Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
SAF Archives  search ask the scientists in the classroom cool science
scientists from previous shows
cool careers in science
ask the scientists
PREVIOUS
SCIENTISTS

Photo of Zandy Hillis-Starr Zandy Hillis-Starr
Turtle Travels


Zandy Hillis-Starr, a biologist with the National Park Service at Buck Island, tells you about how she captures and tracks hawksbill turtles as part of a successful program to save this endangered species.


q We saw some footage of Hurricane Hugo going through your island. I am wondering about the most recent hurricane, Georges. Did it do damage to the turtle nesting areas?

A In my eleven years with the National Park Service working at Buck Island Reef National Monument, I have had the "honor" to witness and document the effects of three hurricanes, Hugo in 1989, Luis/Marilyn in 1995, and as you mentioned most recently, Georges in 1998. Hurricane Georges passed directly over the east end of St. Croix and Buck Island Reef, taking almost exactly the same path as hurricane Marilyn -- luckily just before Georges reached our island the storm force winds dropped from 145 knots to 110 knots. Otherwise things would have been much worse! Over the last three weeks we have been assessing the damage to the entire island ecosystem including the sea turtle nesting areas. So far, things look great compared to the other hurricanes. As you saw on the show when a sea turtle lays her nest in a hurricane erosion zone we relocate that nest within the first hour after it is laid to "safer" ground. By the time Georges arrived we had relocated about 25 nests out of the 80 or so laid so far this season. We have determined that only three known hawksbill turtle nests were lost to the storm. We are happy to say that our nest relocation efforts seem to be working! Storm waves eroded the shoreline, dead trees were blown down and blocked access to some nesting areas, and lots of debris was piled up on the big sand beach. Only a few live trees were damaged and already the beaches are changing shape. Over all nothing the turtles can't crawl around or over.



q Can you please give more details about the relationship between nest temperature and whether the turtles will be male or female. I am also curious how this relationship was discovered.

A Although I do not know how the original study was conducted which determined that incubation temperature determined the sex of sea turtle hatchlings, I can tell you what I do know. Temperature-dependent sex determination (TDSD) in hatchling turtles was discovered in 1971 studying Testudo graeca, the spur-thighed tortoise, a common pet. This was a surprising finding because the sex of vertebrates was assumed to be genetically controlled. More studies have found that TDSD is widespread in reptiles -- it is found in all crocodiles, widespread in turtles, and found in only a few geckos. The sex of hatchling turtles is determined during the second third of incubation and the "average" temperature during that period regulates whether the gonads will become ovaries or testes. Incubation above or below the average temperature will cause the hatchlings in that clutch of eggs to be predominantly either male or female. In turtles, females develop at high temperatures and male at low ones. No one is really sure what the adaptive significance of TDSD is, but we still looking. Many studies have been conducted on TDSD in sea turtles and I can recommend these authors and articles to you:
The Biology of Sea Turtles, edited by Lutz and Musick, 1996;

Mrosovsky and Yntema, Temperature dependence of sexual differentiation in sea turtles: implications for conservation practices, Biol. Consv, 1980;

Limpus, Reed, Miller, Temperature dependent sex determination in Queensland sea turtles in Biology of Australasian Frogs and Reptiles, Grigg, Shine, Ehmann, editors, Surrey Beatty and Sons, Sydney, Australia, 1985.



q How do you monitor the nest temperature - remotely or by visiting each nest? What do you do if it seems to be too hot or too cool?

A We monitor the temperature in each nest using automatic temperature datalogging devices. The ones we use are about 2" x 2" x 1/2" in size. Inside each one is a computer memory chip, temperature sensor, circuit board, and small battery. We hook the unit up to our computer and using a special software tell the temperature recording device how often to measure the temperature, usually once every 2 hours, and then "launch" the unit. Each unit is packaged in a waterproof bag with a small amount of desiccant granules, sealed, and then taken to the beach. We place one temperature unit in each hawksbill turtle nest that we see laid at about 30 centimeters from the surface. Usually we tie one end of a piece of fishing line to the datalogger and the other end to a tree root or strong bush just in case the nest gets dug up by another turtle or washed away in a storm. Once the datalogger is in the nest we won't go back to it until the nest hatches. After it hatches we'll take the datalogger into the office and hook it up to the computer again and download the data; only then will we see just what the incubation temperatures were like in the nest. If the nest is laid in the open sun and dark soil we now know that the temperature will be too hot and most likely the developing hatchlings will die before they hatch -- so we move the nest to a more shaded area. We learned this the hard way and now can prevent loosing these nests by moving them when their laid. So far we haven't had any nests that were too cool.



q Do all baby turtles make it in the wild? Of the 140 or eggs in the nest, about how many will survive?

