Distress calls are commonplace in the animal kingdom, from the deafening calls of howler monkeys to the “maydays” of a captain stuck at sea; vocalizing distress is often the quickest and most effective way to get others attention. While you may think animals are limited in the range of information they can communicate vocally, minor differences can often drastically change the meaning. If you think about it, hearing the word “help” yelled at a fairly monotone voice is not nearly as distressing as hearing a “help” yelled in an inconsistent and erratic voice. That is the essence of the “nonlinearity and fear hypothesis” that was proposed by Daniel T. Blumstein and Charlotte Récapet in 2009. Essentially, they predicted that distress calls that were nonlinear come from more distressed animals and would result in more fearful responses from others. Nonlinear means that the frequency of the sound waves in these calls change quickly and in unpredictable ways.
Researchers Mario Ruiz-Monachesi and Antonieta Labra decided to study if this hypothesis was accurate in weeping lizards (Liolaemus chiliensis). Weeping lizards emit calls that are both linear and nonlinear, making them a perfect option for testing this theory. Do these calls actually send different information? And do the lizards respond differently to them?
Before they got started, Ruiz-Monachesi and Labra had to account for one other thing: weeping lizards are known to behave differently in relation to different scents in their environment. To see if this did have an effect, they alternated between testing lizards in their own container, and using a container previously occupied by a different lizard. The first had the lizard’s own scent in it, and previous research suggests this would be seen as more of a “home” habitat that could represent a safe space. The latter had another lizard’s scent, signaling an unfamiliar habitat. Theoretically, there could be different behavior responses to a distress call depending on the familiarity with the environment.
Then came one of the funniest part of the experiment. The scientists needed to record the lizard’s distress calls in order to replay them back for the experiment. To do this, they reportedly “gently grasped a lizard with the hand and touched its snout slightly with a finger for 2 min[utes].” Adorable and effective! The beginning of those calls often had “deterministic chaos” followed by more linear sequences. Basically, at first there were a lot of random changes of frequencies (nonlinear) followed by stable linear frequencies. The researchers used the full recordings for the complex calls, and replaced the deterministic chaos with white noise to create the simple calls.
To put it all together, lizards were placed in a container (either the one they lived in or one previously occupied by another lizard) and the actual experiment began only once the lizard had done its first “tongue flicks” in the new container. After that, filming started, and conditions were maintained for four minutes. After the four minutes had passed either the complex or simple sound was played for three seconds, then the lizard was given another four minutes of peace afterwards. Later, the footage was analyzed for time in motion, time escaping, number of tongue flicks (which shows chemical exploration), time spent in the front of the enclosure (where the sound was played), and the latency after stimulus (how long the lizards stayed still after the sound was played). They compared the number of events before, during, and after the calls to compare them. Any difference they found compared to the baseline indicated a response to the calls, and the more drastic the difference was, the more fearful the response was.
Both types of calls resulted in the lizard moving less, but the complex call caused a greater decrease in motion than the simple calls. Simple calls resulted in about the same amount of latency after stimulus regardless of which scent they were assigned. However, when complex calls were used, lizards with their own scent stayed still for much longer, and lizards not with their own scent started moving pretty soon afterwards. Lizards with another lizard’s scent also did more tongue flicking when complex calls were played than when simple ones were played. These results support the “nonlinearity and fear hypothesis,” as the lizards could tell the difference between the simple and complex calls, and did indeed react more fearfully. We can see this because of how differently they reacted compared to their baseline.
This is exciting information, because it shows that not only can animals communicate different levels of stress, but that other animals respond differently to these levels. So while it all may sound like noise to you, animals could in fact be communicating pretty sophisticated messages.
Works Cited
Blumstein, D. T., & Récapet, C. (2009). The Sound of Arousal: The Addition of Novel Non‐linearities Increases Responsiveness in Marmot Alarm Calls. Ethology, 115, 1074-1081. doi:https://doi.org/10.1111/j.1439-0310.2009.01691.x
Ruiz-Monachesi, M. R., & Labra, A. (2020). Complex distress calls sound frightening: The case of the weeping lizard. Animal Behavior, 165, 71-77. doi:https://doi.org/10.1016/j.anbehav.2020.05.004
