4i8 



THE NATURE BOOK 



ming quadruped, who dispenses with 

 the use of his fore-hmbs, will be found 

 to balance the loss of power in one of 

 two ways. Either he uses his tail as a 

 propeller, in which case one or both 

 pairs of limbs act simply as fins (balancers) 

 with an occasional sweep to help him 

 through a quick turn ; or else he employs 

 his hind-limbs frog-fashion, a method 



WATER RAT. 

 The white oily deposit round the eyes can be well seen in this picture. 



which requires a specialisation of form 

 totally different from that of the Water 

 Rat, and which results in a progression 

 by successive jerks. 



One feature of a Water Rat's swimming 

 is that his tail is functionless ; another 

 is the smoothness of his action. He 

 glides across the surface with his back 

 bone-dry and his sides barely awash. 

 Apart from the twin ripples which spread 

 outwards from his muzzle, there is little 

 disturbance of the water. One can liardly 

 avoid the conclusion that his stroke is a 

 smooth dog's-paddle, during which the 

 alternate movements of his four limbs 

 are in parallel perpendicular planes. 



In diving from the surface he arches 

 his back and no doubt strikes with all his 

 feet at once, but the movement is so 

 quickly executed (he seems to drop into 

 a hole) that it is impossible to follow 

 it with certainty. Usually the dive is 



complicated by a turn, and he reappears, 

 if he reappears at all, in quite a different 

 direction from the one in which he was 

 originally heading. I ha\'e written " if 

 he reappears at all " advisedly, for the 

 resighting of a Water Rat who has dived 

 either from the bank or from the surface 

 is a difficult matter. 



In connection with this difficulty I ex- 

 perimented at length, 

 some years ago, with 

 Water Rats in a large 

 tank. My chief object 

 was to determine how 

 long a Water Rat 

 naturally remained 

 under water, and how 

 long he could remain. 

 The tank was so situ- 

 ated that its interior 

 could be viewed from 

 a distance, and that 

 one could approach to 

 the edge of it noise- 

 lessly and unseen. It 

 was tilted to ensure 

 six inches to a foot of 

 water at one end, and 

 a dry nesting place at 

 the other. The bot- 

 tom was of rough con- 

 crete, and the whole 

 interior was open to 

 the sky, so that leaves 

 blew freely into it. The longest time 

 during which any Water Rat remained 

 submerged — I experimented with five 

 — was a minute and a half. The 

 normal time proved to be half a minute. 

 As the Water Rat dived, he in\ariably 

 turned to right or left, and sometimes 

 completely round. Often his di\'e was 

 so suddenly determined on that he was 

 unable to expel much air from his lungs, 

 and, since his swimming area was 

 restricted, he war:- compelled on these 

 occasions to devise some means of keeping 

 himself submerged. This he effected by 

 securing a hold on the bottom with his 

 fore feet, and it not infrequently happened 

 that, while the greater part of him was 

 thus anchored, his feet and tail lay 

 astraddle on the surface. He would re- 

 main in this ostrich-like position for a 

 considerable time, evidently unconscious 

 of anything being amiss. When the neces- 



