Evaluation of Automated Radio Telemetry for Quantifying Movements and Home Ranges of Snakes
We evaluated an automated telemetry system that can dramatically increase the amount of activity and spatial data collected for snakes. We developed methods for analyzing data from single automated receiving units (ARUs) and ARU arrays, compared results from ARUs with conventional hand tracking, and assessed previously untested assumptions used in conventional telemetry, using data from ratsnakes (Pantherophis spp.) in Texas and Illinois. ARU data indicated that ratsnakes spent most of their time in small home ranges (mean = 25 ha) but engaged in forays of up to 1.5 km from their core-use areas, suggesting this species may engage in central place foraging. Forays inflated home-range sizes greatly if areas were estimated using minimum convex polygons rather than 95% kernels. Large numbers of locations generated by ARUs produce more reliable home-range estimates than those from hand tracking. ARU data indicated that snakes moved in response to observers during hand tracking. Daily hand tracking produced reliable estimates of distances moved but underestimated distances by a factor of 4 when snakes were tracked every 5 days. Drawbacks of ARUs are that the error associated with individual locations exceeds that for hand-tracked locations and that the costs exceed those for hand tracking. Automated receivers can increase data greatly from radio-tracked snakes, providing novel insights unavailable from conventional hand tracking. There are drawbacks to this technology, some of which will vary among study species; therefore, researchers should evaluate the appropriateness of the technology for both the study species and the questions being asked.Abstract
The study site in east–central Illinois. Stars represent the ARU towers. The darker gray is the core area where spatial resolution averaged 28.6 m. The lighter gray is the area with an average accuracy of 77.1 m. The white portion of this 625-ha area represents the area where we can potentially detect a snake, although at the maximum distances the snake would probably have to be in the top of a tree. Spatial accuracy within the study area was highly variable with an average of 142.9 m.
(A) Locations for snake 164.305 on the afternoon/evening of 14 June 2011, illustrating variation largely resulting from error rather than from actual movements. (B) Locations for the same snake from the afternoon of 30 June 2011, illustrating some movement within the core home range and a foray away from and back to the core. Boxes represent the first location, and triangles represent the last location recorded for the snake over the given time frame. The X- and Y-axis are the Universal Transverse Mercator (UTM) coordinates (Northing, Easting).
Typical data associated with the activity of a snake illustrating how movement involves the simultaneous change in both signal strength and bearing. In this case, snake 172.099 moved 13 times between 0940 h and 1845 h on 29 May 2006.
The correlation between the estimated foray distance as determined via the ARUs and the 24-h, 48-h, and 5-day simulated conventional hand tracking. The dashed line represents a 1 : 1 relationship. All simulated hand tracking underestimates the distance moved when compared to the ARU.
Home-range kernel estimate for snake 164.880 from ARU data for 2010 (A) without individual locations, (B) with individual locations, and (C) estimated from hand-tracking locations (shown as circles). The home range determined using ARU data is smaller than that from hand tracking (11.3 vs. 24.0 ha) because the former does not include forays in the 95% kernel.
Contributor Notes
