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