Friday, August 09, 2013

Discovery in the Night Garden

Even at night, the yard is filled with biodiversity to be discovered. This common orb weaver spider was getting an early jump on Halloween and building a foot-wide web along the stairway. This wasn't a chance event. This spider specifically was building its web across an area busy with insects attracted to our porch light.

There was another more unexpected visitor at dusk–a young raccoon.

Its glowing eyes prompted us to call it "zombie coon."

The flash from my friend's camera reflected perfectly off of the raccoon's tapetum lucidum. This is an adaptation that allows night-time creatures to see well in the dark. Light that did not directly strike the retina is reflected off of a substance in the back or along the side wall of the eyeball so that it has a second chance to pass through the retina. This allows the greatest amount of available light to reach the optic nerve for vision. 

Tapetum come in a variety of shapes, are located in a variety of places in the eye and are made of a variety of reflective substances. The amazing thing is that this adaptation has evolved in many different nocturnal animals. Alligators and crocodiles, canines (dogs, wolves, coyotes), felines (cats, big and small), ungulates (prey animals like cattle, antelope, deer), rodents and rabbits, sharks, owls and dolphins all have tapedum. 

As members of Order Carnivora, we think of dogs and cats as being closely related to each other. What fascinates me is that the tapetum in a cat (Tabby or African lion) is formed by riboflavin in the tissue along the sides of the inner eyeball, while a dog's tapetum (terrier or wolf) is formed by zinc crystals in the back of the eye. This is a wonderful example of evolution in these two different branches of Order Carnivora.

Raccoons are in the Suborder Caniformia with canines (the same evolutionary branch) and therefore what you see in the photo is the white light of the flash reflecting off the zinc crystals at the back of the eye and streaming back through the pupil. Amazing isn't it? 

In another example of evolution, the tapetum in members of Order Artiodactyla (hippos, deer, antelope and bovines) is formed by collagen in the back of the eye. Whales and dolphins have a tapetum with the same collagen structure. Fossil evidence and DNA have now demonstrated the close evolutionary relationship between whales and hippos.

Most primates, including humans, are one of the few groups of mammals that do not have tapetum. Without fire, or man-made light, humans do not see well in the dark.

Discovery is all around you!

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