Mockingbird

To Hear A Mockingbird: The Plight of the Iguana

jgoldman — Mon, 23 Aug 2010 08:00:00 +0000

To Hear A Mockingbird: The Plight of the Iguana

Predator-prey interactions are often viewed as evolutionary arms races; while predators improve their hunting behaviors and their ability to sneak up on their prey, the prey improve upon their abilities to detect and escape from their predators. The problem, of course, is that there is a trade-off between maintaining vigilance - the attention necessary to be consistently aware of others in the environment takes quite a bit of physical and mental energy - and doing all the other things that an animal must do, such as finding its own food. As a result of this trade-off, many social species, especially mammalian and avian species, have developed alarm calls. Alarm calls are specific vocalizations that signal the presence of a danger in the environment to nearby conspecifics, and sometimes contain additional information about the type of threat or predator.

As we've discussed before, subsequent to the introduction of predatory birds, howler monkeys on Barro Colorado Island near Panama rapidly developed an alarm call specific for those birds that indicated the presence of an avian predator; something like "danger from above!" That is, they did not merely adapt an already existing alarm call to the new predator, they developed an entirely new one.

In certain cases, prey species have developed the additional ability to eavesdrop on the alarm calls of other species, gaining access to an additional source of information relevant to the presence of danger in the environment. This ability could be the consequence of a learned association between the alarm calls of another species and the presence of the predator, or it could be due to certain auditory properties common to the alarm calls of both species, and innate. More research is required to tease apart these possibilities. However, until recently, it was thought that the ability to identify and react to the alarm calls of other species was only possible in species that already had vocal communication. Several years ago, however, researchers from Princeton University observed this behavior in an unlikely species - a non-vocal reptile - the Galapagos marine iguana (amblyrhynchus cristatus).

Figure 1: The Galapagos Marine Iguana.

Prior to this observation, it was thought that non-vocal species, who did not have alarm calls themselves, would not be able to associate complex auditory stimuli with the presence of a predator. The Galapagos iguana does not have any vocalizations; instead, they communicate by using visual and olfactory signals. However, they live among the Galapagos mockingbirds (buteo galapagoensis), a species that does have auditory vocalizations and specific alarm calls. Further, since iguanas primarily live on the rocky shoreline, they are often unable to view hawks (their main predator) until it is too late. If they had the ability to eavesdrop on the alarm calls of the mockingbirds, they would be able to engage their anti-predator behaviors significantly earlier and increase their chances of survival.

The researchers recorded two types of vocalizations from the Galapagos mockingbirds: their song calls and their alarm calls, and edited them into soundtracks each containing two or three examples of either type of call. The researchers would find a cluster of juvenile and female iguanas (the hawks mostly hunt for juvenile and female iguanas) in three different sites on the coast of Santa Fe Island, and would play back the various mockingbird calls through a portable speaker system.

Previous observational research indicated that upon detecting a hawk, iguanas who were aggregated in a cluster would scatter in different directions, perhaps in an attempt to confuse the predator. Upon playback, the researchers noted the behavior of the iguanas - and those behaviors were coded as "non-response," "alert," (head raise, orienting towards the sound), or "escape" (walking or running away).

Figure 2: The Galapagos Mockingbird.

Forty-five percent of iguanas showed vigilance behavior during the playback of the alarm call, compared with only 28.1% of iguanas during the playback of the mockingbird songs, which was a statistically significant difference. This suggests that the Galapagos iguanas are able to eavesdrop on the alarm calls of the mockingbirds and respond accordingly. However, in addition to type of playback, the time of day and data collection site were also significant factors in predicting the proportion of iguanas responding to the playbacks.

Figure 3: Proportion of iguanas responding, according to type of call and site.

At each site, the iguanas successfully differentiated between the alarm calls and the songs, responding with escape behaviors more often subsequent to the alarm call. This is the first experimental evidence that a non-vocal species associated the alarm call of a different species with the presence of a nearby predator.

What might account for the differential responses among recording sites? The authors speculate that differences in ambient noise could have contributed, resulting in changes in volume or sound quality. As each recording site was on the rocky coast of the island, the sound quality would be subject to wind and the sounds of the ocean. An alternative explanation, of course, is that there are true differences between the sites, perhaps owing to slight variations in predator behavior or mockingbird vocalizations. The fact that the hawks on Santa Fe island begin their hunts each day on the northern side of the island and proceed south throughout the day might indeed result in higher predation rates at site three. If the hawks are successful at site three on a given day, they might not have any reason to continue on to sites one or two.

Figure 4: Recording sites on the island, with the general hunting flight trajectory of the hawks indicated with an arrow.

It was also noted that the iguanas were more responsive to the playbacks earlier in the day. One possibility is that these cold-blooded reptiles are more responsive earlier in the day, when their body temperature is lower, and they therefore require more time to escape due to their reduced agility. As the day continues, the sun passes overhead, and their body heat increases, their ability to escape more quickly might improve. I'm not quite satisfied with this explanation, as I would expect the opposite, actually - for evasive responses to be higher when the iguanas have higher body temperature.

It is particularly impressive that these iguanas appear able to capitalize on the alarm calls of another species, the Galapagos mockingbird. And by considering the environmental constraints placed on these iguanas by their specific location on the coast of Santa Fe Island, the elegance of evolution and natural selection becomes apparent.

