Have you ever wondered if stingrays can hear noise? Sharks, skates, and rays have multiple sensory organs to help then navigate their environment and hunt for food; for example, jelly-filled pores on their body allow them to sense electric fields in the water. However, less is known about how well these animals can hear. The authors of this paper demonstrate the first field responses to sound in the marine environment for Hypanus americanus, more commonly known as the southern stingray.
Sound System Setup
Stingrays are generally sedentary animals, so during this experiment any behavioral changes other than resting are considered to be in response to the sound stressor. At the Bimini Biological Field Station in the Bahamas, a circular net pen was constructed and twenty wild caught stingrays (9 male and 11 female) were exposed to five low-frequency tones between 50 – 1000 Hz. During the acclimation period before the experiment, the stingrays exhibited resting behavior as they had settled to the bottom of the pen. The behaviors observed during this experiment are: time spent swimming, resting, side swimming (which refers to a stingray swimming vertically along the pen while flapping its pectoral fins), and surface breaches (where a stingray’s head comes out of the water). This means that any behavior changes aside from resting that occurred are in response to sound and can be considered as a stress response.
Two low-frequency underwater speakers were placed adjacently along the perimeter of the pen. The sequence of frequency exposure were randomized and played in a stepwise pattern with 1 minute of sound followed by 5 minutes of silence until all sounds were presented.
How Did the Stingrays Respond?
Both male and female stingrays exhibited changes in behavior when exposed to low-frequency tones. However, female stingrays responded to quieter sounds when compared to males. This could be attributed to the sexual dimorphism between them, where the females are larger than the males. In another stingray species, female thornback rays have more cells in their ears that are responsible for capturing sounds when compared to their male counterparts. If female southern stingrays also have more auditory cells, it might explain why they respond to quieter sounds! These sex differences in the rays could be attributed to assisting in the location of prey, mate detection, or other reproductive processes.
The effects of human caused sound in the marine environment (noise pollution) on organisms has been of concern worldwide and has already been demonstrated to have a negative influence on boney fishes. This experiment exhibits the first behavioral evidence to support that rays can hear within the range of 50 – 1000 Hz. Humans on the other hand can hear between 20 – 20,000 Hz. As people get older, they tend to lose the ability to hear the higher frequencies. The lowest note for a cello is like the low sound exposure that the stingrays have a greater sensitivity to (You can check out this link to experience some of these sound waves for yourself: https://en.wikipedia.org/wiki/Audio_frequency). Noise pollution can be a threat to stingrays by having them expend more energy swimming in response to sound instead of foraging for food or avoiding predators. The family of sharks, skates, and rays are threatened in all the world’s oceans, and they are highly impacted by habitat destruction and overfishing. However, there is very little knowledge available on the impact of sound stressors on sharks and rays.
Paper: Mickle, M. F., Pieniazek, R. H., & Higgs, D. M. (2020). Field assessment of behavioural responses of southern stingrays (Hypanus americanus) to acoustic stimuli. Royal Society Open Science, 7(1), 191544. https://doi.org/10.1098/rsos.191544