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From: Henry Spencer <>
Subject: Re: Martian Light
Date: Mon, 26 Jan 1998 17:19:24 GMT

In article <>,
Craig Bingman <> wrote:
>...Other vertebrates may have a
>visual sensorium substantially different than humans.  Predators (like
>dogs and cats) may be especially sensitive to motion.  A few years ago
>there was a nature paper on some simple signal processing that can be
>done to make things that move "stand out" from a static background.

If memory serves, rabbits are wired to detect motion as slow as that of
the Sun across the sky.  (Humans can *almost* see that as motion, but
not quite.)

>My cats seem to be able to navigate just fine in daylight, but they
>definitely do better at low light levels than I do.  Part of the reason
>for that might lie in the properties of the adaptive molecules in their
>retinas, and part of it may be just mechanical.  Their iris can go from a
>tiny slit to a wide-open light bucket...

The illumination difference between near-total darkness and bright
sunlight is much too large for any sort of mechanical iris to deal with it
effectively.  The bulk of the difference between the two, in humans or in
cats, is handled by changes in sensitivity in the retina.  (And that's why
human light adaptation can take a few seconds and full dark adaptation can
take quite a while -- the retinal changes are slow.)

>Most of the adaptations of humans that enable us to go from the Actic to
>the tropics are cultural and technological.  Devprived of those
>accumulated and shared products of our minds, we are really rather sad

Not really.  It's just that our physical specialties aren't as conspicuous
as those of some other animals.

> ...  We have a low range of thermal tolerance because of lack of fur...

Our tolerance for *cold* is poor, but the combination of lack of fur and
presence of very large numbers of sweat glands gives us outstanding heat
tolerance.  We do better on sustained exertion in hot weather than any
other animal.  It has even been suggested that our large brains got
started to provide extensive redundancy of brain function for sustained
operation under severe heat stress (which brain cells do not handle well).

>a back that has never quite gotten up to speed with upright posture and

Our adaptation to upright posture is definitely incomplete.  Another
example of this is that we are much more prone to respiratory infections
than most animals, because the lining of the air passages evolved to drain
into the nose and outward, but in an upright creature, most of it has to
drain inward (downward!) into the lungs instead.

However, the penalties of incomplete adaptation to upright posture have
to be balanced against the rewards, such as manual dexterity (the hands
don't have to double as feet any more).

>...a really pathetic top running speed...

Really fast runners are specialists.  We specialize differently, for
endurance.  Running the Hawaiian Marathon would kill almost any other
animal; humans do it for fun.  There are primitive tribes that still hunt
large fast animals by simply chasing them, hour after hour, until they
collapse, and this method was probably much more common before effective
projectile weapons were developed.

>...much less brute strength
>than our simian relatives (that issue is largely mechanical and has to do
>with the leverage of muscle insertions relative to the position of joints)

Hot-weather endurance runners are thin and stringy for quite fundamental
reasons, which does limit our abilities in other specialty areas.

>Biochemically, humans are about as sad.  The only non-primate animal that
>is defective in vitamin C synthesis is the guinea pig, as far as I know.

Doing such biochemical comparisons fairly is difficult, not least because
most of the work on identifying crucial nutrients has been human-centered.
It's still quite possible that, say, dogs need unidentified vitamins that
we don't.

In fact, we don't understand human vitamin deficiencies as well as we'd
like to; it's quite possible that there are important fine points which
are still unknown.  For example, certain aspects of scurvy -- vitamin C
deficiency -- are still rather mysterious, and some early attempts to
experiment with it had well-documented results which are frankly puzzling.
(It's no longer a high research priority, and proper experiments are
problematic because scurvy in guinea pigs is not the same as scurvy in

Caffeine kills many non-human mammals; does that make us biochemically
superior in some way?
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

From: Henry Spencer <>
Subject: Re: Martian Light
Date: Sat, 24 Jan 1998 01:00:19 GMT

In article <6ab40p$>,
Dwayne Allen Day <> wrote:
>Is the overall range of the human eye the best available for any animal?
>I know that many animals have specialty eyesight--better infrared, higher
>accuity (for birds), etc.  But usually they pay for this in some way like
>color blindness.  Is the human eye generally the most versatile?

It's at least close; we don't know all the details of animal vision in
general, nor do we understand all the tradeoffs.

Another example of superiority with a price is that cats really do see
better in dim light than humans, because they've got a reflective layer
behind their retina, which bounces the light back for a second chance at
hitting one of the light-sensitive cells.  The price for that is inability
to focus as well as we can, because the light takes two different paths
and you can optimize the focus for only one path length.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

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