Reference: Wheeler, B.C., Fahy, M., Tiddi, B. (2019). Experimental evidence for heterospecific alarm signal recognition via associative learning in wild capuchin monkeys. Animal Cognition, 22(5), 687 – 695.

Could a monkey understand if you gave a warning that a leopard was nearby? What if the monkey was exposed multiple times to your warning sound before the leopard appeared? Many species will run from potential danger, even if they cannot see it, if they hear an alarm call: a warning that a potential threat is nearby. Interestingly, some species like capuchin monkeys will react appropriately to alarm calls from other species. But when a capuchin monkey runs from a bird’s alarm call, does it actually understand what that call means? 

Some scientists believe that responses to alarm calls are reflexive and automatic. If you heard a person yelling in another language, you may start running simply because the yells are loud, not because you understand what the yell means. But maybe you could learn what a specific sound means through associative learning: hearing the warning sound in combination with the thing you are being warned about. If you heard a particular yell and saw a leopard at the same time, you would associate both together and learn that the yell means “Leopard! Run away!” This associative learning for alarm calls appears to happen in the animal world and has been shown in various birds and mammals.

The strongest evidence that associative learning can shape responses to alarm calls comes from work on responses to heterospecific alarm calls, or alarm calls coming from a different species. Many birds and mammals react to alarm calls from other species as if it was from their own, which suggests that animals connect heterospecific alarm calls with the presence of predators through associative learning. However, no one has explicitly tested if these responses are actually the result of learning.

Wheeler, Fahy, and Tiddi (2019) investigated whether black capuchin monkeys (Sapajus nigritus, see image above) could learn a novel alarm call. Capuchins learn new information well and respond with anti-predator behavior to various alarm calls from other species, thus making them an ideal study species to test for alarm call associative learning. The researchers hypothesized that if responses to heterospecific alarms were shaped by learning, then capuchins should react stronger to unheard (novel) sounds they heard associated with predators than sounds that were not. That is, can they learn that a rooster’s crow, a human laugh, or a bell means “Watch out! A predator is nearby”?

How to train a monkey
Wheeler and their team conducted their study in the Iguazú National Park in northeastern Argentina on three separate wild capuchin groups. Each group was trained on one novel sound that these monkeys presumably had not been exposed to before: a rooster’s crow, a human laugh, and a bell. Each novel sound was paired with a decoy ocelot (visual signal, see image below) or a puma call (auditory signal). The novel training sounds were projectected to the capuchins by a speaker that was physically close (10 meters) to the decoy ocelot or played close in time to the puma call.

To test whether the capuchins had learned the alarm call, all three sounds were played back to each of the groups in the absence of a predator stimulus. If the capuchins learned the alarm calls, they would respond to the sound that their group was trained on but not the other sounds; if no learning occurred, then they would not react to any of the sounds.

Can monkeys learn new signals?
Yes! Capuchins were more likely to run away after hearing sounds that had previously been associated with a predator than after hearing sounds that had never been played with a predator cue. This reaction happened in the absence of a predator cue during the test phase, which demonstrates the capuchins are responding strictly to the sound. This response to the sound persisted over time without reinforcement from the researchers: capuchins responded to the new alarm call (rooster’s crow, human laugh, or a bell) three years after the training period.

So can animals glean information from novel signals?
This study demonstrates that capuchin monkeys respond to sounds based in part by their prior experience with that sound. So if you could find the appropriate group of capuchins, you might be able to warn them that a puma or ocelot is nearby. You could laugh, ring a bell, or imitate a rooster’s crow and the monkeys would know to run away. This learning can go both ways: you could learn to associate a particular monkey call with the presence of a leopard in the bushes and know to start running!

Taken with responses of birds and mammals to heterospecific alarm calls, this study suggests that anti-predator behavior in response to heterospecific alarm calls may be based on associative learning between the sound and the presence of predators. This in turn suggests that alarm calls may convey meaning, as receivers are able to use the association between alarm calls and predator presence to make predictions about the environment. Instead of simply reacting to noises, this study shows that some animals have the ability to learn alarm calls and respond appropriately. 

Although alarm calls may not be as complex as human language, animals can learn from other species’ language to avoid predators. What other types of communication are occurring in the animal world? With enough exposure, could animals eavesdrop on our conversations and gain information from human language? 

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Images:
(1) Wikipedia Commons. Accessed 8.26.2019. Link.
(2) Wikipedia Commons. Accessed 8.26.2019. Link.
(3) Wikipedia Commons. Accessed 8.26.2019. Link.