TROPHY HUNTING - Wildlife Conservation in China: Preserving the Habitat of China's Wild West - page 174

horns was heritable from father to son. But research on North American bighorn sheep
has recently confirmed that horn size is indeed heritable, although only partly so. 21 We
now know that there is a strong (albeit imperfect) statistical correlation between the type
of horns borne by fathers and their male offspring.
But horn growth is also known to be substantially influenced by nutrition, particularly
when animals are young. 22 Large horns may be an important (if indirect) avenue to later
reproductive success, but they won't help a youngster if he doesn't live long enough to
use them. Growing large horns comes at a cost, and thus is an evolutionary luxury com-
pared with the imperative of growing a healthy body and acute sensory organs that will
be necessary to keep the young animal alive during its vulnerable early years. Resources
are allocated to horn growth only when they are relatively abundant: an animal faced
with poor forage will sacrifice on horn growth in order to prioritize staying alive. Even
faced with identical vegetation, animals in a dense population must compete with each
other for it, whereas those in a sparse population can more easily select and appropriate
the most nutritious patches. Thus, the animal's environment, including the density of
conspecifics, acts to modify the characteristics of horns ultimately grown by the young
male from those genetically inherited from his ancestors.
A second factor acting to disrupt the correlation between horn characteristics of fathers
and their male offspring should be obvious: mothers. Females of most of these species
also grow horns (albeit much smaller ones), but these animals are not subject to the same
selective hunts as are males. 23 Further, horns are most likely controlled by a large number
of genes (it being unlikely there is a single “large horn gene”), and females contribute
an equal share of these to male offspring. So any selectivity imposed on the identity of
fathers is tempered by the lack of a similar selectivity on mothers.
Third, horns grow with age, becoming continuously larger each year. So if hunters
shoot males with larger horns, they may not necessarily be selecting those with genes for
larger horns but rather those that are simply older. If all males shot were at identical ages,
then clearly the largest ones would be those with some built-in advantage, and artificial
selection against them would be strong. But a large-horned animal may simply be one
that is older than the others, even if it is not particularly well disposed toward growing
large horns genetically.
Fourth, although animals with the largest horns normally achieve the highest rank
within the male hierarchy and obtain the most matings, it does not follow that younger,
smaller animals make no reproductive contribution at all. Recent work has shown that
subordinate males often find ways to obtain matings that are frequently not evident from
observational studies. 24 Thus, even if males with genotypes conducive to growing large
horns are selectively killed and thus obtain fewer mating opportunities than in the ab-
sence of trophy hunting, they are unlikely to have been shut out of the breeding lottery
entirely.
These four facets of reproductive biology among animals with fancy horns act to temper
the magnitude of artificial selection. And for many years, they were sufficient to convince
managers that such artificial selection was impossible among species of trophy interest.
However, biologists working with an intensively studied herd of bighorn sheep in Alberta,