) the capture
and study of turtles outside of the nesting season, (2) accurate identification of
reproductive condition, and (3) an inability to distinguish successful from unsuccessful courtship events. In this chapter we have sought to do three things: (1) to
review and summarize the available literature regarding reproductive cycles of sea
turtles, (2) to identify gaps and controversial areas in the literature, and (3) to
document the conservation implications of the compilation and extension of reproductive information.
5.2 GAMETOGENESIS
Reproductive cycles generally refer to the series of anatomical and physiological
events that lead to the production of male and female gametes, fertilization, and
production of offspring. In adults of both sexes, the process of gametogenesis
involves primordial germ cells undergoing further mitotic and meiotic divisions
within the gonads. These processes (termed spermatogenesis in males and oogenesis
and vitellogenesis in females) are presumably controlled by proximal or ultimate
events that switch on a cascade of physiological processes that act upon reproductive
1123 book.book Page 136 Thursday, November 14, 2002 11:50 AM
The Biology of Sea Turtles, Vol. II
5.1 INTRODUCTION
Reproductive biology and some aspects of endocrinology in sea turtles have been
widely investigated and reviewed over the last two decades (Owens, 1980; 1982;
Ehrhart, 1982; Owens and Morris, 1985; Miller, 1997; Owens, 1997; also see
Kuchling, (1999) for a review on turtle reproduction). Similar to most ectotherms,
sea turtles are seasonal breeders, although in some populations nesting occurs year
round (Witzell, 1983; Marquez, 1994; Hirth, 1997). Most populations have reproductive cycles constrained by proximal environmental conditions, aiding both
survival of the parents and offspring while allowing maximal reproductive effort
(Miller, 1997). A percentage of males from at least some populations can breed
annually in the wild (Limpus, 1993; Wibbels et al. , 1990; FitzSimmons, 1997).
This is not usually the case for most females, with the exception of both ridley
species ( Lepidochelys olivacea and L. kempii ) (Miller, 1997) and captive Chelonia
mydas (Wood and Wood, 1980). Female C. mydas appear to be incapable of
breeding on annual cycles in nature (see reviews by Ehrhart, 1982; Miller, 1997),
but a small percentage of female Caretta caretta and Natator depressus breed in
consecutive years (Hughes, 1974; Limpus et al., 1984a; Parmenter and Limpus,
1995; Broderick and Godley, 1996). In at least one species ( C. mydas ) breeding
rates are regulated to some extent by regional climatic events driven by El Niño
southern oscillation (ENSO) (Limpus and Nichols, 1988; 2000), and it appears
that levels of endogenous energy reserves may play a vital role in both intra- and
interannual reproductive effort in both sexes.
Although significant breakthroughs in these areas have been and continue to
be made, less attention has been given to developing an understanding of the
mechanisms involved in gametogenesis, ovulation and egg production, and factors
regulating the timing of reproductive cycles. These shortfalls in our understanding
of sea turtle biology most probably reflect logistic difficulties in (1) the capture
and study of turtles outside of the nesting season, (2) accurate identification of
reproductive condition, and (3) an inability to distinguish successful from unsuccessful courtship events. In this chapter we have sought to do three things: (1) to
review and summarize the available literature regarding reproductive cycles of sea
turtles, (2) to identify gaps and controversial areas in the literature, and (3) to
document the conservation implications of the compilation and extension of reproductive information.
5.2 GAMETOGENESIS
Reproductive cycles generally refer to the series of anatomical and physiological
events that lead to the production of male and female gametes, fertilization, and
production of offspring. In adults of both sexes, the process of gametogenesis
involves primordial germ cells undergoing further mitotic and meiotic divisions
within the gonads. These processes (termed spermatogenesis in males and oogenesis
and vitellogenesis in females) are presumably controlled by proximal or ultimate
events that switch on a cascade of physiological processes that act upon reproductive
1123 book.book Page 136 Thursday, November 14, 2002 11:50 AM