Six new species of clawed frog have been discovered by a Canadian-led team of researchers hunting for the mysterious "lost ancestors" of some of their cousins.

All six new species are relatively small (around five centimetres) clawed frogs from Central and West Africa, related to the species that was once shipped around the world for use as living pregnancy tests. (When female frogs were injected with the urine of a pregnant woman, they would lay eggs.)

Ben Evans

The study was led by McMaster Unviersity zoologist Ben Evans. He found many of the new species of African clawed frogs while looking for the 'lost ancestors' of other, related species. (McMaster University)

Ben Evans, a zoologist at McMaster University in Hamilton, had previously discovered two other new species of African clawed frogs and found that they were polyploid — they had multiple sets of DNA that suggested they formed from the merger of two different species.

Most animals, like us, are diploid — we have two sets of chromosomes, half our mother's and half our father's.

Instead of getting half of its mother's genes and half of its father's genes as we normally do, the founding member of a new polyploid species gets all its mother's genes and all its father's genes. That might be advantageous, for example, if both species had immune genes that made them resistant to different things, Evans suggests.

A species that arose from a diploid mother and a diploid father would have four sets of genes, making it tetraploid.

Multiple mergers

With African clawed frogs, "what's bizarre is that the duplication hasn't happened just once," Evans said.

There exist not just tetraploid species, but also octaploids (eight sets of chromosomes) and dodecaploids (12 sets), suggesting some arose from multiple species mergers.

Evans thinks the independent species that merged to give rise to the new species "could still be hopping around."

Because of that, he has spent recent years hunting for those "lost ancestors." The six new species were discovered in the process. Four are tetraploid species and two are dodecaploid species.

Mating pair of new species

This is a mating pair of one of the six new species of African clawed frog described in the new paper. (Vaclav Gvozdík)

Genetic analysis shows that none of the new tetraploid species are ancestors of known octoploids or dodecaploids.

"Basically, my efforts have resulted in a greater understanding of the diversity that's out there," Evans said, "but I still haven't found the species that I predicted should be out there if they haven't gone extinct."

African clawed frogs are a group of frogs that are unusual in that they usually spend their entire lives underwater, even as adults, Evans said. He has found them in a huge range of environments ranging from remote, pristine lakes to sewage ditches full of human feces. Some of the new species were discovered in cities where people had dug trenches and filled them with water to keep fish in.

While the group is very diverse, with species that have widely different ranges and habitats, they're very difficult to tell apart.

"They all look the same," Evans said.

Even species that are as distantly related as humans and orangutans are almost identical. They can only be distinguished by their DNA, subtle differences in bone structure and their calls.

Those characteristics of the six new species are described in a new paper published today in the journal PLOS ONE. The study was co-authored by Timothy Carter at the University of Guelph, along with researchers in the U.S., Czech Republic, Belgium and the U.K.

Evans hasn't given up on his original goal of finding some of the lost ancestors, however. He's going back to Ghana again next year to look for more frogs.