Gone are the days when wildlife researchers went afield armed only with binoculars and note pads. Today’s researchers benefit from advances in technology which help them gather information about nature. One of these is telemetry – a way to extend one’s power of observation far beyond the limits of the five senses.
Telemetry is one method used by researchers to gather information about animals that they cannot observe directly. It requires a transmitter, which is attached to the animal being studied, and a receiving device, usually held by the observer.
One form of telemetry uses radio waves to link transmitter and receiver. The transmitter emits a series of radio pulses, which are translated by the receiver into audible beats or tones or into readings on a meter. Changes in the beats or the meter readings indicate how close the observer is to the animal being studied.
Wildlife researchers have used radiotelemetry for more than 30 years, but recent technological advances have made the tool useful in new ways. Improved electronic parts have made transmitters light and tiny – some small enough to implant in an animal’s body. Small, powerful batteries have paved the way for transmitters that are compact and long-lived. Some transmitters get their power from solar panels instead of batteries.
Other types of transmitters are used on animals that travel long distances. They send their data to satellites orbiting hundreds of miles above the earth. Some transmitters that are used to study aquatic creatures emit high frequency sound instead of radio waves which are easily blocked by seawater. This is ultrasonic telemetry.
Although usually thought of as a homing device, telemetry can answer many other questions about wildlife. Staff from the Endangered Species and Nongame and Habitat Units of DEC’s Division of Fish and Wildlife are using telemetry to uncover vital and interesting information about several threatened and endangered animals in New York State.
Rats and Radios
Radiotelemetry is the tool that implicated disease as a possible cause of the decline of the Allegheny woodrat (Neotoma magister) in New York State.
The Allegheny woodrat, a grayish-brown rodent about the size of a squirrel, is on the endangered list in New York State. This creature shares part of its common name and a superficial resemblance with the reviled European species of rat, but not much else. Its living habits differ markedly from what you would expect of a rat.
The woodrat lives in talus slopes – piles of rock rubble that collect at the base of some hills and mountain ranges. Deep within the crevices between these fallen rocks and boulders, woodrats build houses of sticks furnished with nests of shredded bark. Entire generatAons of these vegetarian rodents may live and die within a few hundred yards of their birthplace. Woodrats have lived in the Shawangunk Mountains in southeastern New York State for thousands of years.
In the late 1970s, a respected naturalist in the region alerted DEC to a possible decline of woodrat populations in the Shawangunks. In response, Endangered Species staff surveyed dozens of suspected woodrat sites but found only five that were occupied. Of those five only one harbored more than a handful of woodrats.
A series of observations over time led to the conclusion that this largest and last known population had disappeared by 1987. Although other, undiscovered, populations might exist, researchers concluded that the fate of the woodrat in New York State was looking pretty grim.
A records search and examination of preserved museum specimens uncovered evidence of a healthy and widespread population of woodrats in southeastern New York State as late as the 1960s. Unfortunately, this research did little to explain the woodrat’s sudden and catastrophic decline. Were the rats dying because of disease? Environmental factors? The records shed little light on the matter.
Endangered Species staff suspected that carcasses of woodrats that have recently died might provide some important clues.
Much easier said than done. Woodrats live in an environment that is very difficult to enter. It would be nearly impossible to spot any that died within the talus slopes. The few woodrats that expired outside their rocky abode would be picked over by carrion eaters before researchers find them. Endangered Species staff decided that they had to develop a reliable way to find dead woodrats.
The staff got a group of woodrats from a healthy population in West Virginia, which they planned to release near former habitat on the Mohonk Preserve near New Paltz.
They equipped the rats with tiny radio collars which were designed to vary the rate of transmitted pulses in response to the temperature of the unit. The warmer the unit, the faster the pulse rate. Drawing on their knowledge of woodrat behavior, staff were able to make some useful interpretations of the information sent by the radio collars. Rapidly repeating audible beats coming from the receiver probably meant that the rat was sleeping in its characteristic curled-up position, thereby encircling and warming the radio collar. An intermediate frequency had died, allowing the transmitter to cool.
With this information, researchers took their receivers afield to pinpoint and collect the carcasses of the radio-collared rats. Although many rats died in inaccessible places, researchers located enough carcasses to develop some intriguing information.
