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Re: Island mammals



At 06:02 PM 2/20/97 -0800, you wrote:
>> Jay Freeman wrote:
>> 
>> >...surely there is another issue, one of transport.  Birds -- >quite large
>> birds -- can easily reach any island in the world.  >Non-marine mammals and
>> non-marine reptiles have it tough.  
>
>then why do the Galapagos have larger-than-normal deserty tortoises?  Or
>iguanas which are now marine, but are related to the green tree-climbing
>iguana?  (I'm out on a limb on that one), or elephants-for gosh sakes?!?
>
>
>-Betty

Reptiles have a big advantage over mammals as long-distance dispersers over
water by means such as rafting - because they do not maintain high
metabolic levels they can go for a long time without food.  A lizard or
tortoise would have a far better chance surviving on, say, a floating log
or mass of vegetation for a few weeks than any similar-sized mammal.  This
may explain why reptiles have reached islands (eg Fiji) that terrestrial
mammals have not (though there are endemic rodents in the Galapagos).

Amphibians are a different story because of their permeable skins and low
salt tolerance, though even they have reached the Seychelles, for example.

As for size - the assumption is that the Galapagos (and Indian Ocean) giant
tortoises evolved large size after reaching the islands, though I am not
sure how good the evidence for this is.  Insular gigantism is not at all
uncommon (as is its reverse).
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net