Saddleback toads are members of the genus Brachycephalus, being very small anurans in the family Brachycephalidae. The genus is further divided into two species groups: the pumpkin toadlets and the flea frogs (or flea toads; once placed in genus Psyllophryne). Some pumpkin toadlets are toxic; their often bright colours are thus considered aposematic. In contrast, the overall brown and more frog-like flea frogs are some of the smallest frogs in the world, measuring around in snout–vent length.
The saddleback toads live among leaf litter in the Atlantic rainforest of southern Bahia to Santa Catarina in southeastern Brazil, ranging from near sea level to an altitude of , with most species restricted to highland cloud forest. They are mostly diurnal, feeding on leaf-litter invertebrates and breeding through direct development, with eggs laid on land and hatching into young frogs, without a tadpole stage.
The majority of the Brachycephalus species have only been discovered and described in the 21st century. Most species have very restricted ranges and so are seriously threatened.
Description
The common name "saddleback toad" refers to the bony "shields" that cover their vertebrae; these have an unclear function in life and may not be present in some Brachycephalus species. The skin of the saddleback toads can be rugose (bumpy) or smooth. Their digits (toes and fingers) are reduced compared to most other frogs, with typically only two to four being clearly visible on each foot/hand. Some of the bones in their skull are also reduced.
The genus can be divided into two main subgroups: the pumpkin toadlets and the flea frogs. The pumpkin toadlet group comprises most species in the genus, overall having a toad-like morphology and being unable to jump well. The second group are the flea frogs or flea toads (B. didactylus, B. hermogenesi, B. pulex, and B. sulfuratus), which resemble leptodactylid frogs in appearance and morphology and are better jumpers. The name "flea frog" is also sometimes used for the unrelated Adelophryne.
Brachycephalus are very small frogs and adults range from in snout–to–vent length. Females tend to be larger than males of the same species. The flea frogs or flea toads are generally the smallest at in adults, but overlapping to some extent with the smallest species of pumpkin toadlets. Adults of the pumpkin toadlets are at least , with most species about , and the largest like B. darkside and B. margaritatus at least . At , B. pulex has the smallest minimum and maximum adult size reported in the genus. B. dacnis is similarly tiny, with a minimum snout–vent length of . This makes them the smallest known frogs, with only the northeast Brazilian Adelophryne michelin, and Cuban Eleutherodactylus iberia and E. limbatus approaching their tiny dimensions. In other continents, the only similar-sized or slightly smaller frogs are a few species in the genera Mini, Paedophryne and Stumpffia. Newly hatched saddleback toads are much smaller than the already tiny adults. For example, they are typically in B. ephippium, a species where adults are at least .
Colour and toxicity
Members of Brachycephalus vary greatly in colour and this is often useful for species identification. Individual variation may also be seen within each species, especially in the details of their pattern.
The flea frogs or flea toads are well-camouflaged, overall being brown with patterns of grey or near-black, and sometimes with a few poorly defined yellowish spots below. The remaining Brachycephalus species are at least partially yellow or orange in color, on both their ventral (underside) and dorsal surfaces; this orange coloration is referenced in the common name of these pumpkin toadlets. In some species, their yellow or orange coloration is almost uniform throughout their body, though many species are only partially yellow/orange. On the underparts, the yellow or orange may form a spotted or mottled pattern on a dull and dark background. On the upperparts, there can be spots, mottling or patches in black, brown, greenish, whitish or reddish. Two particularly dull-coloured species, B. brunneus and B. curupira, are overall brown, while a few others are essentially all-brown or greenish above (sometimes with whitish on the mid-back), limiting yellow or orange to the underparts. In species where known, including ones that are bright orange-yellow as adults, the newly hatched juveniles are mostly brown overall and may have white markings below.
The bright yellow, orange or reddish colours of the pumpkin toadlets are generally considered aposematic (warning colours), but toxicity has only been studied in a few species. The brightly coloured B. ephippium and B. pernix have tetrodotoxin and similar toxins in their skin and organs, whereas B. nodoterga, which has a much more subdued colouring but still with some yellow-orange, appears to be non-toxic. It is unknown how certain Brachycephalus species attain the strong neurotoxins, but they are possibly absorbed from the small invertebrates they eat, as known from some poison dart frogs and mantella frogs, or produced by bacteria inside their body.
At least two brightly coloured species, B. ephippium and B. pitanga, have bones that are fluorescent, which is visible through the skin of the toadlet when exposed to UV light. It was initially speculated that the fluorescent colour also is aposematic or that it is related to mate choice (species recognition or determining fitness of a potential partner), but later studies indicate that the former explanation is unlikely, as predation attempts on the toadlets appear to be unaffected by the presence/absence of fluorescence.
Voice and hearing
Although not very loud for a frog/toad (even relative to their tiny size), Brachycephalus are most often located in the field by their call. The primary callers are the adult males, but females and young may also call. As many as five calling males may be found in , and at a single location one may hear tens of males at once. Males will defend their small territory from other males, but in most cases voice or visual displays are sufficient and fights are avoided.
The advertising calls by males are performed either from a hidden or an exposed position. Whether a species is dull or brightly coloured does not accurately predict if it prefers an exposed or hidden calling position. In addition to advertising calls, there are aggressive calls directed towards other males when close. The calls of the different species differ to various extent in frequency, speed, structure and other features.
