How miniaturised are frogs in Australia and southern Africa?
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Naturalists familiar with the anuran faunas of Australia and southern Africa will know of the presence of surprisingly small-bodied frogs (e.g. Microbatrachella capensis, https://en.wikipedia.org/wiki/Micro_frog, Snout-Vent length 18 mm) on these landmasses.
However, there have been many new discoveries worldwide in the last few decades. The following re-assessment shows that the smallest-bodied frogs in Australia and southern Africa are not particularly diminutive after all.
We now know that extreme miniaturisation has evolved in at least five families and 15 genera of frogs (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3256195/table/pone-0029797-t005/?report=objectonly and https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0213314).
According to https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3256195/ and https://en.wikipedia.org/wiki/Brazilian_gold_frog and https://en.wikipedia.org/wiki/Paedophryne_amauensis, the smallest-bodied frog - and the smallest-bodied of all vertebrates - on Earth is:
Paedophryne amauensis (https://en.wikipedia.org/wiki/Paedophryne_amauensis).
This species of Microhylidae lives in New Guinea and has SV length 7-8 mm.
The next-most diminutive species of frogs remain a work-in-progress, but are all tropical.
They include, for example:
- Microhylidae: Mini mum (female SV length 11.3 mm), Rhombophryne proportionalis, and Anodonthyla eximia (https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0213314) of Madagascar,
- Eleutherodactylidae: Eleutherodactylus limbatus (https://amphibiaweb.org/species/3021) and E. iberia (https://en.wikipedia.org/wiki/Monte_Iberia_eleuth) of Cuba, and
- Brachycephalidae: Brachycephalus pulex and B. didactylus (https://en.wikipedia.org/wiki/Brazilian_gold_frog) of Brazil.
Where does this leave the frog fauna of southern Africa, much of which is extratropical?
The most diminutive frogs in southern Africa belong to the pyxicephalid genera Cacosternum (Channing et al. 2013, https://www.biotaxa.org/Zootaxa/article/view/zootaxa.3701.5.2) and Arthroleptella (https://en.wikipedia.org/wiki/Arthroleptella).
The species are:
- Cacosternum striatum (Pyxicephalidae), which lives in eastern South Africa, and has female SV length up to 13.7 mm, and
- Arthroleptella subvoce (https://en.wikipedia.org/wiki/Arthroleptella_subvoce), which lives in the southwestern Cape of South Africa, and has female SV length 14 mm.
These are smaller-bodied than the most diminutive species of Mantellidae in Madagascar, namely Wakea madinika, which has female SV length 15-16 mm (https://en.wikipedia.org/wiki/Wakea_madinika and https://en.wikipedia.org/wiki/Mantidactylus and https://en.wikipedia.org/wiki/Paedophryne_amauensis).
The next smallest-bodied species in southern Africa are:
- Cacosternum platys of the Cape Peninsula, with female SV length up to 14 mm but also confusingly stated to be 15.3 mm,
- Cacosternum parvum of the northern Drakensberg (https://en.wikipedia.org/wiki/Mariepskop), with female SV length up to 16.5 mm, and
- Arthroleptella rugosa (https://en.wikipedia.org/wiki/Arthroleptella_rugosa) of the southwestern Cape, female SV length 15.5 mm.
These are about like-size to the smallest-bodied mantellid mentioned above.
What emerges is that the smallest-bodied frogs in southern Africa can no longer even qualify for the most diminutive category on a global basis.
However, most of the miniaturised frogs on Earth depend on tropical rainforests, where consistent humidity protects small animals with permeable skin from dehydration. The southern African species remain remarkable because their habitat is not rainforest but marshland or grassland, subject to considerable desiccation in the dry season, plus frequent wildfires.
Turning now to Australia:
The smallest-bodied species of frog here belongs to Microhylidae and is restricted to small patches of tropical rainforest. This is Cophixalus exiguus (https://en.wikipedia.org/wiki/Scanty_frog), with SV length about 16-19 mm.
