Owls and eye-shine

Subject: Owls and eye-shine
From: Andrew Taylor <>
Date: Mon, 17 Sep 2007 14:03:26 +1000
To add a bit, when vertebrates have strong eye-shine, its said
to be produced by a reflective layer, the tapetum lucidum, behind
the photoreceptive layer of the retina.  This layer improves visual
sensitivity by reflecting light back and providing a second chance for
it to be detected.  This layer takes various forms in the vertebrates,
probably with multiple origins, but is also absent from many vertebrates
including humans and most birds.

The scattering as light is reflected back by this layer is thought to
reduce visual acuity.  So if humans had a tapetum lucidum as well having
much stronger eye shine, we'd find it easier walking around at night
but we'd find it harder to read or see fine detail even in good light.

Presumably as a consequence of this tradeoff between acuity & sensitivity,
almost all verebrates with tapetum lucidum have no fovea.  The fovea is
a part of the retina whose function seems to be sharper central vision.
Almost all birds have fovea and many raptor retinas have multiple fovea.

Southern boobooks haven't made this tradeoff. They have fovea and
no tapetum.  Any eye-shine should just be the retina.

The birds without fovea are mainly nocturnal. Of these only some
caprimulgiformes are known to have a tapetum - causing presumably 
the huge eye-shine in some species.  No one seems to have dissected
owlet-nightjar eyes but  the absence of eye shine presumably
implies no tapetum.

Tapetums are also found in inverts, producing the bright blue eye-shine
of wolf spiders visible at 100+m and the lovely iridesence of prawn eyes.


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