Peculiarly, at least two species of pumpkin toadlets (B. ephippium and B. pitanga) are unable to hear the frequency of their own advertising calls, as their ears are underdeveloped (though they can still hear in other frequencies). They instead appear to rely on other communication methods, such as the vocal sac that inflates when calling, mouth gaping, and waving of their arms. It is speculated that their calling behavior is a vestigial behavior from their ancestral form, whereas their reduced hearing ability is a novel change in these species. Sound production may make them more vulnerable to predators, but there has likely been little direct evolutionary pressure to lose it because of their toxicity. Unlike B. ephippium, B. pitanga and some others, many species in the genus always call from a hidden position, making any signalling by movements inefficient, and it is likely they can hear their own calls. Direct studies of the medium-brightly coloured (mostly dull above, bright below) pumpkin toadlet B. actaeus and the dull-coloured flea frog B. hermogenesi strongly suggest that they are able to hear their own calls.
Classification Brachycephalus lacks a Bidder's organ, but were historically included in the true toad family Bufonidae, where one of the defining features is its presence. Brachycephalus was subsequently placed in Atelopodidae or Brachycephalidae, with the latter being the generally accepted treatment in recent decades. Initially some authorities placed numerous genera in Brachycephalidae, while others limited it to only contain the genus Brachycephalus. Today, Brachycephalus, along with Ischnocnema, comprise the family Brachycephalidae, which is a member of the larger clade Brachycephaloidea. Brachycephaloids are uniquely characterized by undergoing direct development, meaning individuals hatch from eggs as tiny versions of the adult, rather than tadpoles. The placement of Brachycephalus within this clade is shown in the cladogram below:More than 40 species of Brachycephalus are recognized. Within Brachycephalus, there are two rather distinctive albeit polyphyletic groups: the pumpkin toadlets, characterized by their brighter colors (red, orange, yellow, and green patterns) and bufoniform (more stout) body blan; and the "flea-toads", characterized by their dull colors and leptodactyliform (more slender) body plan. Until 2002, the latter group, which is relatively widespread, was regarded as a separate genus, Psyllophryne, but it is now considered a synonym of Brachycephalus. The pumpkin toadlets, which largely are restricted to highlands and foothills, can be further divided into two subgroups: the southern B. pernix subgroup (Paraná and Santa Catarina) and the northern B. ephippium subgroup (São Paulo and north). Among the pumpkin toadlets, colours alone are not a good indicator of phylogenetic relationships. For example, the brown B. curupira is closely related to the golden B. izecksohni and the brown B. brunneus is closely related to the golden B. leopardus, but these species pairs are more distant from each other.
Species
According to Amphibian Species of the World, the following species are recognised in the genus Brachycephalus:
Brachycephalus actaeus Monteiro et al., 2018 Brachycephalus albolineatus Bornschein et al., 2016 Brachycephalus alipioi Pombal & Gasparini, 2006 Brachycephalus atelopoide Miranda-Ribeiro, 1920 Brachycephalus auroguttatus Ribeiro et al., 2015 Brachycephalus boticario Ribeiro et al., 2015 Brachycephalus brunneus Ribeiro et al., 2005 Brachycephalus bufonoides Miranda-Ribeiro, 1920 Brachycephalus clarissae Folly et al., 2022 Brachycephalus coloratus Ribeiro et al., 2017 Brachycephalus crispus Condez, et al., 2014 Brachycephalus curupira Ribeiro et al., 2017 Brachycephalus dacnis Toledo et al., 2024 Brachycephalus darkside Guimarães et al., 2017 Brachycephalus didactylus (Izecksohn, 1971) Brachycephalus ephippium (Spix's saddleback toad) (Spix, 1824) Brachycephalus ferruginus Alves et al., 2006 Brachycephalus fuscolineatus Ribeiro et al., 2015 Brachycephalus garbeanus Miranda-Ribeiro, 1920 Brachycephalus guarani Clemente-Carvalho et al., 2012 Brachycephalus herculeus Folly et al., 2024 Brachycephalus hermogenesi (Giaretta & Sawaya, 1998) Brachycephalus ibitinga Condez et al., 2021 Brachycephalus izecksohni Ribeiro et al., 2005 Brachycephalus leopardus Ribeiro et al., 2015 Brachycephalus lulai Bornschein et al., 2025 Brachycephalus margaritatus Pombal & Izecksohn, 2011 Brachycephalus mariaeterezae Ribeiro et al., 2015 Brachycephalus mirissimus Pie et al., 2018 Brachycephalus nanicus Nune et al., 2025 Brachycephalus nodoterga (Serra Cantareira saddleback toad) Miranda-Ribeiro, 1920 Brachycephalus olivaceus Ribeiro et al., 2015 Brachycephalus pernix Pombal, Wistuba & Bornschein, 1998 Brachycephalus pitanga (Red pumpkin toadlet) Alves et al., 2009 Brachycephalus pombali Alves et al., 2006 Brachycephalus pulex Napoli et al., 2011 Brachycephalus puri Almeida-Silva et al., 2021 Brachycephalus quiririensis Pie & Ribeiro, 2015 Brachycephalus rotenbergae Nunes et al., 2021 Brachycephalus sulfuratus Condez et al., 2016 Brachycephalus tabuleiro Mângia et al., 2023 Brachycephalus toby Haddad et al., 2010 Brachycephalus tridactylus Garey et al., 2012 Brachycephalus verrucosus Ribeiro et al., 2015