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140 Biobehavioral Resilience to Stress

availability; combustion requirements—oxygen availability, temperature;
physical strain—biomechanical stress; oxidation and rust—oxidative stress
from reactive oxygen species). Less well understood is the uniquely human
plasticity of responsiveness to environmental challenge that permits excep-
tional performance in some individuals but not in others who share the same
essential physiological attributes. Th is flexibility of response (resilience) to

environmental challenge is moderated by factors such as training, acclimati-
zation, behavior, and psychosocial cues.
Common environmental stressors include thermal strain, hypoxia, inad-
equate rest, high levels of physical work, and a variety of psychological factors
such as traumatic exposure, overcrowding, or isolation. When environmen-
tal stressors persist without relief, human operators may experience lapses in
mental performance and motivation, fatigue or collapse, injury, illness, or, in
extreme cases, death. Cortical brain processes (psychological resilience) play

a critical role both in mitigating the effects of environmental stress and in
determining how human operators respond to it.

Coutu (2002) has identifi ed specific abilities associated with resilience and
success in business organizational structures. These include the ability to face

reality, to find meaning in life, and to improvise (Coutu, 2002). These same abil-

ities are important for survival in harsh environments such as prisoners of war
(POW) camps, concentration camps, mountainous terrain, cold environments,
arid environments, and settings that require high-endurance performance with
sustained workload and extreme fatigue. Resilience to environmental adversity

is thus a combined effect of psychological factors (outlook, fl exibility, personal-
ity) and underlying physiological phenotype. The champion marathoner pos-

sesses innate and trained physiological advantages but probably cannot succeed
without also having a high degree of psychological resilience.
The relative and interactive effects of behavior and physiology on human

performance and survival are not yet well understood. It is particularly dif-
ficult to evaluate such potentially complex effects in terms of their impact

upon performance in settings that are moderately stressful but not suffi -
ciently challenging to press human functional capacity beyond tolerable lim-
its. Thus, we have organized this chapter around several unique examples

of extreme performance or survival. In each case, we will consider specifi c
physiological mechanisms known to promote survival, mechanisms that
may link psychological factors to physiological outcomes, and current eff orts
to expand the understanding of such effects and relationships.

Extremes of Human Endurance

Ernest Shackleton’s last adventure to the Antarctic is widely used as a model
of leadership, supported by a long list of teachable skills that can be used

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