A soaring Galápagos hawk will prompt shrieks of alarm from mockingbirds, but other creatures pay attention, too. Marine iguanas eavesdrop on those warnings to prepare themselves to flee the hawks, new research shows. It's the first time that a mute animal has been observed responding to alarm calls from another species, and the discovery hints at hidden complexity in the world of animal signaling.
Marine iguanas (Amblyrhynchus cristatus) graze on algae attached to submerged rocks, alternating dives of as long as 30 minutes with shore-time to warm their bodies. While basking, the lizards are vulnerable to hawks. The sight of one flying overhead sends the iguanas scurrying for cover. However, Maren Vitousek, a graduate student in ecology at Princeton University noticed that the low-slung reptiles also raise their heads and pay attention even before hawks are in sight. Mockingbirds were often around and calling when the iguanas perked up, so Vitousek and her colleagues decided to see if the lizards were taking early warning cues from the high-perched birds.
The researchers recorded both the song and the alarm call of a native mockingbird and played them to hawk-accustomed iguanas on the archipelago's Santa Fe Island. Vitousek and colleagues found that the alarm call prompted increased iguana alertness--measured by the number of lizards that raised their heads to look around--60% more of the time than did a normal song, they report online 3 October in Biology Letters. The difference was most pronounced at the study site closest to the local hawk nest, where predation was most intense, suggesting that lizards under the greatest threat have learned to pay more attention to alarm calls. Little is known about the iguanas' hearing, partially because they don't use sound to communicate with each other, Vitousek says; however, "it appears to be more sophisticated than we thought."
Other species are known to eavesdrop, but the find in iguanas is "particularly impressive because they do not vocalize," says Chris Templeton, an ecologist at the University of Washington, Seattle. Making sense of an auditory alarm may be a bigger step for a nonvocal animal like an iguana than for a monkey or squirrel that routinely makes its own warning noises, Templeton says. Daniel Blumstein, a behavioral ecologist at the University of California, Los Angeles, adds that such a leap illustrates how animals under threat from predators "are clawing for any information that can reduce that risk."