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NORTH AMERICAN FAUNA 68 



similar to other mammals but in other respects it may be unique. 

 Among mammals in general (but with exceptions) total body 

 weight increases at a greater rate than organ weights. It is pre- 

 sumed in the following comparisons that the reader will keep this 

 generalization in mind. 



Liver. — To remain healthy the sea otter must consume about 

 one-sixth to one-fourth of its body weight in food daily. Ap- 

 parently the air blanket in its fur is a less efficient means of 

 insulation in its chilly habitat than the blubber of other marine 

 mammals. The need for a relatively large amount of food in- 

 dicates an unusually high metabolic rate in the sea otter. Slijper 

 (1962) believes that marine mammals, and particularly cetaceans, 

 have a high metabolic rate. Since the liver is an important organ 

 for the production and storage of energy-producing substances, 

 it is not surprising that the sea otter's liver is relatively very 

 large ; the mean is about 5.7 percent of body weight. In this respect 

 it surpasses fur seals of comparable body size (mean 3.1 to 3.5 

 percent; Scheffer, 1960), porpoises (3,2 percent), and dolphins 

 (2.2 percent; Slijper, 1962), and the river otter (4.85 percent; 

 Jensen, letter, 30 Nov. 1964), although the river otter is a smaller 

 animal. 



Heart. — The heart of the sea otter constitutes about 0.66 

 percent of body weight. The ratio is similar to that of fur seals 

 of comparable size (about 0.6 percent of body weight; Scheffer, 

 1960), and to dolphins (0.6 percent; Slijper, 1962). As Jensen 

 (1964) points out, sea water gives more support than fresh water 

 and therefore the sea otter may not need as large a heart as the 

 river otter (0.98 percent of body weight). 



Kidney. — The marine environment may account for a large 

 difference in relative size between the sea otter kidney, mean 2.01 

 percent of body weight, and that of the river otter, 0.85 percent 

 of body weight. The sea otter appears to have overcome the physi- 

 ological problem of the marine environment by developing a 

 lobulate kidney relatively twice as large as that of the river 

 otter. I have observed sea otters in captivity drinking sea water.^ 

 A study of the physiology involved has not been undertaken. That 

 the sea otter obtains liquids of appreciably less salinity than sea 

 water from food is improbable, since body fluids of invertebrates, 



2 The captives were held for 2 months on dry bedding at Amchitka. Frequently when I held 

 a pan of sea water before them they placed their mouths in the water and sucked and lapped 

 it up with their tongues. In their eagerness to drink, they also placed their forepaws in the 

 dish and eventually spilled and splashed so much of the water that I did not measure the quan- 

 tity that was actually drunk. When given a choice of sea or fresh water they drank either 

 unselectively. Except for one experimental offering of fresh water the captives were given only 

 sea water. 



