Old thread, but this seems to be the newest on this topic so I will wake it up
This topic seems to pop up both here and on various tortoise facebook groups etc and it seems people cannot agree on what/if tortoises can hear.
I came upon this research article just now (it is from 2013 though so not that new) and found it interesting, thought perhaps people here would find it interesting as well if you have not read this before.
Turns out someone actually made a systematic examination of what tortoises can actually hear and wrote a scientific article about it, much better than the pure speculation on the subject that seems to go around.
The study was made on Hermann's tortoises but is probably valid for other similar species.
Hermann's tortoises do hear, but they hear only a very limited frequency range compared to humans. Specifically it seems to be in the range 10 to 182 Hz, while humans (young ones with good hearing) will hear approximately 20 to 20.000 Hz.
This means they will hear more infrasound than us. Some vibrations we can only feel, they will hear as sound. While they will miss out on everything above 182 Hz.
What does this mean? Will they hear human voices.
Definitely yes. But they very little chance of recognizing words or any meaning.
The fundamental frequency of human voices, particularly male voices, are usually below 182 Hz. However. all the information is typically in the harmonic frequencies well above 182 Hz. For comparison phones usually limit the frequency range to 300 to 3400 Hz. (Landlines, not sure about cell phones but I think not likely they are much wider in range, more probably even narrower).
I often see people suggesting testing the tortoise by talking through a phone to see if they recognize their owner. This will always fail because the phone will not transmit sound in the range the tortoise can hear while the actual frequency range of human speech goes well below 300 Hz.
The reason for this cutoff is that humans cannot tell there is missing low frequencies. The human auditory equipment will actually synthesize the missing frequencies from the harmonics and to us it will appear that the phone is also transmitting frequencies below 300Hz, this is a well known auditory illusion that the telecom industry always have taken advantage off to simplify circuit requirements and cost.
If you wonder what human speech sounds like to a Hemann's tortoises? I have made a few samples using a free audio editing software (audacity) where I applied a very sharp cutoff filter for frequencies above 180 Hz.
Please note: This is probably not scientifically accurate as the Tortoise will also have a frequency response curve within that range that needs to be applied and I know nothing about and it probably does not match the microphone I used to sample the raw sound before filtering. But it should be a reasonable indication of what human speech sound like to the tortoise.
Bonus points if you can recognize what is being said should be very hard.
Some people also claims the tortoise only reacts to specific people talking.
I think this is not completely impossible. At least the fundamental frequency is quite different between male/female/child and I would expect the tortoise might also learn to recognize personal patterns in speech like rhythm, speed, pitch variations, spacing between words etc.
I guess the fundamental frequency of a voice is also personal depending on the shape and form of your larynx, however I would think the difference between two males for example is not very significant. But it can definitely hear the difference between the husband and the wife in an household.
Come on guys, this has been studied in the field, in labs, on live animals, during dissections, and more. Lots of good, if sometimes hard to read, articles and books on it. Also- why are we debating personal experience under non-controlled conditions when there is so much real research already done on it?
Do they 'hear'? Depends on how you define it, but they do not depend on sounds received by their ears much. Considering how much time they spend in their shells, underground, etc., AND that their food is not found by sound, AND that their main predators are pretty silent, why should they depend on their ears?
Can they sense 'sounds' that are low enough frequency to create vibrations in the water, ground, or their shells? Yes, absolutely- and they are really good at it. In some studies, desert tortoises have been shown to make subsonic sounds that travel for long distances through the ground and that others far away respond to it.
Can they hear noises being made by hatching eggs, fighting males, and mating couples? Yes, with lots of variation over distance they can 'hear' it, and no good controls over whether it is primarily received through the ear or shell.
Do they respond under controlled conditions to sounds from loudspeakers, speakers, etc. that may be loud, but do not cause good ground vibrations, and are usually at normal human conversation frequencies? No.
Seriously, you should read that paper, it is interesting.
Here is a small sample from the Results and Discussion section:
»Tortoise keepers have again and again been
reporting that their tortoises came running
from their outdoor enclosures when they
played the piano. Studying the overview of
frequencies in Fig. 9 shows that the piano is
one of the musical instruments that can also
produce very deep sounds. The above results
also explain why tortoises will respond more
readily to the deeper male voice than to the
higher voice of a woman»