A Unfortunately no, it is felt that less than one percent of all the hatchlings that leave the beach live to reach maturity. From the time the eggs are laid on the beach, they are on their own. The number of eggs that will successfully hatch out is different in each nest and on every beach. At Buck Island Reef NM usually 80 percent of the eggs develop into hatchlings and of those 60 percent leave the nest and make their way to the sea. Their first crawl down the beach is the most dangerous trip they'll ever make - they can be eaten by almost anything, ghost crabs, rats, night heron, and once they reach the water they are prey for a large number of fishes and sea birds including pelicans, frigates, and gulls. The hatchlings only chance is to crawl as fast as they can to the sea and swim like crazy away from the island. Once they are out in the open ocean they drift with the currents and winds and eat anything they can catch. No one really knows exactly what a hatchling sea turtle's life is like after it leave the nesting beach -- these are still called the lost years, and scientists are working on ways to track hatchlings after they leave the beach.



q In the program you mentioned that the mature female turtles return to their nesting site when they are 20 years old to lay their eggs. How do they manage to find this same spot after all those years?

A We, sea turtle scientists, are still not entirely sure. Mother nature works in mysterious ways. What we do know and suspect is at some point toward the end of incubation, possibly beginning when the sea turtle hatchling is still inside its shell or just after it first "pips" out of its egg, that a very small portion of the hatchling sea turtle's brain is orienting and identifying with the magnetic field under their nesting beach. The continental plates and island plates all have specific magnetic fields that sea turtle may be able to detect as they swim over them through the ocean. Studies have been conducted tracking leatherback turtles leaving their nesting beaches off the Pacific coast of Mexico. Each female left the beach independent of other turtles yet each female followed approximately the same route away from the beach and headed off to the southern seas. This is a very deep part of the world's oceans and there are no features on the ocean bottom for the turtles to follow. But just possibly they can sense the magnet fields 1000s of feet below them and use this to guide them away from and back to their nesting beaches each season they nest. It is not always precise and some turtles do change beaches during a nesting season, but overall most females sea turtles return only to their nesting beach.



q In the television show, when you rebuilt the turtle nest, how exactly did you get the temperature in the new nest to match the temperature in the old one?

A When we relocate a nest it is primarily because it is threatened in some way. Either it is too close to the sea and might wash away, or predators like rats have found it, or it is in the middle of a picnic area and will get stepped on by visitors. We pick a "safer" location based on all our past experience and find a location where the nest will survive to hatch. We select an area where we know the temperature regime will produce a healthy nest. We also relocate nests to a wide variety of habitat types to ensure the beach produces a percentage of female and male hatchlings. The temperature through incubation may or may not match the one it would have had had we left it where the turtle laid it, but it will however survive to hatch. We are continuing to measure the temperature in many of the nests laid at Buck Island Reef to better understand what is required to help hawksbill turtles survive.



q How old is the oldest turtle that was ever alive and how old is the oldest turtle that you have caught?

A I am not sure how old the oldest turtle alive is, but I do remember a story about a Galapagos tortoise that Charles Darwin took back to Europe with him after his voyage on the Beagle in 1840s. I heard that this tortoise was still alive today and that it was going to be returned to the Galapagos Islands! If this is true that turtle would be over 150 years old. I am really not sure how true this story is but it is well know that tortoises do live to a very old age. Sea turtles don't reach maturity until they are about 25 to 35 years old and could live to be 40 or 50 years old. No one really knows. I began tagging nesting hawksbill turtles in 1988 and at least two of these females returned to the island to nest this year. If they were 25 when I first tagged them then they are well over 35 years old today. I hope to continue seeing some of our older nesting turtles far into the future.



q Are the hawksbill turtles going to be extinct soon, or do you think programs like yours will save them? Are there laws to protect them and to stop people from killing them for their shell or meat?

A I don't think hawksbill turtles will be going extinct soon, but what is soon? Sea turtles, like hawksbill turtles, have been here swimming in our world's oceans for over 100 million years and through hunting, pollution, and loss of nesting and foraging areas most of the world's sea turtle populations are in serious trouble. This means where once there were 100,000s sea turtles there are now only 1000s alive. Possibly too few to enable the species to survive. I hope that programs like ours will help save the hawksbill turtle and other species. We are working very hard to help the hawksbill turtles at Buck Island Reef NM survive - we protect the nesting females and their nests and hatchlings and hope we are providing a safe coral reef area where young hawksbill turtles can grow to maturity. Here in the United States there are many laws to help protect sea turtles; the most important is the Endangered Species Act under which all sea turtles are protected. Another law which helps to protect sea turtles around the world is CITES, the Convention on International Trade in Endangered Species, which prevents other nations from catching and selling endangered species like hawksbill turtles and exporting or selling these animals or parts of them to the US or other countries. Unfortunately not all nations have signed on and agree to the rules of CITES, it is their choice, we can only provide them with the facts and hope they agree to protect critical species. We all continue to work toward a global protection for all sea turtles species and programs like Scientific American Frontiers "Science in Paradise" are helping.



q Why are the transmitters used to track the turtles so large? Do they make it awkward for the turtle to swim? How long do they stay on and what kind of information do they send to you?

