Cetacean sleep: an unusual form of mammalian sleep.

TLDR: The suggestion is made that the selection pressure necessitating the evolution of cetacean sleep was most likely the need to offset heat loss to the water from birth and throughout life.

About: This article is published in Neuroscience & Biobehavioral Reviews.The article was published on 2008-10-01 and is currently open access. It has received 232 citations till now. The article focuses on the topics: Unihemispheric slow-wave sleep & Slow-wave sleep.

Figures

Fig. 12. Photomicrographs of coronal sections stained for myelin in two species of pinniped and one sirenian, demonstrating the morphology of the posterior commissure. (A) Northern fur seal (Callorhinus ursinus), a member of the Otariidae family and a presumed unihemispheric sleeper when in water. Note the generally typical mammalian appearance of the posterior commissure. (B) Harbor seal (Phoca vitulina), a member of the Phocidae family and a presumed bihemispheric sleeper. Note again the typical mammalian appearance of the posterior commissure. (C) Florida manatee (Trichechus manatus), a species that exhibits both unihemispheric and bihemispheric sleep. Note again the typical mammalian appearance of the posterior commissure. Scale bar = 1 cm, applies to all three photomicrographs. All photomicrographs in this plate are taken from sections housed within the Comparative Mammalian Brain Collections of the University of Wisconsin, Michigan State University and The National Museum of Health and Medicine.

Fig. 7. Photomicrographs ofmyelin stained coronal brain sections demonstrating themor (Vombatus ursinus) scale bar = 3 mm. (B) Tammar wallaby (Macropus eugenii) scale bar = (Marmotamarmota) scale bar = 1 mm. (E)Woodchuck (Marmotamonax) scale bar = 2 mm bar = 5 mm. (H) Sheep (Ovis aries) scale bar = 3 mm. (I) North American badger (Taxidea t black bear (Ursus americanus) scale bar = 5 mm. (L) Chimpanzee (Pan troglodytes) scale ba the Comparative Mammalian Brain Collections of the University of Wisconsin, Michiga

Table 2 Characteristic of sleep in Cetaceans

Fig. 13. Diagrammatic representation of the possible manner in which cetacean sleep ph and brain to maintain body and brain temperature in the thermally challenging aquat

Fig. 8. (A) Photograph of gross coronal slice through the brain of a Commerson’s or piebald dolphin (Cephalorhynchus commersonii), showing the enlarged posterior commissure situated immediately anterior to the superior colliculi, and dorsal to the septal nuclei. The coronal plane in cetaceans is different to other mammals due to the maintenance of the cephalic flexure in the adult (from the collection of Sam Ridgway). scale bar = 1 cm. (B) Photomicrograph of a horizontal section through the brain of an adult bottlenose dolphin (Tursiops truncatus) stained for both Nissl substance and myelin. The myelin dense, horizontally projecting posterior commissure can be seen anterior to the superior colliculi investing into the gray matter of the dorsal thalamus. scale bar = 1 cm. (C) Photomicrograph of a coronal section through the brain of an adult bottlenose dolphin (T. truncatus) stained for myelin. Note the thickness of the posterior commissure, lying immediately ventral to a slightly thicker corpus callosum, and projecting horizontally into the gray matter of the dorsal thalamus. Scale bar = 1 cm. The photomicrographs in this plate are taken from sections housed within the Comparative Mammalian Brain Collections of the University of Wisconsin, Michigan State University and The National Museum of Health and Medicine.

Fig. 9. (A) Gross dissection of the bottlenose dolphin brain with the third and fourth ventricles revealed. This dissection demonstrates the location of the posterior commissure (pc) in relation to the habenular trigones (hb), the superior colliculi (sc), pineal gland (pg) and the inferior colliculi (ic). mcp—middle cerebellar peduncle. Scale bar = 1 cm. It should be noted here that the dolphin dissected in this preparation died while heavily pregnant and that in the normal situation the pineal gland is microscopic or absent in cetaceans (Oelschläger et al., 2008, see text for further details). (B) Photomicrograph of tyrosine hydroxylase immunopositive axons coursing through the posterior commissure of the bottlenose dolphin. Scale bar = 100mm.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Cetacean sleep: an unusual form of mammalian sleep" ?

Their knowledge of the form of lateralized sleep behavior, known as unihemispheric slow wave sleep ( USWS ), seen in all members of the order Cetacea examined to date, is described. Three suggested functions of USWS ( facilitation of movement, more efficient sensory processing and control of breathing ) are discussed. Lastly, the possible selection pressures leading to this form of sleep are examined, leading us to the suggestion that the selection pressure necessitating the evolution of cetacean sleep was most likely the need to offset heat loss to the water from birth and throughout life. 

Q2. What is the role of serotonergic neurons in the regulation of sleep?

The serotonergic neurons may also play a role in the maintenance of arousal, in regulating muscle tone, and suppression of phasic events during waking (Wu et al., 2004). 

Q3. What is the role of the noradrenalin-producing neurons in the REM sleep?

These noradrenalin-producing neurons are for the most part inactive during REM sleep and it has been suggested that the inactivity of these neurons during REM sleep is related to the loss of muscle tone (John et al., 2004; Siegel et al., 1991; Lai et al., 2001). 

Q4. What is the importance of keeping close contact with the mother and calves?

Considering that killer whales are amongst the top predators in the ocean, maintenance of close contact would be of even greater importance for mothers and calves of other cetacean species. 

Q5. What is the role of the histaminergic neurons in REM sleep?

Some of these neurons are selectively active in REM sleep and are involved in the suppression of muscle tone during this state (Sakai and Koyama, 1996), presumably preventing the acting out of dreams. 

Q6. What is the effect of the discharge rate of the other hemisphere?

The other hemisphere, having been asleep for some time, and thus having cooled down (see above), would have a decreased discharge rate of these neurons, leading to cortical arousal. 

Q7. Why is it difficult to identify the hypothalamus in the cetaceans?

The hypothalamus is readily identified, but due to the small size of the fornix, it is difficult in Nissl stained sections to demarcate hypothalamic regions in the cetaceans. 

Q8. How long does it take for dolphins to display REM sleep?

an adaptation period of 2–3 weeks after the implantation of electrodes may not be enough time for dolphins to display REM sleep. 

Q9. What is the reason for the lack of REM sleep in dolphins?

Adaptation to the experimental conditions was another factor considered by Mukhametov and colleagues that may be related to the lack of observation of REM sleep in dolphins. 

Q10. What is the reason for the absence of REM sleep in dolphins?

According to Mukhametov et al. (1997), this observation may also explain the absence of REM sleep in cetaceans in its traditional form, as a loss of muscle tone typically coincides with REM sleep in terrestrial mammals. 

Q11. How can the authors monitor the eye state of dolphins?

The eye state can be monitored via video cameras but continuous monitoring is possible only if the animals are limited in their swimming (i.e. restrained), as the authors did in their recent studies in one beluga whale and one bottlenose dolphin (Lyamin et al., 2002a,b, 2004).