Dolphins

Dolphins are a widely distributed and diverse group of fully aquatic marine mammals. They are an informal grouping within the order Cetacea, excluding whales and porpoises, so to zoologists the grouping is paraphyletic. The dolphins comprise the extant families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the new world river dolphins), and Pontoporiidae (the brackish dolphins). There are 40 extant species of dolphins. Dolphins, alongside other cetaceans, belong to the clade Cetartiodactyla with even-toed ungulates, and their closest living relatives are the hippopotamuses, having diverged about 40 million years ago.
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Size

Dolphins range in size from the 1.7 metres (5.6 ft) long and 50 kilograms (110 lb) Maui's dolphin to the 9.5 metres (31 ft) and 10 metric tons (11 short tons) killer whale. Several species exhibit sexual dimorphism, in that the males are larger than females. They have streamlined bodies and two limbs that are modified into flippers.

Sleeping

Dolphins have to be conscious to breath. This means that they cannot go into a full deep sleep, because then they would suffocate. Dolphins have "solved" that by letting one-half of their brain sleep at a time. This has been determined by doing EEG studies on dolphins. Dolphins sleep about 8 hours day in this fashion. 

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A dolphin's behavior when sleeping/resting depends on the circumstances and possibly on individual preferences. They can either:

1. Swim slowly and surface every now and then for a breath

2. Rest at the surface with their blowhole exposed 

3. Rest on the bottom (in shallow water) and rise to the surface every now and then to breath. 

Sources

1. S.H Ridgway (1990) The Central Nervous System of the Bottlenose Dolphin, in S. Leatherwood and R.R. Reeves: The Bottlenose Dolphin, pp. 69-97, Academic Press

2. Mukhametov, L. M.; Supin, A. Ya. (1978). "Sleep and vigil in dolphins". Marine mammals. Moscow: Nauka.

3. Mukhametov, Lev (1984). "Sleep in marine mammals". Experimental Brain Research. Experimental Brain Research Supplementum. 8 (suppl.): 227–238. doi:10.1007/978-3-642-69554-4_17. ISBN 978-3-642-69556-8.

4. Dallas Grasby (1994). "Excerpts from "Sleep in marine mammals", L.M. Mukhametov". Experimental Brain Research. 8 (suppl.). Retrieved February 11, 2008. [Link]

Source

Intelligence

Brain size was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the ⅔ or ¾ exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalization quotient that can be used as another indication of animal intelligence. Killer whales have the second largest brain mass of any animal on earth, next to the sperm whale. The brain to body mass ratio in some is second only to humans.

In 1995, Marten and Psarakos used television to test dolphin self-awareness. They showed dolphins real-time footage of themselves, recorded footage, and another dolphin. They concluded that their evidence suggested self-awareness rather than social behavior. While this particular study has not been repeated since then, dolphins have since passed the mirror test. [Source 1, Source 2, Source 3]

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Research Example

At the Institute for Marine Mammal Studies in Mississippi, Kelly the dolphin has built up quite a reputation. All the dolphins at the institute are trained to hold onto any litter that falls into their pools until they see a trainer, when they can trade the litter for fish. In this way, the dolphins help to keep their pools clean.

Kelly has taken this task one step further. When people drop paper into the water she hides it under a rock at the bottom of the pool. The next time a trainer passes, she goes down to the rock and tears off a piece of paper to give to the trainer. After a fish reward, she goes back down, tears off another piece of paper, gets another fish, and so on. This behaviour is interesting because it shows that Kelly has a sense of the future and delays gratification. She has realised that a big piece of paper gets the same reward as a small piece and so delivers only small pieces to keep the extra food coming. She has, in effect, trained the humans.
Then one day, Kelly managed to grab a gull that flew into the tank. When she delivered it to her keepers, she got an especially large fish reward. The next time Kelly was fed she hid the fish at the bottom of the pool, and later brought it to the surface to lure more gulls into the pool. The strategy proved so successful that she taught her offspring, who went on to teach others. (Source 1, Source 2, Source 3, Source 4)