Reaching New Heights: Arboreal Camera Trapping Provides New Insights on the Ecology of Gray Treefrogs (Hyla versicolor)
The ecology of many arboreal ectotherms is poorly understood because of access constraints to the forest canopy. For example, despite their common name, Gray Treefrogs (Hyla versicolor) are understudied in arboreal habitats. Advancements in camera trap technology and tree-climbing techniques have created new opportunities to study arboreal species. Using arboreal camera trapping and time-lapse photography, we implemented the first study of H. versicolor behavior in the canopy. We examined seasonal and diel patterns of activity, microhabitat use, and effects of relative humidity and temperature on H. versicolor activity in eastern white pine (Pinus strobus) canopies. We installed camera traps at three research sites in northern Wisconsin, USA, spanning a 57-km north–south transect, and maintained them for multiple years. We detected H. versicolor events, of all sex–age classes, at two of our research sites. Use of time-lapse photography significantly improved our ability to detect H. versicolor in the canopy. Hyla versicolor individuals were active in the canopy every year of the study (2015–18), but predominantly during the summer, and peak diel activity occurred predominantly during the night (between 2100 and 2300 h). Hyla versicolor activity significantly increased as relative humidity and temperature increased. Hyla versicolor individuals were significantly more active in the upper canopy compared with other canopy locations and selected for the top side of branches. Our results highlight that H. versicolor frequent arboreal habitats of P. strobus canopies. Although further research is needed, our work lays a foundation of H. versicolor ecology within the canopy. This research also demonstrates the feasibility of using arboreal camera traps with time-lapse settings to study ectotherms in forest canopies.Abstract
Camera trap placement and heights with images of H. versicolor. (a) Camera trap image of two different H. versicolor at the same location at the same time. (b) Camera trap image of an individual H. versicolor on the bole of a tree from a “close-focus” trail camera (i.e., focal distance was set for close proximity). (c–d) Two different-sized H. versicolor using the same spot at different times. Estimated SVL for Treefrog in (c) was 27.8 mm (95% CI = 23–32.6 mm), classified as a juvenile. Estimated SVL for Treefrog in (d) was 45.7 mm (95% CI = 42.9–48.5 mm), classified as an adult.
(a) Diel activity pattern (hour of day, CST) of arboreal behavior for H. versicolor in old and tall P. strobus canopies, showing peak activity between 2100 and 2400 h. Note that the graph is centered on midnight (gray area indicates a 4-h density plot overlap window, i.e., gray portion on the right side of the graph represents the density plot from 0800 to 1200 h for the left side of the plot and 1200 to 1600 h for the right side of the plot). Dashed vertical lines correspond with sunset and sunrise for the extremes of our sample period (orange, 21 June; purple, 31 October). (b) Seasonal activity patterns (months of the year) of H. versicolor for all sites showing peak activity in September (2014–18, in northern Wisconsin, USA). Gray dashed lines correspond with dates of initiation and cessation of time-lapse photography.
A collection of hourly weather points from all nights (2100–0600 hours, CST) from June to September (open circles) (2014–18, in northern Wisconsin, USA) and from hours where H. versicolor were present (filled circles) in the canopy of a relatively old and large P. strobus at site 1. Data show a general temperature range of H. versicolor occurrence from ∼10 to 24°C, with the majority of H. versicolor events occurring during times with higher relative humidity. Density ellipses of 90% show considerable overlap between conditions with or without H. versicolor, but a logistic regression showed significant differences in both relative humidity and temperature during H. versicolor events (dark gray–shaded ellipse), relative to nonevents (light gray–shaded ellipse).
Heat density map of H. versicolor habitat use in the upper canopy of site 1, created using the image overlay application on http://www2.heatmapper.ca, with a Gaussian radius multiplier of 0.35, contour smoothness of 200, heatmap opacity of 0.2, and number of shades of 30.
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