Imagine someone asked you to make the noise a tiny chihuahua would make. You would
probably make a high pitched “yip.” But if someone asked you to make the sound of a large St.
Bernard, you would probably make a low pitched “woof.” This is an example of a “crossmodal
association.” This means that you are using a clue from one of your senses (visualizing the size
of the dog), to make a guess about another piece of the puzzle (the pitch of the sound the dog
makes). The assumption here is that smaller objects like the chihuahua will make higher pitched noises than larger objects like the St. Bernard.
The ability to make crossmodal associations is a useful skill for animals to have because
it usually corresponds with having a better memory, and making more accurate predictions.
Scientists think that crossmodal associations evolved a long time ago, because many studies have shown that many different animals have this ability. The goal of the present study is to see whether tortoises have the crossmodal association of pitch and size (like the chihuahua and St. Bernard example). Humans, chimpanzees, and dogs have this ability but we don’t know about tortoises yet. We do know that tortoises can respond to sounds between 10 and 940 Hz, and that males make special sounds to attract females, so maybe they also use sounds for crossmodal associations.
First, the researchers raised ten male tortoises to adulthood. They made an “arena” out of
wood that had three areas, one as a starting place, and two for the tortoise to choose between. They trained the tortoises to hear a certain pitched sound and go towards it to get a strawberry treat. The sound would be either high or low pitched, and would be played on a speaker in one of the areas the tortoise could choose between. The tortoise learned that if it went into the room that was playing the sound, it would get a strawberry treat. Thus, the tortoises learned to follow the sound into one of the two choice areas.
After the tortoises learned to go into the area that made the sound, the scientists added
another layer to the experiment. This time the sound would be played on speakers in both areas, so the tortoise could not use its learned skill of just going towards the sound. Instead, the tortoise would have to use another clue to make a decision which area to go into. The scientists put a small yellow disk on one of the areas’ doors and a large yellow disk on the other areas’ door. The turtles saw the size of the disks and made their choices based on that. When a high pitched sound was playing, the tortoise went towards the area with the small disk on the door. When a low pitched sound was playing, the tortoise went towards the area with a large disk on the door.
Since the tortoises correctly deciphered that they should go towards the small disk when
hearing a high pitched sound, and towards the large disk when hearing a low pitched sound, this experiment shows that tortoises have the ability to form crossmodal associations for the size of an object and pitch of the sound it makes. This is a skill that humans have, as well as dogs and chimpanzees. This means that the skill of forming crossmodal associations could have evolved in a common ancestor of both reptiles and mammals.
Loconsole, Maria, et al. “Crossmodal Association between Visual and Acoustic Cues in a
Tortoise ( Testudo Hermanni ).” Biology Letters, vol. 19, no. 7, July 2023, p. 20230265.
DOI.org (Crossref), https://doi.org/10.1098/rsbl.2023.0265.