Smallest tetrapod

Tetrapods are four-limbed vertebrates (i.e., mammals, birds, reptiles and amphibians, but not fishes), and the world's smallest tetrapod species is the Brazilian flea toad (Brachycephalus pulex). Adult males have an average snout-to-vent length of only 7.1 mm (0.27 in); the smallest individual measured was just under 6.5 mm (0.25 in). Adult females are slightly longer, with an average snout-to-vent length of 8.15 mm (0.32 in). It is native to the Serra Bonita mountain range in Bahia, on Brazil's Atlantic coast, hence its alternative name of Serra Bonita toad. The measurements were set out in a paper published in the journal Zoologica Scripta in February 2024.

This means that B. pulex is also the world's smallest species of frog and amphibian, and among the smallest of all vertebrates. According to research published by Professor Theodore W Pietsch (USA) of the University of Washington on 25 August 2005, the smallest vertebrates are male Photocorynus spiniceps anglerfish, with one sexually mature adult measuring 6.2 mm (0.24 in) long collected from a depth of 1,425 m (4,675 ft) in the Philippine Sea.

This species was first discovered in 2009, and was formally described and named in 2011, but the significance of its tiny dimensions was not realized until 2024, when a detailed study of its body size was published by biologists from the Universidade Estadual de Santa Cruz (Brazil). It was first thought that this species was a toad rather than a frog, explaining its common name.

The world's previous smallest-known tetrapod was also a frog: Paedophryne amauensis is a species of microhylid frog native to eastern Papua New Guinea first discovered in 2009. With an average length when fully grown of 7.7 mm (0.3 in) when measured from snout to vent, it is about the size of a housefly. Its measurements were revealed in a study published in PLoS ONE on 11 January 2012.