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pulling sleds by fighting or entering into other social displays. Any dog
exhibiting dominance hierarchy behavior which escalates repeatedly to
aggression is culled. Other causes for not making the team include problems
defecating while running or the tendency to collect snow and ice on their feet.
Evolution
Sled dogs were hybridized from a non-local population of dogs. The ances-
tors of sled dogs were individuals of hunting, sheep, and household “breeds”
of dogs. In the beginning, size was the major criterion for choosing an ani-
mal. Differential mortality resulted from culling animals that did not perform
well, and from choosing superior performers for breeding. The resulting
divergent form has a specialized and uniform morphology which is an adap-
tation to the specific behavioral performance. “Choosing” is used here to
avoid confusion with the word “selecting”; chosen animals are already “fit.”
The dogs of early freight handlers tended to have little value. It wasn't until
dog racing became lucrative that individual dogs became valuable for their
abilities to win.
There was no recognition cognizant by humans about details of the adap-
tive morphology; for example, they did not know the thermodynamics of
heat load ( Phillips et al., 1981 ) or the histology of foot pad sweat glands
( Sands et al., 1977 ). Early attempts to breed greyhounds to racing sled dogs
belied recognition that the 32-second sprint races of greyhounds are run on
liver glycogen and the leaping gait is inappropriate for a harnessed animal.
Very quickly, the human selectors of good running sled dogs abandoned
such Darwinian (1858) “capriciousness,” and selected for performance. The
resulting divergence toward sled dog morphology, although orchestrated by
humans, was an adaptation “of the whole internal organization” based on the
sorting of varieties.
Hybridization provides the genetic diversity from which the new form
was chosen. With sled dogs, the divergent morphology and behaviors were
created in perhaps 20 years. This rapid evolution can be accounted for by the
tremendous variation among the founding stock. There is not a gradual shift-
ing of any particular character. Hybridization is rather a recombining and
re-sorting of characters plus the creation of novel combinations of characters.
After the initial hybridizations, choosing only those dogs that perform a task,
restricting breeding to the best performers, and choosing among the offspring
over generations, sounds like natural selection, but it should not be confused
with gradualism.
Hybrids are often thought to be a blending or averaging of characters.
However, mongrelization also creates unpredictable outcomes and saltatory
events where there are sudden changes in the dog's morphology ( Arons and
Shoemaker, 1992 ). Hybridization is a common methodology of breeding for
new and novel working dogs. Cattle dogs and truffle hounds are recombinations
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