Biology

Hawksbill on the Beach

Hawksbill on the Beach

Seven species of marine turtles are generally recognized worldwide, and six of these species are found in and around the Caribbean: the Leatherback, Green, Loggerhead, Kemp’s ridley, Olive ridley and Hawksbill. The hawksbill, renowned for its beautiful golden, reddish-orange, and brown mottled shell, swims throughout the tropical and subtropical waters of the Atlantic, Indian and Pacific Oceans. The turtle is named for its distinctive beak-like bill, which is adapted for picking sponges from coral reefs. Other distinguishing characteristics include the two pairs of scales between the eyes (called prefrontal scales) and overlapping scutes (keratin plates) on the carapace, or shell. The hawksbill is a rather small sea turtle; at Jumby Bay, adult females weigh between 40 and 60 kg (88 to 130 lbs), and shells range in length from about 75 to 95 cm (30 to 37 in).

Hawksbills are associated with coral reefs and feed primarily on sponges that grow there. These sponges contain glass needles called “spicules” — it’s one of nature’s wonders that hawksbills can subsist on a diet of glass! The unique hawksbill feeding habits help to maintain the delicate balance between corals and sponges in reef communities.

Female hawksbills in the Caribbean are believed to reach sexual maturity at 15 to 30 years of age. They mate while swimming and are able to store sperm for an extended period. Amazingly, they will travel hundreds of miles from their foraging grounds and return to the region (although not necessarily the exact same beach) from which they hatched decades earlier to deposit their eggs. Hawksbills prefer to nest on small, isolated “pocket beaches” with vegetation. Because they are small and agile, hawksbills are able to access maritime forest and conceal their nests within dense beach shrubbery, a unique behavior among marine turtles.  Nesting hawksbills show very strong fidelity at Jumby Bay, returning to the same beach – and often the same section of the beach – nesting season after nesting season.

Hawksbills do not reproduce every year, instead skipping a year or more to build up the resources necessary to produce eggs and make the long journey from foraging grounds to breeding sites.  Remigration intervals – the time that elapses between successive nesting seasons – average about 3 years at Jumby Bay. While most females return every two, three or four years, we have recorded 7 or more years elapsing between an individual female’s consecutive nesting seasons!

Female Turtle Laying Eggs

Female Turtle Laying Eggs

When the time is right, gravid females (those carrying eggs) crawl onto the beach and excavate a 50 cm (~1.5 ft) deep chamber shaped like an upside-down light bulb, using their rear flippers as scoops. Depending on the nesting substrate and surrounding vegetation, digging the egg chamber may take from about 20 minutes to more than an hour.  Some females need to make multiple digging attempts before they are satisfied with the site and able to deposit their eggs. Once the turtle has completed the digging process, she situates her tail over the chamber. Her ovipositor withdraws from beneath her tail, and she begins laying about 150 ping-pong ball-shaped eggs. Depositing this large clutch of eggs only takes around 15 minutes! After egg-laying is complete, she uses her rear flippers to scoop sand over the eggs and gently tamp the surface. The front and rear flippers are used to camouflage a huge swath around the egg chamber so that a hungry predator would have a difficult time pinpointing the exact site. The entire nesting process takes, on average, close to 1 hour and 30 minutes to complete; however, we’ve seen some turtles successfully nest in about an hour and others remain on the beach for more than 6 hours! Witnessing this ancient nesting ritual is a truly magical experience.

Hawksbills generally lay four to six nests per season at two-week intervals. For example, if a turtle is observed nesting on June 1, we’d expect to see her again on or around June 15, June 29, and so forth. A hawksbill nesting five times lays about 750 eggs in a single nesting season. That’s a lot of eggs, but very few hatchlings reach maturity: some estimates suggest that only about 1 in 1,000 hatchlings survive to reproductive age. Such low survival rates mean that an individual turtle must reproduce for 3 nesting seasons in order to replace both herself and her mate just once.

Hatchlings at Night

Hatchlings at Night

Hawksbill nests incubate for about 60 days, though the actual range varies from 55 to 70 days. A fascinating characteristic of hawksbills and all sea turtles is termed temperature dependent sex determination: the temperature of the sand in which eggs incubate determines whether a turtle will be male or female. More females are produced at higher temperatures, while males are more common with lower incubation temperatures.

Upon hatching, the tiny turtles take a day or two to rest, straighten their shells, and wait for their siblings to hatch: the digging power of many hatchlings to successfully emerge from the sand. A drop in temperature, perhaps brought about by rain or nightfall, serves as a cue to hatchlings to start digging. Upon emergence, hatchlings crawl towards the brightest horizon. In the absence of artificial lights, the brightest horizon is where the ocean meets the sky. Reaching the sea and orienting their swimming against the waves, hatchlings begin their perilous journey.