The Darwin Diaries #4: The Imps of Darkness

If Darwin and I were to meet, there would be one glaring difference between us. No, not our gender, or nationality, or socioeconomic classes, but rather, our opinion on the marine iguana.

See, for some reason, Darwin absolutely despised these creatures, he referred to them as ‘the imps of darkness’, and called them ‘disgusting’, ‘clumsy’, and ‘stupid’. He even went so far as to physically abuse them by tossing them from the land into the sea to see their response. He claimed that no matter how many times he threw one away from land they would always come back to the shoreline. He concluded that they were urged by a ‘fixed and hereditary instinct that the shore is its place of safety’. He figured these reptiles could be easily out-competed and predated upon in the oceans by large sharks or on land by terrestrial enemies. That, for the most part is true, the marine iguanas make their homes on the lava rocks on the shore where they can easily soak up sunlight and conserve energy. They venture deeper onto land only when laying eggs, and they swim mostly when they’re trying to find food or suitable mates.

Despite Darwin’s hatred, after seeing these guys in action they became my favorite part of the islands. Not only was Darwin wrong about these guys, he completely discounted their importance in supporting his postulations on evolution by natural selection. The marine iguanas found on the Galapagos are the only aquatic lizards in the world, their ability to dive up to 98 feet in search of food for over an hour is the product of millions of years of adaptation to the very specific island environments.

For Darwin’s sake, they’ve even evolved a nasal gland which filters salt from the iguanas circulation system that was accidentally ingested while diving! Sea turtles have a similar adaptation, but they excrete the excess salt through their tear ducts (if you’ve ever heard about turtles crying after getting out of the ocean). I suppose turtle crying is a little cuter than what the iguanas do to expel salt, they basically snot rocket it… and it often lands on their head… I think Darwin’s hatred probably comes from being snotted on when getting too close to these guys.

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They’re also the only (as far as I know) animal with all tri-cuspid teeth, this allows them to efficiently scrape algae off of rocks.

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Darwin also must not have seen these guys during mating season, when the color pops and the head bobbing starts. See, most of the year these guys are dark in color (black or grey) which helps them absorb the sunlight and stay warm in between their dives into the cold ocean. However, much like the sally lightfoot crabs, sex changes everything. During mating season the males start to produce vivid pigment on their bodies. While in the islands I saw every color on them, from deep purples to brilliant golds.

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Not only are their colors attractive, they are also helpful in distinguishing the native individuals from travelers. Every island has their own unique ‘flavor’ of marine iguana, or more scientifically, their own subspecies. In Santa Cruz there’s Amblyrhynchus cristatus ssp. hassi, these guys are fairly dull in color, they’re mostly black with touches of yellows and greens…

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Photo by Bailey Day

The subspecies Amblyrhynchus cristatus ssp. venustissimus are found on the Floreana and Española islands, however, these marine iguanas have their own nickname from the locals, they’re called ‘Christmas iguanas’ as their colors are usually bright red and green.

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I got the majority of my great camera shots while on Isabella island as it has the largest population of marine iguanas, though the Amblyrhynchus cristatus albemarlensis subspecies is regarded as this island’s, I found many individuals that must have traveled from near by islands in search of food during a particularly harsh El Niño year. A. c. albemarlensis individuals are recognized by their gold, orange, and red tones.

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Let’s not forget that there are also land iguanas! These guys are native only to a couple of the islands, for example, back in Darwin’s day Santiago Island was so overrun with these guys that he couldn’t find a place to pitch a small tent. I got to see three individuals at the Charles Darwin Research Station on Santa Cruz island. Unlike the marine iguanas, these guys a yellow all year round, our native guide claimed that this was because they primarily ate native flowers (which are all yellow due to the only pollinators preference for yellow hues). Though there’s no denying these guys love their flora snacks, I haven’t been able to find any research connecting diet to color in iguanas.

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Unfortunately, Darwin didn’t like these guys any better he wrote, “(they are) ugly animals, of a yellowish orange beneath, and of a brownish-red colour above: from their low facial angle they have a singularly stupid appearance”. Well, there goes my snot on Darwin theory, I guess he just really didn’t like reptiles. But look at the similarities between these guys and their hydrophilic brethren, they have much shorter tails and claws and their crests are much less pronounced. If you’re comparing these two types of iguanas and picking out phenotypic differences, then congratulations, you’re an evolutionary biologist! Now begin to postulate why their different habitats have selected for their different traits. For example, the long tails help marine iguanas navigate choppy waters, but for land iguanas they probably only slowed them down when running from predators.

What’s even cooler than looking at the diversity of colors of the Galapagos iguanas is postulating how this diversity occurred. Researchers have hypothesized that the common ancestor between a land iguana and a marine iguana (let’s call this Ancestor A) came from South America to one of the islands presumably swimming or by ‘rafting‘. Once arriving to their new homes the progeny of ‘Ancestor A’ began to diverge into a marine group and a land group, 8 million years of evolution later we now have these two distinct species. Even though, it’s important to know that they can still reproduce fertile offspring between them, this is somewhat common and ‘hybrid iguanas‘ which have the struggles and the benefits of both worlds.

After the divergence from Ancestor A both group continued to diverge further into subspecies, some of these subspecies we’ve already discussed and generally differ based on size and color. This suggests that some colors do better in certain environments than others, for example, perhaps the dully colored Santa Cruz species requires more heat absorption for digestion of certain food items found on that island. Where as, the brightly colored Española iguanas have more energy to devote to reproduction and sexual selection. Land iguanas have also diverged, for example, Conolophus marthae is a pink land iguana only found on Isabella island. Recent genetic testing has actually found that this population (of only 100 individuals) is a completely different species than other land iguanas, suggesting a second divergence 5.7 million years ago.

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It’s been 100+ years after Darwin first met and described these creatures, and their circumstances have changed a lot. Human introduction of livestock/domestic animals and invasive species have caused the populations of iguanas to plummet. Most species described in this blog post are classified as vulnerable and threatened and many conservation programs led by international scientists and locals are attempting to eliminate unnatural and introduced predators and encourage reproduction. Please consider learning about these conservation issues and donating to the cause. I know that even Darwin would love to adopt an iguana.

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