What if every animal were the same size?

A bee hummingbird is about as long as a grown-up's thumb. An elephant is taller than a grown-up.

What if every animal were the same size? What would happen?

The bee hummingbird of Cuba is the smallest bird: about two and a quarter inches long and less than 2 grams, lighter than a dime. A big male African elephant can weigh up to about 6,800 kilograms. The largest animal ever known, the blue whale, can grow to 110 feet long.

What if every animal were the same size, say the size of a dog? In this thought experiment, you will see that size is not just about looks. It decides what animals eat, how they stay warm and even how fast they live.

Animal body masses span a vast range: from a bee hummingbird of under 2 g to a male African elephant of up to about 6,800 kg on land, and a blue whale of up to about 150,000 kg (330,000 lb) in the sea.

This thought experiment collapses that range to a single size and asks what would follow. It draws on three ideas: feeding depends on body size, body plans have size limits, and the ratio of surface area to volume controls heat loss and metabolic rate. The outcomes are reasoned ideas, not predictions.

The biggest animal, the blue whale, eats tiny animals called krill.

An elephant eats lots of plants every day. A tiny hummingbird visits flowers.

Animals of different sizes eat in very different ways. The blue whale, the largest animal ever, eats almost nothing but krill, tiny shrimp-like animals. It swims into a big swarm with its mouth open and strains them out with plates called baleen. The biggest blue whales can eat up to 6 tons of krill a day.

An African elephant eats 75 to 150 kilograms of plants a day and spends about 16 hours a day eating. A bee hummingbird is so small that people often mistake it for a bee as it hovers at flowers.

Feeding strategies scale with size. NOAA Fisheries describes blue whales as filter feeders that eat almost exclusively krill, engulfing swarms and sieving them through baleen plates; the largest may eat up to 6 tons a day. San Diego Zoo Wildlife Alliance gives an elephant's daily intake as 75–150 kg, about 4 to 6 percent of its body mass, over about 16 hours of feeding.

At the other extreme, the bee hummingbird, under 2 g, beats its wings about 80 times a second in flight (Audubon). Each of these lifestyles depends on a size match between the animal, its food and the way it gathers it.

What if a whale were small? It might not eat enough krill.

What if an ant were as big as an elephant? Its hard shell might be too heavy.

Make every animal dog-sized and food chains would get muddled. A dog-sized blue whale could not gulp a whole swarm of krill, and if krill grew to dog size too, there would be no tiny food to strain out of the water at all. Animals that now eat much smaller animals would need new ways to feed.

Body plans would struggle too. Insects and spiders wear their skeleton on the outside. A thicker shell is needed to hold up a bigger body, and that is one reason most animals with an outside skeleton are small. A dog-sized ant might be too heavy for its own shell to work well.

Collapsing the size range might break many feeding relationships built on size differences, such as filter feeding on small prey, and might force very different diets. Which food chains would survive is unknowable; the point is that size and diet are linked.

Body plans also scale unevenly. OpenStax notes that exoskeletons must be molted to grow and must thicken to bear more weight, which limits most animals with an exoskeleton to a relatively small size. A dog-sized arthropod might face heavy, cumbersome armor and risky molts. Endoskeletons, with muscles attached outside the bones, cope better with large size.

Small animals lose their warmth quickly. They need to eat lots to stay warm.

Big animals keep warm more easily. Elephants flap their big ears to cool down.

Imagine a small cube and a big cube. The big cube has much more inside it for each bit of outside surface. Animals are the same: a small animal has lots of skin for its weight, so it loses body heat fast and must burn food quickly to stay warm.

Scientists measured this. Gram for gram, a mouse uses about 12 times as much oxygen as an elephant, because its body runs much faster. Big animals have the opposite problem: they can struggle to lose heat. An elephant's ears work like a radiator: when it flaps them, the blood in their many vessels gives off heat.

As an object grows, its volume rises faster than its surface area, so its surface-to-volume ratio falls (for a sphere it is 3/r). OpenStax Biology 2e explains that smaller endotherms therefore have more surface for their mass, lose heat faster and have a higher basal metabolic rate per unit of body weight; some small endotherms save energy with daily torpor.

OpenStax's figure compares a mouse of about 35 g using about 890 mm³ of oxygen per gram per hour with an elephant of about 4,500 kg using about 75, roughly a twelvefold difference. Large animals face the reverse problem of shedding heat; elephant ears act as radiators. In a one-size world, these trade-offs would all collapse to one setting.

Tiny animals and huge animals each have their own way of living.

A bee hummingbird's egg is the size of a coffee bean!

Real animals come in many sizes. Tiny krill feed the biggest animal that has ever lived. Elephants munch huge amounts of plants. A bee hummingbird lays eggs about the size of a coffee bean in a nest barely an inch across, small enough to tuck onto a twig.

If every animal were the same size, the jobs that only tiny or only huge animals can do might not get done. An animal's size is tied to what it eats, how it is built and how it keeps the right temperature.

Body size links to almost every part of an animal's biology: what it can eat, how it is eaten, how it handles heat and how fast it lives. The bee hummingbird's coffee-bean-sized eggs and inch-wide nest (Audubon), the elephant's 16 hours of daily feeding and the blue whale's krill diet each reflect size.

A world of one size is impossible to model precisely, but reasoning from surface area, skeletons and feeding shows why the real range of sizes is not an accident of looks. It is tied to how each animal makes a living.