Dolphins Remain Alert for Weeks Without Sleep
Modern society seems to wage a constant battle with the biological need to sleep. We have arrays of caffeinated beverages, 24-hour stores at our disposal, and technology the provides round-the-clock entertainment sources. If you have ever been a shift worker, a new parent, a desperate student during finals week, or have traveled across multiple time zones, it’s likely that you have experienced just how mentally and physically overwhelming it can feel to be in a state of sleep deprivation. Although the average human spends roughly a third of their life in a state of slumber, sleep is an enigmatic phenomenon that we are still striving to understand. Clearly, it must have some benefit, if we sacrifice so much of our time to such dormancy. How universal is this requirement, though?
Evidence suggests that there is substantial variation in the need for sleep between species, and dolphins are a prime example: electroencephalography has shown that these marine mammals appear to “sleep” with only one hemisphere of their brains at a time (Lyamin 2008), and there is mixed evidence as to whether they actually experience REM activity (Siegel 2005).
So, dolphins are lucky enough to escape the hefty time burden of sleep required by humans. How do they manage this? Nearly everyone has noticed, at some point, a precipitous decline in their function and efficiency at day-to-day tasks after being deprived of sleep. Studies have shown that driving tired can be as dangerous as driving drunk. This leads us to the obvious question: how long can a dolphin go without sleep before experiencing performance declines? The lifestyle of a dolphin requires an incredible amount of vigilance and constant monitoring of the surrounding water via echolocation. A new study published in the journal PLoS ONE, led by Brian Branstetter of the National Marine Mammal Foundation, addresses this question in a study of vigilance in the bottlenose dolphin (Tursiops truncatus).
As is common in studies of marine mammals, the sample size was admittedly limited: a single male and single female dolphin were tested. Due to the difficulties of studying marine mammal behavior in the wild, research on dolphin behavior is commonly conducted with small sets of captive subjects. The researchers monitored the dolphins’ performance in a set of trials that involved detecting targets—generated by electronic “phantom echoes” that could be detected by echolocation—around their enclosure and reporting detections by pressing a paddle. Dolphins were rewarded for correct detections, and “false” detections were not punished, but yielded no positive reinforcement. The dolphins were put through their paces over the course of three 5-day sessions during which they did not sleep.
Both dolphins showed remarkable accuracy in target detection during the entire five days without sleep. The male’s accuracy ranged from 75.3-86.2%, and the female earned straight A’s with a correct detection rate of 96.3-99.6%. Although performance decreased over the course of each session (via increases in missed detections and/or false reports), the dolphins’ accuracy remained high despite the potential for fatigue suggested by these incremental declines. Interestingly, performance increased in the final sampling session, indicating that the dolphins were learning to perform the tasks more efficiently as the study progressed.
In addition to her exceptional accuracy, the female also proved to be nearly inexhaustible. The researchers planned a 30-day trial to see how long she could maintain a high performance outside the constraints of the 5-day sessions. She kept up her impressive showing for 15 straight days, and it seems that she could indeed have lasted longer if the researchers hadn’t been forced to cancel the session due to a winter storm.
This study demonstrates the amazing degree to which sleep requirements vary between species, and raises myriad questions about what the functions and benefits of sleep actually are. Dolphins are not just any mammals—they are some of the most encephalized species on the planet, and have been demonstrated over and over again to exhibit highly complex and even creative behaviors, whether it be through cooperative hunting or performing trained tasks such as those used in this study. The fact that they can maintain these cognitive faculties on so little sleep challenges some current hypotheses about the role of sleep in knowledge consolidation. Branstetter et al. suggest that the dolphin's adaptation to function on so little sleep is a necessity for survival in the ocean, to avoid either drowning (an issue not faced by the fish that share the same marine habitats) and to minimize the risk of being caught unawares by predators such as sharks. Further behavioral and physiological research should yield further insights into exactly how the dolphin's brain manifests these adaptations to allow for such prolonged vigilance.
Branstetter BK, Finneran JJ, Fletcher EA, Weisman BC, & Ridgway SH (2012). Dolphins Can Maintain Vigilant Behavior through Echolocation for 15 Days without Interruption or Cognitive Impairment. PloS one, 7 (10) PMID: 23082170
Lyamin O.I., Manger P.R., Ridgway S.H., Mukhametov L.M., Siegel J.M. 2008. Cetacean sleep: an unusual form of mammalian sleep. Neuroscience and Biobehavioral Reviews 32:1451–1484.
Siegel, Jerome M. 2005. REM sleep. Principles and Practice of Sleep Medicine 4:120-135.