Red knots give up flight capacity and defend food processing capacity during winter starvation

During the last phase of starvation, animals depend mainly on protein breakdown. All organs are a potential protein source. Do starving animals prevent particular organs from being catabolized in order to defend certain functions? In this study we investigated if starving birds maintain locomotion and digestion capacities, both essential for the recovery process. We compared body composition data of healthy wintering and winter-starved red knots (Calidris canutus islandica), a long-distance migrating shorebird that breeds on High Arctic tundra in Canada and Greenland, and winters in temperate coastal areas such as the Wadden Sea and the British estuaries. Throughout the wintering period they eat hard-shelled molluscs ingested whole. Our results showed that winter-starved knots had catabolized 60.5% of their pectoral muscles. This was much more than the decrease in overall body mass (32.5%). As a result, their flight capacities will have been reduced. Winter-starved knots defended the muscular gizzard, which lost only 21.2% of its mass. As knots crack the ingested shellfish with their gizzard, the organ is essential for food processing. The intestines and liver were not defended; their atrophy equalled that of the pectoral muscles (60.6% and 61.3%, respectively). Comparison with data from the literature led to the conclusion that starving birds only defend organs that are essential to either obtain or process food. These organs are maintained at the minimal level of normal capacity. Other organs decrease below this level and may lose much of their functional capacity. Even in near-death situations, with low fitness prospects, organisms show interpretably adaptive changes in organ size.