| In
Context #6
(Fall, 2001, pp. 12-14); copyright 2001 by
The Nature Institute
What forms an animal? A likely answer these days is "genes." Or perhaps:
"genes and environment." Such high-level abstractions reveal how little
we actually know and tend to discourage further inquiry. When I hear "genes
and environment" I yearn for something more concrete, something I can
mentally take hold of. And the only way I know to develop such saturated
concepts is to get back to the things themselves—to look carefully
at what nature presents and inch my way toward a more full-toned understanding.
Wild and Captive Lions
A few years ago I came across a remarkable article written in 1917 by N.
Hollister, then superintendent of the National Zoo in Washington, DC. (See
end of this article.) He was studying
the lion specimen collection at the National Museum, which encompassed over
100 lion skulls and skins. Hollister noticed marked differences between
wild-killed specimens and those that had lived for a number of years at
the Washington zoo. He proceeded to make a more detailed comparative study.
Since lions from different areas of the world and also different regions
of Africa differ substantially from one another, Hollister focused on
one subspecies—the Masai lion (Panthera leo masaica) from
East Africa. Five of the zoo-reared animals were Masai lions and had been
captured as small cubs near Nairobi, Kenya. Hollister compared these specimens
with wild-killed lions from the same area. He thus had animals from the
same subspecies and one regional population. He knew, in other words,
that he was comparing fairly close relatives and not genetically and geographically
distinct populations.
When the five lions were brought from Kenya to the Washington zoo, they
already stood out through their very pale, grayish buff-colored fur. This
is the typical coloration of wild-living Masai lions, but contrasted starkly
with the much more darkly colored lions at the Washington zoo. Over a
period of years the fur of these imported animals darkened considerably,
becoming like that of the other lions at the zoo. Moreover, the captive
male lions grew much longer manes than wild Masai lions and they also
had longer and fuller hair tufts at their elbows.
Immediately we ask, "Why?" But an easy answer is not forthcoming. Hollister
was cautious. He believed the higher humidity and precipitation in Washington
might have played a role in fur darkening, since humidity has been correlated
with darkening of fur, and also feathers in birds. But he also recognized
that the quality of light as well as metabolic changes due to the abnormal
life and diet in the zoo might have contributed to the differences.
The Skulls of Wild and Captive Lions
Since an animal's fur is in direct contact with the external environment,
we can imagine that it might somehow change in relation to changing environmental
conditions. But the solid and complexly formed skull, hidden from the world
by skin and muscles, is another matter. And yet, surprisingly, the most
striking differences between the wild and zoo-reared animals were in their
skulls (see Figures 1, 2 and 3).
Figure 1. Top view of a zoo-reared (left) and a wild-killed
(right) lion, both adult males. Drawn to same scale. (Drawings by Christina
Holdrege. After Hollister 1917.)
The skulls from the zoo-reared animals are much shorter and broader
than in the wild animals. They appear compact compared to the more sleek
skulls of the wild lions. When I first saw the photographs of the skulls,
I thought they had been incorrectly labeled, expecting the more stocky,
massive skull to have belonged to a wild animal. But they were correctly
labeled and I needed to consider the matter more closely. (A good exercise
in overcoming prejudice!)
The skulls from the zoo-reared animals—whether male or female—are
not only broad but also thicker-boned. One can see this in the prominent
cheekbones (zygomatic arches, see Figure 1). The arch sweeps out further
to the sides and consists of much thicker and more rounded bone. Figure
2 shows a cross section through the bone of the zygomatic arch in a zoo-reared
and a wild animal. The difference is glaring. The zoo-reared animal's
bone is triangular in cross section with convex surfaces and rounded corners.
It consists largely of porous bone material (spongiosa). In contrast,
the wild animal's arch is narrower and has one concave and one convex
surface that meet at the top of the arch, forming a sharp ridge. The arch
has little porous bone, consisting mainly of the outer layer of strong
compact bone.
Figure 2. Cross section through the zygomatic arch of a wild-killed
(left) and a zoo-reared (right) lion. Adult males of same age; natural
size. (From Hollister 1917.)
Similar differences are visible at the rear of the skull (see Figure
3). Not only is the skull of the zoo-reared animal much broader, the surfaces
and forms are more rounded with gradual transitions from convex to concave.
The skull of the wild-reared lion has much sharper, more defined edges
and angles.
Figure 3. View from the rear of a wild-killed (top) and a zoo-reared
(bottom) lion, both adult males. Drawn to same scale. (Drawings by Christina
Holdrege. After Hollister 1917.)
One further interesting contrast between the skulls pertains to the
braincase (see Figure 1). Measured externally, the braincase in the skull
of a wild lion is smaller than in the zoo-reared lion. When, however,
one measures the internal cranial capacity—which is a direct indicator
of brain size—the wild lion skull is considerably larger (40 to 50
cubic centimeters greater in size). This apparent paradox is resolved
when one considers bone thickness. As in the other parts of the skull,
the bones of the braincase are substantially thicker in the skull of the
zoo-reared animal. Therefore the braincase appears externally larger but
internally leaves less room for the brain. The larger brain of wild lions
is covered by thinner, but solid, compact bone.
