If you’re a baby bird, your species has two options: either do a lot of your maturing in the safe, warm environment of your egg and be born more or less ready to survive, or start out totally helpless and play catch-up by developing quickly after hatching. The challenge you face is the trade-off between using energy to grow and devoting it to staying warm. For species living in harsh environments like the Antarctic, it is especially vital to get this strategy figured out. A team of scientists revealed for the first time how young Adélie penguins strike a balance between growth of and heat production by the pectoral muscle, in order to mature in time to head out to sea.
Cet article est également disponible en français, traduit de l'anglais par Clio Bayle : Grandir ou avoir chaud : telle est la question pour les manchots Adélie.
Flickr / Iain B. of Over
Like young Scouts around the world, ducklings take the “Be Prepared” approach. The young of such species known as “precocial” are born relatively independent and reach adult levels of functioning for certain processes, like controlling body temperature, before hatching or soon thereafter. This both gives them the liberty and forces them to grow slowly, as they devote their energy to staying warm. Adélie penguins, on the other hand, are a semi-altricial species, meaning they opt for the born-helpless-grow-fast method. Chicks are unable to keep their own temperature up, relying on their parents for warmth and to stuff them with nutritious krill while they concentrate on growing. In a study published in September in PLOS ONE [available on MyScienceWork], scientists revealed for the first time how changes in gene expression alter the birds’ physiology and biochemistry, letting them go, within two months of hatching, from utterly dependent on their parents for survival in the cold to heading out to sea.
Adélie chicks experience a mega-growth spurt after hatching that puts human teenagers to shame. Their body mass increases five times in just the first six days and more than 40 times over the first two months. As a result of this growth, their surface-to-volume ratio decreases significantly. Along with better insulation from thickening down, this new body form helps them to conserve heat.
The biggest factor in turning these hatchlings into whopper chicks is the growth of their pectoralis muscle: in the first month, it balloons to 80 times its original size! Essential for diving, it also becomes the main heat generator in adult Adélie penguins. The cold triggers shivering, and this movement of the muscle produces heat that warms the bird. How the body conducts this transition from muscle growth to muscle use for thermogenesis (and, consequently, from ectothermy to endothermy) was shown in the study by researchers led by Mireille Raccurt of the Université de Lyon (France). The team worked over three breeding seasons during the Antarctic summer on the Pointe Géologie archipelago, nesting grounds for some 34,000 pairs of Adélie penguins.
The scientists looked at factors involved in growth and in thermogenesis, analyzing which genes were being expressed at different points in the Adélie chicks’ life. After hatching, growth hormone (GH), through its stimulation of or interaction with other hormones, is necessary for the chicks to grow. How sensitive a tissue is to GH depends on the number of receptors for this hormone that are present. Looking at the production of the GH receptor, the Franco-Canadian team found that the chicks were born with high levels in the pectoral muscle, which then decreased during the first 15 days of life. This alters the initial explosion of muscle growth: local production of another hormone receptor can then ensure the end of growth and maturation of the pectoral muscle.
Once the important pectoral muscle has achieved much of its growth, it needs to attend to its future role as heat engine. At day 15, when growth stops dominating, the researchers found a sudden rise in the expression of mitochondrial genes involved in thermoregulation, which continued on through two months of age. This, along with studies of the muscle fibers themselves, showed signs of the muscle reaching its mature state, when it will be capable of burning energy-rich lipids to efficiently produce heat for the now-autonomous penguin.
These molecular controls that let one species of polar bird manage the conflict between needing to grow quickly and to develop the ability to produce heat may also be used by other species in a similar environment. The authors note, though, that comparative studies have so far suggested that each species has its own strategy. Every muscle may even develop differently, according to the importance of its role in a young bird gaining independence. Understanding more about the links between animal physiology and developmental trade-offs like the Adélie penguin faces will give us a more complete view of the way evolution works on animals in their natural environment.