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

354 REPROOUCTIVE BIOLOGY ANO EMBRYOLOGY OF CROCODILIANS
REPROOUCTIVE BIOLOGY
355
Cooper, 1982, 1984) and again for mound nests of C. porosus (Grigg, unpublished
data). The data do not differ significantly. Shortly after egg
laying, the pOz in the nest is 147 torr (ambient =: 150), and the pC02is 5.0
torr (ambient =: 0.3). These values reflect the properties of the nest as the
same values are obtained whether or not eggs are present (Grigg, unpublished
data). Embryonic arterial blood is approximately 86% saturated and
venous blood about 40%: arterial pOz is 52 torr (range 42-57), arterial pC02
is 12 torr (range 8-14), and venous pOz is 22 torr (Grigg, unpublished
data). Clearly, the eggshell and shell membrane represent a significant
barrier to gaseous diffusion (see Section III.E). As incubation proceeds,
nest p02 falls and pCOz rises (Lutz and Dunbar Cooper, 1982, 1984; Grigg,
unpublished data). Crocodilian embryos can tolerate extremely low p02
levels and recover; furthermore, they are very resistant to raised pC02
levels. Even pCOz levels as high as 60 torr do not inhibit the embryonic
metabolic rate (Grigg, unpublished data).
Flooding of crocodilian nests has long been known to cause embryonic
death (Reese, 1915a; McIlhenny, 1935; Joanen, 1969; Joanen et aI., 1977;
Fleming et aI., 1976; Hines et aI., 1968; Goodwin and Marion, 1978; Nichols
et aI., 1976; Voeltzkow, 1891, 1892, 1893, 1899; Webb, 1977a; Webb et al.,
1977, 1983b and e; Magnusson, 1982; Deraniyagala, 1939; Cott, 1961, 1969;
Pooley, 1962, 1969a). Joanen et al. (1977) tested the effects of simulated
flooding on hatchability by immersing the eggs of Alligator mississippiensis
for single periods of two, six, 12, and 48 hours at different stages of development.
Immersing eggs of any age for two to six hours had little effect on
subsequent hatchability, nor did submerging eggs for 12 hours if done
before day 30 (after laying). Thereafter, 12 hour submergence killed all
embryos. Submergence of the eggs for 48 hours killed all the embryos at all
ages. Similar results are recorded for experiments on Crocodylus porosus
eggs (Magnusson, 1982). Any moisture on the surfaces of crocodilian
eggshells drastically reduces their gas conductance and may totally inhibit
oxygen diffusion. It therefore seems likely that flooding inhibits gaseous
diffusion, and thus causes embryonic death: a similar mechanism operates
in chicken eggs (Kutchai and Stean, 1971). All embryos could utilize air
within the eggshell and so withstand short periods of flooding; whereas
younger embryos would have a lower O 2 requirement and relatively more
air within the eggshell, so that they could withstand longer periods of
flooding than older embryos.
Factors that influence the site selected by females for nest construction
are complex and incompletely understood but probably include social interactions
with other animals, vegetation type at site, proximity of the site
to water, its temperature, degree of exposure to sunlight, and height above
water level. Whereas investigations of these parameters are, by definition,
ecological and behavioral, there can be no doubt that these factors affect
fecundity within a population. Further studies are likely to contribute not
onlv to our understanding of crocodilian reproductive strategies, but may
shed light on the role of incubation temperature in determining the sex
ratios of hatchlings (Section VILO).
E. Maternal Behavior
Those behavioral repertoires of female crocodilians that improve reproductive
efficiency are not restricted to nest selection and construction (Section
11.0), but include subsequent nest protection and opening, egg opening,
transport of hatchlings, and parental care. They have been described for
Crocodylus acutus (Campbell, 1973; Garrick and Lang, 1977), C. cataphractus
(Waitkuwait, 1982), C. johnsoni (Dunn, 1981; Webb et aI., 1983e), C.
moreletii (Hunt, 1975, 1980), C. niloticus (Cott, 1961, 1909, 1971, 1975; Garrick
and Lang, 1977; Pooley, 1962, 1969a, 1974a, b, 1976, 1977; Pooley and
Gans, 1976; Voeltzkow, 1891, 1892, 1893, 1899; Bohme, 1977; Guggisberg,
1972; Neill, 1971), C. novaeguineae (Neill, 1971), C. palustris (Symons, 1918;
Deraniyagala, 1939; Whitaker and Whitaker, 1977a, b), C. porosus (Deraniyagala,
1936, 1939; Webb, 1977a; Webb et aI., 1977; Magnusson, 1980;
Bustard and Kar, 1981), Alligator mississippiensis (Reese, 1915a, 1931a;
McIlhenny, 1935; Giles and Childs, 1949; Lee, 1968; Joanen, 1969; Campbell,
1973; Kushlan, 1973; Herzog, 1975; Garrick and Lang, 1977; Garrick et
aI., 1978; Kushlan and Kushlan, 1980; Kushlan and Simon, 1981; Deitz and
Hines, 1980; Hunt and Watanabe, 1982), Caiman crocodilus crocodilus (Del
Toro, 1969; Campbell, 1973; Garrick and Garrick, 1978; Staton and Dixon,
1975, 1977), Osteolaemus tetraspis (Tryon, 1980), and Cavialis gangeticus
(Singh and Bustard, 1977; Bustard, 1980b,c,d; Bosu and Bustard, 1981).
Maternal and prehatching vocalization of crocodilians are reminiscent of
avian behavior (Vince, 1969, 1973; Gottlieb, 1973; Oppenheim, 1973). In
birds, they are supposed to accelerate and synchronize late embryonic
development in the clutch of eggs. The same effects have been suggested
for crocodilians (Lee, 1968). However, a single experimental study (Magnusson,
1980) showed that advanced embryos of Crocodylus porosus called
in response to a tape recording of wild hatchling vocalizations, but neither
developed faster nor hatched more nearly synchronously than did control
eggs. How embryos, enclosed within the fluid environment of an egg, emit
such sounds is unknown. Vocalizations are also involved whenever the
females excavate nests, open eggs, or transport hatchlings in their cavernous
jaws. All these activities require a high degree of oral sensitivity and
muscular control. Numerous domed sensory receptors on the palate and
jaw margins (Figs. 3A and B) may facilitate these and other (e.g., courtship
and feeding) behaviors (Ferguson, 1981b, 1982b). All available reports are
phenomenologic, but it is obvious that further investigations of the neuralendocrine
mechanisms underlying these complex behaviors are warranted,
not merely for their intrinsic interest, but also for their potential
contribution to our understanding of the evolution of parental care in
birds and mammals.