Hollister writes that even an untrained observer would group the skulls
into wild and zoo-reared specimens, so apparent and uniform are the differences.
He suggests that if one were dealing with only specimens from wild animals
(or fossils), a biologist or paleontologist would think that he or she
was viewing specimens of different species (a remark that makes one wonder
about the accuracy of fossil classifications). Where does this contrast
come from?
Activity that Sculpts
A primary activity missing from the life of a captive lion is the hunt and
kill. A hungry lion in Africa's savannah crouches in the grass, all muscles
tensed and its senses focused on the movement of a herd of antelopes or
zebras. It stalks slowly and silently toward the herd and then suddenly,
in a forceful burst of speed, sprints toward an animal, leaps, grabs onto
the neck, and pierces through blood vessels and the wind pipe with its long,
pointed canines. It pulls the prey down—using head and paws—and
holds it until it dies. If the lion is a female with young cubs, she may
drag the prey, locked into her jaws, toward the place where she's hidden
them.
All this activity is missing from the life of a captive lion. And this
activity forms the skull. The lion uses powerful muscles to grip, bite
into and hold the prey in its jaws. The masseter muscle is especially
important for the gripping power exercised in using the incisors and canines
to pierce and hold the prey. This muscle attaches to the zygomatic arch
and to the mandible (lower jaw). A powerful muscle must be rooted in strong
bones. As the lion exercises its muscles, they not only grow but also
put tension and stress on the bones. Although we tend to think of bones
as inert structural elements of the body, they are, in fact, alive and
adaptive. With an increase in stress and tension the bones change form
and structure to meet the demands of the activity. The zygomatic arch
remolds to form a sharp ridge of compact bone as the ideal attachment
for the masseter muscle. In the same way the mandible forms thinner, more
compact bone with ridges and rougher surfaces for the strong muscles attached
to it. In contrast, the rounded, smooth zygomatic arch and mandible in
the zoo-reared lions reveals a lack of activity. The bones grow and billow
out, being hardly influenced by muscular stress and strain. Hollister
notes their juvenile appearance, which reflects the lack of change due
to inactivity.
Likewise, the sculpting of the rear of the wild lion's skull discloses
activity. The wild lion uses its neck muscles in holding, pulling, lifting,
shaking, and dragging prey. At least seven different neck muscles attach
to the rear of the skull and every contraction sculpts the bones these
muscles are rooted in. As in the jaw, the rear of the skull forms defined
ridges and rough surfaces where the muscles attach. The little-used neck
muscles of the captive lion leave the rear of the skull largely unaltered;
the bones become more rounded and have smoother surfaces.
The Formative World
In the life of an animal, activity is a key formative factor. The active,
hunting lion takes on a modified form compared to the inactive zoo lion.
The muscle-orchestrated movement of the lion shapes the bones. This movement,
in turn, is stimulated internally by the animal's drives (hunger) and externally
by the perception of the antelope or the zebra. In this sense the antelope
and the zebra form the lion. A remarkable thought. We all know that the
flesh of these animals nourish the lion, but now we can recognize that the
activity these animals call forth in the lion sculpts the lion's very bones.
We can go even further and say that the savannah—its soil, light, warmth
and moisture, its grasses and trees, its other animals—forms the lion.
But it becomes increasing difficult to say precisely how this larger
world influences the lion.
The outer world that forms the lion points us to the lion. By "lion"
I mean the specific way-of-being that, for example, is open to and reacts
to antelopes and zebras in a particular way. A lion doesn't see the grass
it's crouching in as something to feed on, as does the antelope. Grass
is something to hide in and move through. In this sense the lion is a
specific world, a way to be and behave. This aspect of the lion is centered
in the bodily form it is born with. This form is given through inheritance
and then molded by activity. The hereditarily given model is something
dynamic and plastic, waiting to be filled and formed by the animal's activity.
This is what we should be picturing when we speak of a "genetic background"
or genes, not some fixed plan.
The vast and rich ecology of the savannah stimulates the lion to activity.
In a sense it brings forth the lion and allows it to unfold its life.
This stimulation influences the whole metabolic activity of the animal,
not only the muscles and the bones. Every sense perception forms nerve
activity and influences the formation and function of the brain. The zoo
lion lives in a world that calls forth little activity. Its bones grow
large and thick, expressing the weight and inertia of its existence, while
muscles and nerves receive little stimulation. One can sense the responsibility
one takes on in having captive animals—knowing that we are cutting
them off from part of the world that enlivens and forms them. How can
we create a surrogate environment that at least to a degree is appropriate
to their needs?
So when you hear that an animal is a product of its genes and its environment,
think of the lion. Think of the most solid part of the body—bone—being
molded by the animal's activity. In activity, the lion's specific anatomical
and behavioral readiness takes hold of a world without—the kill at
a watering hole at dusk. The antelope shapes—and so is part of—the
lion.
* Hollister,
N. 1917. Some effects of environment and habit on captive lions. Proceedings
U.S. National Museum 53: 177-193.
Original source: In Context (Fall, 2001, pp. 12-14); copyright
2001 by The Nature Institute
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