Can allometric growth by juvenile marine turtles thwart gape-limited predators? (A morphological test of that hypothesis)

Background/Question/Methods

Female marine turtles lay thousands of eggs over a long reproductive life but only a small fraction of their offspring survive to adulthood. Most offspring mortality occurs during the earliest growth stages (hatchlings, juveniles) that survive in the open ocean by hiding until they become too large to be consumed.  Many predators of small turtles are “gape-limited” (fishes and seabirds) and must swallow their prey whole.  We reared 2 species of marine turtle hatchlings (loggerheads, Caretta caretta; green turtles, Chelonia mydas) for several months to determine if how they grew might shorten their exposure time to gape-limited predators.  We documented through weekly measurements that both species grow wider more rapidly than they grow longer.  We also showed that in loggerheads, rudimentary spines in hatchlings grew faster and were proportionally larger in juvenile turtles.  Both patterns of differential (positive allometric) growth might make the turtles more difficult to swallow at a smaller length than if they grew larger and retained their hatchling proportions (isometric growth).  We tested that hypothesis by measuring the gape of a known oceanic predator on small turtles (the dolphinfish, Coryphaena hippurus) across its effective predatory size range (50 – 110 cm in body length).  

Results/Conclusions

We found that allometric growth shortened the time that the turtles remained vulnerable to dolphinfish by as much as two weeks, compared to an isometric growth schedule.  These results support the hypothesis that allometric growth, previously known but an enigma functionally, may be an important antipredator strategy in marine turtles.