In the 19th century, German zoologist Christian Bergmann pondered a simple question: why are some animals so small? His answer, that the size of a warm-blooded animal increases as its habitat cools, remains a rule in biology to this day.
“Bergmann highlighted that smaller species tend to live in warmer climates. This pattern has to do with surface area and volume: smaller animals lose heat more quickly and have difficulty maintaining their body temperature when it is very cold,” says Dr Simon Loader, lead curator of vertebrates at the History Museum Natural. “Whatever the reasons, these little species are fascinating,” he says.
With much of life on Earth still unknown, scientists discover new tiny organisms every year, redefining what is considered the smallest of their species – and some claims about who is the smallest of all are hotly contested.
Tiny creatures can sometimes struggle to get the same conservation attention as their larger, charismatic counterparts. “Small species often go unnoticed or overlooked,” says Paul Rees, nursery manager at Kew Gardens. We asked scientists to tell us about the smallest creatures of their kind.
Smallest reptile: Brookesia nana nano-chameleonMadagascar
First described as a species (discovered by science) in 2021, a male Brookesia nana it is just 20 mm (0.8 in) long – the size of the head of a matchstick – and is found in the tropical forests of northern Madagascar. Females are larger, growing to almost 30 mm. Researchers believe the world’s smallest reptile is critically endangered, found in an area that has been severely degraded by deforestation.
Despite its overall size, Brookesia nana is considered notable for its disproportionately large male genitals, known as hemipenes in snakes and lizards.
“Miniaturized males may need larger hemipenes to allow for better mechanical fit with larger females,” says Loader.
Madagascar is famous for its small animals, including several miniaturized frogs and Madame Berthe’s mouse lemur, the world’s smallest primate.
Smallest bird: the bee hummingbird, Cuba
The bee hummingbird (Mellisuga helenae) weighs as much as a paperclip and measures just 5 to 6 cm (1.9 to 2.3 in) long. Found mainly in dense forests and on the edge of woodlands in Cuba, its eggs are the size of a coffee bean and its wings beat 80 times per second. Due to habitat destruction on the Caribbean island, scientists are concerned about its survival.
“Its population is believed to be declining at a rate of 20-29% per decade due to forest loss and degradation, and it has already disappeared from many areas where it was previously widespread,” says Dr Ian Burfield, Global Scientific Coordinator from BirdLife International. .
“Like other hummingbirds, it feeds on the nectar of a variety of flowering plants and plays an important ecological role as a pollinator, so its decline is doubly worrying,” he says.
Smallest insect: Dicopomorpha echmepterygis, a parasitic wasp, USA
The world’s smallest insect is so small that it is smaller than some single-celled organisms. Just 0.139 mm long, the US parasitic wasp spends most of its life inside its host, the bark louse.
“A species of ‘fairy fly’, they are small wasps that develop as parasitoids, with eggs laid within lice eggs, which are not very large,” says Dr. Gavin Broad, chief curator of entomology at the History Museum Natural. .
“A female wasp develops in the host egg, eating most of the contents, accompanied by one to three males, which are wingless, have a rudimentary head and are generally streamlined because they never emerge from the egg; they’re just there to fertilize the females,” says Broad.
Smallest amphibian: Paedophryne amauensis frogPapua New Guinea
This little frog is so small that it doesn’t have tadpoles. Described in 2012, the frog lives in the leaf litter of the rainforest, feeding on ticks and mites.
“Unlike many other frogs, its life cycle does not include the aquatic tadpole phase. Instead, tiny frogs hatch directly from eggs that are deposited in damp leaves on the forest floor,” says Dr. Jeff Streicher, chief curator of herpetology at the Museum of Natural History. “Adults feed on small invertebrates found in the same leaf litter. This lifestyle is common in other species of small frogs, which highlights the essential role that leaf litter microhabitats play in the survival of these small amphibians.”
