Much of what we know about temperature-dependent sex determination
(TSD) in reptiles stems from constant temperature incubation studies in the
laboratory. In recent years, as TSD studies moved into the field it became evident
that TSD was much more complex than previously thought. The present study
attempted to reveal the complexity of TSD, as it relates to other features of the
species' biology and physical characteristics tractable only in the field, such as
fluctuations in incubation temperature and reproductive life history. To this end I
studied the ecology of the turtle Carettochelys insculpta, a TSD species inhabiting the
wet-dry tropics of northern Australia from 1996 to 1998. I tested hypotheses
associated with movements, activity, behaviour, reproduction, nest site choice, nest
temperatures, embryonic survival, embryonic aestivation, hatch-ling sex ratios, and
emergence in the species. Each of these was also considered in the context of the
influence of the wet-dry tropics.
Compared to other turtles inhabiting lotic habitats, C. insculpta occupied
considerably larger home ranges, covering up to 10 km of river. Of previously
published factors influencing home range size, low productivity of the (micro) habitat
may best explain the extensive home ranges in C. insculpta. Patchiness and low
nutrient value of the chief food (aquatic vegetation) of C. insculpta may force turtles
to cover large expanses of river to acquire sufficient energy for growth and
reproduction. Females were more active, moved farther, and occupied larger home
ranges than males. Home ranges of females comprised 1-4 activity centres, many of
which were associated with thermal springs. I suggest that females may exhibit
increased activity and movements relative to males because of sexual inequality in
parental investment, where food is particularly limiting (e.g., in species with biennial
reproduction). Biennial reproduction in the population allowed the examination of the
influence of reproductive condition on home range size, movements, and activity.
Reproductive condition did not influence home range or activity, but gravid turtles
moved father between successive sightings than non-gravid females. Individual data
corroborate these findings, with females moving farther between successive sightings
while gravid compared to while spent. Contrary to previous reports, turtles did not
appear to move into estuarine areas or lowland flood plains during the wet season, but
moved into the riparian forest and possibly into wetlands adjacent to the main channel
in the vicinity of their dry season home ranges.
During the study I documented the turtles' use of small, localized thermal
springs discharging from the river bottom. Dataloggers attached to the carapace to
monitor ambient water temperatures recorded the frequency and duration of thermal
spring use by individuals. Turtles used the thermal springs frequently during the
winter (4-6 months) when river temperatures were lower than that of the thermal
springs (8 = 29 � 0.52� C). Turtles often utilized thermal springs for several
consecutive hours, leaving the springs only to surface for air. Thermal springs may be
derived from ground water (which maintains a temperature equivalent to the annual
mean air temperature), rather than from a specific geothermal heat source. Nine of 19
radio-telemetered adult females were seen to use thermal springs, of which seven
were gravid and two non-gravid. Thus, gravid turtles may seek thermal springs more
than non-gravid turtles. Frequency, duration, and timing of usage collectively suggest
active thermoregulation as the primary function of thermal spring use. Utilization of
thermal springs probably permits turtles to be more active in cooler months, which
may enhance growth rates and accumulation of energy for reproduction. Onset of
nesting along river stretches with thermal springs preceded nesting in a stretch not
known to have thermal springs by 24 days. Thus, I speculate that by warming
themselves on thermal springs in the months prior to nesting, turtles may have
accelerated follicular development and nested earlier.
Female C. insculpta matured at ca. 6 kg body mass (38.0 cm carapace length,
30.5 cm plastron length). Turtles produced egg sizes and clutch sizes similar to that
of other turtle species of similar size. Turtles reproduced every second year, but
produced two clutches in each breeding year, ca. 40 days apart. Thus, it appeared that
females were energy limited, possibly due to the low available energy content of the
dry season diet (aquatic vegetation). Life history theory predicts that if some costly
behaviour is associated with reproduction, skipping years could reduce that cost and
allow savings to be directed into future reproduction. The present study revealed no
obvious accessory behaviour in the population. Within years, clutch mass did not
differ between early (first) and late (second) clutches. However, earlier clutches
tended to have more and smaller eggs per clutch but than later clutches, a new finding
for turtles that has been demonstrated in lizards and other animals. Because the study
spanned both years with 'big' and 'small' wet seasons, I was able to examine how the
magnitude of the wet season influenced reproductive characteristics. Following big
wet seasons turtles produced larger, heavier, and more eggs per clutch than they did
after small wet seasons. Relationships among body size, egg size, and clutch size
were evident after two big wet seasons but not apparent after two small wet seasons.
