Life in the tropics tends to have a more relaxed pace than at more extreme latitudes. According to Joe Williams and his colleagues from Ohio State University, USA, the basal and peak metabolic rates of tropical birds are significantly lower than those of their more temperate cousins. Knowing that some internal organs account for a disproportionate amount of an animal’s net metabolic energy consumption, Williams, Popko Wiersma and Brittany Nowak decided to find out whether tropical species have scaled down the size of any of their major internal organs to account for their reduced metabolic demands (p. 1662).
Scouring the literature for details of the organ masses of 408 species and collecting an additional 32 tropical species in Panama and 17 temperate species in Ohio, the team compared the masses of the birds’ organs, ranging from the liver, kidneys, heart and lungs to the flight and leg muscles. Analysing the masses of these tissues, the team found that the heart, lungs, flight muscles, liver, kidneys, ovaries, testes and feathers of tropical species were significantly lighter than the same tissues from temperate birds. However, the masses of the leg muscles, gizzards, intestines, skin and brain did not seem to be affected by the birds’ latitude.
Suggesting that the tropical birds, which do not require as much insulation, conserve energy by reducing their plumage and cutting down on the size of some organs, the team suspects that warmer tropical conditions have driven the evolution of smaller organs in tropical species. Williams also adds that the cells of tropical birds may also run at a slower pace than those of birds from higher latitudes, and he and his team are currently testing this idea.