Population differences in responses of red-legged frogs to introduced bullfrogs - Rana aurora

Ecology, Sept, 1997 by Joseph M. Kiesecker, Andrew R. Blaustein

All animals were maintained on a 14:10 light: dark photoperiod at 15-20 [degrees] C. Rana aurora eggs from each population were kept separate and were placed in aerated 38-L aquaria filled with dechlorinated water. After hatching, tadpoles were fed ground rabbit chow ad libitum. Water was changed every 5-7 d. Rana catesbeiana tadpoles were reared in 38-L aquaria and were maintained on a diet of ground rabbit chow. Adult R. catesbeiana were reared in 38-L aquaria and were fed earthworms ad libitum.

Experiment 1: Detection of larval Rana catesbeiana chemical cues by larval Rana aurora

The ability of syntopic and allotopic R. aurora tadpoles to detect R. catesbeiana was evaluated by exposing them to chemical cues of R. catesbeiana. Testing occurred from 11 January 1995 to 26 March 1995. We tested tadpoles in a gravitational flow-through system modified from Petranka et al. (1987) that was composed of three 25-L plastic tubs 51 x 37 x 21 cm deep. The tubs were placed at different heights so that water flowed from one to another at 0.6 L/min. The two lowermost tubs had both input and output openings and never contained [greater than]12 L of water. The upper container was filled with 23 L of water. We thoroughly rinsed the tank with tap water before each test and used clean, dechlorinated tap water for each test. We added stimulus animals (R. catesbeiana tadpoles for experimental treatments, nothing for controls) to the middle tub. From the middle tub, water flowed to the lowermost tub, which contained five R. aurora tadpoles and a line that divided the tub into widthwise halves. A refuge made of opaque plexiglass, measuring 25 x 37 cm, was present on the output side of the lower tub. The water in the lower tank was at a depth of 10 cm. The refuge was placed at a depth of 5 cm. Thus, tadpoles had a choice of swimming above or below the refuge.

A test began 10 min after flow was initiated. Each test included two 5-min trials (an initial and final response), separated by a 5-min pause. An observer concealed behind an opaque blind measured tadpole activity and distribution. As a measure of activity level, we counted the number of times a test animal crossed the center line during each trial. To assess avoidance of stimulus animals, we counted the number of test individuals under refugia at 30-s intervals. These 30-s counts were then averaged for each test. The treatments were presented in random order. Twenty tests, 10 controls and 10 experimentals, were conducted with animals from the four allotopic and four syntopic populations, (for a total of 160 tests in this experiment). Test individuals were never used more than once.

Stimulus animals (R. catesbeiana) used in tests were arbitrarily drawn from stock tanks and then were returned after testing. During testing, all stimulus animals were size matched between tests (6.3 [ or -] 0.033 g mean mass [ or -] 1 SE; range 4.27-8.47 g). In all tests, three stimulus animals were used. Test animals (R. aurora tadpoles) used in this experiment were all of the same developmental stage (Gosner stage 25; Gosner 1960) and approximate size (0.07 [ or -] 0.0044 g, mean mass [ or -] 1 SE; range 0.043-0.095 g).