Mila calls Eva to come over – Watch and listen!
Above: In this video (1) Left, Paola cues Eva to “listen” and Eva then submerges. (2) Right, Juan Carlos cues Mila to “tell” Eva to come over to her and Mila then submerges. At this point the soundtrack changes to the hydrophone (underwater) recording of the dolphin vocalizations, also see spectrogram below. (3) Mila vocalizes underwater to Eva. (4) Eva then vocalizes and joins Mila for a whistle “duet”. (5) As the “duet” continues, Eva swims over to Mila. (6) They then surface together. (7) Both dolphins are thanked by their trainers. At the end, Juan Carlos and Mila share an exuberant high five.
Below: Spectrogram of the hydrophone recording from the above video of Mila “telling” Eva to come over to her. Eva replies using whistles that match Mila’s. As Eva swims over to Mila the two dolphins chorus together with the matching, overlapped whistles. The faint whistle on the far left was produced by Mila just before she submerged.
Experimental Framework
We have designed an experimental framework in which the dolphins who are in acoustic but not visual contact must communicate in order to accomplish one of a set of specific tasks. In essence, for a task to be completed successfully one dolphin must “tell” another dolphin what to do. We can then analyze underwater recordings to see what mechanisms the dolphins used to communicate the necessary information.
In those past experiments2-4 closest to what we are doing now, dolphins were trained to communicate for only one particular task. In contrast, our dolphins have learned a more general “tell and listen” concept. That way a hand signal for “tell” can be prepended to the hand signal for any one of the behaviors known by two dolphins. A dolphin given such a two-part hand signal (i.e. “tell” followed by the signal for a specific behavior) knows that, rather than performing the behavior herself, she should “tell” the other “listening” dolphin to perform that specific behavior. Because the requested behaviors can be varied, we can investigate behavior specific differences in the communication methods used by the dolphins.
The most difficult hurdle, and the one critical to the success of all the experiments in the ongoing series, was the development of a training plan through which the “tell and listen” concept could be conveyed to the dolphins. This critical step, along with many other important contributions, was accomplished with our collaborators at Dolphin Adventure in Puerto Vallarta, Mexico.
First Experiment
Two female 
dolphins, Eva and Mila, are the first dolphin participants in our research. For the formal trials of this first experiment, Mila was the “teller” and Eva was the “listener”. For these trials: (1) Eva is cued by her trainer to be the “listener” and submerges. (2) Mila is cued by her trainer to “tell” Eva to perform a particular task. (3) Mila submerges and communicates with Eva underwater. (4) If Mila has successfully conveyed the information to Eva, Eva can perform the behavior.
We selected four behaviors, previously known by both dolphins, to be communicated. They ar
e:
- “Clap” – The dolphin waves her pectoral fins.
- “Come” – One dolphin calls another to come over to her, as shown in the video at the top of this page.
- “Sing” – The dolphin, with her head out of the water, produces rapid, pulsed vocalizations in front of her trainer.
- “Splash” – The dolphin splashes water with her rostrum.
Formal Trials
After completion of the training phases, which were usually performed with both dolphins at opposite ends of the same pool, Mila and Eva were transitioned to working in separate pools (see video at top of page). Although the dolphins could hear each other, a plywood wall prevented Eva from seeing either Mila or Mila’s trainer and vice versa. The wall also prevented the trainers from seeing each other or the opposite dolphin. Consequently, the timing of actions necessitated a third person to act as coordinator, as can be heard in the video. An open gate between pools allowed Eva to swim over to Mila for the “come” behavior.
Formal trials were conducted in 10 sessions between the 8th and 22nd of October, 2018. Each session consisted of twelve randomized task behaviors, with each behavior tested exactly three times per session. Consequently, for the entire set of formal sessions, communication of each behavior was tested thirty times (10 sessions x 3 trials/session). The communication success rate for each task behavior over the course of all sessions is as follows:
- “Clap” – 15 correct, 15 wrong – 50%
- “Come” – 30 correct, 0 wrong – 100%
- “Sing” – 25 correct, 5 wrong – 83.3%
- “Splash” – 22 correct, 8 wrong – 73.3%
- ——————————————–
- Total – 92 correct, 28 wrong – 76.7%
Whistle overlap matching, as found in the vocalizations of Mila and Eva during the “come” behavior, has also been found to occur in wild bottlenose dolphins in association with a type of food-related call during which one dolphin swims rapidly towards a calling dolphin’s location.5 Mila and Eva’s perfect score for this behavior in our formal trials may be attributable to its perhaps innate use in this species as a social signal to facilitate dolphin to dolphin approaches. In contrast, the somewhat lower scores for the other behaviors in our trials may result from the absence of a clear-cut signaling mechanism in the dolphin vocal repertoire, necessitating the invention of some other method (e.g. imitation) to convey the necessary information.
Footnotes and References:
1. The communication of identity between dolphins already is well established in signature whistle studies. See the following for a comprehensive description of signature whistle research: Janik V.M., Sayigh L.S., 2013. Communication in bottlenose dolphins: 50 years of signature whistle research. J Comp Physiol A, vol. 199, no. 6, pp. 479–489. https://doi.org/10.1007/s00359-013-0817-7
2. Bastian, J., 1967. The transmission of arbitrary environmental information between bottle-nose dolphins. In Busnel, R.G., ed. Animal Sonar Systems, Biology and Bionics. Jouy-en-Josas, France: Laboratoire de Physiologie Acoustique, pp. 803–873.
3. Bastian, J., Wall, C. & Anderson, C., 1968. Further Investigation of the Transmission of Arbitrary Environmental Information Between Bottle-Nose Dolphins., Naval Undersea Warfare Center, San Diego, TP 109.
4. Zanin, A.V., Markov, V.I. & Sidorova, I.E., 1990. The ability of bottlenose dolphins, Tursiops truncatus, to report arbitrary information. In Thomas, Jeanette A and Kastelein, Ronald A, ed. Sensory abilities of cetaceans. New York: Plenum Press, pp.685–697.
5. King, S.L. & Janik, V.M., 2015. Come dine with me: food-associated social signalling in wild bottlenose dolphins (Tursiops truncatus). Animal Cognition, vol. 18, no. 4, pp. 969-974. https://doi.org/10.1007/s10071-015-0851-7
Additional Notes:
All black and white spectrograms on the website were created using Raven Pro. Color plots were generated using our own custom software.
Because the frequency range of the overlapped whistles found in the vocalizations of Mila and Eva during the “come” behavior was at the higher end of human perception (or beyond that for some people), the underwater audio soundtracks of the videos for “come” were pitch lowered into a more human sensitive range using the digital audio editor Audacity.
Spectrogram of Mila “singing” underwater to Eva. The x-axis is time and the y-axis is frequency.