WA and NSW cane toads are bigger, stronger and can travel further, but they lag behind Queensland toads in one significant feature — testicle size.

Key points: Scientists believe Queensland cane toads have bigger testicles because the species is well established

Scientists believe Queensland cane toads have bigger testicles because the species is well established They think invading toads in NSW and WA don't have as much energy to invest in testes size

They think invading toads in NSW and WA don't have as much energy to invest in testes size Invading toads have less competition to fertilise eggs, so don't require large testicles compared to Qld toads

In a study published today in Biological Letters, Chris Friesen from the University of Wollongong, working with Professor Rick Shine from the University of Sydney, measured the size of testicles in toads from Queensland, New South Wales, and Western Australia.

The testicles of Queensland toads were found to be a whopping 30 per cent bigger than their southern and western rivals.

Dr Friesen is putting the difference down to Queensland toads being well established while Western Australian and New South Wales toads are on the frontline of invading new territory.

"Those toads at the edge of their distribution actually have smaller testes, about 30 per cent smaller, than those in the core," Dr Friesen said.

The bigger and more active toads on the invasion frontline have smaller testicles. ( Supplied: M G Swan )

Testicle trade-off

Researchers have previously found these frontline toads are generally bigger, stronger, and able to travel greater distances.

But it seems that the superior physical prowess comes at a cost, and that cost is the size of their testicles.

"There seems to be a trade-off between having long legs and being very mobile, and potentially your testes size," Dr Friesen said.

"It could be that investing in that highly mobile lifestyle leaves you with less energy to invest in testes."

Compared to other animals, cane toads have fairly big testicles with each one being about the same size as their kidneys.

One reason is thought to be because females produce a lot of eggs, so there is a lot of fertilising to be done.

But the other reason is that being amphibians, cane toads have external fertilisation and that introduces the potential for "sperm wars".

"We have seen cases of this where there are multiple males on top of a particular female," Dr Friesen said.

"You can think of it as like a lottery, and the more lottery tickets you have, or sperm in this case, the more likely you are to fertilise her eggs."

But sperm wars are less likely for the big toads that are good at travelling to new territory where there is less chance of competition to fertilise eggs.

"When you get out to the frontline, it might be harder to find a mating partner, so there's less likelihood for that sperm competition," Dr Friesen said.

A cane toad's testicles are found inside its body. ( ABC News: Georgia Hitch )

Testicle size a window on promiscuity

Before you start looking for the added swagger of a large-testicled Queensland toad, keep in mind that unlike humans they wear their testicles on the inside.

This study of toad testicles was not so much cup and cough as it was cut and weigh; toads were dissected and their testicles were placed on scales or photographed and measured.

Dr Friesen said testicles were widely studied by evolutionary biologists because of what they revealed about reproductive strategies.

"A bush cricket has the largest testes on the planet for their body size, and their testicles are 14 per cent of their body mass, where ours are well below 1 per cent," he said.

"There's great variation, and that variation we correlate with mating systems or how promiscuous the sexes are and how much sex they have with other individuals."