Variation in Sea Turtle Life History Patterns
245
9.3 SEA TURTLE LIFE HISTORY PATTERNS
Although there are only seven extant species, sea turtles exhibit a surprising
diversity of life history traits that make them good subjects for comparative life
history studies. Aspects of this diversity are illustrated by reproductive extremes
from arribadas to solitary nesting, dietary specializations from seagrasses to
sponges to jelly organisms, and metabolic adaptations from hibernation to endothermy ( sensu lato ). Despite this high diversity in life history traits among sea
turtle species, there have been few comparative analyses or syntheses of their life
history patterns. Hendrickson (1980) was the first to attempt to summarize the
ecological strategies of sea turtles. However, his conclusions were limited by the
lack of information at the time of his synthesis in both life history characteristics
of the different sea turtle species and their taxonomic relationships, which are now
better understood through molecular techniques. In another analysis, Van Buskirk
and Crowder (1994) developed a dendrogram to compare the reproductive characteristics among the seven species. This chapter focuses on an analysis of the
variation in the juvenile developmental stages.
Once hatchlings emerge from their nests, crawl down the beach, and enter the
sea, post-hatchlings embark upon one of three basic developmental life history
patterns:
• Complete development in the neritic zone (Type 1, Figure 9.1, top panel)
• Early juvenile development in the oceanic zone and later juvenile development in the neritic zone (Type 2, Figure 9.1, middle panel)
• Complete development in the oceanic zone (Type 3, Figure 9.1, bottom
panel)
9.3.1 T YPE 1: T HE N ERITIC D EVELOPMENTAL P ATTERN
The Type 1 life history pattern is characterized by developmental and adult stages
occurring completely in the neritic zone (Figure 9.1, top panel). The Australian
flatback turtle ( Natator depressus ) apparently has a completely neritic developmental
pattern (Walker and Parmenter, 1990; Walker, 1994) and is the only extant example
of the Type 1 life history pattern. Walker (1994) suggests there may be increased
food resources in the neritic zone, but with a tradeoff of increased predation. Flatback
hatchlings are larger than those of other cheloniid sea turtles (60 vs. 41–50 mm
carapace length [Van Buskirk and Crowder, 1994]; 39 vs. 15–25 g hatchling mass
[Miller, 1997]). As would be expected from the evolutionary tradeoff between the
size and number of offspring, clutch size is smaller in flatbacks (53 vs. 100–182
[Hirth, 1980; Van Buskirk and Crowder, 1994], although East Pacific green turtles
[ Chelonia mydas ] have small clutch sizes from 65 to 90 [Hirth, 1997]). Flatbacks
thus produce fewer, larger progeny, a pattern typical of marine species with shorter
dispersal distances relative to species that disperse more widely, such as Type 2 and
3 sea turtles.
Researchers have speculated that the larger hatchling size of flatback turtles may
reduce predation in the neritic zone (Hirth, 1980; Walker and Parmenter, 1990;
1123 book.book Page 245 Monday, November 11, 2002 11:11 AM
245
9.3 SEA TURTLE LIFE HISTORY PATTERNS
Although there are only seven extant species, sea turtles exhibit a surprising
diversity of life history traits that make them good subjects for comparative life
history studies. Aspects of this diversity are illustrated by reproductive extremes
from arribadas to solitary nesting, dietary specializations from seagrasses to
sponges to jelly organisms, and metabolic adaptations from hibernation to endothermy ( sensu lato ). Despite this high diversity in life history traits among sea
turtle species, there have been few comparative analyses or syntheses of their life
history patterns. Hendrickson (1980) was the first to attempt to summarize the
ecological strategies of sea turtles. However, his conclusions were limited by the
lack of information at the time of his synthesis in both life history characteristics
of the different sea turtle species and their taxonomic relationships, which are now
better understood through molecular techniques. In another analysis, Van Buskirk
and Crowder (1994) developed a dendrogram to compare the reproductive characteristics among the seven species. This chapter focuses on an analysis of the
variation in the juvenile developmental stages.
Once hatchlings emerge from their nests, crawl down the beach, and enter the
sea, post-hatchlings embark upon one of three basic developmental life history
patterns:
• Complete development in the neritic zone (Type 1, Figure 9.1, top panel)
• Early juvenile development in the oceanic zone and later juvenile development in the neritic zone (Type 2, Figure 9.1, middle panel)
• Complete development in the oceanic zone (Type 3, Figure 9.1, bottom
panel)
9.3.1 T YPE 1: T HE N ERITIC D EVELOPMENTAL P ATTERN
The Type 1 life history pattern is characterized by developmental and adult stages
occurring completely in the neritic zone (Figure 9.1, top panel). The Australian
flatback turtle ( Natator depressus ) apparently has a completely neritic developmental
pattern (Walker and Parmenter, 1990; Walker, 1994) and is the only extant example
of the Type 1 life history pattern. Walker (1994) suggests there may be increased
food resources in the neritic zone, but with a tradeoff of increased predation. Flatback
hatchlings are larger than those of other cheloniid sea turtles (60 vs. 41–50 mm
carapace length [Van Buskirk and Crowder, 1994]; 39 vs. 15–25 g hatchling mass
[Miller, 1997]). As would be expected from the evolutionary tradeoff between the
size and number of offspring, clutch size is smaller in flatbacks (53 vs. 100–182
[Hirth, 1980; Van Buskirk and Crowder, 1994], although East Pacific green turtles
[ Chelonia mydas ] have small clutch sizes from 65 to 90 [Hirth, 1997]). Flatbacks
thus produce fewer, larger progeny, a pattern typical of marine species with shorter
dispersal distances relative to species that disperse more widely, such as Type 2 and
3 sea turtles.
Researchers have speculated that the larger hatchling size of flatback turtles may
reduce predation in the neritic zone (Hirth, 1980; Walker and Parmenter, 1990;
1123 book.book Page 245 Monday, November 11, 2002 11:11 AM