The weird sex life of NZ's tiny bat just got weirder

New Zealand's endangered lesser short-tailed bat is one of just two bat species in the world known to use the weird "lek breeding" mating system. Photo / File

The weird mating rituals of our native bats just got weirder, with new findings showing how the little guys beat the big boys when it comes to wooing the ladies.

Few Kiwis may be aware New Zealand has bats - our only native land mammals - but fewer still would know that some use their own urine as a seductive cologne, with a serenade or two thrown in to turn up the heat.

Only by observing the lesser short-tailed bat's odd antics deep in the Central North Island's Pureora Forest were scientists able to confirm the ancient, endangered species uses the complicated system called "lek" breeding, from the Swedish term for play.

They were amazed to watch how males would find a suitable tree cavity, sing a tune - and sometimes pee on themselves to smell good.

Once a bat had lured a passing female and mated, they left the spot and another male came in - making the tree cavities something of a time-share apartment split between blokes.

If the tiny species wasn't already unique - it can deftly tuck its wings away so it can walk on its elbows and scuttle across the forest floor, chasing weta and foraging for grubs - the study made Mystacina tuberculata one of only two types of lek-breeding bats on the planet.

The lesser short-tailed bat exists today in two sub-species - one considered nationally endangered and the other declining - while the greater short-tailed bat hasn't been seen since 1967 and is probably now extinct.

A new study, led by a former University of Auckland researcher, picked up where the first left off.

Again, Dr Cory Toth and his colleagues ventured into the bush with video cameras and microchip-tag readers to take a closer look at these quirky "time-share" roosts.

They made new recordings of their songs to pick apart differences between those males that shared their pads and those that didn't.

Back in the lab, they also carried out genetic work on samples to see how successful each was at mating.

"For each singing male we calculated how related it was to every other individual in the population that we had genetic information for," said Toth, now based at Boise State University in the US.

New Zealand has two species of bat - the endangered lesser short-tailed bat, and the greater short-tailed bat, which is probably now extinct. Photo / File

"The thinking behind it is, the more successful you are, the more offspring you'll have over the years, who will then have their own offspring, and the more relatives you'll have in the wider population.

"Think of Genghis Khan and how many people can trace some ancestry back to him - because he had tonnes of kids."

Later, they took all of the values and averaged them together, with equations showing the higher the value, the more relatives, and more successful their mating had likely been.

"This also helped because we could calculate how related timeshare males were to one another, because one hypothesis we had was that they're using kin selection."

The results were surprising, Toth said, and suggested that size mattered - although bigger didn't mean better.

Smaller males - which spent more time in their singing roosts, sang more songs, and had higher relatedness - were more successful at fathering a greater number of offspring.

"This suggests that smaller males may have higher energy reserves than larger males," Toth said.

"Further, solitary males were significantly smaller than timeshare males, and individually, solitary males spent more time in their singing roosts than timeshare males."

Yet, overall, timeshare roosts were occupied longer each night than solitary roosts - and the two roost types didn't differ in relatedness.

"So, to put this all together: smaller males seem to out-compete larger males one a one-to-one basis, but timeshare roosts overall have higher lek attendance," Toth said.

Because New Zealand's short-tailed bats were threatened - rat plagues are rapidly sending our bat species toward extinction - gaining more insights around their breeding behaviour could help conserve them.

"What we think may be happening is that timesharing is an alternative strategy for larger males; they are at a competitive disadvantage, and so they form timeshare roosts to increase their presence."

If these larger bats were to sing solitarily, they wouldn't be able to compete with smaller males, and females would be less likely to hear their songs.

"But in a timeshare, more songs will come out of that roost across the course of a night compared to a solitary roost," he said.

"This may translate into higher reproductive success, as their relatedness values are the same as solitary males."

While fascinating, the study ultimately raised more questions than it answered.

Researchers still didn't know what it was that females were looking for - whether it was longer songs, or something hidden within the tunes, or something else entirely - and how the timeshare arrangements specifically worked.

"Do some males in the timeshare roosts get more matings than others, or do the females spread out across the males, or is it proportional to the amount of time that males spend singing?

"It's also possible that timeshare males are co-operating, in the scientific sense of the word."

To show that, Toth and his team would have to demonstrate how timeshare males would do worse by themselves, or with fewer roostmates, than they would in their normal timeshares.

"If we were to show co-operation, it would be the first cooperative display shown in mammals, and the first co-operative display that is conditional on body size," he said.

"The fact that Mystacina is potentially unique in this sense is very interesting."

Most importantly, because our bats were threatened - rat plagues are rapidly sending our bat species toward extinction - learning more about their breeding behaviour could help conserve them.

"It allows us to know what traits individuals need to be successful in current population - but also for any potential re-introductions of new populations."