The smallest mammal living today is the Etruscan shrew; the smallest bird is the bee humming bird. Each weighs about 2 g (0.07 oz) - less than a bumble bee.The creatures are at the limit of miniaturisation for warm-blooded animals. This is because warm-blooded animals lose heat from the surface of their bodies, and at this size their skin area is so large in proportion to their volume that virtually all the heat the animals generate is lost.
Cold-blooded animals can be much smaller, because their body temperature varies with the environment and they do not need to retain heat. The smallest vertebrate (creature with a spinal column) is the pygmy goby, a fish just 8 mm (0.3 in) long - about the width of a pencil. It tips the scales at 4 mg (0.0014 oz), not a catch a fisherman would boast about. Insects may be even tinier - the smallest of all being the fairy fly, at 0.4 mm (0.016 in) long.
The creatures are made up of a vast number of individual cells. Early forms of life, the protozoa, consisted of a single cell. Since the average human body contains 50,000 million cells, you might expect all protozoa to be microscopic in size. But some extinct species were almost 25 mm (1 in) across. Living forms reach 15 mm (0.6 in) in diameter.
Eggs, too, whatever their size, are just single cells. Those of some extinct birds and reptiles were enormous. The largest yet discovered is the egg of the aepyornis, or elephant bird, which lived on the island of Madagascar between 2,000,000 and 10,000 BC. This laid an egg 375 mm (15 in) log. One aepyornis egg would have provided 180 people with the equivalent of a hen's egg each.