Collectively, annual variation in reproductive characteristics and current life history
theory suggest that a big wet season is a plentiful time for the turtles.
I investigated beach selection of nesting pig-nosed turtles (Carettochelys
insculpta) along a 63 km stretch of river in 1997 and 1998. I used three classes of
beaches to examine beach choice: beaches with nests, beaches with only crawls, andbeaches without nests or crawls. Across these beach types I compared aspect, solar
exposure, temperature, substrate moisture, height, water depth at approach, and the
height of cohesive sand. I located 82 nesting beaches with 221 nests, and identified
171 potential nesting beaches based on previously published criteria. Beaches with
nests had a greater substrate moisture content and corresponding higher cohesive sand
line (hereafter CSL) than beaches without nests. Beaches with nests also had a higher
CSL than beaches with only crawls. Apparently, turtles could not excavate a nest
chamber above the CSL due to loose substrate consistency causing sand to fall in on
itself. Turtles could only nest at low elevations below the CSL on beaches with lower
substrate moisture. Turtles apparently avoided nesting on these beaches due to the
higher probability of nest flooding, as corroborated by a concurrent study. Beach
temperatures increased with a seasonal increase in air temperatures, and were
influenced by aspect and total angle of solar exposure. Temperatures did not differ
among beaches with nests, beaches with only crawls, and beaches without crawls or
nests. Therefore, there was no indication that turtles were manipulating offspring sex
through choice of nesting beach. However, turtles may be manipulating sex by
nesting in areas with particular thermal characteristics within beaches.
Two related aspects of hatchling emergence were studied. Using emergence
phenology data, nest temperatures, historical weather data, and a developmental
model, I tested the hypothesis that delayed hatching occurred in C. insculpta, and that
such a delay would allow hatchlings to time their emergence to match the onset of the
wet season. Hatchling C. insculpta emerged, on average, 17 days later than dates
predicted from a developmental model. Combined with observations of hatchlings
remaining in eggs until emergence, these results confirmed delayed hatching in
nature. This delay was synchronized with initial river rises associated with the onsetof wet season rains, and is consistent with published criteria for embryonic
aestivation. On a diel scale, I generated predictions of two potentially competing
models for nocturnal emergence in hatchling turtles, based on the knowledge that air
temperatures decrease with season during the emergence period. A test of those
predictions for C. insculpta produced ambiguous results. However, further analysis
indicated that C. insculpta, and probably other nocturnally emerging turtle species,
respond to a decline in diel temperature rather than an absolute temperature. The
former would ensure nocturnal emergence, while the latter is experienced during the
day as well as at night. Nocturnal emergence may be associated with nesting in open
microhabitats.
The 'decision' of when and where to nest can influence both offspring survival
and hatchling sex ratios in animals with temperature-dependent sex determination
(TSD). Knowledge of how these maternal attributes influence the incubation
environment is an important first step in hypothesizing why TSD evolved in a
particular species. 1 studied the influence of nest site choice and timing of nesting on
embryonic survival and hatchling sex ratios. Predation and flooding were the major
sources of embryonic mortality. Embryonic survival was influenced by both lay date
and nest site choice: In one year when nesting began later, nests laid later and at lower
elevations were destroyed by early wet season river rises. In other years early nesting
precluded flood mortality. However, turtles did not nest at the highest available
elevations. I hypothesized that turtles were unable to nest at higher elevations
because the sand was dry and not cohesive. A field experiment demonstrated that
turtles were constrained to nest at lower elevations where they could construct a nest
chamber. A mathematical model predicting hatchling sex from fluctuating
temperatures was applied to temperature data from 102 natural nests. Resultsconfirmed a type la pattern of TSD, whereby males are produced from cooler
temperatures and females from warmer temperatures. The principal determinant of
hatchling sex was lay date. Clutches laid earlier in the season produced mainly males,
while later clutches yielded mostly females, due to seasonal ramping of air and sand
temperatures. However, nest site choice also exerted an influence on hatchling sex.
