It's not fair. At the same time that we enjoyed ample Christmas dinners and complained about the consequent weight gain, many animals remained hungry as their prey vanished during the cold winter months. Even long-distance migrating shorebirds, such as red knots (Calidris canutus islandica),are affected by food deprivation if the food reserves in their winter homes become scarce, ultimately facing starvation in the worst case. In the final critical phase of starvation, when fat reserves are already depleted, animals eventually resort to breaking down protein stored in organ systems that are essential for survival. But do animals equally catabolise all organs during starvation or are certain organs protected from consumption as fuel for metabolism?
To answer this question, Maurine Dietz and Theunis Piersma from the University of Groningen in the Netherlands have ventured out into the field,patiently collecting and compiling body composition data from both healthy wintering knots and starved individuals over a 20 year period. Their samples consisted of birds in good health that had died accidentally during capture or after crashing into lighthouses. Secondly, they used starved knots that were found dead when their mudflat homes froze, rendering their mollusc diet inaccessible beneath the ice. Dissecting the birds, the team quantified a number of parameters relating to the animal's body condition, before drying the bird's organs and muscles to determine their fat and water content.
By comparing healthy over-wintering birds with starved ones, Dietz and Piersma found that the hungry birds were one-third lighter than the well-fed knots, and they had lighter pectoral and leg muscles, smaller gizzards and lighter intestines. Correspondingly, the intestines of starved birds were about 5 cm shorter than in healthy wintering conspecifics. All of these changes in organ morphometry were attributed to prolonged fasting. Dietz and Piersma also found that the organ fat content of the two groups of knots differed greatly, with healthy knots having higher fat contents than winter-starved birds. When including these factors in their statistical analysis, the authors found their most interesting result. It turned out that only pectoral muscle and liver mass were lower in the starved knots compared with healthy wintering birds.
From this, Dietz and Piersma concluded that food-deprived knots `burned'their pectoral muscles and liver tissue while starving, but maintained their gizzard and intestines, which are essential for digestion when the food supply returns. By doing so, knots apparently sacrificed flight capacity to preserve their food-processing capability. However, as knots walk while foraging, it is still adaptive to defend the essential digestive system components at the expense of flight capability, which can be restored during times of plenty. Finally, this interesting result again points to the tremendous plasticity of the gastrointestinal tract. Not only can it be elongated and shortened according to the bird's physical demands but also it is maintained at the expense of organs, such as skeletal muscle, that are less essential for survival.
