477 



CHEIROPTERA 



The bats show many primitive features which relate them to 

 the insectivores, such as the possession of five digits, small 

 cerebral hemispheres, and undescended testes, and often of a 

 duplex uterus. Their command of warm blood is imperfect, for 

 their temperature falls to that of their surroundings whenever 

 they are inactive. On waking, they restore their warm-blooded 

 state by muscular activity. Their chief specialisation is obviously 

 the possession of the power of true flight, in which they are unique 

 amongst mammals. Their wing is a patagium or fold of skin 

 stretched from the second finger to the tail and supported chiefly 

 by the radius, the second to fifth metacarpals, and the tibia. 

 There are many specialisations in the skeleton ; not only is the 

 hand enormously enlarged, but the ulna is reduced and fused 

 to the radius, the clavicle is large, and the sternum bears a 

 keel, although this is small compared with that of birds. The pelvic 

 girdle has long ilia and is attached to a long, strong sacrum, 

 but there is only a loose ventral symphysis, or none at all ; there 

 is little movement between the vertebrae, and the ribs are flattened 

 and form a rigid thorax. It will be noted that although the 

 structure of the wing skeleton is very different in the birds and 

 in the bats (Fig. 552), the other skeletal pecuharities are very 

 similar, and look as if they were either produced by, or were 

 a necessary condition for, successful flight. Other features of 

 obvious importance in flight are the large heart and lungs. 

 The first digit of the fore-Hmb and all those of the leg bear claws, 

 by means of which the animal can hang ; it spends most of 

 the day hanging upside down by its toes, and hibernates in the 

 same position. The bats share with a number of other mammals a 

 pecuHar separation of the season of sexual activity and birth, 

 for which no satisfactory explanation in terms of function is 

 known. In the bats copulation occurs in the autumn, but the 

 sperms are stored within the female until spring, when fertilisa- 

 tion occurs. Another pecuHarity is the great sensitivity of their 

 hearing and the use to which this is put. When flying, they avoid 

 obstacles— and they are notoriously good at doing this—by 

 making high-pitched squeaks and picking up the echoes which 

 are returned from sohd objects. As they can pick their way 

 rapidly between quite comphcated sets of wires their co-ordination 

 must be both rapid and good. 



