AQUATIC MAMMALS 



As previously mentioned, it is significant that the capybara is the 

 only aquatic rodent without a tail and the only one in which the hind 

 feet are not indicated as the primary means of propulsion. 



LAGOMORPHA 



Leporidae 



Sylvilagus palustris and S. aquatkus. The marsh and swamp rabbits 

 of the southern United States are quite aquatic, readily taking to the 

 water when dismrbed and swimming freely. They have no discernible 

 aquatic modifications, however, and the hind feet are even smaller 

 and narrower than in the majority of rabbits. It is not meant to im- 

 ply that their swimming propensities have resulted in a reduction of 

 foot size, but it is not unlikely that they are descended from some 

 stock of small-footed running rabbits (as is Romerolagus) rather than 

 of hopping rabbits, like most other sorts. 



UNGULATA 



Artiodactyla: Hippopotamidae 



This family is the most aquatic of any of the existing ungulates. 

 The animals usually spend the day either in the water or basking upon 

 some segregated sandbar, repairing to the land at night to feed along 

 the banks, and sometimes foraging to considerable distances or travel- 

 ing overland from one river to another. 



Hippopotamus — the common hippopotamus of Africa is highly 

 aquatic in habits. It evidently swims with all four feet but these are 

 not especially modified for the purpose. External specialization for an 

 aquatic life consists of the elevation of the nostrils and the peculiar 

 orbits, which allow the eyes as well as the nares to be kept above 

 water while the remainder of the animal is submerged. The shortness 

 of the legs may, to some extent, also be due to this influence. The 

 short tail is exceedingly flattened laterally, but it is too small to as- 

 sist in swimming and the reason for its shape is not clear. 



Choeropsis — the pigmy hippopotamus of parts of west Africa is much 

 smaller than its cousin and its head is much less modified in the direc- 

 tion which the other genus has taken, the orbits not being excessively 

 elevated. 



The hippopotamus is related to the pigs and remains that are hardly 

 distinguishable from the existing species have been found in the Euro- 

 pean Pliocene. The wonder is that this animal is not considerably more 



[38] 



