Smallest Animals in the World: Tiny Mammals, Birds and Frogs

How do scientists decide which animal is smallest?

“Smallest” sounds simple, but scientists measure tiny animals in different ways, so the answer depends on the method. Mammals can be compared by weight or by body length. Frogs and lizards are usually measured by snout–vent length, from the tip of the nose to the vent (the rear body opening), which leaves out the tail. Fish are measured by standard length, which leaves out the tail fin.

Sex matters too. In many tiny species the females are larger, because they have to carry eggs, so a record may refer only to adult males. Finally, the animal must be fully grown and able to breed: a newly hatched larva does not count. New, even smaller species are still being discovered, especially in leaf litter and on tropical islands, so several of these records are recent and may change again.

The smallest animals at a glance

1. Myxozoans: the smallest animals of all

The smallest known animals are myxozoans, a group of parasites that live inside fish and worms. Surprisingly, they are cnidarians, the same group as jellyfish and corals, but they have been reduced to just a few cells. Many species never grow larger than 0.02 mm, and Myxobolus shekel is no more than about 8.5 micrometers (thousandths of a millimeter) when fully grown.

Myxozoans are far too small to see without a microscope, and most people only meet them indirectly: one species causes whirling disease in trout. Because they are so unusual, most “smallest animal” lists skip them and focus on animals with eyes, legs or fins.

2. Fairyflies: the smallest insects

Fairyflies are tiny wasps in the family Mymaridae that lay their eggs inside the eggs of other insects. The males of Dicopomorpha echmepterygis, described from Illinois, are the shortest insects known: eight measured males ranged from 0.139 to 0.24 mm long. They are blind and wingless, mate with their sisters inside the host egg, and die without ever leaving it. The females are larger, at about 0.55 mm.

The smallest insect that can fly is another fairyfly, Kikiki huna, at about 0.15 mm long. Its name comes from Hawaiian words meaning “tiny bit.” At this size, air feels thick and syrupy, so these insects have bristly, feather-like wings instead of flat ones.

The smallest insect that is not a parasite is the featherwing beetle Scydosella musawasensis from Central and South America. Specimens collected in Colombia in 2015 measured 0.325–0.352 mm long, and they feed on fungi.

3. Patu digua: the smallest spider

The smallest spider is usually said to be Patu digua, a tiny orb-weaving spider from Colombia whose males have a body only about 0.37 mm (0.015 in) long. That is roughly a fifth of the size of a pinhead. A close relative, the Samoan moss spider (Patu marplesi), is almost as small, and experts still debate which species holds the record.

4. Brazilian flea toad: the smallest frog and vertebrate

The title of smallest vertebrate (animal with a backbone) now goes to the Brazilian flea toad, Brachycephalus pulex, from the Atlantic Forest of Bahia, Brazil. Adult males average about 7.1 mm (0.28 in) from snout to vent, females about 8.2 mm, and the smallest adult measured was just 6.45 mm. It could sit on a fingernail with room to spare.

Until recently, the record belonged to Paedophryne amauensis of Papua New Guinea, which averages about 7.7 mm (0.30 in). It was found in 2009 by researchers who tracked its insect-like calls through the leaf litter and was formally described in 2012. Tiny frogs like these have fewer bones and toes than larger relatives, and they hatch straight from the egg as miniature frogs, with no tadpole stage.

5. Paedocypris and the smallest fish

The smallest fish include Paedocypris progenetica, a see-through relative of carp from the acidic peat swamps of Sumatra and Borneo, where mature females can be as small as 7.9 mm (0.31 in). Its skull never fully develops, so the brain is not covered by bone. The stout infantfish (Schindleria brevipinguis) of Australia’s Great Barrier Reef is similar in length, and one specimen weighed only 0.7 milligrams.

A deep-sea anglerfish, Photocorynus spiniceps, has mature males just 6.2–7.3 mm long, shorter than any other adult fish. However, these males live permanently fused to females that grow up to about 5 cm (2 in), so many scientists do not count them as free-living record holders.

