Reduced Metabolic Cost of Locomotion in Svalbard Rock Ptarmigan (Lagopus muta hyperborea) during Winter

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.

Neither season nor sex affects the cost of terrestrial locomotion in a circumpolar diving duck: the common eider (Somateria mollissima)

“…Locomotor activity consumes a significant proportion of an animal's daily energy budget (Goldstein and Nagy 1985;Elliott et al 2013), meaning the energetic cost of locomotion (CoL) is likely to be under selection pressure. Balancing these costs is paramount, particularly in variable climates where energy conservation is essential to survival or reproduction (Tolkamp et al 2002;Lees et al 2010). For many species of bird, terrestrial locomotion is of critical importance to fitness at some point in their life history, for example during mating, feeding, incubation or wing moult when these animals may be restricted to moving on the ground Lees et al 2011;Portugal and Guillemette 2011).…”

“…Furthermore, changes in climate may influence predation pressures, resource acquisition and thermoregulation, which may affect energy expenditure (Weathers and Sullivan 1993). Birds are known to experience seasonal variation in daily energy budget, but only one study (Lees et al 2010) has considered the effect of changing season upon the CoL. The Svalbard rock ptarmigan (Lagopus muta hyperborea) almost doubles in M b prior to winter, acquiring significant fat stores that serve as thermal insulation and as an energy source (Stokkan et al 1995).…”

“…The Svalbard rock ptarmigan (Lagopus muta hyperborea) almost doubles in M b prior to winter, acquiring significant fat stores that serve as thermal insulation and as an energy source (Stokkan et al 1995). Interestingly, despite the additional weight in winter, ptarmigan have a reduced mass-specific CoL compared to summer, perhaps as an adaptation for energy conservation in cold and resource-limited Arctic winters (Lees et al 2010). As this represents the only study to date, the ubiquity of seasonal influences on the CoL in birds is unclear.…”

“…Furthermore, egg formation, incubation and parental care require significant metabolic investment from only females (Parker and Holm 1990). It is therefore surprising that of the many studies on the CoL in such birds, only two have considered sex differences (Brackenbury and Elsayed 1985;Lees et al 2011). Male domestic chickens (Gallus gallus domesticus) sustained higher maximum U than females (Brackenbury and Avery 1980;Brackenbury and Elsayed 1985), and both sexes shared an identical mass-specific CoL, despite the males being 56 % heavier (Brackenbury and Elsayed 1985).…”

“…Similarly, male Svalbard rock ptarmigan, only 5 % heavier than females, sustained higher maximum U than females but, surprisingly, had a significantly lower massspecific CoL, which could not be explained by differences in hindlimb kinematics . Lees et al (2011) suggested that the lower mass-specific CoL in male ptarmigan might be an advantage for territorial behaviour, which may influence male reproductive success if females select males based upon their ability to defend territories.…”