Biology Reference
In-Depth Information
appetite and therefore require a more concentrated ration for optimal well-
being and productivity, it would be penalized if compared with other stocks
having greater appetites if all are fed a ration more suitable for the latter
stocks.
BEHAVIORAL DIFFERENCES AMONG POPULATIONS
Behavioral differences, usually of a quantitative nature, are found among
domesticated stocks of chickens. Most of these can be demonstrated to be of
genetic origin. Often it is difficult or impossible to know how these differ-
ences arose. Likely contributors consist of genetic differences in foundation
populations, random genetic drift in relatively small and isolated populations,
adaptation to local environments by natural selection, and different goals
under artificial selection. The impact of selection by poultry breeders is clear
when comparing breeds developed specifically for beauty, fighting ability,
meat production, or table-egg production. As an example of how selection
for an economic trait can affect behaviors,
Siegel (1989)
described how
long-term selection for heavy juvenile body weights in broilers resulted in
their having more docile behavior, greater appetite, and reduced motor activ-
ity when compared with egg-production stock.
Comparisons among stocks are complicated in some cases because of
developmental stage effects. Studies by
Tindell and Craig (1959)
and
Bellah
(1957)
are especially instructive in this regard. Tindell and Craig found dif-
ferences in peck-order status among hens of four breeds (White Leghorn,
Australorp, Rhode Island Red, and White Plymouth Rock) when placed
together in intermingled flocks and observed to determine relative peck-
order status at about 5 months of age. Bellah used hens of the four breeds
from the same experiment that had been kept in separated-strain flocks and
determined their relative social dominance ability beginning at about 8
months of age. Strikingly different results were obtained from those of
Tindell and Craig when relative social dominance ability of the breeds was
compared at the later age. In the most extreme case, Rhode Island Red hens,
which ranked lowest among the breeds when compared in intermingled
flocks at 5 months, were the most dominant when tested in pair contests at
8 months of age.
Inconsistencies have also been found for social dominance ability among
experimental stocks within the White Leghorn breed when compared at
different ages. Using two populations subjected to long-term selection for
number of eggs laid before 40 weeks of age, mostly in competitive floor-pen
environments, and the unselected control from which they were derived,
Craig et al. (1975)
found that the selected stocks had decreased age at sexual
maturity and increased aggressiveness and social dominance ability during
the adolescent period. However, for one of the selected strains, rank for the
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