Technically known as the Delphinidae, this was named by John Edward Gray in 1821 as part of one of the earliest formal lists of mammal families. Gray's original definition basically included all toothed cetaceans other than narwhals and sperm whales, encompassing four genera, only one of which is still placed in the family today - Delphinus, from which it takes its name. He didn't list how many species he thought that genus contained, but it would certainly have included the two named by Linnaeus in 1758, and probably at least three others that had been described in the interim. Over the following decades, the number expanded considerably, with Gray himself identifying several of them, notably when he catalogued the observations and specimens collected by the Ross Expedition of 1839-43.
An important point to note here is that while I'm referring to the Delphinidae as the "dolphin family", not all animals we would necessarily call "dolphins" are really delphinids, because some of the stranger ones turned out not to be closely related. Furthermore, porpoises are not dolphins, with Gray having given them a family of their own as early as 1825. So what does define a dolphin?
In a general sense, we can say that dolphins are small to medium-sized cetaceans, with slender bodies, a prominent dorsal fin, and a notch in the middle of their tail fluke. Unlike porpoises, most, though not all, have a distinct beak and their teeth are conical, rather than spade-shaped. Of course, there's rather more going on, especially if we consider the features that separate cetaceans more generally from other mammals.
The dolphin skeleton is built from the same elements as that of any other mammal but highly modified to fit their unusual shape. They have no hind limbs, even internally, and the pelvis is greatly reduced and no longer attached to the backbone. The front limbs are shaped into paddles, with the arm bones shortened and flattened, the elbows and wrists inflexible, and the fingers present internally, but fused together by flesh and ligaments.
The neck is unusually short although, just as in giraffes, it has the same seven vertebrae that we would find in almost every other mammal. The rest of the backbone is a different matter, having many more vertebrae than we would normally expect, with considerable variation in the total number between individuals, never mind different species.
For most mammals, one of the surest ways of determining the family to which it belongs is to look at the number and arrangement of the teeth, especially the fine details of the shape of the molars. Dolphins, however, like all living cetaceans, have entirely lost the usual distinctions between incisors, molars and so on; all their teeth are conical and near-identical. Most have far more teeth than mammals typically do, although there are some exceptions.
The jaw is also oddly structured, partly to form in the long beak that most species possess, but also because of some weird asymmetry that probably relates to their ability to use sonar and possibly also their ability to swallow prey underwater without choking. In most mammals with a snout, the front part of the upper half is formed by a pair of "premaxillary bones" that run up the side to join with the lower edge of the bones that form the bridge of the nose. (In humans, lacking a snout, these have fused with the maxillary bones that form the rest of the upper jaw/lower face). In dolphins, the nostrils are so far back that the premaxillary bones effectively form the top of the beak and only the one on the right-hand side meets the associated nasal bone, the asymmetry leaving a gap between those on the left.
Other changes to the skull include the lack of sinuses, something found in all cetaceans, presumably because the thin bony walls would crack during a deep dive. Instead, these are replaced by flexible air sacs that can easily expand and contract as the surrounding water pressure changes. While such structures are also found in other toothed whales and porpoises, a distinguishing feature of dolphins is that the bones of the middle ear remain separate from the skull, something that may improve their sense of hearing compared with their relatives.
Dolphins, like all toothed whales and porpoises, have a single nostril pushed so far back that it's on what we would consider the forehead - something that's obviously useful when you're swimming near the surface and still need to breathe. The larynx projects partly through the throat and is held rigidly in place, while the nose/blowhole requires voluntary effort to open. The trachea is short and unusually stiff, presumably as another adaptation to deep diving.
All dolphins are purely carnivorous, although exactly what they eat varies significantly between species. Unlike terrestrial carnivores - and seals - their stomach has three distinct chambers, although the detail is different from that of ruminants. As in ruminants, the first chamber is really an extension of the oesophagus, lacking the usual digestive glands that the stomach has in other mammals. This chamber is muscular, not only storing food but squeezing it and mashing it up, something that's essential in an animal that can't chew.
After this forestomach, the food enters the second, fundic chamber, where it is digested with stomach acid and all the usual enzymes we would expect to find in a mammalian stomach. This connects with the final, pyloric chamber, through a muscular sphincter, and this probably serves to regulate the amount of food that passes through the next sphincter and into the small intestine - which itself begins with a small expansion that looks vaguely like a fourth stomach chamber but is not separated off.
Hair is often considered one of the defining features of mammals, but it is almost completely missing in dolphins, where only a few tiny remnants of whiskers can be seen in very young individuals. Otherwise, the skin is entirely smooth and hairless, also lacking any trace of sweat or sebaceous glands. The epidermis is ten to twenty times thicker than it is in most terrestrial mammals and overlies a thick layer of insulating blubber.
Given how good water is at conducting sound, it's hardly surprising that dolphins' primary mode of communication is vocal, even before we address their abilities at using sonar. Obviously, they have no external ears, and the eardrum appears to be only protective, having nothing to do with sound conduction. On the other hand, the lower jaw, which is hollow, does apparently act as an additional organ for collecting sound and transmitting it to the middle and inner ear, which remain similar to those in other mammals.
Their eyesight is probably good underwater, allowing dolphins to identify each other visually at close range, and explaining why so many species have distinct colour patterns. They also have the same eyeshine-creating tapetum lucidum inside the eyes that cats (and many other mammals) do, enabling them to see in dim light.
It's unclear whether they have much of a sense of taste, and their tongue is less mobile than that of terrestrial mammals, if more so than that of most other cetaceans. Structures that resemble taste buds are present on the tongues of at least some species, but there are very few, and they appear vestigial; it's possible that they have been co-opted to provide only a sense of touch. So far as we know, dolphins have no sense of smell at all, with the relevant brain structures being missing and there being no holes in the skull through which sensory nerves could pass into the nose. When you spend so much time underwater, smell is likely an unnecessary luxury.
All species of dolphin give birth to just a single individual at a time. Of necessity, this occurs in the water, and the young dolphin is therefore unusually well-developed at birth, allowing it to swim immediately. This requires a long pregnancy on the part of the mother, with few species requiring less than twelve months. Growth after birth is relatively slow, and young dolphins may not be fully weaned for years. This extended maternal care probably allows for the 'teaching' of relatively complex social traits and skills, but has the disadvantage that, even considering their unusual longevity, dolphins cannot give birth to many young during their life.
Female dolphins appear to experience oestrus ("coming into heat") several times a year when they aren't caring for young, something they signal by tactile behaviour towards males rather than smell. On the other hand, at least some species appear to engage in sexual activity when they aren't ready to get pregnant, presumably as a means of social bonding not unlike that seen in humans and bonobos.
Although, as is so often the case, the exact number of species is debated, today over 30 species of delphinid are recognised to exist, not counting those known only from fossils. Over the next nine months, I am going to look at all of them, starting with some of the most familiar...
[Photo by Paul Harrison, from Wikimedia Commons.]
Do other aquatic mammals have functioning senses of taste? Do, for that matter, fish?
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