The Dolphin-Hippo Connection
by Susan Lumpkin
At first thought, it seems a stretch that cetaceans—dolphins, whales, and porpoises—and hippopotamuses come from the same branch on the tree of life. True, hippos spend much of their time submerged up to their snouts in rivers, but they feed on land, as no cetaceans do, and their bulbous bodies bear little resemblance to the cetaceans’ sleek, streamlined forms.
Moreover, hippos are members of the mammalian order Artiodacytla—the even-toed ruminant ungulates—a group that includes deer, cows, sheep, antelope, and the like. Cetaceans are . . .? Well, we know they are not fish. Although people long labeled them fish, Aristotle recognized that dolphins were not fish, although he did not associate them with land-living mammals, which he called quadrupeds. Linnaeus, father of modern taxonomy, officially classified them as mammals in 1776. According to biologist Phil Clapham, in Whales of the World, the Swedish naturalist solemnly stated: “I hereby separate the whales from the fish.” And that was that.
Knowing that mammals evolved on land raised questions about which land animals the cetaceans evolved from. On morphological grounds, from comparative analysis of the bones of fossil and modern mammals, scientists concluded that cetaceans were related to ungulates. Artiodactyls are ungulates; so too are horses, rhinos, and other members of the Perissodactyla (odd-toed ungulates), and some extinct groups. Traditionally, elephants (Order Proboscidae), hyraxes (Order Hyracoidea), and manatees and dugongs (Order Sirenia) are included in the ungulates as well. However, it was not clear which ungulates were the cetaceans’ next of kin.
Recently, two opposing views emerged, one based on morphology and one based on molecular genetic evidence. The morphologists, looking at the ankle bones of fossils of early whales, artiodactyls, and an extinct group called the mesonychians, argued that the cetaceans were “sister taxa” to the mesonychians, which were land-living carnivorous ungulates. Sister taxa, in the jargon of stystematics, share a common ancestor.
Genetic evidence painted a different picture, instead showing that cetaceans were most closely linked to the artiodactyls, and, more specifically, were sister taxa to the hippopotamuses. Today, only two species of hippos survive, both in Africa, but the family Hippopotamidae once included more species in Africa as well as Europe and Asia.
Now, new fossils have emerged that tend to support the genetic results. These fossil cetaceans from Pakistan were first to be found with ankle bones—and the form of the fossil cetacean ankles matches the unique form of artiodactyl ankles. This confirms, at least, that cetaceans are artiodactyls—the group has been called “cetartiodactyla” —but morphologists aren’t ready to concede the cetacean–hippo connection because fossil evidence is still lacking.
To bolster their case, molecular biologists point to similarities among modern cetaceans and hippos. Animals in both groups lack hair and sebaceous glands, and both are capable of vocalizing underwater. These were once believed to be the result of the two groups’ evolving similar adaptations to aquatic environments, but it may turn out that the similarities are due to their sharing a common ancestor.
Molecular biologists are actually re-casting the relationships among all the mammals in light of genetic analyses, with some surprising, not to mention controversial, results. They divide the mammals into four large groups, whose divisions can be related to the location of continents in geological time.
Elephants, hyraxes, and manatees and dugongs have been split from the other ungulates, and lumped with aardvarks, golden moles, tenrecs, and elephant shrews to form a group dubbed Afrotheria. The group’s origins are believed to be African because the earliest fossils were found there and some, such as the elephant shrews, never left. More certainly, their origins are in Gondwana, the supercontinent that once consisted of Africa and South America and other southern hemisphere land masses. The Afrotheria may have included the first true mammals. Also originating in Gondwana, and perhaps left behind when Africa and South America broke apart, is the group called Xenartha, the armadillos, anteaters, and sloths that are confined to South America.
A third group, the Laurasiatheria, is so named because they were first found in Laurasia, the conjoined northern-hemisphere landmass of North America, Europe, and Asia. This groups includes cetartiodactyls, perissodactyls, carnivores, pangolins, bats, and insectivores. Finally, the Euarchontoglires unites rodents, rabbits, tree shrews, flying lemurs, and primates—also with northern-hemisphere origins.
The ancient Greeks had different ideas about the origins of dolphins. In one myth that explains dolphins’ apparent affection for people, the god Dionysus transformed evil men into good dolphins. Dolphins symbolized the benign aspects of the ocean and, in several myths, a dolphin rescues a drowning person. The god Apollo, appearing in the form a dolphin, led a ship out of danger and named the sailors priests at his temple in Delphi (delphis is the Greek word for dolphin). Some South American myths give dolphins the ability to turn into people and help cure the sick; others claim that dolphins inherit the souls of the drowned. While science puts rats and rabbits among our nearer relatives, undoubtedly many of us would prefer the mythical alliance with intelligent, attractive dolphins, an unusual case of water being thicker than blood.
ZooGoer 32(5) 2003. Copyright
2003 Friends of the National Zoo.
All rights reserved.