Smallest mammals: the Etruscan shrew and the bee bat
For mammals, it is difficult to separate two small competitors. The Etruscan shrew, found in parts of Eurasia and northern Africa, weighs between 1.2g and 2.7g on average. It is solitary and active mainly at night while feeding on invertebrates. The little shrew lives a short life, rarely surviving a second winter, says Paula Jenkins, senior curator of mammals at the Natural History Museum.
The other miniature mammal considered the smallest in the world is the bee bat, also called the hog-nosed bat, found in two isolated populations in Thailand and Myanmar. It also weighs around 2g, with a wingspan of up to 145mm and a body length between 29mm and 33mm.
“It roosts in extensive caves in limestone outcrops near rivers,” says Jenkins. “Individuals roost separately, at some distance from each other. They hunt invertebrates in the upper forest using echolocation to detect their prey in flight, and they can also catch prey in the foliage.”
Smallest flowering plant: Wolffia globosanative to Asia but found throughout the world
Sometimes known as duckweed, Wolffia globosa it has the fastest known growth rate of all plants and can quickly cover entire bodies of water. Despite not having common plant organs, such as leaves, roots and stems, it produces the smallest known fruits and is highly nutritious.
Tom Pickering, senior display greenhouse manager at Kew’s Royal Botanic Gardens, says The plant is similar to a weed in appearance and nature. “This vigorous, buoyant aquatic plant is grown in tanks at the Kew Tropical Nursery and is used globally for animal feed, medicine and food. Despite its size, Wolffia is from the same plant family as titan arum, a flowering plant with the largest inflorescence in the world”, he states.
Smaller fish: depends on who you ask…
The title of smallest fish in the world is hotly contested. According to Guinness World Records, it is the 6.2mm male Photocorynus spiniceps, a species of deep-sea anglerfish found in the Philippine Sea that is sexually parasitic. It attaches itself to the much larger female – a common trait in anglerfish – effectively turning her into a hermaphrodite. It feeds, swims and ensures its survival – it only cares about reproduction.
But given the female Photocorynus spiniceps is several times greater than the male, other researchers say that the title belongs to the Paedocypris progenetics from Sumatra, which swims in peat bogs, growing up to 7.9 mm when adult. The tiny Indonesian fish was scientifically described in 2006 and proclaimed the smallest – a claim quickly disputed by researchers who have studied anglerfish.
Smallest cactus: Blossfeldia liliputanaArgentina and Bolivia
O Blossfeldia liliputanaThe epithet comes from the word ‘lilliput’, which means little person or being, explains Paul Rees, nursery manager at Kew Gardens. “It also refers to the imaginary country inhabited by tiny people in Gulliver’s Travels,” he notes.
Found growing on rock faces and in crevices at high altitudes in Bolivia and Argentina, it can withstand extreme droughts, losing up to 80% of its moisture content. Despite its versatility, the smallest cactus in the world is increasingly threatened by collectors.
“Over the years, this species has been desired by collectors and, due to its very slow growth rate, several plants have been hunted in the wild. Although its wide distribution means it is listed as ‘least concern’, poaching remains the main threat to this species.”
The smallest fungi: waiting to be discovered
After the little one Mycena subcyanocephala was photographed in Taiwan last year and was went viral on social media, some incorrectly said it was the smallest in the world. However, with around 2 million species of fungi waiting to be discovered, there are likely to be many microscopic organisms waiting to be found, says Ester Gaya, senior research leader in mycology at Kew Gardens.
“Mycena subcyanocephala is one of the smallest species of fungi in the world. Regardless of its diminutive size and ethereal appearance, this species of fungus plays its role in nature’s complex recycling system. Mycena species are saprobes, meaning they live on decaying organisms, helping to clean up unwanted ‘trash’ from our forest.”
With important functions ranging from nutrient recycling to carbon sequestration, she says, “As with all small things, it is often the coordinated work of multiple small fungi that has a big impact on our ecosystems.”