Friday, October 31, 2014

Eyes in The Night!

In honor of all things spooky, we've been running a feature called 'Eyes In The Night' over on our facebook page. So far, we've seen raccoon, deer, opossum, bobcat, hare, great horned owls, skunk, great crowned night herons, and bullfrogs. We've watched them forage, hunt, eat, and very occasionally interact with one another under IR light, which they can't see. It's fascinating to see just how many animals are busy after dark!

Animals can be classified as nocturnal (primarily active at night), diurnal (primarily active during the day), cathemeral (sporadically active during the day or night), and crepescular (primarily active at dawn and dusk). Whether nocturnal, cathemeral, or crepescular, animals that are active under low-light conditions may have bigger eyes and ears, darker colors, more distinct vocalizations, and quieter movements than their strictly diurnal counterparts.

Seeing
Opossum at Eaglecrest 
Why do the animals and birds we watch after dark have such large glowing eyes? Because their eyes:
  • Tend to be larger and more protruding than those of diurnal or daytime animals. 
  • Often have pupils that open more widely than those of diurnal animals.
  • Often have slit rather than round pupils. Slit pupils are more efficient at opening and closing quickly.
  • Usually have more rods than cones. Rods work better in low light, detecting motion and basic visual information, while cones work better in bright light, registering detail and detecting color. Some nocturnal animals have no cones at all.
  • May have a special light gathering structure called a tapetum, which amplifies the amount of light that reaches the retina. The tapetum reflects light that has already passed through the retina back through the retina a second time, giving it another chance to strike the light-sensitive rods. The animals we watch can't see IR light, but their widely dilated eyes certainly reflect it!

Hearing
Deer are crepuscular, or active at dawn and dusk.
Their large, moveable pinna give them excellent
directional hearing. 
Why do the animals we watch after dark spend so much time listening? Many nocturnal animals rely on their hearing as much or more than their vision. How are their ears different from those of diurnal animals? They might have:
  • Enlarged pinnae (the outer part of the ear), facial disks, or other features that help them gather and condense sound.
  • Directional location. Think of ears as a microphone. In some animals (bobcats, for example), pinnae can be moved to funnel sound into the inner ear, helping localize the source of sound. Other animals, like owls, have offset ears that provide a precise x/y prey point based on a sound’s location.
  • Inner ear structures that maximize high frequency sounds (like mice rustling in the leaves). This might include denser, shorter auditory hairs, and denser auditory nerves. All the better to hear you with, my dear! 
  • Ultrasonic hearing. Bats are famous for echolocation, but some moths and nocturnal butterflies have ultrasound-sensitive ears on their wings to help them escape. 

Smelling 
Opossum flehvening at Eaglecrest
Why do the animals we watch spend so much time smelling things? In a low to no-light world, scent conveys important information about potential mates, rivals, predators, and dinner. Nocturnal animals:
  • Often have a larger olfactory epithelium, which improves their sense of smell. The sense of smell is most highly developed in nocturnal mammals, although some nocturnal birds also have a sense of smell. An improved sense of smell helps nocturnal animals find food and avoid becoming dinner under low-light to no-light conditions.
  • Often communicate via scent marking and smell. Scents intimidate rivals, attract mates, convey information about health, and provide information about numbers and types of animals in any given place. 

Touching
Nocturnal animals are very sensitive to touch. Many of them have special adaptations to help maximize touch in low-light or no-light situations, including:
  • Vibrissae, or whiskers that serve as tactile organs. While many animals have vibrissae, they tend to be more prominent on nocturnal animals or animals that hunt in low light. Vibrissae may be found on an animal’s face, forelegs, or feet. 
  • Sensory maps that maximize the input from vibrissae, antennae, and other tactile sensors. A study by biologist Dennis O’Leary found that a mouse’s sensory map is dominated by clusters of neurons that process whisker signals. A cluster for one whisker is bigger that the cluster of neurons dedicated to the mouse’s entire foot. 
  • A higher percentage of the cerebral cortex dedicated to touch. For example, almost two-thirds of the area responsible for sensory perception in a raccoon's brain is specialized to interpret touch. 
In a dark world, special adaptations help animals find food and mates, avoid predators, and defend their territories. I am grateful to have a chance to see them, spooky or not! Every time I step outdoors after dark now, I think about all of the animals that might be active around me.
Did you know...
While we don’t understand all of the ways touch works, image maps called cortical homunculi help map tactility. Check it out:
Links that helped me understand this:


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