To the Editor:
Howard et al. (1) are to be complimented on their article “Bispectral Index Monitoring of Unihemispheric Effects in Dolphins.” They present a humane nonsurgical method for study of unihemispheric bispectral index (BIS) readings in nonmedicated dolphins and in dolphins given propofol, atropine, and/or diazepam. The dolphins they studied were Tursiops truncatus. This dolphin has a brain on average slightly larger than the brain of humans (2), providing an interesting comparative animal model for study of unihemispheric effects in humans.
As pointed out by Howard et al. (1), there are many studies, including their article, which show unihemispheric electroencephalogram (EEG) changes in dolphin sleep. A key question is whether the Tursiops truncatus dolphin can sleep simultaneously with both brain hemispheres, thus losing the ability to monitor the open water environment (3).
As mentioned by the authors, I have observed Tursiops truncatus dolphins “resting at the surface, virtually immobile, with both eyes closed and breathing in an ‘automatic fashion’ for periods of an hour or more” (4). One of the animals in this state failed to give a previously learned response to a command to swim to an observer. Another did not respond to a flash camera through a Plexiglas® viewing port in the dolphin tank. Others did not respond to dolphins swimming near them (Figs. 1–3 and 4B). The photographs (Figs. 1–6) I shot during this study (4) of dolphin sleep behavior have never been published and are presented here to give the reader a better appreciation of sleep behavior in Tursiops truncatus in a large open water tank, as opposed to the laboratory setting used by Howard et al. (1).
Figure 5 depicts Tursiops truncatus dolphins resting on the bottom of their home tank, shown in Figure 1. This bottom “catnapping ” behavior may or may not progress to the deeper surface sleep behavior shown in Figures 2, 3, and 4B. While exhibiting catnapping behavior on the bottom, the dolphin has one or both eyes open, may blink the eyes alternately, and is responsive to the movement of observers outside or nearby animals. Between bottom cat-napping periods lasting approximately 4 min each, the dolphin will swim to the surface, breathe several times, and then slip tail-first back down to the bottom to continue the nap (4).
I made these observations (4) during anesthetization of 35 dolphins (Tursiops truncatus and Lagenorhynchus obliquidens) and during the study of sleep behavior in two Lagenorhynchus obliquidens, seven Tursiops truncatus, and 10 Phocoenoides dalli. Two additional Tursiops truncatus were tested with trifluomeprazine, and their behavior was reported in my article (4). More details of this trifluomeprazine work are presented in an article by Dr. Ridgway (5).
In their article, Howard et al. also discuss the similarities of BIS and EEG readings in dolphin sleep and drug-induced states. Ridgway and I (6) found that during halothane induction of anesthesia in Tursiops truncatus, the dolphin goes through the same swimming motion on the operating table (Fig. 6) as seen in my photographs presented here for dolphin “surface” sleep behavior (Figs. 2, 3, and 4B). Just as with the depth of behavioral surface sleep, the swimming motion of the dolphin’s tail subsides according to the depth of anesthesia, coming to a complete stop with attainment of surgical anesthesia. On recovery from anesthesia, the dolphin starts this swimming motion again on the operating table in the process of regaining consciousness. Rapid induction of anesthesia in the dolphin with injectable drugs such as sodium thiopental administered IV usually happens too fast to elicit the swimming response on the operating table (6).
Dr. Ridgway also found (4) that doses of 1 mg/kg trifluomeprazine can tranquilize Tursiops truncatus without a depression of respiration, and without having the animal sink to the bottom of his home tank. With trifluomeprazine injection, the dolphin goes through the same surface sleep behavior seen in Figures 2, 3, and 4B, culminating in a surface position with only gentle strokes of the tail with each respiration, and both eyes closed for 2 h or more. The dolphin’s eyes do not open when one gently touches the animal. In 24 h, recovery from the trifluomeprazine is complete (4).
The behavioral observation figures presented here give added evidence to the supposition that the dolphin Tursiops truncatus not only is capable of unihemispheric sleep, but also is capable of bihemispheric sleep with complete insensitivity to the immediate surroundings. Further, the similarity of swimming motion change in surface sleep, trifluomeprazine injection, and gas induction of anesthesia suggests the possibility of a basic reflex mechanism in the dolphin brain which is triggered in sleep, sedation, and anesthesia.
James G. McCormick, PhD
Department of Anesthesiology
Wake Forest University School of Medicine
Winston-Salem, North Carolina
1. Howard RS, Finneran JJ, Ridgway SH. Bispectral index monitoring of unihemispheric effects in dolphins. Anesth Analg 2006;103:626–32.
2. Ridgway SH, Flanigan NJ, McCormick JG. Brain-spinal cord ratios in porpoises: possible correlations with intelligence and ecology. Psychon Sci 1966;6:491–2.
3. Ridgway SH. Asymmetry and symmetry in brain waves from dolphin left and right hemispheres: some observations after anesthesia, during quiescent hanging behavior, and during visual obstruction. Brain Behav Evol 2002;60:265–74.
4. McCormick JG. Relationship of sleep, respiration, and anesthesia in the porpoise. Proc Natl Acad Sci USA 1969;62:697–703.
5. Ridgway SH. The bottlenosed dolphin in biomedical research. In: Gay WI, ed. Methods of animal experimentation. New York: Academic Press, 1968:387–446.
6. Ridgway SH, McCormick JG. Anesthetization of porpoises for major surgery. Science 1967;158:510–12.