Most carcasses of rats collected at the Mohonk site were infected with a roundworm parasite that lives in the intestines of raccoons. The intestinal parasite lays eggs which the raccoon excretes in its feces. It’s not certain how wo/drats come into contact with infected feces, but the behavior of this species offers at least one possibility.
The woodrat, like the packrat – its counterpart m the western United States – picks up and stores objects that catch its eye. For reasons unknown, feces hold particular interest.
The mystery of the decline of the Allegheny woodrat has yet to be fully solved, but radio telemetry has provided a clue that may not have been discovered any other way.
The Tale of the
Scientists used three types of telemetry to make surprising discoveries about the endangered Kemp’s ridley sea turtle (Lepidochelys kempii) – and about the important role that the waters surrounding Long Island play in their lives.
The numbers of dead and cold-stunned sea turtles washing ashore on Long Island during the winter raised questions in the minds of scientists. They had always considered these turtles to be creatures of tropical waters. Why were so many of them coming here in the first place? Were they carried here by ocean currents? Did illness or some other factor cause them to wander far from their normal habitat? The mystery went begging for a solution.
Kemp’s ridleys begin life on a few beaches on the Gulf Coast of Mexico. Each spring, over the course of a few nights, waves of gravid females lumber ashore to lay clutches of about 100 eggs, each the size of a Ping-Pong ball. Their business concluded, the females immediately turn tail and head back into the ocean leaving their sandy nests to the mercies of predators and the elements.
These turtles have a tough childhood – even before they hatch. The eggs are often infected by fungus, crushed by heedless mother turtles on their way to and from the sea and dug up by foraging mammals such as raccoons and coyotes.
Some of the eggs – less than 10 percent – produce young turtles which are small enough to hold in the palm of your hand. Fresh out of their eggs, they follow their instincts in a desperate dash to the sea. Those turtles lucky enough to escape hungry birds on the beach begin a new struggle – evading a host of predators in the ocean.
Kemp’s ridleys that meet these challenges will grow to weigh 100 pounds, to measure 30 to 40 inches in length, and to spend the next 50 years or so cruising the warm Caribbean Sea. Until recently, however, scientists have known relatively little about their adult lives.
Volunteers to the Rescue
In the early 1980s, the Okeanos Ocean Research Foundation, located in Hampton Bays, Long Island, formed a partnership with DEC. They established the New York Marine Mammal and Sea Turtle Stranding Network, a group of citizens who rehabilitate, when possible, and gather biological and ecological data from marine wildlife that washes up on Long Island beaches. Their sea turtle observations eventually piqued the interest of the scientific community and DEC.
As a result, DEC’s Return a Gift to Wildlife (RAGTW) program began, in 1988, to fund a five-year study project with Okeanos. The goal of the project was to develop information about the population of Kemp’s ridley sea turtles in New York’s coastal waters.
New Study, New Methods
Researchers knew they needed to gather more detailed information than they were likely to get from the usual study method – tagging turtles and drawing conclusions from recaptured specimens. They planned to study the turtle’s diets by collecting feces from captured specimens. They also turned to radio, ultrasonic and satellite telemetry to provide detailed answers to other specific questions about Kemp’s ridley sea turtles.
The study began with cold-stunned turtles collected by the stranding network during the winter months. Researchers kept these turtles for a short observation and recovery period, and then fitted them with radio transmitters. They released these turtles and tracked them from boats, aircraft and from shore.
Radiotelemetry studies conducted in local waters showed that these turtles eventually returned to normal patterns of behavior. These early studies also provided the first bits of evidence that these turtles were preparing to migrate southward from Long Island at the onset of winter.
Intrigued by this new information, researchers wanted to further test the assumption that this migration was a normal part of the turtle’s behavior. To do so, they set out to find a source of turtles that had not been cold-stunned.
They turned to Long Island fishermen, who often pull live sea turtles aboard in their fishing nets during the summer and fall months. By agreement, fishermen contacted Okeanos whenever they hauled in a live sea turtle.
Researchers fitted many of these turtles with radio and ultrasonic transmitters. These units helped scientists to follow closely the daily movements of these turtles, and to form a detailed picture of how they dove, fed and moved about in the shallow, warm water that surrounds Long Island. Ultrasonic tracking also allowed divers to enter the water, pinpoint the exact location of a turtle and capture it. Then it was studied and released again.
