Tracking Elephants with Conventional Radio Collars
We used to think that elephants undertook seasonal long distances migrations between their wet season and dry season feeding grounds. So to test this we radio tracked 18 elephants with VHF collars in Sri Lanka. None of them migrated!
Previously, based on the idea that elephants migrated, elephant conservation plans were designed to limit elephants to protected areas (their presumed wet or dry season feeding grounds) and linking them with long narrow corridors - so that they could move between them. However, we also found that the home ranges (the area an animal needs to move about in to fulfill its resource requirements such as food, water, mates etc.) of many elephants had no relation to protected areas - they ranged completely outside protected areas. Other elephants used both protected areas and areas outside, while yet others ranged entirely inside protected areas.
While many animals used different parts of their home ranges seasonally, in no instances were these areas a significant distance from each other, requiring them to migrate long distances. The home ranges were generally 50-150 km2 . So what this says is that elephants in Sri Lanka are able to find their resource requirements within a very small area, throughout the year.
With VHF collars, the transmitter on the elephant emits a signal continuously, which sounds like "beep -beep-beep". We use a directional antenna - much like your TV antenna to locate it. When the antenna is pointed in the direction the elephant is in, the signal is obtained very clearly. When it is pointed away, the signal becomes faint or inaudible. By using this method we can basically walk up to the elephant. This type of tracking is called 'homing in'.
However, this works only if you are already within range of the elephant - the transmitters have a range of 3-4 km, which decreases even more if the elephant is in thick scrub forest. Now if we don't have any idea where the elephant is within to around 3 km, then we have to find it! For that we use a method called triangulation. Although the signal gets weak quickly when it travels through the forest, once it comes out of the forest canopy, it can be detected 10-15 km away. It is just like your TV signal - if you set up an antenna at ground level among trees, you get a weak signal, if you raise it above the trees the signal gets stronger.
So what we do is climb a high rock or mountain in the region the elephant is in, and scan for the signal. Once we find the signal, we figure out the direction of the signal from that point, mark the location on a map and draw a line from that point in the direction of the signal. Then we move to another mountain or rock some distance away and repeat it. Then where the lines cross will be where the elephant is. However, it is easy to make mistakes in figuring out the exact direction the elephant is. If it is very far away, an error of a degree or two will mean many km difference. So we do this process from a minimum of three locations, which gives us an 'error triangle', where the elephant should be located. If the triangle is approximately more than 1 km2 we repeat the process till we get a smaller triangle. This process is called triangulation. Once we detect the location the elephant is in through this manner, then we can go closer to the place and 'home in' if we want to.
This sort of tracking using VHF collars is very labor intensive. If we track around 5 animals, it generally takes a full day to find each animal. So in a month, we get only 4-5 data points for each animal. This sort of data is fine to answer questions such as, do elephants migrate and how big are their annual home ranges. However, it is insufficient to answer questions such as, what are the micro habitats elephant use most? Are there differences in movement patterns between night and day? How important is water availability etc. So to answer such questions which require detailed knowledge of the habitats and resources elephants use, we are now using a different system that incorporates GPS or Global Positioning System technology.
Read more about tracking elephants with GPS.
Department of Wildlife Conservation, Sri Lanka
Fernando P, Wikramanayake ED, Janaka HK, Jayasinghe LKA, Gunawardena M, Kotagama SW, Weerakoon D & Pastorini J (2008) Ranging behavior of the Asian elephant in Sri Lanka. Mammalian Biology 73:2-13.
Fernando P & Lande R (2000) Molecular genetic and behavioral analyses of social organization in the Asian elephant. Behavioral Ecology and Sociobiology 48:84-91.