erion.
In  The Expression of the Emotions in
Man and Animals (1872), Charles Darwin
(17) laid an important foundation for viewing the defensive patterns of other species as
essential evolutionary precursors to human
fear and anxiety reactions (18). More recently, a number of clinical accounts have
conceptualized human anxiety disorders as
disorders of defence (e.g., 19-22), in which
the key feature concerns inappropriate activation of defensive behaviour arising from
erroneous assessment of danger. As such, an
understanding of the neurobiology of defensive behaviour assumes particular importance in anxiety research and, in this context,
recent studies have pointed to a remarkable
similarity among vertebrates in the neural
systems (e.g., amygdala, periaqueductal gray
matter) involved in detecting danger and
producing defence responses (23,24). As this
system is so strongly conserved in evolution,
it has been convincingly argued that we can
learn much about human defence or fear
reactions by studying other creatures (25).
At the behavioural level, it has been
known for quite some time (26) that animals
are capable of displaying diverse defensive
reactions in response to external threats, e.g.,
a predator or an aggressive conspecific. In
mammals (e.g., rats), such behaviours classically comprise freezing, flight, defensive
threat/attack and even death-feigning, and
are dependent both upon threat imminence
(27) and escape opportunity (28). This repertoire is most clearly observed in feral animals, although several features (e.g., freezing) may be elicited by appropriate stimulation of laboratory rats (29). However, recent
research has shown that the rodent defensive
repertoire is even more elaborate than suggested by this classical description. Thus,
laboratory rats will bury dangerous objects
(30), and emit ultrasonic vocalizations when
injured (31), separated from their mother
(32) or exposed to a natural predator (33).
Furthermore, in potentially dangerous situations (for example, when a predator has been
seen but is no longer present), laboratory rats
(34) and mice (35) have been reported to
engage in a cluster of behaviours collectively referred to as risk assessment. These
responses, originally identified in specially
constructed visible burrow systems, are characterized by cautious approaches to a surface area where a predator (cat) has briefly
been presented and include i) scanning the
danger area from tunnel openings, ii)
stretched attend, or flatback, postures directed towards the danger area and iii)
stretched, or flatback, locomotion upon initial re-entry into the danger area