Ethology Neighbor–Stranger Discrimination in Concave-Eared Torrent Frogs, Odorrana tormota Albert S. Feng*, Victoria S. Arch , Zulin Yuà, Xin-Jian Yu§, Zhi-Min Xuà & Jun-Xian Shenà * Department of Molecular and Integrative Physiology & Beckman Institute, University of Illinois, Urbana, IL, USA Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA, USA à State Key Laboratory of Brain and Cognitive Science, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China § Shanghai Institutes of Biological Sciences, Chinese Academy of Sciences, Shanghai, China Correspondence Abstract Albert S. Feng, 2355 Beckman Institute, University of Illinois, Urbana, IL 61801, USA. The dear enemy phenomenon in which animals discriminate familiar E-mail: [email protected] neighbors from unknown strangers, and respond more aggressively to strangers is well established in various social animals, especially in song- Received: March 6, 2009 birds and mammals. So far, very few studies of neighbor–stranger dis- Initial acceptance: March 30, 2009 crimination have been carried out in amphibians and the results have Final acceptance: April 29, 2009 been mixed. Thus, it is unclear whether this phenomenon exists com- (L. Ebensperger) monly in frogs and toads, and whether it is exhibited by species with doi: 10.1111/j.1439-0310.2009.01666.x large vocal repertoires. We conducted acoustic playback experiments with male concave-eared torrent frogs (Odorrana tormota, a species with an unusually large vocal repertoire) kept in a tank in a quiet room to investigate whether or not they can discriminate strangers from neigh- bors acoustically. Nine of the 14 males tested showed evoked vocal responses to the calls from strangers, but none to calls from neighbors; vocal response to the calls from strangers was accompanied by aggres- sive motor activity. These results demonstrate that male O. tormota pos- sess the ability to discriminate neighbors from strangers acoustically. Odorrana tormota therefore joins three ranid species (Rana catesbiana, Rana clamitans, Rana dalmatina, all known to have a comparatively small and stereotyped vocal repertoires) as the only anurans demonstrated to have this ability. Given the difference in signaling complexity between these frog species, the salient acoustic features used for discriminating neighbors and strangers are likely to be quite distinct. wise be expended in unnecessary territorial defense Introduction (Falls 1982; Temeles 1994). A variety of territorial animals (e.g., birds, mammals, Sound communication plays an important role in lizards, fish, amphibians, and insects), especially the reproduction of anuran amphibians (Kelley those maintaining multi-purpose breeding territories, 2004; Gerhardt & Bee 2007; Wells & Schwartz exhibit the ‘dear enemy phenomenon’ (see Temeles 2007). In most species, the male produces long- 1994; Briefer et al. 2008), in which they discriminate range advertisement calls that serve to attract mates familiar neighbors from unknown strangers using a and keep competitors at bay. Typically, a single male variety of sensory cues and respond more aggres- will begin to call at dusk, exciting nearby males to sively to strangers. By recognizing neighbors and call leading to the formation of a chorus. All males respecting established territorial boundaries, individ- call to out-signal their rivals to attract females, thus uals can conserve time and energy that would other- inter-male vocal interactions are intense and often Ethology 115 (2009) 851–856 ª 2009 Blackwell Verlag GmbH 851 Neighbor–Stranger Discrimination in Odorrana A. S. Feng et al. quite complex. Each male is behaviorally responsive quencies, the call duration, and the durations of the to changes in local chorus density and to the subharmonic and chaotic segments of the frogs’ long approach of conspecifics (Wells & Schwartz 2007). calls are individual specific. Moreover, individual In amphibians, three ranid species with small vocal males produce calls from their own calling sites (A.S. repertoires and stereotypic advertisement calls, Ameri- Feng, T. Riede, V.S. Arch, Z.L. Yu, Z.M. Xu, X.J. Yu, can bullfrogs (Rana catesbeiana), green frogs (R. clamitans), J.X. Shen, unpublished data) where male–female and agile frogs (R. dalmatina), have been shown to dis- mating pairs (i.e., engaged in amplexus) can be play the dear enemy phenomenon (Davis 1987; Owen found during the reproductive season (Shen et al. & Perrill 1998; Bee & Gerhardt 2001a,b,c; Bee 2004; 2008), suggesting that these are multi-purpose Lesbarre`res & Lode´ 2002); they can use acoustic and breeding territories. Based on the possession of indi- location cues to discriminate neighbors which are vidually distinct calls and apparent defense of breed- accepted as ‘dear enemies’ from strangers that are ing territories, we hypothesized that O. tormota males attacked. In contrast, the Strawberry dart-poison frog display neighbor–stranger discrimination, despite (Dendrobates pumilio), which also has a small