Pregnant pause: Researcher ultrasounds tiny bats in amazing breeding study

Nicola Bail from the University of Adelaide is using the unique approach of ultrasounding tiny Southern bent-wing bats in order to estimate the proportion of females reproducing each year.

“Sometimes you have to put a little pencil under their back so that their stomach is sort of popped up, which is very cute. And we put some ethanol on their stomach, and then we put the probe on and have a look around and just diagnose the pregnancy there,” she says. 

The critically endangered bat species, and Cosmos 2022 Australian Mammal of the Year, lives only in south eastern South Australia and south western Victoria.

Bail’s research is focused on the bat population based in and around Naracoorte Caves National Park in South Australia. The park is home to the Bat Cave, one of two main maternity caves for the Southern bent-wing bats.

The bats roost in caves and forage over large distances, mainly for moths which they catch while flying. Then, in Spring and Summer breeding female bats, juveniles born the previous year, and male bats all begin returning to their maternity sites.

In 2021-22 and 2022-23, Bail – with support from a volunteer veterinary sonographer from Adelaide – used ultrasound to determine the pregnancy status of “a big sample” of around 550 bats.

“We do this in September and October when we know that they’re pregnant, but not heavily, heavily pregnant. Because we don’t want to further stress them out while their body is strained,” she says.

In September, the foetus just looks like a tiny 3mm dilated uterus and a heartbeat. By early October, a bit later into the pregnancy, it looks like a tiny baby bat – maybe 4 or 5mm high – but you can see the full skeleton, Bail says.

Knowledge about the bat’s reproduction is “a really big knowledge gap”, Bail says. She knows of only one previous study done on their reproductive biology. 

By analysing the data gathered, Bail will be able to determine the proportion that are pregnant, and look at the factors that lead to successful outcomes. The results of her study are yet to be published. 

Using ultrasound has advantages over other methods of determining pregnancy, such as abdominal palpation, which only identifies later pregnancy where the foetus has ‘calcified’, Bail says.

“We can get a lot more detail. We don’t need to catch and test the bats again […] It’s just a lot more sensitive to determining early pregnancy, which is the main thing that you miss with the other methods.” 

Southern bent-wing bats are pregnant from May through until November or December, but “not in the same way that a human would be pregnant,” she says. 

The bats have two “reproductive delays”. They get pregnant in late autumn/early winter, but twice during the pregnancy the bats “just sort of hold it in a period of stasis for a while”, especially to get them through winter, when there is not much food available.

“They have to just slow that pregnancy right down. And then they resume it around September, in spring. That’s when they start returning to the maternity site.”

The maternity cave is warm, humid and insulated, which allows the mums to leave their pups on the roof and fly out for the night. “It’s kind of like a humidicrib for a human baby,” Bail says.

Southern bent-wing bats usually give birth between November and December. 

The pups are born – within a couple of weeks of each other – on the cave ceiling and are about 20-30% of the mum’s weight.

“The pups are really dependent on mass birthing, they cluster on the cave ceiling and huddle together, like penguins do. That conserves their body temperature.”

The mothers leave the pups in the ‘creche’ to go out and feed during the night, returning to find their specific baby. Bail says that’s incredible to watch via the cameras in the caves.

“And it’s just like a writhing mass of little baby naked bats. 

“The mother lands on there. And she walks around and just sort of walks over all these pups, calling a little bit. We think they use smell, and a lot of bats have like a genetic call that the mother and the pup can recognize.”

The clip shows an ultrasound of a Southern bent-wing bat pregnant. The scale bar on the left is 1cm. The foetus’ head is on the left, ribcage and spine on the right. Bail says, “if you look at the head/left you can actually see this one roll over and yawn, it’s pretty cute” / Credit: SonoVet – Dr Manhei Ma