On Santa Fe Island, the hawks routinely approach on their hunting paths from the north. It might make sense, therefore, that the iguanas generally orient towards the north so that they would be able to see the hawks coming. However, marine iguanas such as these must orient their bodies directly towards the sun, or at 180 degree angles to it, in order to survive. Without this behavioral thermoregulation, they would quickly overheat and die. The realities of their environmental requirements are directly at odds: face north to anticipate predation, and risk overheating, or face the sun to avoid overheating, and risk being eaten by an unseen hawk. The ability to capitalize on the auditory alarm vocalizations of another species - especially a species less constrained by thermoregulatory requirements - could therefore provide significant benefit. The evolution of this ability would allow the iguanas to simultaneously maintain their body temperature while maintaining awareness of potential threats of predation.

In order to confirm this evolutionary explanation, more research would be required to identify whether naive Galapagos iguanas, never exposed to the threat of predation or the calls of the mockingbirds, would recognize and respond to the alarm calls. The other possibility is that this is simply the result of associative learning of complex auditory information. In either case, the eavesdropping ability of this non-vocal reptile species is remarkable. For these iguanas, it truly is a sin to kill a mockingbird.

Vitousek MN, Adelman JS, Gregory NC, & Clair JJ (2007). Heterospecific alarm call recognition in a non-vocal reptile. Biology letters, 3 (6), 632-634. PMID: 17911047

jgoldman Mon, 08/23/2010 - 04:00

City mockingbirds can tell the difference between individual people

edyong — Tue, 19 May 2009 06:30:57 +0000

City mockingbirds can tell the difference between individual people

While the rapid expansion of human cities has been detrimental for most animals, some have found ways of exploiting these brave new worlds and learned to live with their prolific inhabitants. The Northern mockingbird is one such species. It's very common in cities all over America's east coast, where it frequently spends time around humans. But Douglas Levey from the University of Florida has found that its interactions with us are more complex than anyone would have guessed.

The mockingbird has the remarkable ability to tell the difference between individual humans, regardless of the clothes they wear. After less than a minute, they can tell one person from another and adjust their responses according to the threat they pose to its nest. This ability suggests that these birds are both intelligent and very flexible in their behaviour - two traits that must surely stand them in good stead in the urban jungle.

It obviously benefits an animal to be able to distinguish between threatening and harmless species, but discriminating between individuals of the same species is a much more difficult task - just think about how difficult you would find it to tell the difference between two mockingbirds by eye.

Levey worked with 24 pairs of mockingbirds that had taken up residence on the university's campus. Hundreds of people walk within five metres of their nests every day and elicit absolutely no reaction. To simulate a greater threat, Levey asked one of his colleagues to approach the nests of birds with fresh clutches, and touch their rim for 15 seconds. When faced with such intrusion, mockingbirds will typically react by rallying from the nest, making alarm calls and diving aggressively at the trespasser.

Initially, the same intruder approached the nest for four days in a row, each time wearing different clothes. Despite their changing garments, the birds seemed to recognise them and reacted more aggressively as the days went by. They flew from the nest when the human was further away, they make more alarm calls and they were more likely to attack.

That could simply reflect a generally rising state of panic, but the events of the fifth day put paid to that idea. At that point, the intruders swapped and a different person approached the nest in exactly the same way. Faced with this new intruder, the birds reduced the degree of their alarm to the level of the first day. To Levey, this simple experiment clearly shows that the mockingbirds quickly learned to recognise people who approached their nest.

Many animals can tell the difference between individuals of their own species. But, anecdotes aside, there are relatively few reports of them pulling off the same trick with members of other species. Chimps can apparently do it from photos, prairie dogs from sound and cattle from smell. And animals from pigeons to honeybees have shown some skill in distinguishing between photos of humans in experimental conditions. But the mockingbirds in Levey's study passed a very realistic and difficult test - they had to tell one individual apart from thousands of others, all of whom changed their appearance on a daily basis.

They also did it with incredible speed. Within just two approaches by the human intruder, each one lasting just 30 seconds, the birds started behaving more aggressively. This means that a mockingbird can learn to recognise a new individual within less than a minute. In virtually all studies where other animals have learned to identify or classify individuals of other species, they have required 10-1000 times more training.

The fact that a songbird like the mockingbird can do this strongly suggests that other birds can do the same. In particular, the intelligentsia of the bird world - crows, jays and parrots - should be all too capable of distinguishing between individual humans.

If mockingbirds can tell the difference between individual humans, it's likely that they can do the same for individual cats, raccoons and other egg thieves. Indeed, it will be interesting to see if rural mockingbirds have the same ability. But Levey doesn't think that the urban populations have evolved a specific ability to tell humans apart. Rather, he suggests that mockingbirds are generally perceptive and learn quickly, traits that allowed them to colonise urban environments. Moving from rural areas to the big, bad city poses many unfamiliar challenges, including many new predators intending to kill a mockingbird.

Reference: Levey, D., Londono, G., Ungvari-Martin, J., Hiersoux, M., Jankowski, J., Poulsen, J., Stracey, C., & Robinson, S. (2009). Urban mockingbirds quickly learn to identify individual humans Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0811422106; images by Calibas and Mdf

More on bird brains:

edyong Tue, 05/19/2009 - 02:30