Eavesdropping of woodchucks (Marmota monax) and eastern chipmunks (Tamias striatus) on heterospecific alarm calls

1. What they did

Many animals give alarm calls when they spot a predator. Animals of other species nearby can listen in (“eavesdrop”) and take cover without calling themselves, which could give away their position. Most studies of eavesdropping have looked at social species. The authors asked whether two solitary rodents that live side by side, the woodchuck and the eastern chipmunk, react to each other’s alarm calls and to the alarm calls of American crows.

• Catching and marking. They live-trapped woodchucks and chipmunks at Gilsland Farm, a wildlife preserve in Maine, gave each animal a unique black hair-dye mark and numbered ear tags, and released them where they were caught.
• Recording calls. They recorded alarm calls from several woodchucks (8), chipmunks (9) and crows (4) calling at a natural threat or an approaching person, and territorial songs from 4 male black-capped chickadees as a non-alarm control. Using calls from several individuals avoided testing reactions to just one animal’s voice.
• Playbacks. From May to August 2008 they played a single call from a speaker 15 to 20 m from a foraging animal and video-recorded the animal before and after. Each of 13 woodchucks (2 yearlings and 11 adults) and 5 chipmunks heard all four call types in random order, no more than one call per day. Four more chipmunks heard only some of the calls.
• Measuring responses. From the videos they recorded whether the animal fled, looked up or did nothing, how quickly it reacted, how long that first reaction lasted, and how much time it spent vigilant (upright and scanning, moving towards a refuge, or hiding in cover) before and after the call.

2. What they found

Woodchucks

• Woodchucks fled only after hearing woodchuck alarm calls (7 of 13). They looked up after most other calls.
• They reacted faster to woodchuck and chipmunk alarm calls than to the control song.
• They stayed alert longer after woodchuck alarm calls than after any other call.
• Their vigilance rose after woodchuck and chipmunk alarm calls, but not after the control song. After correction for multiple tests, the rise after crow alarm calls was not significant.
• The increase in vigilance after chipmunk and crow calls was much smaller than after woodchuck calls, and not significantly different from the control.

Chipmunks

• Chipmunks usually looked up after a playback, whatever the call. With only 5 animals hearing all four calls, none of the tests showed significant differences.
• Effect sizes for the larger group of 9 chipmunks pointed to faster and longer reactions to woodchuck and chipmunk alarm calls than to crow calls and the control, and the greatest overall change in vigilance after woodchuck alarm calls.
• Many chipmunks disappeared before they could hear the full series of calls, which kept the sample small.

3. What they concluded

• Woodchucks can apparently recognize other species’ alarm calls, but they react to them less strongly than to their own species’ calls, suggesting they process the two differently.
• Woodchucks may recognize chipmunk alarm calls without treating them as immediate danger. All woodchucks stopped feeding and looked up at chipmunk calls; the authors suggest they may then have seen only a person and gone back to feeding. Chipmunks seemed to call more often than woodchucks (the first author’s observation), so woodchucks may also have become used to their calls.
• The chipmunk results were less clear, but effect sizes suggest chipmunks also respond to other species’ alarm calls.
• Differences in call length might explain some responses; the authors suggest a control call closer in length to chipmunk and crow calls for future studies.
• The study adds to evidence that nonsocial species, not just social ones, eavesdrop on other species’ alarm calls. Animals may need to learn what other species’ calls mean.

6. Other technologies, skills or innovations

• Live traps baited with apples and peanut butter, hair-dye marks for recognizing individuals at a distance, and numbered metal ear tags.
• Field recordings with a directional microphone and digital audio recorder, edited on a laptop and played back from an iPod through a portable speaker.
• Frame-by-frame video analysis (25 frames per second) and the JWatcher program for timing behaviour.