Q. You wrote that blue whales are the largest mammals. What are the largest and smallest animals of other groups? What ecological problems do animals confront by being too big or too small?
A. Body size affects two basic ecological issues all animals must deal with – getting enough food and not becoming food themselves. Temperature and moisture are vital to certain bodily functions. Animals must deal with various environmental considerations related to seasonal changes and extreme weather conditions. Effective strategies depend on the animal’s size.
Of all known animals, past and present, the blue whale is indisputably the largest. The most obvious ecological problem for such a creature is how to get enough to eat. The bulk of this behemoth’s diet is a small zooplankton, krill, the ocean’s most abundant prey. Some resemble tiny shrimp. A 90-foot-long blue whale may eat several tons of krill in a week.
Because of its size, a blue whale has little to worry about with regard to predators. Babies start life at more than 20 feet in length, weighing more than 2 tons. A 150-ton mother nearby probably deters most would-be predators. Sadly, humans have been the greatest threat to blue whales. The 19th and 20th centuries saw unregulated killing of these magnificent creatures by whalers with explosive harpoons. According to some estimates, the whaling industry has reduced the world’s blue whale population by more than 90%. Humans: the most destructive predator the world has ever known.
The world’s smallest mammal is probably the Etruscan shrew found in the Mediterranean region. A big one is only about an inch and a half long. Adults weigh less than a dime. The American pygmy shrew is slightly larger. A major problem confronting these small, warm-blooded animals is an exceedingly high metabolic rate that results in a voracious appetite. Most shrews eat insects and other small animals; they are constantly on the move in search of prey. When trapped at night for scientific purposes, shrews starve to death before morning if no food is left in the trap. Almost any carnivorous animal, including owls and raccoons, might be a predator, but shrews are small enough to stay out of sight by traveling underground through root tunnels and passageways made by insects.
African ostriches are the largest birds now on Earth. They solve the food requirement by eating pretty much any plant or animal they can get their big beaks on. Mammals, reptiles and insects are always on the menu, as well as certain plants and seeds. Being flightless birds, escaping predators by flapping their wings is not an option. But they can run very fast. Potential predators, including lions and leopards, unlucky enough to catch an adult ostrich risk having their guts ripped out by the bird’s enormous claws. I recall seeing a herd of six ostriches moving silently and confidently through a South African scrub habitat. I did not need the warning from our guide. I knew instinctively “those giant birds could hurt you.”
The world’s smallest birds are the hummingbirds. A full-grown male of the calliope hummingbird, the tiniest U.S. species, attains a length of under 4 inches and weighs less than a nickel. Their diet consists of small insects, tree sap and nectar from flowers. To escape seasonal changes in vegetation and flowers they depend on for food, they migrate to Mexico from as far north as Canada. These pixies are in constant danger because of their small size. However, they fly so rapidly they never make an easy target. They will even chase larger birds from their proclaimed territory.
Every animal must succeed at two vital tasks to survive: successfully feeding itself and avoiding becoming prey. An animal’s size and behavior coupled with its environment affect its chances in the game of survival. Big or small, they have evolved to give the species its best chance at survival.