Skeleton of a mysticete (right whale) and an odotocete (dolphin, not to scale). Taken from Dr. Thewissen's website, The Thewissen Lab
Although the ancestors of whales started their evolutionary journey back to the oceans as land living mammals, the body of their extant descendants is now extremely derived and perfectly adapated to a fully marine existence.
The streamlined body plan of modern cetaceans arose about 34 million years ago at the end of the Eocene. The number of adaptations to an aquatic lifestyle it comprises is vast, but some of the most important include:
- lack of a neck
- evolution of forelimbs into paddle-like flippers without any external digits
- an increased number finger bones (hyperphylangy)
- extreme reduction or absence of hindlimbs and pelvis
- detachment of the pelvis from the spine
- loss of fused sacral vertebrae
- a horizontal tail fluke, which serves the animals as their main means of propulsion
- absence of hair and development of a thick layer insulating fat (blubber)
- nasal openings located on top of the head to make breathin possible during submersion in water; consequently, facial bones are displaced relative to the braincase ('telescoping')
- absence of external ears
- absence of (continuously) external genitalia
Nasal drift in early cetaceans, showing the position of the nostrils in Eocene Pakicetus (left), Eocene Rodhocetus and an extant gray whale, in which it has developed into the blowhole. Note the displacement of the facial bones ('telescoping'). Adapted from: National Geographic's The evolution of whales. Open source licence CC ASA 2.5.
The streamlined body of a fin whale as seen from the air. Note the absence of external hind limbs or a discernibke neck region. Photo: Protected Resouces Division, Southwest Fisheries Science Center, La Jolla, California.
Though fish-like in body shape, cetaceans are mammals. As such, they are warm-blooded, suckle their young and even possess
remnants of hair, even though only as an embryo or in very reduced form in the region around the mouth. One of their most impotant adaptations to an aquatic lifestyle
in this respect is the development of a thick layer of fat (blubber), insulating them from the cold surrounding water. The
reason why a layer of fat, rather than a waterproof fur has been favoured by natural selection may lie in the more
streamlined body shape that a smooth, hairless body surface confers. Alternatively, it has been suggested that the
hairlessness of cetaceans might indeed share a common origin with that of hippopotami and would thus be a trait predating
the development of the superior cetacean swimming capabilities.
Another matter of some importance to any sea-living creature is the ability to survive in salt water. Because they
have no access to sources of drinkable freshwater, cetaceans have to obtain water either from their food or from metabolic processes.
They have also been reported to occasionally ingesting salt water, especially when fasting.
However, finding drinkable water is not the only problem to be addressed. Water rich in salt (NaCL) tends to draw out
water from cells with a lower concentration of dissolved salts in a proses called osmosis. All marine mammals are
hypotonic, i.e. the salt level of their blood is below that of sea water. For the same reason that lettice becomes soft
an disgusting when it is left in salty water, all marine mammals thus have to be able to control the amount of
water and/or salt entering or leaving their body, a capability called osmoregulation. They normally do this by concentrating
their urine to a very high level, so as preserve their body water and excreet excess salt. Cetaceans,
however, appear to not always show this behaviour and sometimes even produce dilute urine during fasting experiments.
It has been suggested that dehydration of the animals may cause an increase in production of metabolic water by oxidising fat,
thus producing an excess of water that is relesed with the urine. Whatever the mechanism, the ability of cetaceans
to osmoregulate in salt water probably first arose in Middle Eocene protocetids, which indeed, owing to this trait, were
the first whales to leave coastal habitats and spread around the globe.
Many of the evolutionary steps that cetaceans have undergone in order to adapt to a fully aquatic lifestyle are repeated,
in a manner of speaking during the development of the cetacean embryo. Studies on embryos have been conducted by the whale
palaeontolgist Dr. Hans Thewissen and his collegues. Working on a unique collection of embryos of the spotted
dolphin Stenella attenuata, they managed to make various developmental stages of cetacean embryos visibe and
widely accessible (Digital Library of Dolphin Development).
They showed that dolphin embryos, just like human embryos, still develop
what can be interpeted as immature gills, an evolutionary vestige inherited from the common ancestor of all
tetrapods. Most interstingly though from an evolutionary
point of view is the development of posterior growth buds that in land mammals would give rise to hind limbs. In cetaceans,
these hind limb buds usually reduce and finally disappear again, but sometimes traces of them are retained and develop into
protrusions or even little flippers visible even in the adult. This sudden reappearance of an ancient trait is called an
atavism and can occur in all animals. In humans, the development of a fur or even of vestigial tails have sometimes
been observed, linking us back to our evolutionary ancestors.
Embryo of a spotted dolphin in the fifth week of development. Note the hind limb buds near the base of the tail. The tail is approximately 1 inch/ 25 mm long. © Prof. J. G. M. Thewissen 2006, used with permission.