call rep- having a large vocal repertoire of highly complex ertoire comprised of stereotypic signals (Pro¨ hl 2003), calls (Feng et al. 2002; Narins et al. 2004). cannot discriminate strangers from neighbors acousti- cally (Bee 2003). Thus, at this time it is unclear Methods whether the ‘dear enemy phenomenon’ is exhibited by anurans other than the three ranid species above, To assess whether male frogs possess the ability to especially those with larger vocal repertoires. Song- discriminate neighbors from strangers on the basis of bird species with medium or large repertoires have vocal signals, we performed acoustic playback exper- been suggested to have a reduced ability to recognize iments investigating the evoked vocal responses individuals (Kroodsma 1976; Falls 1982); whether or (EVRs) of male frogs kept in a plastic tank. The not this negative correlation between song repertoire experiments were carried out between May 22 and and acoustic discrimination (i.e., the repertoire con- 30 2006 (which corresponds to the mid-period of straint hypothesis, sensu Kroodsma 1976) applies to the breeding season of O. tormota), in Huangshan anurans remains to be tested. Hot Springs, China (30°06¢N, 118°10¢E). Experiments We carried out acoustic playback experiments were performed between 20:00 and 24:00 h, within with male concave-eared torrent frogs (Odorrana 48 h of capture. Prior to EVR tests, frogs were tormota; previously Amolops tormotus) in the labora- housed individually in 1-gallon transparent plastic tory to determine whether they can discriminate a bags in a room exposed to ambient light. The experi- stranger’s call from a neighbor’s call. Odorrana mental protocol was approved by the Institutional tormota represents an interesting test case because it Animal Care and Use Committees in the Chinese has the largest vocal repertoire among anurans; their Academy of Science and the University of Illinois at vocal signals are broadband containing spectral Urbana-Champaign. energy that extends into the ultrasonic range (Feng For EVR experiments, 14 male O. tormota from five et al. 2002; Narins et al. 2004). One of the eight acoustically isolated areas (2–4 frogs per area) were call-types of O. tormota (the long calls) shows pro- captured and placed in a transparent plastic tank nounced frequency modulation and contains various (width · depth · height = 22 · 12 · 15 cm) which non-linear phenomena [NLP: subharmonics, deter- was setup indoors, approx. 1 km away from the ministic chaos (characterized by noisy signals that frogs’ natural habitat. The room was quiet and dark- are not random noise, but are generated by a chaotic ened. The top cover of the tank had slated ventilat- process within the vocal production system; Fitch ing openings that facilitated sound transmission in et al. 2002), frequency jumps and biphonation] – and out of the tank. A Polaroid loudspeaker (diame- these acoustic features vary greatly from one call to ter of 3.8 cm and a pass band of 22–120 kHz; roll- another within the same individual (Fig. 1; A.S. off: 10 dB ⁄ octave below 22 kHz) was attached to a Feng, T. Riede, V. S. Arch, Z. L. Yu, Z. M. Xu, X. J. tripod and placed 12 cm above the tank. A com- Yu, J. X. Shen, unpublished data). In spite of the puter-based playback system (‘Play-unit’; Feng et al. extensive intra-individual variability, a quantitative 2006) broadcast the calls through the loudspeaker. A analysis of long calls of marked frogs showed that custom-made ultrasonic microphone with a fre- males have consistent spectral and ⁄ or temporal call quency response of 15–120 kHz (roll-off: 6 dB ⁄ features that are individual specific; in particular, the octave below and above this range) was mounted on average, maximum and minimum fundamental fre- a separate tripod and placed 10 cm above the 852 Ethology 115 (2009) 851–856 ª 2009 Blackwell Verlag GmbH A. S. Feng et al. Neighbor–Stranger Discrimination in Odorrana Fig. 1: Twelve representative long calls of a male Odorrana tormota showing the variabil- ity in the frequency modulation pattern and the time and frequency of occurrence of non- linear phenomena (chaos, frequency jumps and subharmonics). Shown are the sound spectrograms of these calls, which were derived using Selena, a custom-designed sound analysis software (Feng et al. 2006). loudspeaker. This microphone was used to record at a distance of 8 m depending on tone frequency, the frog’s vocal response as well as the acoustic play- and by an additional 20–24 dB at 32 m for tones up back stimulus. Recordings were made digitally with to 20 kHz (or 40–45 dB for tones >25 kHz). There- PCTape, a computer-based data acquisition system fore, calls emitted at the border of an acoustically (Feng et al. 2006). isolated site would have a sound pressure level of Calling sites of male O. tormota extend over a vast >60 dB SPL at the opposite border – this is above area of Huangshan Hotsprings, along the banks of the predominantly low-frequency ambient noise the Tao-Hua Creek.