HIGHER CHORDATES 



295 



nothing stop them. Many perish from drowning 

 when attempting to swim lakes, from predation, from 

 disease, and other causes. The last lemming popula- 

 tion remnants may reach the sea, swim out, and soon 

 drown. 



Migration, a homing behavior, does not present 

 so confusing a picture. It is the seasonal regular 

 departure from and return to a region. This phe- 

 nomenon is most marked in birds, but is also found 

 in some mammals, amphibians, fish, and inverte- 

 brates. The incentives for many migrations seem to 

 be to obtain food and water, to avoid unfavorable 

 climate, and to reach the breeding grounds. In con- 

 sidering a few mammalian examples, one sees that 

 migration can be extremely localized or can involve 

 great distances. 



The California gray whale spends early winter to 

 late spring along the California coast where the young 

 are born. All whales then move northward to con- 

 gregate in the Arctic Ocean and Okhotsk Sea. Later, 

 cool weather and reproduction cause their return to 

 the California coast. 



All fur seals except the old bulls winter as far south 

 as California — the old bulls winter either south of the 

 Aleutian Islands or in the Gulf of Alaska. As the 

 breeding season approaches, the old males move to 

 the Pribilof Islands breeding grounds, where the bulls 

 obtain a territory and await the females. Shortly 

 thereafter the females complete their 3000 mile trip 

 to the Pribilofs and within a few hours or days their 

 pups are born. The females then are impregnated 

 and spend much of their time in the sea, returning 

 every two days to nurse their pups. During the entire 

 three-month period until early August, the breeding 

 males do not feed; they are too busy defending their 

 harem and breeding ground against the "stag line" of 

 nonbreeding males. In early August the males return 

 to their wintering grounds, but the females remain 

 until early November when their young of that year 

 can accompany them to the female wintering 

 grounds. 



The hoofed animals display less remarkable mi- 

 grations. The caribou show a circular counterclock- 

 wise movement in the winter and strike north in the 

 summer. Their movement is then somewhat erratic 

 before they return to the wintering grounds. Elk and 

 our deer summer in high mountain valleys and usu- 

 ally move in the winter to the climatically less severe 

 lowland areas where food is more abundant. Even 



the bison showed a migration pattern, one seemingly 

 related to available food. 



Local migrations are shown by the Norway, or 

 brown, rat and certain meadow mice. Some brown 

 rats spend the spring breeding period in fields, mead- 

 ows, or ditches and with the coming of winter move 

 into human habitations. Meadow mice spend the 

 rainy season in fields and the summer drought period 

 in meadows and other damp places. 



In addition to the above conspicuous movements, 

 there are occasional periodic movements on the part 

 of many mammals. Most of these movements can be 

 related to reproduction, climate, overpopulation, 

 and/or available food. 



ADAPTATIONS 



Mammals have many adaptations to their environ- 

 ment. Many of these adaptations are independent 

 of ancestry or relationship of the animals involved; 

 these features are a response to ecological factors. 

 For example, in Australia, the marsupials, pouched 

 mammals, have structures similar to those of our 

 placental mammals. Adaptations of Australian 

 marsupials are indicated by carnivorous, herbivorous, 

 burrowing, jumping, and gliding forms. Also, two 

 distantly related groups comprise the whales. In 

 spite of similar adaptations in form, due to their en- 

 vironment, the toothed and baleen whales are be- 

 lieved to have separate origins. Therefore, in the 

 following discussion of mammalian ways of life 

 neither ancestry nor relationships are considered. 



Aquatic mammals have certain features that are 

 related to their mode of life. Heat loss is reduced 

 either by a heavy layer of fat called blubber, or a 

 heavy coat of fur. Swimming modifications are re- 

 lated to the form of the animals. The fish-like whales 

 use body and tail undulations and the limbed mam- 

 mals usually depend on the limbs. Most truly aquatic 

 animals have a streamlined, often fish-like, shape. 

 Also, in direct proportion to the aquatic nature of 

 certain animals, there is a reduction of hair, increase 

 in blubber, and increase in size. 



Extreme aquatic specialization is seen in whales, 

 which like all aquatic mammals have a land mammal 

 ancestry. Whales have near perfect adaptation to a 

 watery environment. They are streamlined; the neck 

 is shortened; the hindlimbs absent, forelimbs paddle- 

 like, external structures are reduced or absent; the 

 tail is a propulsion organ; the single nostril is high on 



