2006).Īccumulating energetic reserves is not the only physiological change taking place. Migrants are capable of storing and mobilizing large amounts of fat: birds that fly long distances can double their body weight prior to departure, and a monarch butterfly can store up to 125% of its lean dry weight as fat (Dingle 1996 Brower et al. Fat stores about 8–10 times more energy than the equivalent mass of either carbohydrate or protein, making fat the fuel of choice (Jenni & Jenni-Eiermann 1998). The main source of energy used for migration is fat. This behaviour is known as hyperphagia, which is triggered by an internal circannual rhythm, and is seen in species as varied as monarch butterflies, caribou, and baleen whales. Most migrants therefore feed intensively prior to departing for their journey to boost their fuel stores. Some migratory animals forage along their migratory routes, while others, such as the bar-tailed godwit crossing the Pacific Ocean, do not. For example, young salmon migrate to sea to take advantage of the great abundance of food and high growth potential available there, but must someday return upstream to the small rocky tributaries required for spawning, a trip sometimes involving thousands of kilometres (Figure 4 Dingle 1996). In other cases, the resources needed in different life stages may be found in different locations. By leaving these areas after breeding, they avoid the northern winters with short days, low temperatures, and low food availability (Pulido 2007). For instance, migratory birds like the bar-tailed godwit that breed at high latitudes (e.g., in the arctic tundra) exploit the extraordinary abundance of food during a few weeks in early summer and profit from long days, which allow them to extend foraging time. Many migrants take advantage of favorable food and weather conditions offered in certain areas during very limited time periods. The annual cycle of the seasons produces large differences in the duration and intensity of solar energy received in each hemisphere at any given time. Migration is an adaptive response to the seasonal or geographic variation of resources (Gauthreaux 1982). Monarchs specialize on milkweeds (Asclepiadaceae), which contain substances that are poisonous to vertebrates and thus protect the monarch from many potential enemies (Dingle 1996). Instead, the females lay eggs during the migration, from which the next generation of migrants emerges. No single individual makes the entire round trip, as the duration of these journeys exceeds their lifespan. The complete migration requires an intergenerational relay. At the end of every summer, more than 100 million individuals sweep across North America to overwinter up to 4,750 km further south in California and Mexico. Perhaps the best-known insect migrant is the monarch butterfly ( Danaus plexippus) (Figure 3a). Tracking individuals using lightweight satellite tags revealed that these birds are able to cross the Pacific Ocean in a non-stop marathon flight lasting over 9 days, making it one of the most impressive feats among migratory animals (Figure 2 Gill et al. baueri subspecies breeds in Alaska and migrates south to its non-breeding quarters in Australia and New Zealand. One of the longest non-stop flights is performed by the bar-tailed godwit ( Limosa lapponica) (Figure 1).
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