180
The Biology of Sea Turtles, Vol. II
6.3.5.2 Temperature
High sand temperatures are an additional stress affecting hatchling behavior as well
as nest success. Although thermal inhibition of movement most likely prevents daytime emergence, preventing additional thermal and dehydration stress and exposure
to daytime predators (Mrosovsky, 1968; Gyuris, 1993), when temperatures are particularly high in nests, embryonic and hatchling deaths may result either as a direct
result of crossing into the upper lethal temperature range or possibly as a result of
behavioral (movement) inhibition to the point of nonemergence. Miller (1985) found
that sea turtle eggs held at temperatures greater than 33∞C for extended periods of
time did not hatch, consistent with the thermal tolerance range for developing sea
turtles proposed by Ackerman (1997) of between 25-27∞C and 33-35∞C; it was noted
by both Cheeks (1997) and Fortuna and Hillis (1998) that higher than normal nest
temperatures in the field decrease sea turtle nest success. In an in situ comparison
between naturally or artificially shaded hawksbill turtle nests in St. Croix and those
exposed to direct sunlight (after Hurricane Hugo removed shoreline vegetation),
Fortuna and Hillis (1998) found that unshaded nests averaged 2.1∞C warmer than
shaded nests in the same location. Unshaded nests also had significantly lower mean
hatch success and nearly three times as many full-term dead embryos, with an
apparent exponential relationship between maximum nest temperature and the percentage of embryos that died late in development.
A similar correlation was noted between extreme temperatures (greater than
33∞C, with some temperatures as high as 37.6∞C) in loggerhead nests relocated to
a Miami Beach, FL, hatchery and low emergence (but not hatching) success. Especially significant were high temperatures during the last 3 days of incubation and
number of pipped dead hatchlings in the nest (Blair, 2001). A significant increase
in the number of pipped dead occurred in nests experiencing maximum temperatures
between 32 and 34∞C. Although high temperatures may be directly lethal to developing embryos, it cannot be determined if hatchlings from nests with high hatching
but low emergence success are affected directly by temperature or indirectly through
temperature effects on behavior. Experiments on newly emerged individuals and
small-group behavior at various temperatures have shown that crawling by newly
emerged loggerhead hatchlings from the Miami Beach hatchery, even in a group, is
significantly inhibited by temperatures above 33∞C (Blair 2001), which may result
in nonemergence of a nest despite high hatch success. Physiological and behavioral
responses to increased temperatures include the well-described thermal inhibition
that prevents hatchling emergence when sand temperatures are high (Witherington
et al., 1990; Moran et al., 1999) as well as reduced swimming speeds at temperatures
above 30∞C and loss of coordinated muscle movement in loggerheads swimming at
temperatures above 33∞C (O’Hara, 1980).
6.3.5.3 Frenzy Swimming
Crawling and frenzy swimming are also metabolically costly; as in emergence, the
hatchlings (D. coriacea, C. caretta, and C. mydas) again exceed their aerobic scope
and blood lactate increases, though to a lesser extent than in digging hatchlings
1123 book.book Page 180 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
6.3.5.2 Temperature
High sand temperatures are an additional stress affecting hatchling behavior as well
as nest success. Although thermal inhibition of movement most likely prevents daytime emergence, preventing additional thermal and dehydration stress and exposure
to daytime predators (Mrosovsky, 1968; Gyuris, 1993), when temperatures are particularly high in nests, embryonic and hatchling deaths may result either as a direct
result of crossing into the upper lethal temperature range or possibly as a result of
behavioral (movement) inhibition to the point of nonemergence. Miller (1985) found
that sea turtle eggs held at temperatures greater than 33∞C for extended periods of
time did not hatch, consistent with the thermal tolerance range for developing sea
turtles proposed by Ackerman (1997) of between 25-27∞C and 33-35∞C; it was noted
by both Cheeks (1997) and Fortuna and Hillis (1998) that higher than normal nest
temperatures in the field decrease sea turtle nest success. In an in situ comparison
between naturally or artificially shaded hawksbill turtle nests in St. Croix and those
exposed to direct sunlight (after Hurricane Hugo removed shoreline vegetation),
Fortuna and Hillis (1998) found that unshaded nests averaged 2.1∞C warmer than
shaded nests in the same location. Unshaded nests also had significantly lower mean
hatch success and nearly three times as many full-term dead embryos, with an
apparent exponential relationship between maximum nest temperature and the percentage of embryos that died late in development.
A similar correlation was noted between extreme temperatures (greater than
33∞C, with some temperatures as high as 37.6∞C) in loggerhead nests relocated to
a Miami Beach, FL, hatchery and low emergence (but not hatching) success. Especially significant were high temperatures during the last 3 days of incubation and
number of pipped dead hatchlings in the nest (Blair, 2001). A significant increase
in the number of pipped dead occurred in nests experiencing maximum temperatures
between 32 and 34∞C. Although high temperatures may be directly lethal to developing embryos, it cannot be determined if hatchlings from nests with high hatching
but low emergence success are affected directly by temperature or indirectly through
temperature effects on behavior. Experiments on newly emerged individuals and
small-group behavior at various temperatures have shown that crawling by newly
emerged loggerhead hatchlings from the Miami Beach hatchery, even in a group, is
significantly inhibited by temperatures above 33∞C (Blair 2001), which may result
in nonemergence of a nest despite high hatch success. Physiological and behavioral
responses to increased temperatures include the well-described thermal inhibition
that prevents hatchling emergence when sand temperatures are high (Witherington
et al., 1990; Moran et al., 1999) as well as reduced swimming speeds at temperatures
above 30∞C and loss of coordinated muscle movement in loggerheads swimming at
temperatures above 33∞C (O’Hara, 1980).
6.3.5.3 Frenzy Swimming
Crawling and frenzy swimming are also metabolically costly; as in emergence, the
hatchlings (D. coriacea, C. caretta, and C. mydas) again exceed their aerobic scope
and blood lactate increases, though to a lesser extent than in digging hatchlings
1123 book.book Page 180 Monday, November 11, 2002 11:11 AM