Abstract
The Svalbard rock ptarmigan, Lagopus muta hyperborea experiences extreme photoperiodic and climatic conditions on the Arctic archipelago of Svalbard. This species, however, is highly adapted to live in this harsh environment. One of the most striking adaptations found in these birds is the deposition, prior to onset of winter, of fat stores which may comprise up to 32% of body mass and are located primarily around the sternum and abdominal region. This fat, while crucial to the birds' survival, also presents a challenge in that the bird must maintain normal physiological function with this additional mass. In particular these stores are likely to constrain the respiratory system, as the sternum and pelvic region must be moved during ventilation and carrying this extra load may also impact upon the energetic cost of locomotion. Here we demonstrate that winter birds have a reduced cost of locomotion when compared to summer birds. A remarkable finding given that during winter these birds have almost twice the body mass of those in summer. These results suggest that Svalbard ptarmigan are able to carry the additional winter fat without incurring any energetic cost. As energy conservation is paramount to these birds, minimising the costs of moving around when resources are limited would appear to be a key adaptation crucial for their survival in the barren Arctic environment.
Highlights
Maintaining an optimal balance between energy acquisition and consumption is paramount to the evolutionary fitness of organisms [1,2]
No significant difference was found between summer and winter birds indicating that they were carrying the additional fat at no additional energetic cost (Figure 1b, ANCOVA: season, F1,64 = 2.81, r2 = 0.025, p = 0.100; U, F1,64 = 47.7, r2 = 0.42, p,0.001)
Svalbard rock ptarmigan undergo a dramatic seasonal change in body mass; the birds being up to 47% heavier during winter in the present study
Summary
Maintaining an optimal balance between energy acquisition and consumption is paramount to the evolutionary fitness of organisms [1,2]. For an animal to move at greater speed (U) they must contract their muscles faster to move their limbs more quickly and reduce the amount of time that the feet are in contact with the ground [5,6]. This requires more metabolic energy, which we can measure as oxygen uptake (V_ O2,max ). The efficiency of locomotion may vary in birds with differing locomotor specialisations, with both morphology and gait influencing the cost of locomotion [13,14,15,16,17,18]. Birds specialised towards non-terrestrial locomotor modes may use differing gait variations, such as the waddling walking gaits seen in penguins and mallards, characterised by extensive lateral movements of the centre of mass (CoM) [16,20]
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