Research uncovers inbreeding in toads

During breeding, the call of the male leopard toads is a deep snoring sound repeated every few seconds and in chorus sounds like a tractor or motorcycle engine.

Traffic and habitat loss are annihilating the endangered western leopard toad, but their numbers are also dwindling because of inbreeding, says Jessica da Silva, a post-doctoral researcher with the SA National Biodiversity Institute (SANBI).

Dr Da Silva has picked up the inbreeding problem from studying dead toads picked on the roads by volunteers and by Sanbi and City staff over the past two decades.

“These roadkill provided the necessary samples to study the genetic diversity and structure of local communities of these enigmatic creatures, which only occur in Cape Town and Agulhas,” she says.

“Genetic diversity provides the capacity for species to evolve in response to environmental change, and measuring it allows us to establish the health and robustness of the populations.”

She has found increased inbreeding over only four generations.

“These changes are likely due to severe impacts on the population with 91% of natural habitat lost in the study area, which covers Constantia, Bergvliet, Tokai, Kirstenhof, Lakeside, Steenberg.”

There is also evidence of at least three “prehistorical bottlenecks” – where populations declined dramatically – that likely contributed to the inbreeding.

“Although declines in gene-pair richness were not detected, the inbreeding and change in genetic structure are early warning signs of genetic attrition (erosion). We cannot just go on as normal.”

Dr Da Silva says this pilot project provides a baseline for future monitoring, with the ultimate goal of tracking long-term trends. This can guide conservation actions to ensure that the species survives into the future.

Sanbi ecologist Professor Tony Rebelo says a possible reason for the decline might be the lack of suitable breeding ponds, due to wetland drainage.

“Increasingly, wetlands are being filled in, and existing wetlands are maturing to reed beds, squeezing out the western leopard toad breeding sites. This allows fewer males to dominate the remaining breeding sites, which may be contributing to the inbreeding detected.”

Professor Rebelo says western leopard toads go courting every year for a few days, from mid-July to September, usually late August.

“You can’t miss them. They sound like a bunch of Hell’s Angels on steroids, heard from over a kilometre away. They’re not called ‘snoring toads’ for nothing!

“We still don’t know what triggers their breeding, but it’s some combination of full moon and rain.”

If you see a toad in the road, safely move it to the side of the road in the direction it was heading and do not move toads around, he says.

Toad-rescue volunteers are desperately needed in Tokai, Constantia and Sweetvalley, according to Professor Rebelo.

If you would like to do evening patrols or help with night rescues, volunteers are also welcome, contact Philippa at 082 630 0187.