142 



NORTH AMERICAN FAL^'A 74 



1941: Amoroso et al. 1951: Rand 1956: Pilson and Kelly 1962; Ashworth et al. 

 1966: Kerry and Messer 1968). The relati\e amount of each fraction \aried 

 among samples, possibh' in relation to the stage of lactation (Br\ den 1968: \'an 

 Horn and Baker 1971). 



The \\ ater content of the walrus milk tended to be some\\-hat greater and the 

 fat content somewhat less than in milk from the other pinnipeds. The protein 

 fraction was comparable in amount to that in milk of the bearded seal but was 30 

 to 50 '^c lower than that of the fur seal. The carboh\-drate content (if an\-) was not 

 detectable b\' the method used on the walrus samples. Absence of lactose in 

 \\ alrus milk was reported earlier by Pilson (1965) and confirmed by Herber and 

 Peterson (in Kerry and Messer 1968). Those imestigators also determined that 

 the enzymes necessary for h\-drolysis of milk disaccharides. such as lactose, are 

 absent or occur onl\- in trace amounts in the intestine of the walrus calf. 



The gross energ\- value of walrus milk, based on the mean composition of 

 samples II. Ill, and I\' of Table 20 and on the standard caloric values for milk 

 constituents (from Kleiber. 1961:312). is about 3.240 kcal L. 



The low fat content of walrus milk ma\' account for the low rate of postnatal 

 weight increase in walrus calves, compared with phocid pups (cf. Laws 1953Z?; 

 Mansfield 195SZ?: McLaren 1958). Nevertheless, its fat content is ver\- high 

 relative to the milk of most terrestrial mammals, and its water and sugar 

 contents are low. These are characteristics of all marine mammal milks, and they 

 seem clearh- adaptive to life in the sea. where fresh water is scarce and there is a 

 high demand for metabolic heat {lr\ing 1968: Depocas et al. 1971). Fat is an 

 efficient source of both heat and water for the calf, for it possesses more than 

 twice the caloric value of lactose per unit volume and liberates large quantities 

 of metabolic water when oxidized (Dukes 1955). Probably the low water and 

 sugar contents are adapti\-e also for the lactating adult, for the synthesis of sugars 

 requires much water, and water conserv ation probably is of critical importance, 

 especially in the first da\-s or weeks after parturition. At that time, the mother 

 seems to do little or no feeding. Of 46 cows with newborn calves that I examined, 

 only 3 (6^c ) had any food in the stomach, and the amount in those 3 was not 

 more than a handful. Conversely. 31 of 89 (35 '~c) other animals taken in the 

 same area, at the same time, had food in the stomach, and in most of them this 

 amounted to se\eral kilograms. 



Rate of Intake of Milk 



The frequency of feeding and \olume of milk consumed by walrus calves 

 under natural conditions have not been determined, but there is a substantial 

 amount of data on this subject from calves reared successfully in captivity-. Those 

 data ma\- be indicati\'e of the feeding rate under natural conditions. I obtained 

 information on milk intake from records of 12 \s'alrus calves that were reared in 

 the Copenhagen Zoological Garden (Re\ entlow 1951). the San Diego Zoological 

 Garden (Pournelle 1961). Marineland of the Pacific (Brown and Asper 1966: 

 D. H. Brown, personal communication), and the New York Aquarium (G. C. 

 Ra\-. personal communication). In each instance, the calves gained weight 

 during the first 4 to 9 months at rates ranging from 0.27 to 0.59 kg per da\' 

 (mean. 0.42 kg per da>"); their bod\' weight doubled in about 6 months. 



Each of these cah'es was reared on a diet of artificial marine mammalian milk. 