Having learned about the activities of these creatures around Long Island, researchers now set out to prove the suspected connection between these turtles and the breeding population in tropical waters. To do this, they employed satellite. telemetry – a tool that can send information even as these creatures migrate over long distances.
Starting in the fall of 1990, researchers began to attach satellite telemetry units to turtles. These satellite units provided detailed information on the paths these turtles followed once they left Long Island at the beginning of winter. Occasional recapture of other tagged turtles served to reinforce the satellite data.
Evidence gathered through the five-year study paints a picture that is in stark contrast to the traditional view of Kemp’s ridley sea turtles.
During the warm weather months, the waters around Long Island are home to young (two-to five-year-old), healthy turtles. They are probably getting experience in hunting several species of slow-moving, bottom-dwelling crabs that are abundant in these waters.
Satellite tracking suggests that there are one or two well-used migration paths between Long Island and southern U.S. waters. Other information developed during the study indicates that changes in water temperature trigger the arrival and departure of the turtles.
Much research remains to be done, but it is now clear that Long Island’s coast is far more than an unplanned stopover. These waters seem to be an important part of the life cycle for many young Kemp’s ridley sea turtles.
A Primer on Telemetry
Radiotelemetry transmitters can be built for many different types of land and aquatic animals.
To lessen their effect on the animal, transmitters are made as small and light as possible. A general rule is that the transmitters weigh no more than two percent of the weight of the animal.
Most radiotelemetry units transmit at frequencies near the VHF television broadcast band. The amount of power produced by subminiature units can be as little as 5 to 10 milliwatts – about 1/500th the power output of a CB radio.
The working life and range of these transmitters depend on the amount of available battery power. Units attached to large animals like moose can carry large batteries, which gives them a range of several miles and a working lifetime of up to five years. Units for small animals like woodrats may last only a few months and have a range of no more than a few hundred yards.
Environmental conditions also affect range. For example, moisture absorbs radio waves, cutting their range considerably. Researchers working with woodrats at the Mohonk site often found that wet vegetation reduced their working range to a matter of yards.
Radiotelemetry can, within limits, be used to track aquatic animals, The practical depth to which radiotelemetry transmitters are useful in fresh water is about 30 feet. Sea water, in contrast, is virtually opaque to these radio waves. Therefore, transmitters used on aquatic animals in marine environments are often encased in a streamlined float which is attached to the animal by a line.
Satellite telemetry is the method of choice when tracking animals over long distances. Satellite telemetry units work very much like radio units. There is, however, one crucial difference – satellite units send their signal to a satellite in earth orbit, not to a hand-held receiver.
The Long Island study used the Argos satellite system to monitor seaturtles. Argos is a group of satellites launched over several years from Europe and the United States. Designed primarily for weather observation and communication purposes, each satellite orbits the Earth in a slightly different way.
Some orbit from pole to pole while the Earth turns beneath them, Others cover more limited areas of longitude and latitude. Together, the group of satellites forms a network that covers the entire surface of the earth.
About six times a day, an Argos satellite that orbits the Earth From pole to pole passes over the turtle’s migration route. If the turtle is at the surface, the satellite will intercept the transmitter signal and record the data. The satellite periodically sends this data to a facility in Maryland. From there, the information is available to sea turtle biologists via computer link.
The satellite need not pass directly over a transmitter to intercept a signal. In general, however, the closer they are to being directly overhead, the more reliable the data, The most reliable satellite data can locate a turtle to within 150 yards.
Satellite transmitters are attached to sea turtles by a break-away lanyard looped through a small hole drilled in the rear of the upper shell. The floating transmitter unit is about 1 1/4 inches in diameter and 11 inches long.
To conserve the battery, an internal timer restricts operation to daylight hours when the turtles are most likely to be on the surface. Even so, battery power gives out after 50 to 65 days. This is usually about the some time the lanyard separates, letting the transmitter break free.
Ultrasonic transmitters generate sound waves in the range of 30,000 to 100,000 hertz (cycles per second). For comparison, the range of human hearing is about 20 to 20,000 hertz.
Quite the opposite of radio waves, ultrasound dissipates rapidly in the air, but travels long distances underwater. This property makes ultrasonic transmitters ideal for use with aquatic animals. The five-ounce ultrasonic transmitters used on Kemp’s ridleys often enabled researchers to monitor them from more than a mile away.