January 2009



cat whiskers

cat's whiskers aid sense of touch

Did you ever wonder why bird-watching is a major pastime but mammal-watching is not? By some estimates,  about 100 million people watch birds in the United States but almost no one watches mammals. Since we are mammals one might think that we should have a great interest watching other mammals. We certainly "people-watch." But we can only easily watch other mammals in zoos and on safaris in Africa where many species, grazers and browsers as well as baboons, are day-active (diurnal). Most mammals are night-active (nocturnal) while we, along with most of our primate relatives, are diurnal.

So, what are your hypotheses about why we are bird-watchers and not mammal-watchers? Hint: How are we more like most birds than like most mammals?



Because humans are diurnal, they are unlike most mammals which are nocturnal. Compared to nocturnal mammals humans have good color vision, a similar sense of taste, but a very poor sense of smell and touch. On the other hand we are like birds in that we are both diurnal. Compared to birds, we have poorer color vision and a similarly poor sense of smell. Let us now compare the details of vision, touch, taste, and smell among humans, diurnal birds, and nocturnal mammals.



All vertebrates have complex "camera eyes" and a light sensitive layer called the retina with rod and cone light receptors. The density of rods and cones on the retina determines visual acuity. Rods function at low light levels and do not detect color. Cones function only at high light levels and different cones detect different colors from ultraviolet to blue-green, yellow, orange-red. Since cones function in bright light they can not detect colors on a dark moonless night.

Nocturnal mammals and nocturnal birds are similar in that they have mostly black and white vision. Both have only 1 - 3% cones and the rest rods. They have only two kinds of cones so they do not see in the orange-red part of the color spectrum. They have 2 - 3 times the densities of rods as their diurnal relatives. And they have a reflecting layer behind their retina called the tapetum lucidum.

panther's eyes shine at night

panther eye shine at night
The tapetum lucidum is responsible for the eye shine you see if you point a flashlight at or take a flash photo of a nocturnal mammal or bird. The positive tradeoff of the tapetum is that it reflects light through the rods a second time and so increases sensitivity to low light. The negative tradeoff is that it gives a low acuity of vision with a fuzzy image.

Diurnal mammals, like humans, and diurnal birds are similar in that they have predominately color vision. Both have 5 - 10 times as many cones as nocturnal species. In addition they, especially birds, have parts of the retina that have almost all cones and this allows acute color vision. Also, diurnal birds have better color vision than humans because they have four kinds of cones instead of three. The fourth kind of cone allows birds to see in the ultraviolet. Overall, birds' vision is even better than our vision. Birds' eyes are the largest, relative to body size, of any group of vertebrates. Someone has described a bird as two eyes with wings!


Baltimore oriole - male and female

  male & female otter - which is which?
baltimore orioles - male and female

male & female otter - which is which?Many species of male birds have bright colors for courtship and territorial behavior. Even human males and females are easy to tell apart if they are not wearing clothes. Males and females of nocturnal mammals look alike to us and have no bright colors.

So how do they tell each other apart in courtship or territorial disputes?



the nose knows



Humans who become blind can learn to hear and touch to navigate. And we use touch to make love in the dark. However humans are never nearly as good with touch and hearing as nocturnal mammals and we can never learn to smell our way around. When our human ancestors came down from the trees and became diurnal, they evolved an exquisite sense of vision and essentially lost the sense of smell. Anyone who has watched dogs, rabbits, or squirrels knows that their noses "know." Watch how their noses twitch, how they "look" for things, and how they interact with each other. Also notice their long facial whiskers that allow them to feel their way even in complete darkness.




Two of Douglas Florian's poems emphasize the relative importance of vision and smell in birds and mammals.

The Hawk The Bloodhound
I stare
I glare
I gaze
I gawk
With keen
Mean eyes
I am the hawk.
All day I pray
For prey to view.
Be thankful if
I don't
My big nose knows
Where my prey goes.
 Fresh tracks I track
 On ground or snows.
 My stride is wide.
 My ears are full.
 My senses are

what would the brain sensory cortex
of a nocturnal mammal look like?

