Islands of the Three-Eyed
Listening to a group of cruise ship tourists recently, I overheard this comment, “They sure have a lot of armadillos here and they’re all bright green”. Bright green armadillos??? This same woman then screeched and ran behind her companion as a Green Iguana sauntered out of the bush. Ah, my next subject for All At Sea…the unappreciated lizards of the Caribbean!
How many species are there in not only the Caribbean but also the world? No one knows for certain since new species are discovered on a fairly regular basis. More interestingly, known species demonstrate the ability to rapidly evolve based on habitat, leading to the creation of even more new species whose appearance is distinctively different but who share quite similar DNA.
Biologists consider the Caribbean Islands a “hot spot” not only for studying currently known lizard species but also a fertile ground for discovering new species and observing rapid evolutionary changes as new species emerge.
In 2001, a lizard touted as the world’s smallest - adults less than ¾” long and fitting on a US Dime- was discovered on a small island off the coast of the Dominican Republic. In 1965, another species of tiny lizard was discovered in the British Virgin Islands. Just this year, a new species of dwarf gecko, brightly colored and patterned in reds, yellows, and browns, was discovered by an amateur naturalist on a small island in St. Vincent and the Grenadines. People who saw it in the past assumed it was an insect and passed it by, due to its small size and bright coloration and the fact that it lives in leaf litter.
While the diversity of lizards in the Caribbean is almost overwhelming and includes dwarf geckos, anoles, iguanas, skinks, curly tails, and the legless glass lizards (distinguished from snakes by their eyelids and external ear openings), many of them have a very interesting common characteristic…a third eye!
Known as a parietal eye, it is found on top of the lizard’s head, sometimes visible as a grayish spot as in the case of iguanas, or it may be invisible, appearing as another color blended opaque scale. But, it’s there and for those insistent on keeping lizards as pets it should never be mistaken for an injury because it serves a critical function in the life of lizards.
The parietal eye does not see shapes as do the ‘regular’ eyes that, in most species, rotate and move independently of the other, giving the lizards excellent visionary coverage of their surroundings. Instead, the parietal, pineal, or third eye senses only light and dark with its limited retina and nerves connecting directly to the pineal area of the lizard’s brain.
This third eye provides protection for the lizards from flying predators and even human hands as one attempts to swoop in from above and snatch the lizard. The parietal eye’s ability to discern a sudden change from light to dark has saved many a lizard from becoming prey; even when it sleeps, since the parietal eye is always “open”. The third eye also assists in triggering reproductive hormone secretions; signaling to the lizard’s brain that it’s time to mate as daylight changes with the seasons.
Perhaps the most important function of the parietal eye is its ability to send thermoregulatory messages, based on changing light, to the lizard’s brain. Since lizards are reptiles rather than mammals, they are “cold-blooded” meaning they must thermo-regulate by moving from sun to shade, from cooling ground to sun warmed rocks and often pavement where they absorb the stored heat.
In addition to its thermoregulatory messaging, the parietal eye is believed by many scientists to contain a circadian clock, a timer so to speak, that tells the lizard it’s time to sleep, wake, and feed; much like our own circadian clocks located in the parietal area of the human brain.
Islands of the Three-Eyed is a work in progress since future articles will present individual species and some of the characteristics unique to them.
Share Tweet