Also small-bodied is Litoria microbelos (https://museum.wa.gov.au/explore/frogwatch/frogs/javelin-frog and https://en.wikipedia.org/wiki/Javelin_frog) in the family Pelodryadidae. This tropical species has female SV length less than 20 mm.
In subtropical or temperate Australia, none of the frogs is small enough to be described as diminutive, by global - or even southern African - standards.
Smallest-bodied among these are four of the species of Geocrinia (https://en.wikipedia.org/wiki/Geocrinia) in the family Myobatrachidae, that live at the southwestern tip of the continent. All have SV length 25 mm or less when fully-grown. The species are:
- Geocrinia alba (https://www.inaturalist.org/taxa/25242-Geocrinia-alba),
- G. lutea (https://www.inaturalist.org/taxa/25244-Geocrinia-lutea),
- G. rosea (https://en.wikipedia.org/wiki/Geocrinia_rosea), and
- G. vitellina (https://en.wikipedia.org/wiki/Geocrinia_vitellina).
These species of Geocrinia, although occurring under similar climates to those of some of the southern African species described above, do not occur in marshland or grassland. Instead, they are restricted to temperate forests subject to enough seasonal desiccation to allow occasional wildfires.
The overall result in the case of Australia is somewhat paradoxical.
On one hand, the frogs hardly show miniaturisation, given that:
- none of the Australian species is particularly small-bodied,
- there are no counterparts for the somewhat diminutive species of temperate-zone marshland and grassland in southern Africa, and
- even in tropical rainforest in Australia the smallest-bodied species retains SV length exceeding 15 mm.
On the other hand, New Guinea - which contains the most diminutive frog of all - has, for most of the Pleistocene, been part of the same landmass (https://en.wikipedia.org/wiki/Sahul) as Australia. In this sense the 'island continent' is the one that, by producing the genus Paedophryne, has surpassed all other continuents in the miniaturisation of frogs.
Furthermore, pelodryadids (https://en.wikipedia.org/wiki/Pelodryadidae) show remarkable phylogenetic plasticity - however one classifies them into rather arbitrary genera - in ranging from large-bodied frogs (https://en.wikipedia.org/wiki/White-lipped_tree_frog) to a species as small-bodied as L. microbelos.
Comments
https://www.smithsonianmag.com/smart-news/three-newly-discovered-frog-species-are-among-worlds-smallest-180971819/
The Cacos come out after heavy rain and breed in temporary and shallow permanent pools. They seem to burrow down at these seasonal wetlands that are often dry for long periods. In more arid areas you will find them in dry river beds under large embedded boulders where there is still soil moisture. So I wonder if they need substrate that maintains moistures throughout the dry season. I don't know much about Australia, but perhaps this sort of habitat is lacking and frogs need to be large enough to protect their water levels (surface to volume ratio scaling).
Interesting thoughts. I wonder if there isn't a second variable in play here other than just desiccation resistance. Of course, my experience with anurans is mostly in North America, so I have limited experience with many of the species you are discussing.
I've always thought of smaller body size in Anurans as an adaptation to the ephemerality (is that a word) of their larval habitat?
The solutions we see appear to be:
metamorphose quickly at a small size and stay small (e.g. Litoria microbelos, Pseudacris ocularis, many species you mention)
metamorphose quickly at a small size and grow in your adult form (e.g. Bufonids, Pelobatids and others)
direct development (e.g. Eleutherodactyline frogs)
breed in a more permanent water source and have a longer slower growing larval stage (i.e. Ranids)
creative solutions to lack of water (e.g. some Dendrobatidae)
Just some thoughts.
@sandboa in southern Africa 'ephemerality' is taken to extremes with Pyxicephalus and Tomopterna, while the small Arthroleptella and Anhydrophryne have specialise to have terrestrial egg nests within wet seeps which tend to stay moist throughout the year (usually with deep peaty soils). Cacosternum are small, but also seasonal and opportunistic breeders, so there doesn't seem to be an obvious pattern in southern Africa.
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