Some exciting recent research shows that humans have only half as many genes for smell as other mammals such as mice. The largest family of genes is in mice, 1400, of their ~25,000 gene total, is devoted to the sense of smell. Mice can detect about 160 of 200 distinct odors. Humans can detect only 5 of 200 odors. When birds are studied I expect that they will be shown to have even fewer genes for smell.

Most of the human genes that had coded for smells are said to be fossilized. Our smell genes have accumulated mutations so that they are not functional. Because we no longer use them, natural selection has not eliminated these mutant smell genes. It is not really a case of "use it or lose it" because these fossil genes continue to accumulate mutations over evolutionary time. Our nocturnal ancestors used smell as their main sense but humans have become diurnal and have evolved color vision as our main sensory system.


Birds are diurnal and they have easily recognized species-specific calls and songs. Almost all bird species have been described.

In the new world tropics, each year, on average, experts find and describe one new genus and ten new species of mammals. This is at least 10 times more new mammal than new bird species. If experts had as good night vision and sense of smell as most mammals, they could find even more new species of mammals!



Since most mammals are nocturnal, we only know they are present from signs, like scat, tracks, trails, and occasional sounds. It is rare to see mammal tracks but we often see their scat. A kids' camp song (author unknown) makes this point.

            Scat Rap

It begins with an S and ends with a T,
It comes out of you, and it comes out of me.
I know what you're thinking but you can't say that,
Let's be scientific and call it scat!

If you want to find out what an animal eats
Have a close look at what it excretes.
Inside of its scat you'll find many clues.
All kinds of food that the body can't use.
raccoon scat with coco plum seeds   coyote scat with possum fur
raccon scat with coco plum seeds

coyote scat with possum furMammal scat often has recognizable stuff that is not completely digested. Rabbit scat has tiny bits of wood that smell like hay and taste like sawdust. Raccoons often have seeds of coco plum or pond apple in their scat. Otters have bones and scales of fish or exoskeletons of crayfish, and their fresh scat is stinky. And bobcat scat often has bones of mice and their own hair that they have self-groomed.

mouse bones in owl pellet

mouse bones in owl pellet

Even though we see many small birds at the Refuge, like hummingbirds and warblers, we do not see the smallest mammals, like mice, moles, and shrews. We do not even see their scat. Not only are these mammals small but they are also nocturnal and live in burrows. Their tiny scat decomposes quickly and its contents are not recognizable except under a microscope. If it were not for owls that hunt at night, we would not know that at least two species of shrew, two species of mole, and five species of mice live at the Refuge. The bones of these small mammals are easily found and identified in the pellets of undigested parts that the owls spit up.

The last verse of the camp song really refers to mammal scat and not to bird scat. Almost all bird scat looks alike with a dark bit of feces and a white smear of uric acid in the dropping. In birds, as in other reptiles, the urinary and fecal wastes always mix in the cloaca before discharge. Some birds may have tiny seeds in their excreta and color stains from different fruits they eat but with their highly efficient digestion they can generally digest almost everything. However no birds eat grasses or leaves because these are not very digestible. Recall that dabbling ducks and moorhens only eat those few kinds of plants that are easily digestible like submerged and floating plants, especially duckweeds.


This month I explained why bird-watching is easier than mammal-watching. Next month I will explain why plant-watching is even easier. Plants do not run away so teachers can bring even large groups to study plants. After we develop hypotheses about aspects of plant anatomy we can go back to test these hypotheses by more careful examination of the same individuals. We will see that plants' adaptations are just as fascinating and amazing as animals' adaptations.

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a. Humans do not use ~95% of known smell genes and so we have lost them over evolutionary time.
b. Bird-watching is a very common hobby because we are like birds. We both are diurnal and both use hearing and color vision as the main senses.
c. Most mammals are nocturnal and get around by very sensitive night vision, an incredible sense of smell, and a fantastic sense of touch.
d. Birds are like mammals in that their scat has clues to everything that they eat.