Physiological aspects of freezing behaviour in willow ptarmigan hens.

When threatened, wild willow ptarmigan (Lagopus lagopus lagopus) hens displayed freezing behaviour, i.e. motionless, crouched with head down and eyes open. But while non-incubating hens showed increased heart (HR) (from 229 +/- 18 to 254 +/- 33 beats min-1) and ventilation rates (VR) (from 25 +/- 2 to 31 +/- 4 breaths min-1), incubating hens showed bradycardia (from 208 +/- 40 to 102 +/- 13 beats min-1) and reduced VR (from 25 +/- 2 to 10 +/- 2 breaths min-1), which could be maintained for as long as 20 min. The bradycardia response became stronger as hatching approached. The bradycardia period was interspersed by short bursts of tachycardia and hyperventilation which presumably supplied oxygen to blood and muscles. The bradycardia period was typically followed by a period of increased HR (276 +/- 104 beats min-1) and VR (32 +/- 3 breaths min-1). Atropine abolished the bradycardia response, which was thus apparently due to parasympathetic stimulation. The bradycardia response observed in incubating ptarmigan hens, during forced diving of birds and mammals and during death feigning in other animals are all examples of how emotional stimuli trigger a basic bradycardia reflex.