SHEDAO PIT VIPERS

      An estimated number of 20,000 venomous Shedao pit vipers, living on the 180-acre Shedao Island, eight miles off the coast of China, have an unusual way of securing prey animals.  Younger, smaller snakes lie still on tree limbs and wait for small, migrant birds to come close.  When birds come close to these vipers, they quickly grab the birds with their mouths and inject poison into the victims with their fangs.  Older, larger vipers lie in wait on the ground for feathery victims that were killed, but too large to be ingested by smaller snakes, and, therefore, dropped to the ground.  That separation of feeding places reduces competition among the snakes for food. 

     Those migrating thousands of birds, the only prey animals on Shedao Island for pit vipers, are going north to Siberia during a couple of weeks in May to nest, and heading south for the winter in September.  Those birds stop on Shedao Island to rest, and feed on invertebrates, making May and September the only times of the year when the snakes can feed.  Theirs' is an extreme adaptation to what prey is available, and when.  The rest of the year they are inactive to conserve energy.  Being cold-blooded reptiles, snakes normally don't expend much energy.  It's been written that one bird will sustain a viper for a year, but each snake probably snares more than one bird per year.  

     It's estimated there is one pit viper in every eleven square feet on Shedao Island, on average.  They probably are descended from a few vipers that somehow managed to get to that island and reproduced over many years.  And Shedao pit vipers live only on that island.  They grow up to 26 inches long, and are camouflaged on tree limbs and the ground; being gray-brown, with darker markings.   

     Many kinds of life have amazing life histories, and Shedao pit vipers certainly are one of them.  But their future hangs from a thread.  If the migrant birds change their paths of migrations because of a lack of food on Shedao Island, the snakes will starve.  Or the island might become flooded.  But for now, it is intriguing to acknowledge how a few individuals of a land species can populate an island to its limit, partly because that species has few predators to keep their populations in check.   

 

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