A A: The satellite transmitters are sized to hold the batteries necessary to power the signal from the sea turtle at the ocean's surface up to the satellites 100 miles overhead. Each transmitter weighs about 700 to 800 grams and should send signals for about 5 to 6 months. The sea turtle is hardly aware that she has anything on her shell, and it doesn't affect her swimming. Many sea turtles, hawksbill, loggerheads, and greens all carry a variety of ectobiota (barnacles, salps, crabs, and all kinds of algaes) on their shells, and the transmitter would just be another kind of hitchhiker that would come along for the ride.

The biggest problem with attaching satellite transmitters to the shells of sea turtles is hoping they stay on! Sea turtles, especially hawksbill turtles, spend a lot of time in the coral reef and could knock the transmitter off at any time. We just hope the transmitters stay on long enough (6 to 8 months) to tell us where the hawksbill turtles go after they leave the nesting beaches on Buck Island Reef NM. While the transmitters are attached to the turtles they will give us their location in the ocean, the longitude and latitude within about 20 miles, how often they are diving below the surface, and the length of the last dive before the signal was sent. This summer National Marine Fisheries Service provided 20 satellite transmitters for deployment on nesting hawksbill turtles all over the Caribbean. Transmitters were put on turtles in Barbados, Antigua, Jamaica, Yucatan Peninsula, Puerto Rico, and Buck Island Reef. All these turtles are being tracked by the Argos wildlife telemetry satellites and we are watching their progress every day.



q What do you look for to tell how old a turtle is? How accurate are your methods for determining the age of a turtle?

A It is very difficult to determine how old a turtle is by just looking at it. To make a good guess we look at several things - how big is it, what is it doing, and where did we find it. If the hawksbill sea turtle is found on the nesting beach laying eggs, you can be pretty sure it's female, usually larger than 65 centimeters long, and at least 20 to 25 years old. If it is a very small hawksbill turtle, less than 25 centimeters long, and found feeding on a coral reef -- it's a good guess it's not much older than 3 to 5 years and has just arrived on that reef from years of floating and swimming along with the ocean currents. We have several young hawksbill turtles that we first found at Buck Island in the reef four years ago. Every year we try and recapture them to get new measurements.

We are looking forward to getting a better understanding of how wild healthy hawksbill turtles grow through this work which will make our age estimates a lot better. A lot is still guess work, but it's getting better. Some scientists are counting the rings in the keratin-like scutes which cover the shell, marking them, and watching the turtle grow to estimate growth rate. Others are injecting the turtle's long bones with tetracycline (an antibiotic which stains the bone cells) to mark the bone. After a year they take a bone biopsy and count the growth rings in the bone see how much the turtle has grown. All of these things will help us better understand wild sea turtle growth and aging.



q What might be the evolutionary advantages and disadvantages of having the gender of a species determined by temperature?

A This is an excellent question and still much debated by sea turtle scientists, and I don't think anyone can give you a definite answer. Considering that sea turtles have been in existence for over 150 million years, whatever the advantages or disadvantages are, they have dealt with them all! I just hope they have the ability to adapt to all the changes they are facing and the speed at which they are occurring -- El Nino, global warming, more frequent and more intense seasonal storms, pollution, degradation of their habitats, development of nesting beaches, overfishing, and on and on, and survive for another 150 million years.



q How can you make a turtle relax by rubbing its belly? How did you discover this? What happens physically to a turtle when it is put into a trance?

A Zandy Hillis Starr passed this question along to Brendalee Phillips because she has "the touch" with hawksbill turtles. This is Brendalee's answer: I rub where the plastron and skin meet near the neck of the turtle. I'm not sure why it works but it does seem to relax the turtle. Perhaps the feeling of the gentle touch on this sensitive area of skin is soothing but I have no studies to verify this, just years of experience with it working.

When I worked with sick and injured stranded sea turtles, we often had to put them on their backs while treating the wounds, When I took genetics in college, my professor showed us how rubbing the beak of a chicken would "put it in a trance", and I had also seen some TV shows where scientists working with alligators would rub the belly when the gator was on its back to calm it down. When you turn the turtles on their backs, they wave their flippers around a lot -- this is a very unnatural position for them to be in -- so one day I decided to try it on a turtle and found that the lightest of touch rubbed at this site eventually calmed the turtle down and it stopped moving so much and we could get the work done faster and thus get the animal back upright sooner. The animal is not in a trance and will sometimes start moving around again but I have found that if you do nothing the turtle will continue moving around, by getting the animal calm it makes the encounter easier on the turtle and the scientists.

The turtle is not really in a trance, it just calms down, takes a depth breath and relaxes its flippers. I'm not sure why this happens. I found this relaxation technique by chance and have no scientific reason for its success.



q Can you recommend some good Websites about turtles?

A Zandy Hillis-Starr has recommended these sites for Turtle Happenings, the Leatherback Task Force and the Marine Turtle Newsletter:
www.turtles.org/happen.htm

www.coas.drexel.edu/environ/leatherback/core.html

www.seaturtle.org/mtn



Zandy Hillis-Starr is also featured in Cool Careers in Science. Check it out!




 

Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.