ECOLOGY OF ISLE ROYALE. 329 



THE COLD-BLOODED VERTEBRATES OF ISLE ROYALE. 



DR. ALEXANDER G. RUTHVEN. 



The collection wliicli has served as the basis for this report was made 

 by the University of Michigan Museum expedition to Isle Royale, in 

 the summer of 190.5. The report should be considered as supplementary 

 to the papers upon the fish, amphibians and reptiles of the island, pub- 

 lished in 1905. (Ruthven, 1905, pp. 107-112.) This, the second expedi- 

 tion to Isle Royale, has added a number of species to the fauna, and 

 has est.iblished the fact that most of the previously known forms ex- 

 tend throughout the entire length of the island, which was, of course, 

 to be expected. 



The amoiint of data on this fauna accumulated by the two expedi- 

 tions is considerable, when it is considered that up to 1904 practically 

 nothing was known of the cold-blooded vertebrates of the island. Our 

 knowledge, however, is still very incomplete. In the case of the 

 fishes this is due to the fact that no systematic attempt was made by 

 the field parties to secure these forms, and the specimens obtained are, 

 in most instances, tliose that came most easily to hand. The list is, 

 therefore, undoubtedly very incomplete both as regards the number of 

 species and their distribution. On the other hand, particular attention 

 was paid to the amphibians and reptiles, and, although there is still 

 much to be discovered concerning the local distribution of the species, 

 the complete list includes nearly all of the species which would be ex- 

 pected to occur on the island. 



Xature of the faunn. — The cold-blooded vertebrate fauna of Isle 

 Royale, as at present known, consists of eighteen fish (exclusive of 

 TrifilOfsis thompsoni. Avhich was taken in deeper waters of Lake Su- 

 l)erior), one toad, one tree toad, three frogs, the nuid puppy, and two 

 snakes.* 



Affinities of the fauna. — Adams, on a previous page, has dwelt at length 

 on the, fact that Isle Royale has never been connected with the main land 

 since glacial times, a fact that is of first importance in discussing the 

 origin of the fauna. ;Most of the fish obtained on the island occur both in 

 the inland waters and in the bays and coves about the shores. Since they 

 are, moreover, forms of general distribution in the Great Lakes drainage 

 system, occurring also in Lake Superior, their presence on Isle Royale 

 is easily explained. To account for the presence of the inland, brook- 

 dwelling forms, however, another explanation must be sought; for such 

 species as the common stickleback, nine-spined stickleback, black-head 

 minnow and Leuciscus ncogaetis can hardly be conceived as able to 

 cross the fifteen miles of open lake intervening between the island and- 

 the nearest mainland. At present we have no data that throw light 

 on this problem. 



The same difficulties arise in attempting to account for the origin of 



* As elsewhere stated (Ruthven, 1905, pp. 109-112) Hyla versicolor a,nii ThamnophU sauritus have 

 been recorded from Isle Royale, but the records cannot be verified. 



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