Centre for Excellence in Basic Sciences, Mumbai
Imagine the era when eyes were yet to be evolved,.. animals racing for survival in pitch darkness. With the accumulation of sensory receptors at the anterior end of the general body plan, a rough sketch for certain senses could be guessed. Gradually along the evolutionary tree, we could see the emergence of senses that capture visual data and vibrations (sound).
Such senses meant to some animals as a better tool to target and, for others, as a tool to spot (the predator). Animals could now precisely analyze their surroundings and thrive better.
However,.. what if an animal was no longer able to see? Having eyes is no good, if the animal is not able to resolve the processed image.
Such a situation for an animal, such as a predatory bird, would surely prove to be devastating. Certain small mammals, which are a usual snack for such birds, also show nocturnal behavior in order to avoid being hunted. In such a case, the predator needs to come up with a better trick, in order to survive.
Who’s got the answer ?
Owls have found an ingenious answer to this.
Owls are famous for performing their night-shifts in the predatory world. In fact, no other bird of prey can hunt in pitch darkness.
So.. why prefer such a difficult time to hunt?
The animal food habit can be studied by drawing certain food webs, for a given ecosystem. Many a times it so happens that two or more species has a liking of a single prey. This gives rise to an inter-specific competition for the meal. To solve the matter, either, the two organisms have to agree on a pact fit for both… which never really happens…, or a smarter move would be to shift to a night-time hunt.
As mentioned earlier, some small mammals prefer being active at night. Hence, there is an ample supply of food available at night (for the owl).
The Night Vision
Owls have characteristically large eyes, which is evident from the fact that a Tawny owl’s eye socket occupies nearly 70% of their skull. Human eye socket occupies only 5% of the skull volume. This allows the bird to gather as much light as possible. Thus, even a faint source of light is trapped enabling scotopic vision.
Image source: skullsite.com
The retina of their eye have a relatively higher density of rod cells. Rod cells are photoreceptor cells that enable perception in dim light. These are a extremely sensitive, even to a single photon of light. Thus, having a higher rod cell presence helps the owl to make a brighter image of what it sees.
Owls are also known to have an excellent memory. They create a mind map of their territory, and so can easily fly through an area of pitch darkness.
These adaptations explain how the nocturnal bird is able to hunt.
Now lets get back to the question asked earlier. How can it hunt, what it doesn’t see??
Owls don’t have ear lobes to collect sound from the surroundings and channel it to the ear drum. In fact their ears are present beneath the feather layer at the side of their face.
The face of an owl is shaped such that it acts as a giant satellite dish, which collects vibration from all around it. Now, to channel the sound waves towards the hidden ears.. they have a ring of stiff feathers around their face.
Now,.. a bigger dish would just allow me to capture more vibrations. It increases sensitivity if the ear by collecting a wide frequency of sound waves… but still, it isn’t something exceptional. So.. what is it that makes the bird target its prey, even if it is beneath a layer of thick ice?
Image source: cdn.birdwatchingdaily.com
Certain owl species have one of their ear openings a bit higher than the other. This makes it possible for the owl to pinpoint the precise location of its target.
Thus, a sound coming from below would hit the lower ear opening earlier than the other. This allows the bird to judge that the object is beneath it. The bird is even capable to estimate, how deep is the prey beneath the surface.
Now, to be sure of the spot, an owl readily changes the orientation of its head.
order to do so, you would have to turn your neck to an extent that you’d eventually hear a cracking sound down your jaw. And that, would be the last one you would hear.
Most animals can turn their heads to a certain angle, beyond which if they tried, they would end up blocking the blood supply to their brain.. which would lead to a tragic death.
Image source: www.ctvnews.ca
Owls have an ability to turn their heads to an extraordinary angle of 270°. To prevent passing out, owl neck vertebrae have ample spaces around the blood vessels. This prevents them from crushing.
Now, what if the “desperation” to locate the prey, in order to settle the urge to eat lead the owl to cross certain limits..? Guess what.. they have a backup planned.
Owls have enlarged segments of arteries below their jaws which acts as a reservoir of blood. Thus, even if the artery is blocked for a moment.. there would be a constant supply of blood to the brain. This level of flexibility allows the owl to focus its senses on the target.
Animals go a great length to remain in the struggle for existence, and with the rapidly evolving ecosystem.. the game gets even better.
Eagles may be sharp eyed, falcons may be faster and dragonflies may be quicker… but nothing stands a chance when the sun goes down. Owls are the unrivalled assassins, howling fearlessly under the roof of the stars.