While human language has an extensive vocabulary, we often rely on more than just the words to ascertain the meaning behind what someone is saying to us. Characteristics such as volume and inflection can tell us much more about the intentions and emotional state of the speaker. For example, a simple rise in pitch at the end of a phrase can turn a statement into a question. However, this sub-communication using inflection is not unique to humans. Recently, researchers Mario Ruiz-Monachesi and Antonieta Labra have been studying the way in which a species of lizards called “weeping lizards” (Liolaemus chiliensis) communicate predator information using different types of distress calls.
Distress calls are sounds produced by many species when experiencing discomfort, especially predation, isolation, and hunger. They not only communicate a need for help, but also inform others of nearby predators, allowing them to respond. Many animals ‘encode’ information in calls, modulating the sound based on the nature of the threat. When conspecific individuals, others of the same species, hear a particular call, they are often able to interpret the level of urgency it communicates and respond accordingly. Studies on alarm calls in a number of species have led to the development of the “nonlinearity and fear hypothesis”, which dictates that nonlinear distress calls – complex calls that include greater vocal modulation and subharmonics – induce a greater level of fear and more urgent anti-predator response than do simpler, “linear” calls. This has been observed in social meerkats, who not only flee, but also reduce their general level of activity in foraging for food when they hear these complex alarm calls. This exceeds the short-term response they exhibit when they hear linear calls, indicating that they can interpret the level of threat based on the complexity of the call. In their recent study, Ruiz-Monachesi and Labra set out to explore whether this hypothesis held true in their weeping lizards.
Using lizards captured at Isla de Maipo in Chile and placed in habitats filled with sand, the researchers designed an experiment to test the level of antipredator response when they exposed their subjects to non-linear vs linear sound. Using sound editing software, they created an example of each type of call by aggregating recordings from each of the lizards and played it back to the animals in their exposures. In order to make things more interesting, they also decided to test the way in which scents modulate the response to alarm calls. Conspecific scents have been known to lessen the intensity of alarm calls in other species, and the researchers predicted that being immersed in their own scent would make the lizards relax to an even greater extent. In order to measure the lizards’ response, the researchers recorded the animals’ time in motion – all activity besides escape – their time escaping, chemical exploration time – time exploring the scents of their environment – and time spent in the front half of their enclosure, which was opposite the sound device and supposedly “less scary”.
The results seemed to support the nonlinearity and fear hypothesis; the lizards spent more time escaping and less time in the front half of the habitat when the nonlinear call was played. However, the scents modulated behavior in an unexpected way. Notably, for lizards exposed to their own scent, instead of tempering the level of stress, the scent exacerbated it, causing greater escape response and lag before the individuals returned to normal behavior. Ruiz-Monachesi and Labra speculated that this may be because familiar scents convey a feeling of safety and control characteristic of ‘home’; as a result, when lizards hear a severe distress call in this safe environment, it catches them especially off guard.
Though animals don’t have the same words we do to communicate with each other, evolution has led to the development of incredible signaling systems that allow animals to encode information in many characteristics of the sound. As more studies continue to observe the signaling habits of various species, researchers hope to continue to understand the many ‘languages’ of animals.
Ruiz-Monachesi, M.R., Labra, A. (2020). Complex distress calls sound frightening: the case of the weeping lizard. Animal Behaviour, 165, 71-77. https://doi.org/10.1016/j.anbehav.2020.05.004.
