ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



143 



which was provided to them in two to four allotments per day. Since they were 

 not fed ad libitum, there is some justification for doubt that their rate of intake 

 was as high as it might have been under less restrictive conditions. From my 

 experiences in the first weeks of rearing seven of those calves, and from 

 subsequent, detailed reports on their treatment (D. H. Brown, G. H. Pournelle, 

 and G. C. Ray, personal communication), however, I am confident that the 

 volume of milk consumed in any 24-h period was governed more by the calves 

 than by their keepers. The calves often were unable or unwilling to accept all of 

 the milk that was made available to them. Because the rate of intake of food by 

 mammals tends to be a function of their nutritional requirements, rather than of 

 the capacity of their digestive tract (Kleiber, 1961:284-285), I assume that the 

 amounts ingested usually were governed by the individual's needs. 



The artificial milk that each animal received was mainly a mixture of heavy 

 cream, minced clams or fishes, and fresh water, in proportions of about 4:5:3, 

 with supplementary vitamins. Although the components were somewhat 

 different in each instance, the nutritive qualities were about the same, judging 

 from the comparable growth of the animals. The gross composition of the 

 artificial milk was different from that of natural walrus milk, having about 60 % 

 less fat, 30 % more water, and more than 100 times more carbohydrate, but a 

 similar protein content (Table 21). The gross energy value of this mixture per 

 unit volume was about 50 % that of natural walrus milk. 



Table 21. Approximate gross composition and energy value of artificial marine 

 mammal milks on which walrus calves were reared in captivity. 



Percentage composition** 



Energy value 



Location 



Water 



Fat 



Protein 



Sugars 



(kcal/L)b 



Marineland of Pacific 



73.4 



12.7 



10.3 



2.4 



1,860 



New York Aquarium 



78.7 



10.6 



6.6 



2.8 



1,480 



'iBased on formulae for liquid diets provided by Brown and Asper (1966) and G. C. Ray 

 (personal communication) and on the composition of their constituents, as given by Spec- 

 tor (1956:190). 



^Based on the energy values of milk constituents, as given by Kleiber (1961:312). 



The rate of intake of artificial milk was about 3 to 6 L per day in the first 

 month, increasing to 13 to 15 L per day in the eighth month (Table 22). Ex- 

 pressed as a function of total body weight (TBW), this amounted to an intake by 

 weight (17 observations) of 6.5 to 9.3% TBW per day (mean, 9.2%), which was 

 a large amount, but about 75% of it was water. The gross energy value of this 

 diet amounted to about 135 kcal/kg TBW per day (60 kcal/lb per day), or about 

 20 % more than the daily gross energy requirement of human infants on a diet of 

 mother's milk (50 kcal/lb per day: Ebbs 1966). Expressed in terms of metabolic 

 body weight (MBW = TBW kg3/4: Kleiber 1961:209), the walrus calves 

 consumed about 250 to 610 kcal/kg3/4 per day (mean, 423 kcal), which was 

 about 20% higher than the maximum rate of intake by domestic animals (253 to 

 452 kcal/kg3/4 per day; mean, 351 kcal: Kleiber 1961:319). These differences 