Female-producing clutches were deposited at higher elevations than male-producing
clutches. The onset of nesting was not influenced by water temperatures, but may
have been related to the magnitude of the previous wet season(s). Turtles nested
earlier after two 'big' wet seasons and later following two 'small' wet seasons. This
pattern indicates that the wet season is a plentiful time for the turtles. Adaptive
'differential fitness' models for the evolution of TSD have recently been reviewed and
clarified. The differential fitness model that best fits C. insculpta is the 'timematching'
model, whereby one sex benefits more than the other from early hatching.
Male C. insculpta hatched 2-3 weeks earlier then females, on average. Benefit to
early hatching males and, therefore, the ultimate selective mechanism (e.g., growth,
time to mature) is unknown. Obtaining such data will likely prove difficult in such a
long-lived species.
A recent adaptive explanation for the evolution and maintenance of temperaturedependent
sex determination (TSD) in reptiles rests upon the assumption that mothers
can predict or manipulate offspring sex. I postulated that four physiological and
behavioural criteria must be met in order for this assumption to be valid: (1) a strong
correlation must exist between substrate temperatures during nest site choice and nest
temperatures during the period of development when sex is determined in the egg
(thermosensitive period = TSP). (2) Assuming that (1) is possible, mothers would need
to be capable of correcting for temporal factors obscuring the predictable thermalcharacteristics of nest sites. This could be accomplished in two ways. By contracting
nesting times mothers could assess the relative temperatures of alternate nest sites with
some accuracy. A protracted distribution of nesting times could greatly reduce a
mother's ability to distinguish between, for example, a cooler nest site at a warmer
time and a warmer nest site at a cooler time. Alternatively, mothers would need to be
able to incorporate temporal changes in nest site temperatures. (3) Sufficient variation
in thermal profiles among nest sites, relative to the breadth of temperatures producing
both sexes (pivotal temperatures), would be necessary. For example, if most nests
produced both sexes, then depth of the eggs would be the deciding factor determining
sex, leaving little opportunity for nest site choice to produce one sex or the other. (4)
Mothers would need access to nest sites spanning a range of thermal profiles in order
to produce either offspring sex. To this end, home range size relative to the number
and location of nesting beaches should be important. I tested these four predictions in
Carettochelys insculpta, a beach nesting turtle with TSD, using three years of field
data on nest site choice, nesting times, thermal characteristics of nests, hatchling sex
ratios, and movements of nesting turtles. A strong positive correlation existed between
assessable substrate temperatures at nest site choice and mean daily TSP temperatures
in all three years. However, the proportion of explained variation was highly variable
among years, and low in 1998. Accordingly, the proportion of nests in which substrate
temperatures at nest site choice predicted offspring sex correctly was low in 1998 (48-
62 %, depending on treatment of the data). Nesting times were normally distributed,
and combined with diel changes in nest site temperatures greatly reduce a turtle's
ability to distinguish between sites that would produce different sexes. Considerable
among-clutch variation in thermal profiles to produce variable sex ratios existed,
agreeing with other studies on turtles. Radiotelemetry indicated that home rangesencompassed several nesting beaches with differing thermal profiles, indicating scope
for producing the desired sex. However, the seasonal increase in air temperatures
resulted in an overriding effect of mostly males being produced in early (first) clutches
and mainly females being produced in late (second) clutches. Collectively, the results
suggest that C. insculpta mothers would find it difficult to predict, and therefore,
manipulate hatchling sex, supporting the conventional notion that TSD mothers have
little or no control over offspring sex.
| Identifer | oai:union.ndltd.org:ADTP/218589 |
| Date | January 2002 |
| Creators | Doody, J. Sean, n/a |
| Publisher | University of Canberra. Applied Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright J. Sean Doody |
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