- University of Toronto
We investigated whether the use of primary or secondary behavioural defences is related to prey sensory thresholds using two species of North American katydids, Neoconocephalus ensiger and Amblycorypha oblongifolia. Male katydids produce intense calling songs to attract mates, and many gleaning bat species are known to use these calls to locate them as prey. Low duty cycle calling (i.e. sporadic calls) is a primary defence against gleaning bats (prevents attacks), and song cessation is a secondary defence (enables survival of an attack), for which these two species show behavioural differences. Echolocation calls of Myotis septentrionalis, a sympatric gleaning bat species, were broadcast to singing katydids and to neural preparations of these katydids to test if differences in behavioural response were related to differences in auditory sensitivity. We measured thresholds and firing patterns of the T-cell, an auditory interneuron involved in predator detection. We hypothesized that low duty cycle calling is the best defence for species not sensitive enough to mount a secondary defence in response to predator cues; therefore, we predicted that N. ensiger (high duty cycle song) would have lower behavioural and T-cell thresholds than A. oblongifolia (low duty cycle song). Although more N. ensiger ceased singing than A. oblongifolia, the number and maximum firing rate of T-cell action potentials did not differ between species for echolocation call sequences. We suggest that the T-cell has divergent functions within the Tettigoniidae, including predator and mate detection, and the function could be context dependent in some species.
Study area and animals
We conducted these experiments during July and August of 2004 to 2007 at the Queen's University Biological Station, near Kingston, Ontario, Canada. The protocols used in this study conformed to the guidelines of the Canadian Council on Animal Care and were approved by the animal care committees of the University of Toronto Mississauga and Queen's University. Permission to capture bats was obtained from the Ontario Ministry of Natural Resources (Wildlife Scientific Collector's Authorization). Bats (Myotis septentrionalis Trouessart) were captured in modified harp traps (Tuttle, 1974) positioned at the entrances of local abandoned mines, and were housed indoors in screened cages (60 cm×40 cm×40 cm; H×W×D). They were provided with water ad libitum and fed mealworms (Tenebrio molitor Linnaeus) after trials. Katydids (Neoconocephalus ensiger Harris and Amblycorypha oblongifolia De Geer) were collected in local fields by following the sound of their song and picking them off grass stems or shrubs. They were housed in plastic and metal mesh cages with water, cat food, grass and pieces of apple. For experiments requiring bats in flight, we used a large outdoor flight room (9.14 m×3.66 m×3.66 m; L×W×H) consisting of a wooden frame with fibreglass mesh panels for walls and ceiling, and an earthen floor.
Acoustic stimuli used in behavioural and neurological experiments
We recorded three stimuli for playback experiments with katydids: two types of echolocation call sequences of M. septentrionalis (gleaning and searching calls, both predator cues) and the calling song of a cricket (non-predatory sound). Recordings of M. septentrionalis echolocation call sequences were made in August 2004. Our initial interest in M. septentrionalis stemmed from observations of several bats gleaning singing N. ensiger in the flight room. We subsequently discovered that naïve M. septentrionalis released into the flight room would land on a speaker broadcasting this insect's calling song. To obtain a suitable gleaning attack sequence, we released four M. septentrionalis individually into the flight room and broadcast the calling song of N. ensiger at 93 dB peak equivalent sound pressure level (peSPL) (Stapells et al., 1982), with reference to a 13 kHz continuous tone, from a laptop via a data acquisition card (DAQCard 6062E; National Instruments, Austin, TX, USA), ultrasonic amplifier (70101; Avisoft Bioacoustics, Berlin, Germany) and speaker (ScanSpeak 60102; Avisoft Bioacoustics). The speaker was positioned on a shelf 1.1 m above ground facing toward the centre of the flight room. An array of 60 infrared-sensitive diodes spaced 3 cm apart on a pole was placed opposite and two metres away from two near infrared light sources (51 W, λ=890 nm) on tripods. The array was positioned such that the axis of light to receivers was perpendicular to the axis of sound originating at the speaker, and the point between the receivers and sources was 2 m away from the speaker. The diodes registered the amount of light received as voltage, and thus when a bat flew between the light sources and the diodes it caused a decrease in the voltage produced by the diode(s) in the shadow of the bat. This signal was transformed by a custom built converter into a 1 s 12VDC pulse, which was recorded onto one channel of a RACAL Store 4D tape recorder at 76 cm s–1. A second channel recorded the echolocation calls of the bat from a 6.35 mm microphone (2200C; Larson Davis, New York, USA) positioned directly above the speaker. This provided a recording of the attacking bat's echolocation calls with a marker at the time it crossed the 2 m light line in front of the speaker. A near-infrared-sensitive CCD camera (VCB-3524; Sanyo, San Diego, CA, USA) placed at the back of the room facing the speaker and recording onto a VHS tape provided information about the horizontal and vertical position of the bat relative to the speaker when it broke the light beam. From these three relative measurements, we could calculate the exact distance of the bat to the speaker when it triggered the light array. We recorded one attack sequence per bat.