Smallest amphibian

The world's smallest species of amphibian is the Brazilian flea toad (Brachycephalus pulex), which was scientifically named and described in January 2011 but not studied in depth until a detailed investigation of its body size, published in Zoologica Scripta in February 2024. Native to the Serra Bonita mountain range in Bahia, on Brazil's Atlantic coast, adult males of this species have an average snout-to-vent length of 7.1 mm (0.27 in), which is smaller than a typical garden pea, with the smallest examined specimen measuring just under 6.5 mm (0.25 in). Adult females are slightly longer, at 8.15 mm (0.32 in) on average.

This means that B. pulex is also the world's smallest species of frog and tetrapod (four-limbed animal), 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.

The world's previous smallest-known amphibian was Paedophryne amauensis, a species of microhylid frog native to eastern Papua New Guinea. 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 documented in the journal PLoS ONE on 11 January 2012.

B. pulex 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.