Variation in Sea Turtle Life History Patterns
251
has probably exhibited the Type 3 pattern for a long time, with resultant physiological
adaptations that have allowed leatherbacks to exploit both tropical and temperate
oceanic realms.
9.4.2 R EPRODUCTIVE T RAITS
Van Buskirk and Crowder (1994) evaluated a number of reproductive traits (e.g.,
female size, clutch size, egg volume, hatchling size, clutch frequency, and remigration interval) and developed a dendrogram illustrating the relationships among the
species (Figure 9.3, right dendrogram). The Van Buskirk and Crowder (1994) dendrogram is not congruent with the phylogeny of Bowen and Karl (1997), but has a
greater similarity to the three life history patterns presented in Figure 9.1. This better
fit would be expected between life history patterns (Figure 9.3, center) and reproductive behavior/demographic traits (Figure 9.3, right dendrogram). The two species
with the greatest difference in reproductive traits (leatherbacks and flatbacks) are
also the two species with the greatest difference in life history patterns.
9.5 A CLOSER LOOK AT THE TYPE 2 PATTERN:
ONTOGENETIC HABITAT SHIFTS
Species exhibiting either the Type 1 or Type 3 pattern commit to either the neritic
or oceanic zone, respectively, for their entire developmental stages as well as for
the adult foraging stage. Only turtles with the Type 2 pattern have a major habitat
change during their development. The Type 2 pattern is the most successful pattern
if success is defined by the number of species with this life history pattern (five of
the seven extant species exhibit the Type 2 pattern). As presented above (Section
9.4.1), the Type 1 pattern is hypothesized to be the ancestral pattern that still exists
today (although presumably secondarily derived) in the Australian flatback. Why
post-hatchling turtles leave the neritic zone for the oceanic, and why, after an
extended development period in the oceanic zone, the turtles return to the neritic to
complete their development, are two intriguing questions.
The existence of an early developmental stage in the oceanic habitat may be a
result of higher predator pressure in neritic habitats and/or intra- and interspecific
competition for food in neritic habitats. Such competition may not be apparent now
because of depleted sea turtle populations, but evidence for density-dependent effects
on growth rates has been reported for a population of green turtles in a neritic
foraging habitat (Bjorndal et al., 2000b).
Even more puzzling is the shift from oceanic to neritic habitats. Why do juvenile
turtles leave the oceanic zone where they have spent the first years of their lives
successfully finding food, growing, and surviving? When they leave the oceanic zone
for the neritic zone, they enter a new habitat with which they are unfamiliar, and must
learn to find new food sources and avoid a new suite of predators. A current hypothesis
to explain why ontogenetic habitat shifts occur is that a species shifts habitats to
maximize growth rates (Werner and Gilliam, 1984). Bolten (in press) presents evidence
for the Atlantic loggerhead population that supports the Werner and Gilliam hypothesis.
The extrapolation of the size-specific growth function for the oceanic stage intersects
1123 book.book Page 251 Monday, November 11, 2002 11:11 AM
251
has probably exhibited the Type 3 pattern for a long time, with resultant physiological
adaptations that have allowed leatherbacks to exploit both tropical and temperate
oceanic realms.
9.4.2 R EPRODUCTIVE T RAITS
Van Buskirk and Crowder (1994) evaluated a number of reproductive traits (e.g.,
female size, clutch size, egg volume, hatchling size, clutch frequency, and remigration interval) and developed a dendrogram illustrating the relationships among the
species (Figure 9.3, right dendrogram). The Van Buskirk and Crowder (1994) dendrogram is not congruent with the phylogeny of Bowen and Karl (1997), but has a
greater similarity to the three life history patterns presented in Figure 9.1. This better
fit would be expected between life history patterns (Figure 9.3, center) and reproductive behavior/demographic traits (Figure 9.3, right dendrogram). The two species
with the greatest difference in reproductive traits (leatherbacks and flatbacks) are
also the two species with the greatest difference in life history patterns.
9.5 A CLOSER LOOK AT THE TYPE 2 PATTERN:
ONTOGENETIC HABITAT SHIFTS
Species exhibiting either the Type 1 or Type 3 pattern commit to either the neritic
or oceanic zone, respectively, for their entire developmental stages as well as for
the adult foraging stage. Only turtles with the Type 2 pattern have a major habitat
change during their development. The Type 2 pattern is the most successful pattern
if success is defined by the number of species with this life history pattern (five of
the seven extant species exhibit the Type 2 pattern). As presented above (Section
9.4.1), the Type 1 pattern is hypothesized to be the ancestral pattern that still exists
today (although presumably secondarily derived) in the Australian flatback. Why
post-hatchling turtles leave the neritic zone for the oceanic, and why, after an
extended development period in the oceanic zone, the turtles return to the neritic to
complete their development, are two intriguing questions.
The existence of an early developmental stage in the oceanic habitat may be a
result of higher predator pressure in neritic habitats and/or intra- and interspecific
competition for food in neritic habitats. Such competition may not be apparent now
because of depleted sea turtle populations, but evidence for density-dependent effects
on growth rates has been reported for a population of green turtles in a neritic
foraging habitat (Bjorndal et al., 2000b).
Even more puzzling is the shift from oceanic to neritic habitats. Why do juvenile
turtles leave the oceanic zone where they have spent the first years of their lives
successfully finding food, growing, and surviving? When they leave the oceanic zone
for the neritic zone, they enter a new habitat with which they are unfamiliar, and must
learn to find new food sources and avoid a new suite of predators. A current hypothesis
to explain why ontogenetic habitat shifts occur is that a species shifts habitats to
maximize growth rates (Werner and Gilliam, 1984). Bolten (in press) presents evidence
for the Atlantic loggerhead population that supports the Werner and Gilliam hypothesis.
The extrapolation of the size-specific growth function for the oceanic stage intersects
1123 book.book Page 251 Monday, November 11, 2002 11:11 AM