190 



PRINCIPLES OF ANIMAL BIOLOGY 



receive little or no parental care or they may be fed and cared for for 

 many weeks or even months. Among the reptiles, for example, the 

 young are left to their own devices as soon as they hatch or are born. 

 Most fishes and invertebrates also throw off all parental solicitude after 

 their offspring leave the eggs. Most birds, on the contrary, must feed 

 and protect their young for a period of days or weeks; and mammals care 

 for their offspring for weeks or years. In these cases, how long the 

 young must receive aid depends on how far they develop before birth. 



Fig. 161. — Recently hatched young of the chimney swift, Chaetura pelagica (Linnaeus), 

 left, and spotted sandpiper, Adiius macularia (Linnaeus), right. These are examples, 

 respectively, of altricial and precocial birds. 



There are great differences in birth stages even in the same group. Thus 

 among mammals the marsupials (opossums and kangaroos) give birth 

 to young in a very immature state and carry them in a pouch (Fig. IGO) 

 until they are well formed; mice are born blind, hairless, and very helpless; 

 rabbits are born blind but covered with hair; and guinea pigs are born in 

 such an advanced stage that they are very shortly independent of the 

 mother. Among birds are to be distinguished altricial and precocial 

 forms (Fig. 161), the former usually, although not always, hatched blind 

 and practically without feathers, thus requiring longer parental care; the 

 latter covered with down and with the eyes open, requiring shorter care. 



