Genetic Influences on the Behavior of Chickens Associated with Welfare and Productivity - Genetics and the Behavior of Domestic Animals - page 326

production; and it is easily identified during the first year of egg production
( Saeki, 1957 ).
Should there be concern about the loss of a previously important behavior
from chickens? We believe the absence of a behavior previously expressed
is weak evidence that suffering results. Craig and Swanson (1994) discussed
this as follows:

With the example of broodiness in mind, one may ask, why should behaviors that are
no longer required be expressed? Presumably, most or all such behaviors could be
eliminated by genetic selection between or within stocks, so that hens would not be
motivated to show them or be frustrated by the absence of conditions allowing their
expression.

Behavioral exhibition is controlled by both internal and external factors.
Internal factors are mainly neuroendocrine factors. Neurons of the central
nervous system have the capacity to change structure and functions (neuronal
plasticity) to adapt to a given environment ( Joseph, 1999; Kolb and
Whishaw, 1998; Smith, 1993 ), which, in turn, affects animals' behavioral
exhibition ( Rosenzweig and Bennett, 1996; Sausa et al., 2000 ). Through
domestication, farm animals are quite different from their wild counterparts
in many ways, including behavior. In addition, the degree of welfare could
be dramatically different between strains of a species and individuals of the
same strain even housed at the same environment, which is dependent on
each individual's behavioral and physiological characteristics (genotypic
and phenotypic variations). Adaptation of an individual, but not another,
to its surrounding environment can be achieved through environmental
stimulation-associated behavioral and physiological plasticity in the animal
(a genetic environmental interaction) and the experiences received during

the animal's lifetime (allostasis, i.e. allostatic load or overload) ( Clark et al.,
2007; McEwen, 2001 ).

Stereotyped Pacing During the Pre-Laying Period
Breeds, strains within breeds, and individuals within strains vary in stereo-
typed pre-laying pacing seen in single-hen cages. Such behavior may be
interpreted alternatively as indicating frustration ( Duncan and Wood-Gush,
1972 ) or the alleviation of stress ( Hughes, 1980; Dantzer, 1993 ). The
absence of nests in single-hen cages is apparently responsible for stereotyped
pacing before an egg is laid in some stocks but not in others ( Mills and
Wood-Gush, 1985 ). Instead of pacing, some stocks show quiet sitting behav-
ior during the pre-laying period. Thus, it may be conjectured that those
stocks showing pacing are frustrated whereas other stocks are not. If con-
vincing evidence becomes available that well-being is compromised in
stocks that pace in the absence of nests, then choices need to be made. For
example, either cages should be modified by providing nests, stocks should