60 The Nature of Biological Diversity 



the birds, produce not urea but uric acid as an alternative way of de- 

 toxicating ammonia. But from some reptilian groups interesting facts 

 of probable evolutionary significance have emerged (Cragg, Balinsky, 

 and Baldwin, 1961). We know little about the crocodiles and alliga- 

 tors except that ordinarily they live in water and excrete nitrogen 

 mainly in the form of ammonia (Table 7). One would like to know 

 what happens during their overland treks in search of new haunts 

 when the water dries up in the dry season. It should be possible to 

 do "desiccation" experiments similar to those tried on Xenopus. 



Table 7. Nitrogen excretion in crocodilian reptiles 



source: Cragg, Balinsky, and Baldwin (1961). 



It is among the tortoises and turtles — perhaps the most primitive 

 reptilian group — that the most interesting results have emerged, 

 though the number of species used was only eight. Three groups have 

 been studied by Moyle ( 1949 ) , one of which lives an amphibious life 

 and never goes far from the water (Table 8). In these the major end 

 product is urea, inherited presumably from ancestral amphibians. A 

 second group comprises two species that, like Xenopus, have made a 

 secondary and apparently permanent return to fresh water. Here 



Table 8. Nitrogen excretion in chelonian reptiles 



Reptile Habitat 



Per cent of total N as 



Lmmonia Urea Uric acid 



Kinosternon subrubrum Aquatic 24.0 22.9 0.7 



source: Moyle (1949). 