6. Nano-chameleon: the smallest reptile

The nano-chameleon, Brookesia nana, was described in 2021 from a single mountain forest in northern Madagascar. Adult males are about 22 mm (0.87 in) long including the tail, with a body of about 13.5 mm, and females reach about 29 mm. It lives on the forest floor rather than in trees, and its tiny home range leaves it vulnerable to deforestation.

Other tiny reptiles include:

• Jaragua dwarf gecko (Sphaerodactylus ariasae): 14–18 mm from snout to tail base and about 0.13 g; it can fit on a US quarter. It lives in the Dominican Republic.
• Barbados threadsnake (Tetracheilostoma carlae): one of the smallest snakes, about 10 cm (4 in) long and as thin as a spaghetti noodle. It lays a single egg.
• Speckled padloper (Chersobius signatus): the smallest tortoise, with a shell of 6–10 cm (2.4–3.9 in), from western South Africa.

7. Bee hummingbird: the smallest bird

The bee hummingbird (Mellisuga helenae) of Cuba is the smallest bird in the world and the smallest warm-blooded vertebrate. Males average about 5.5 cm (2.2 in) long and 1.95 g (0.07 oz), lighter than a US penny, while females are slightly bigger at about 6.1 cm (2.4 in) and 2.6 g. Its two eggs are each about the size of a coffee bean.

Despite its size, it is a strong flier that feeds on nectar and small insects, and it may visit around 1,500 flowers a day. Read more in What is the smallest bird in the world?

8. Etruscan shrew and bumblebee bat: the smallest mammals

Two mammals share the title, depending on whether you measure weight or length:

• Etruscan shrew (Suncus etruscus): the lightest mammal, usually about 1.8 g (0.06 oz) and 1.3–2.5 g in total, with a body 3–5.2 cm (1.2–2 in) long plus a 2.4–3.2 cm tail. It lives from Mediterranean Europe and North Africa to Southeast Asia. To stay warm, its heart can beat up to about 1,500 times a minute, and it eats 1.5–2 times its own body weight in insects every day.
• Kitti’s hog-nosed bat (Craseonycteris thonglongyai): also called the bumblebee bat, it is the shortest mammal at 29–33 mm (1.1–1.3 in) long and weighs about 2 g. It roosts in limestone caves in western Thailand and southeast Myanmar and is listed as Near Threatened.

Very small mammals and birds lose body heat quickly because they have a lot of skin surface for their volume. That is why the smallest ones must feed almost constantly, and why no warm-blooded animal is as small as the tiniest frogs or fish.

Other tiny record holders

Many familiar animal groups have their own miniature champions:

• Smallest primate: Madame Berthe’s mouse lemur of western Madagascar, with a body about 9.2 cm (3.6 in) long and a weight of around 30 g (1 oz).
• Smallest wild cat: the rusty-spotted cat of India, Nepal and Sri Lanka, at 0.9–1.6 kg (2–3.5 lb), rivaled by Africa’s black-footed cat.
• Smallest carnivore: the least weasel, whose females can weigh as little as 29 g (1 oz). See What is the smallest carnivore?
• Smallest shark: the dwarf lanternshark of the Caribbean coast of South America, which reaches only about 20 cm (8 in).
• Smallest crustaceans: tantulocarids, parasites of other crustaceans, some just 0.085 mm long.

For the opposite end of the scale, see the largest animals in the world.

Why are some animals so small?

Scientists call the evolution of very small body size miniaturization. It often happens in places where tiny animals can find plenty of food and shelter, such as leaf litter, moss, peat swamps and small islands. Small animals can live on tiny prey such as mites and springtails, hide in narrow spaces and need less food in total.

Being tiny has limits, though. Miniature frogs lose bones and toes, the smallest insects lose wings, eyes or even parts of their nervous system, and the smallest birds and mammals live close to the edge of how fast an animal can burn energy. Many of these species also live in a single forest or island, which makes them especially vulnerable to habitat loss.