Exploring Our Fluid Earth

Mammals vary greatly in terms of body size and shape. The largest mammal species—and the largest individual organism ever to have lived on Earth—is the blue whale (Balaenoptera musculus; Fig. 6.10). It can grow to 30 meters long and 170 metric tons in weight. Among the smallest mammals are shrews and some bat species, weighing less than five grams and growing to less than five centimeters long.

Fig. 6.10. (A) Blue whale (Balaenoptera musculus)

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

Fig. 6.10. (B) Blue whale (Balaenoptera musculus) skeleton on display at the Long Marine Laboratory, University of California, Santa Cruz

Image courtesy of Bronwen Lea, Wikimedia Commons

Fig. 6.7. (C) Sperm whale tooth

Image courtesy of Ryan Biracree, Flickr

All mammals possess hard bony skeletons composed of calcium phosphate (Ca₁₀(PO₄)₆(OH)₂; Figs. 6.7 C and 6.10). They all possess thick protective skin without scales. All mammals have hair, although the amount of hair varies greatly among the various mammal groups. Cetaceans and sirenians appear almost hairless while the marine carnivorans—pinnipeds, polar bear, sea otter, and marine otter—are covered with thick fur.

Along with the amphibians, reptiles, and birds, all mammals are considered tetrapods (tetra- + -pod meaning “four-legged”), even if they do not possess four limbs. As discussed previously, several marine mammal groups adapted to ocean habitats from terrestrial ancestors. Cetaceans and sirenians share recent evolutionary ancestry with hippopotamuses and elephants, respectively. Both cetaceans and sirenians bear remnants of their evolutionary history in their anatomy.

Image

Image caption

Fig. 6.11. (A) Sperm whale skeleton

Image copyright and source

Image courtesy of National Oceanic and Atmospheric Administration (NOAA) & Richard Lydekker, retrieved from Wikimedia Commons

Image

Image caption

Fig. 6.11. (B) Adult and juvenile manatee skeletons

Image copyright and source

Image courtesy of Sklmsta, Wikimedia Commons

Image

Image caption

Fig. 6.11. (C) Manatee fore limb toenails

Image copyright and source

Image courtesy of Fritz Geller-Grimm, Wikimedia Commons

Fig. 6.4. Generalized phylogenetic tree diagram of extant mammals. Groups containing marine mammal species are indicated by asterisks. Monotremes are shaded red. Marsupials are shaded blue. Placentals are shaded yellow.

Image by David Lin, adapted from Fred Hsu, Wikimedia Commons

Figure 6.11 shows the vestigial structures in cetaceans and sirenians. Both cetaceans and sirenians lack hind limbs. However, all species from both groups possess pelvic bones (Figs. 6.11 A and 6.11 B). In closely-related mammals such as hippos and elephants, these same pelvic bones attach to massive hind limb bones that support standing and walking on land. The remaining non-utilized pelvic bones in whales and manatees are examples of vestigial structures. Vestigial structures are organs or anatomical structures that have been retained during evolution even though they have lost some of their ancestral functions. Another common example of vestigial structures in mammals is found in the tailbone (coccyx) of humans and other ape species. The presence of such vestigial structures among different groups of organisms suggests their common evolutionary ancestry. The toenails on manatee fore limbs (Fig. 6.11 C)—along with molecular phylogenetic evidence—illustrates their shared evolutionary heritage with elephants (Fig. 6.4). Both molecular and fossil evidence has verified that whales, hippos, manatees, elephants, monkeys, and humans all share some common ancient mammalian ancestor (Fig. 6.4).

One major defining characteristic for mammals is lactation, or the ability to produce milk. All mammal species are capable of producing milk to nourish their young and increase their chances of survival (Fig. 6.12 A). The female of the species produces milk from mammary glands. Cetaceans and sirenians are capable of nursing their young underwater.

Fig. 6.12. (A) Galápagos sea lion (Zalophus wollebaeki) mother feeding her pup through lactation, Galápagos Islands

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

Fig. 6.12. (B) Dense hair or fur covering a sea otter

Image courtesy of Daniel H. Friese, Wikimedia Commons

Fig. 6.12. (C) Fur covering a polar bear

Image courtesy of 4028mdk09, Wikimedia Commons

Fig. 6.12. (D) Placentas offer additional protection and nourishment for developing embryos.

Image courtesy of Magnus Manske, Wikimedia Commons

The other defining characteristic of mammals is hair (Figs. 6.12 B and C). Hair is a protein filament that grows from the skin. In non-human mammals, hair is often referred to as fur. All mammals have hair on their bodies although the amount of hair varies greatly between species. Dolphins appear to be hairless but they are actually born with tiny sensory hairs near their mouths, almost like whiskers. Hair can serve a variety of functions in mammals. Dense fur can provide heat insulation and keep mammals warm in cold climates. Nerve receptors at the base of hair follicles can also detect touch and movement. Many marine mammals have sensory hairs on their faces called whiskers that can aid in feeding. Hair coloration patterns can also serve to camouflage mammals for defense or ambush hunting.

Another major innovation found in most mammal groups is the placenta. The placenta is the organ that connects the developing embryo to the mother (Fig. 6.12 D). Approximately 94 percent of all extant mammal species are considered placental mammals.

Fig. 6.13. Internal anatomy of a dolphin

Image courtesy of WikipedianProlific, Wikimedia Commons

Not all mammal species possess placentas, however. Monotremes are a basal branch of mammals known for laying eggs. Only five species of monotremes remain today: four species of echidnas in New Guinea and Australia and the platypus.The other, larger group of non-placental mammals is the marsupials. Marsupials are a diverse group of mammals characterized by their front pouches used to protect their newborn offspring. Examples of marsupial mammals include kangaroos, wallabies, wombats, the koala, and several other smaller-bodied species. Although we typically think of the Australian continent when we think of marsupials, only about 70 percent of the world’s marsupials occur in Australia. Common marsupials found outside Australia include the opossum in North America. Like other mammals, monotremes and marsupials have hair-covered bodies and produce milk to feed their offspring.