xxvi REPRODUCTION 673 



'dancing' has been observed and also homosexual behaviour, in cap- 

 tivity. Many species migrate, for example, the humpbacks (Megaptera) 

 and others spend the summer in the Antarctic feeding on krill and 

 then come north to tropical waters to breed. 



The reproduction shows various modifications for aquatic life. The 

 testes do not descend into sacs, but to a position just below the body 

 surface. The penis is very long, and curled when not erect. The uterus 

 is bicornuate, but only one young is carried and is retained for a long 

 time (more than a year in large whales), so that it is as much as a third 

 the length of the mother at birth. The placenta is diffuse but with a few 

 villosities like cotyledons. Its structure is epitheliochorial and there is 

 a large allantois. There is a pair of teats in the inguinal region and the 

 mammary glands are provided with a special receptacle and muscle 

 so that milk is pumped into the mouth of the young. Some species of 

 dolphins migrate to shallow protected water at the time of parturition. 



It is clear that many factors have collaborated to concentrate the 

 biomass of whale life into large units. Indeed, whales include the 

 largest known animals, either fossil or recent. The blue whale, 

 Balaenoptera musculus (Fig. 438), reaches nearly 150 tons, with a 

 length of 100 ft. This is one of the whalebone whales, which are in 

 general larger than the odontocetes, perhaps because of the immense 

 sources of food directly available in the plankton; they have grown 

 fat by eliminating the 'middle-men' upon which all toothed whales 

 must feed. These mysticetes appeared in the Oligocene and radiated 

 in the Miocene and since into a relatively small number of types, all 

 of large size. Balaena, the right whale of the Arctic, is now very rare. 

 The chief modern prey of the whalers are the fin whales {Balaenoptera 

 physalus) of the Antarctic. 



The odontocete whales are a more varied group; their history can 

 be traced back to the late Eocene. The squalodonts of the Oligocene 

 and Miocene were like the porpoises, but with triangular teeth. Most 

 of them disappeared in the Miocene, but the river-porpoise Platanista 

 of the Ganges, and related forms in the Amazon and in China may be 

 descendants. The modern porpoises (Delphinoidea) are a very success- 

 ful and numerous group of relatively small animals, with a dorsal fin 

 and teeth in both jaws. They are all active predators, but the habits vary 

 from those of the killer-whale Orchitis (Fig. 439), which is a fierce and 

 cunning hunter, attacking even the largest whales, to the omnivorous 

 porpoise Phocaena (Fig. 440), whose food includes Crustacea as well as 

 fishes and cephalopods. This is the commonest and smallest British 

 cetacean, the largest individuals reaching 6 ft; the jaws are rather short, 



