Reproductive Biology and Embryology of the ... - Seaturtle.org

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REPROOUCTIVE BIOLOGY ANO EMBRYOLOGY OF CROCOOILIANS
taken up by the follicles (Lance, 1983). Immature alligators of either sex can
be induced to produce vitellogenin by injection of estradiol (Dessauer,
1974; Lance, 1983). The oviduct is also stimulated by ovarian oestrogen and
hypertrophies during the follicular growth phase (Lance, 1983). After
oviposition, plasma estradiol levels remain low to nondetectable, and there
is no evidence for a prehibernation onset of vitellogenesis (Lance, 1983).
Nonbreeding females have low to nondetectable estradiol levels (Lance,
1983) and show nO evidence of follicular growth (Joane n and McNease,
1980). Likewise, immature females have low estradiol levels and small
ovaries that consist of clear cortical tissue with numerous clusters of 00­
cytes distributed evenly throughout the organ. Reproductively senescent
or "barren" females (usually over 2.7 m in length) have few growing 00­
cytes, no corpora lutea, black hemoglobin deposits, and their ovaries frequently
contain medium-sized resorbing ovarian follicles and retained
oviducal eggs undergoing resorption (both in Alligator mississippiensis,
Joanen and McNease, 1980; Lance, 1983 and in Crocodylus niloticus,
Graham, 1968). The occurrence of senescence in the Crocodilia suggests
that there is a limit on the number of oocytes a female can produce, despite
the fact that oogonial proliferation continues throughout much of adulthood.
Macroscopic changes similar to those detailed above have been described
in the male and female reproductive systems of Crocodylus niloticus
(Cott, 1961; Graham, 1968). However, the breeding season of Alligator
mississippiensis is shorter and more clearly defined than is that of the more
tropical C. niloticus (Graham, 1968), C. porosuS (Webb et al., 1983b), and C.
palustris (R. Whitaker, personal communication). This may reflect environmental
temperature; the cold winters in Louisiana make the alligator inactive
from October through February, and it must complete its reproductive
cycle within a restricted time frame. ThuS for C. niloticus in Kenya, sperm
production extends from April through December, and although there is a
peak breeding season (October-December), follicular development and
some reproduction occur throughout the year (Graham, 1968). Moreover,
15% of the sexually active females of C. niloticus have two sets of follicles
developing concurrently (Graham, 1968). This means that maturation of
one batch of ova may be accompanied by development of a younger batch
that will near maturity soon after oviposition of the first; the second set can
then be fertilized and two clutches of eggs may be produced in one season.
This "double clutching" has also been reported for some C. palustris (in
captivity, R. Whitaker, personal communication) and is suspected in some
C. porosus (Webb et al., 1983b). The controlling mechanism remains uninvestigated.
However, given the amount of yolk protein necessary to produce
two clutches of approximately 40 eggs weighing apprOXimately 60­
70 g each, not to mention the energy expended in mating behavior, nest
building, and maternal care, it may be assumed that only females in an
REPROOUCTIVE BIOLOGY
343
optimal environment (in terms of temperature and food) may pursue this
strategy.
The data on the peak nesting times of various species presented in Table
III generate many unanswered questions: What environmental factors trigger
ovulation, spermatogenesis, reproductive behavior, and egg laying,
and how do they operate?
Numerous reports for various species imply a possible correlation
among breeding times and changing water levels in the various habitats.
The data do not reveal consistent trends across all species and the interested
reader is referred to the original publications (Cott, 1961, 1975;
Modha, 1967; Graham, 1968; Pooley, 1969a; Staton and Dixon, 1977; Deraniyagala,
1939; Webb et al., 1977, 1983e; Joanen, 1969; Joanen and
McNease, 1975a, 1979b, 1980; Kushlan and Kushlan, 1979).
A strong relationship exists between peak nesting times in Alligator
mississippiensis and average air temperatures. An analysis of nesting data
for ten years reveals a highly significant correlation between the time of
nesting and the ambient temperatures for March-May (Joanen and
McNease, 1979b). Nesting occurs earliest when these temperatures are
highest. Rainfall is significant only when nesting effort is reduced during
extremes of accrued surface water levels; females apparently respond to
higher water levels by laying eggs at higher levels within the nest (Kushlan
and Kushlan, 1979). Oviposition occurs on the longest days of the year
(Joanen and McNease, 1979b). Temperature appears to be the major regulator
of male and female reproductive cycles (Lance, 1983); as in other
reptiles photoperiod plays only a minor, or at best synergistic, role (Fischer,
1974; Crews and Garrick, 1980). It is known that males in an
artificially heated pond in Georgia had spermatozoa in the penial groove
one month earlier than their counterparts inhabiting ponds at ambient
temperatures (Murphy, 1980). Moreover, motile sperm have been recovered
in January from males kept in artificial hot houses (Cardeilhac,
1981). Whereas adult A. mississippiensis do respond to experimentally altered
photoperiods (Lang, 1976), such experiments have not been conducted
in relation to the cueing of reproductive events. Experiments involving
altered photoperiods at constant high temperatures would be
interesting, especially for tropical species in regions with minimal annual
variations of temperature.
It is currently impossible to state categorically which of the potential
extrinsic regulating factors, such as drought, temperature, water level,
photoperiod, or the psychological effect of available breeding grounds,
stimulate ovulation and spermatogenesis, and whether these factors are
similar in all crocodilians. Clearly more information is required, not only
on the stimuli themselves, but also on their perception and the way in
which they achieve their effects.
The hormonal regulation of the reproductive cycle is poorly understood.