g2U ANIMALS AND THEIR ENVIRONMENT 



The extent to which the taxonomic schemes of parasites and their 

 hosts agree can be used as an indication of the age of the association 

 between parasites and hosts. The conclusion that ancestral camels were 

 infested with ancestors of Microthoracius, together with the age of 

 camel fossils, indicates that this association has existed for at least 30 

 million years. The Australian fauna was isolated about 75 million 

 years ago, and the Australian marsupials were separated from their 

 relatives in America. The tapeworms of these two marsupial groups 

 are similar, suggesting that tapeworms were parasitizing them before 

 their separation. On the other hand, their internal trematodes are not 

 similar and it is concluded that these parasites have infected marsupials 

 for less than 75 million years. The same conclusion is reached for the 

 sucking lice, which are found on all the American marsupials but 

 which are entirely absent from Australia. 



Concordance in the evolution of parasites and their hosts is often 

 marred, however, by occasional "jumps" to new hosts. Most of the 

 species of sucking lice in the genus Linognathus are found on ungulates, 

 and the genus is believed to have evolved with this mammalian group. 

 One species, however, is found on the fox and dog. This does not sug- 

 gest that the latter evolved from ungulates, but rather that the lice 

 established a new beach-head on the predators of their usual hosts. 

 These changes are often associated with ecologic relationships. One 

 species of a genus of rabbit fleas is a parasite of birds that nest in 

 rabbit holes. The relationship is less obvious in the case of malarial 

 parasites of the genus Plosinodium. Some species are foiuid in man and 

 a few other primates, while other species are found in several different 

 groups of birds. All of these parasites use mosquitoes as the alternate 

 host, and it is the mosquitoes that provide the ecologic link, sucking 

 the blood of warm-blooded birds as well as that of mammals. Since 

 jumps to new hosts of parasite groups with extreme host specificity 

 are known to occur occasionally, agreement of taxonomic relationship 

 among parasites and their hosts can never be used as absolute proof 

 for the course of evolution implied in the taxonomy. 



368. Social Parasites 



Animal societies may be subjected to a kind of parasitism in which 

 the parasite does not feed on individuals but intrudes itself into the 

 social economy. The American cowbird and European cuckoo are 

 examples of this. These birds lay their eggs in the nests of other 

 species where the involuntary foster parents obligingly feed and care for 

 the young. The rightful nestlings are often smaller and less vigorous 

 than the social parasites and may be crowded out of the nest. These 

 parasites successfully invade the social family life of the host birds. 



Insect societies are invaded by a variety of beetles and wasps that 

 in one way or another become accepted as a part of the colony. Some 

 of these parasites are food-robbers, masquerading as colony members 

 while they actually do nothing but steal food when hungry. Others 

 enter into the trophallaxis of the colony, offering secretions in return 



