THE BREEDING HABITS OF ANIMALS 



201 



o/,..._ 



brd. 



around the hind logs of the male, or arc held in the vocal sacs. One frog, 

 the marsupial frog (Fig. 154), has a pouch formed of a fold of the skin 

 on the back in which the eggs are carried. This habit is again found in 

 the pipe-fish and sea horse (Fig. 155) which carry the eggs in a ventral 

 pouch. Eggs thus carried in pouches may perhaps receive oxygen from 

 the parent, but little is known on this subject. Either the male or female 

 may carry the eggs, but usually only one sex does this in any given species. 



Care of the Young After Birth or Hatching : Birth Stages. — After birth 

 in viviparous forms and after hatching in oviparous species the young 

 may or may not require protection and assistance in getting food. This 

 is partly dependent upon the stage of development which the offspring 

 has attained at the time of birth, but not 

 entirely so. 



The animal may leave the egg complete in 

 all its parts and needing only the growth of the 

 body and the maturity of the sex cells to attain 

 the climax of. its development. Among these 

 forms the young may 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 de- 

 vices 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 pro- 

 tect their young for a period of days or weeks; 

 and mammals care for their offspring for weeks 

 or years. How long the young must receive aid 

 depends on how far they have developed before 

 birth. There are considerable 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. 156) until they are well formed; mice 

 are born blind, hairless and very helpless; rabbits are born blind but 

 (hovered 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. 157), the 

 former usually, although not always, born blind and practically without 

 feathers, thus requiring longer parental care, the latter covered with 

 down and with the eyes open, requiring shorter care. The common 

 song birds are all altricial, while domestic fowls, partridges, most wading 

 birds, and the various ducks are precocial. 



There are also animals which escape from the egg so early that they 

 lack important organs and must undergo extensive changes to attain 



Fig. 155. — Hippo- 

 campus, the sea horse, male 

 specimen showing brood 

 pouch. The male of this 

 species of fish carries the 

 eggs in the pouch until they 

 hatch. br. ap, branchial 

 aperture; brd. p, brood 

 pouch; df. dorsal fin; op, 

 opening of brood pouch; 

 pet. /, pectoral fin. 



