EPOW - Ecology Picture of the Week

A Tale of Two Iguanas

left:  Land Iguana (Conolophus subcristatus), Urbina Bay, Isabela Island
right:  Marine Iguana (Amblyrhynchus c. cristatus), Pta. Espinosa, Fernandina Island
Galapagos Islands, Ecuador

Explanation:  Behold the result of evolution:  two iguanas, one island archipelago, and two very different head shapes ... but why?

We are on the amazing Galapagos Islands in the equatorial eastern Pacific Ocean.  The nearest land is some 600 miles (960 km) to Ecuador in South America, and 684 miles (1,100 km) to Costa Rica in Central America where roam the more familiar Green Iguanas (whose scientific name -- Iguana iguana -- is not difficult to remember) of the tropical jungles.  

But here in the Galapagos roam not Green Iguanas, but rather a number of other lizards, including the Land Iguana and the amazing Marine Iguana.  Both are endemic to these islands, found nowhere else.  Both likely evolved from some initial species (perhaps Green Iguanas) that rafted over from the mainland in some unlikely incident long ago.  

But why do the Land and Marine Iguanas have such different head shapes?

It all has to do with where they live, especially how and where they feed, and what they feed on.  

Land Iguanas are strictly terrestrial and feed mostly on cactus fruits as well as some other plants and some insects (the young feed mostly on insects and other arthropods).  For such a diet (not dissimilar to that of Green Iguanas, by the way), the long snout that their ancestors likely sported serves them well enough to be retained through this evolutionary time.  

But Marine Iguanas -- the only sea-swimming iguana in the world -- mostly inhabits the islands' rocky coastlines where they feed almost solely on red and green algae growing on exposed rocks and also on those submerged in the intertidal zone.  Their snout has evolved to be short and blunt, allowing the lizards to better scrape the algae from the rock surfaces.   
  

Check out the following screen captures of a Marine Iguana
scraping algae from a rock surface along the shoreline
(from a video I took):

Closeup of a Marine Iguana -- note the very blunt face as well
as the unusually large upper and lower labial ("lip") scales that
provide greater strength than those of the Land Iguana.
Perhaps, too, the horned scales on the forehead (not present in the 
Land Iguana) may provide some degree of protection as the lizard 
rubs its head against the rock surface while feeding ...
and might also play another role in their territorial head-pushing behavior.