Thursday, March 5, 2015

Factors effecting precision of radio telemetry in indigo snakes

When researching snake ecology, you might want to track it via radio telemetry. This means that you can know where the snake is and what it's doing without the snake knowing, and you can track it with a computer. But, for other animals, like birds, triangulation is used, which involves doing bearings of direction at two locations, and then using said bearings to estimate where the animal is. But that introduces a much larger chance of error. But, telemetry in snakes has many ways it could go wrong. If your snake is on flooded habitat or dense vegetation, you might not be able to track it, and many snakes spend a lot of time underground or in very thick vegetation, or even underwater! And what if your snake is brumating? And, not to mention, for triangulation, snakes are down-to-earth literally, and having a transmitter that's very low could cause issues in triangulation. Even though, apparently, triangulation is not commonly used in studies of reptiles and amphibians, it may be critical to determine just how accurate triangulated telemetry can be for snakes.

A radio telemetry study was done on Eastern Indigo Snakes as a part of a movement and resource selection study. During the study, a lot of snakes ended up on private land the landowner did not allow access to. During those instances, typical triangulation was used. To determine said snakes' locations, multiple estimators and beacon tests were used and linear error was predicted.

The study was done on the southern 40 kilometers of Lake Ways Ridge in Highlands County, Florida. The study area was a mix of natural habitats including scrub, scrubby flatwoods, mesic flatwoods, forested wetlands, and non-forested wetlands. There were also cattle ranches, citrus groves, and rural and urban development. Eastern Indigo Snakes were searched for using road-cruising and visual encounter surveying around burrows belonging to Gopher Tortoises. However, about 90% of captures were done when tracking other snakes or traveling from field site to field site. Snakes were chosen for radio transmission in a way that distributed the transmitters as evenly as possible among the sites. Smaller snakes got smaller transmitters and larger snakes got larger transmitters. However, at no time did the transmitters exceed 2% of the snake's body weight. Snakes were taken to the Small Animal Hospital for the necessary surgery for transmitter installment. Once the snakes had recovered, they were released into the nearest burrow or brush pile to where they were captured. Each snake was located every two days using a Yagi antenna and a R-1000 receiver. Beacon tests were done throughout the study on telemetered snakes that had accessible locations and did not appear to be moving based on the transmitter signal. 

Radio transmitters were implanted into 32 Eastern Indigo Snakes. Two individuals were lost less than 30 days after release, probably because their transmitters failed. The results had shown that distance to the estimated location of the animal had the strongest effect on linear error, which was consistent with triangulation studies done in the past. Angular error was increased with magnified distance from the transmitter. Reducing the distance between the scientist observing and the animal is often necessary to maintain reasonable accuracy. The low vertical height of the transmitters may have also reduced accuracy. When animals spend large amounts of time underwater or in thick vegetation, it also may reduce the accuracy of transmissions. The position of the snakes had very little effect on the error, except for beacon tests, in which most located snakes were moving. Either way, the results suggested that triangulation may not be sufficient for snake radio telemetry studies. It was suggested that triangulation should be avoided in said studies and beacon tests should be used instead to measure linear error of triangulation.

Bauder, Javan, Barnhart, Patrick. "Factors Affecting he Accuracy and Precision of Triangulated Radio Telemetry Locations of Eastern Indigo Snakes (Drymarchon couperi)" Herpetological Review, 45(4), 590-597. 2014. 

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