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Animal lists · 12 entries

The Smallest Animals on Earth

The smallest known animals are microscopic — certain parasitic wasps and rotifers measure fractions of a millimetre — but among vertebrates the title goes to species such as the Paedocypris fish and the bee hummingbird, ranked here by minimum adult body length rather than egg or larval size.

Updated 25 July 2026

Smallest-animal lists often mix eggs, larvae and adults, or compare a parasitic worm to a mammal as though the measurement were the same thing. A fertilised egg is not an animal in the same sense as the adult that hatches from it, and many invertebrates have life stages that differ in size by orders of magnitude.

This list ranks adult body length for multicellular animals, with separate notes where the record depends on sex or life stage. Single-celled organisms are excluded — they are not animals. Where the smallest known individual belongs to a little-studied group, that uncertainty is stated rather than glossed over.

How this list was compiled

Entries are ranked by minimum reliably documented adult body length, not egg size, larval size or weight. Single-celled organisms are excluded. Sexual dimorphism is noted where females or males are consistently smaller. Figures from taxonomic descriptions of one or two specimens are flagged as provisional. Parasitic and free-living animals are included together because the question is about size, not lifestyle.

The Smallest Animals on Earth

  1. Dicyemid mesozoans

    ~0.1–0.5 mm

    Among the smallest multicellular animals. Simple parasites of cephalopod kidneys; rarely encountered outside specialist studies.

  2. Rotifers

    ~0.05–2 mm

    Many species are well below a millimetre. Bdelloid rotifers are fully functional at sizes invisible to the naked eye.

  3. Tardigrades

    ~0.05–1.2 mm

    Commonly called water bears. Fully grown adults are typically under half a millimetre.

  4. Paedocypris fish

    ~7.9 mm (mature female)

    Among the smallest known vertebrates by minimum mature length. Males can be slightly larger.

  5. Bee hummingbird

    ~5.5 cm; ~1.6–2 g

    The smallest living bird. Endemic to Cuba.

  6. Etruscan shrew

    ~3.5 cm body; ~1.8 g

    A contender for smallest mammal by mass. Skull length is sometimes used instead of body length, which changes the ranking.

  7. Kitti's hog-nosed bat

    ~2.9–3.3 cm; ~2 g

    Also called the bumblebee bat. Among the smallest mammals by length and mass.

  8. Speckled padloper tortoise

    ~6–10 cm shell

    The smallest tortoise species. Endemic to South Africa.

  9. Brookesia micra chameleon

    ~29 mm (male)

    Among the smallest reptiles. Discovered in Madagascar; total length including tail is slightly greater.

  10. Pygmy goby

    ~8–9 mm

    Marine fish that complete their life cycle at sizes comparable to Paedocypris.

  11. Fairyfly wasps

    ~0.14–0.5 mm

    Parasitic wasps in the family Mymaridae. Some are smaller than many single-celled protists in volume but are multicellular animals.

  12. House dust mites

    ~0.2–0.4 mm

    Arachnids, not insects. Included because they are among the smallest animals people encounter domestically.

Key facts

  • The smallest animals are microscopic — rotifers, tardigrades and parasitic wasps measure fractions of a millimetre as adults.
  • Among vertebrates, tiny fish, shrews and the bee hummingbird compete for the title depending on whether length or mass is used.
  • Egg size is not adult size; many smallest-animal lists conflate the two.
  • New smallest-vertebrate records appear whenever poorly surveyed regions are studied — current holders should be treated as provisional.
  • Sexual dimorphism matters: the smallest mature female of a fish species may be smaller than the smallest male of another.
  • Single-celled organisms are excluded because they are not animals in the taxonomic sense used here.

The Smallest Animals on Earth: frequently asked questions

What is the smallest animal in the world?
Several microscopic animals qualify, including some parasitic wasps and rotifers under half a millimetre long as adults. Among animals visible without a microscope, tardigrades and dust mites are among the smallest.
What is the smallest mammal?
By mass, the Etruscan shrew at around 1.8 grams is a leading candidate. By length, Kitti's hog-nosed bat at roughly three centimetres competes closely. Rankings depend on which measurement is used.
What is the smallest bird?
The bee hummingbird of Cuba, at around 5.5 centimetres long and roughly two grams. It is smaller than many insects but is a fully warm-blooded vertebrate.
What is the smallest fish?
Paedocypris progenetica, with mature females around 7.9 millimetres, is among the smallest known vertebrates. Several pygmy gobies are comparable in size.
Are tardigrades the smallest animals?
No, though they are among the most famous tiny animals. Several parasitic wasps and rotifers are smaller. Tardigrades are typically 0.3 to 0.5 millimetres as adults.
Why exclude single-celled organisms?
Because this list ranks animals — multicellular organisms in the kingdom Animalia. Single-celled protists can be smaller but belong to different groups entirely.

Sources

How we check facts: our sourcing standards →

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