Many animals use complicated behavioural defences, such as alarm calls, to warn their fellows of a predators' presence. However, some mammalian and avian species have learned to respond to the alarm calls of other species. The ability to recognize and respond to other species' alarm calls has only been described in species with vocal communication. In a recent issue of Biology Letters, Maren N. Vitousek and co-workers from Princeton University, USA, and University of Bath, UK, examined whether the silent Galapagos marine iguana(Amblyrhynchus cristatus) could recognize and respond to the Galapagos mockingbirds' alarm calls, and found that they do.
Vitousek and co-workers tested the iguanas' responses to mockingbird alarm calls by recording Galapagos mockingbirds' song and alarm calls in response to the Galapagos hawk (Buteo galapagoensis), which predates both species. Then they played the mockingbirds' song and alarm call sequences to clusters of juvenile and female-sized marine iguanas at three distinct sites on the island and recorded the reptiles' responses.
The team found that significantly more iguanas (45%) exhibited alert behaviour (where they raised their heads in response to the recorded alarm calls) in response to the mockingbirds' alarm calls than when the birds simply sang (28%). The team realized that marine iguanas are able to differentiate between, and respond accordingly to, the mockingbirds' alarm calls and songs. This is the first demonstration that a species that lacks vocal communication can associate the auditory alarm signals of another species with the threat of predation.
There was also a significant difference in the percentage of iguanas that exhibited anti-predator behaviour during playbacks at the various sites. This could be due to differences in the volume of ambient noise at each of the sites, caused by different wind speed or the distance from the ocean. However,the authors suggest an alternative explanation for the different responses at each of the three sites. Vitousek explains that the hawks on Santa Fe Island have a highly predictable flight trajectory along the island's east coast. They appear first at the northern end of each site and then proceed south along the island's coastline until they capture prey. The study sites were positioned on this axis – and the team found that the iguanas were most responsive to both types of mockingbird recordings at northern sites where the predation rates are highest.
The team also explain that the energetic cost of escape behaviour can be substantial, so gleaning additional information about a predator's presence in response to the mockingbirds' call may enable the iguanas to respond more effectively. Energetics could also explain the iguanas' relative infrequency of running-and-walking escape responses. Instead, they make the most of the mockingbirds' signals by looking up to confirm the predator's location and the risk posed, consequently saving energy by not responding if the risk appears low.
This is the first time that a non-vocalizing species has been shown to recognize another species' calls in order to acquire information about a common predator's movements. However, how the marine iguanas recognize the mockingbirds' alarm calls isn't clear. The team suggest that it may involve associative learning of the complex auditory signals, and further research is required to establish whether this ability is learned, or whether naive iguanas are capable of recognizing and responding to the mockingbirds' alarm calls. The team are also keen to know whether all marine iguanas are capable of eavesdropping, or whether it is a specific trait of the residents of Santa Fe Island.