How do fruit bats survive in the city? Sheer chutzpah

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“He gets it from you,” one parent will often grouse to the other every time the kid brings home a stray bear or jumps off the roof. Where do the kids get it from? In the case of baby fruit bats, it turns out they get their cojones from their mother, whether biological or adoptive – indicating that courage isn’t primarily an inherited trait, Tel Aviv researchers have discovered in groundbreaking new research.

Israel is thronged with Egyptian fruit bats, Rousettus aegyptiacus. They live in the cities. They live in the country. Beloved by many, others regard them as some Londoners regard pigeons: aviating excreters.

However one feels about them, the question remains: How do the bats adapt to the bustle and roar of city life? A key trait in any animal adapting to city life is sheer daring, explains the Tel Aviv University fruit bat team led by Prof. Yossi Yovel, with Dr. Lee Harten, Nesim Gonceer, Michal Handel and Orit Dash, in collaboration with Prof. H. Bobby Fokidis from Rollins College in Florida in BMC Biology.

Adult city fruit bats are far more brash than adult country fruit bats, the team has discovered. But what does it mean to be bolder when one is a bat?

A fruit bat enjoying some dates.Yuval Barkai

City bat, country bat

Wild fruit bats are sensibly loath to walk into just any box they find on the ground. So first the team tested risk appetite in the adult bat by putting food inside a box. The bats had to land on the box and enter it to get the food.

The bats’ risk appetite and boldness was measured as the proportion of times they would land on the box and enter it. City bats solved the problem immediately. Country bats hemmed and hawed and took hours, the researchers observed.

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“Similar results were observed in past experiments with birds,” Yovel says. City birds take more risks than country birds. This was the first study to test this trait in bats.

On the other hand, city bats were more leery of exploring than rural ones. Exploration was defined as the proportion of unique boxes entered by a bat during the experiment. Country bats were better at exploring.

Until they learn to fly, baby fruit bats spend most of their time clinging to their mothers’ bellies. The mother has a fruit in her mouthYuval Barkai

So, the urban adults are bold. The next step was to test whether the trait is hereditary or taught. Researchers devised a unique cross-adoption experiment, testing 86 mother-pup pairs. Pups born to urban fruit bats were adopted by rural mothers and vice versa. The bat babies were pre-flight: They had not yet experienced their environment independently. Pre-flights babies spend their time clinging to their mothers’ tummies.

The experiment was conducted under lab conditions because gauging the boldness of baby bats in the wild is quite the challenge.

Country pups fostered by urban mothers were bolder and took more risks than city-born pups raised by rural mothers. This argues for a stronger influence of nurture over nature. “Our findings suggest that this trait is passed on to pups by the mothers that nurse and raise them, even when they are not their biological mothers,” Yovel said.

Stressed in the city

Since 1950, the global urban population has increased sixfold and as the cities spread, the animals can either crowd into their shrinking habitats or adapt. Raccoons learn to open garbage cans; feral cats just stroll into people’s homes and help themselves. Handing down boldness could be key to the bats’ success in the anthropogenic epoch, which is characterized by constantly expanding urbanization.

How these personality traits are passed along through the generations is unclear. Likely there is some combination of nature (genetics), with nurture (teaching) – and some biochemistry.

The team may have closed in on a significant biochemical side-effect of the rat race. Lactating urban fruit bats have significantly higher cortisol levels in their milk than their rural counterparts. Cortisol is a stress hormone.

An Egyptian fruit bat. Country bats may be less bold, but they showed a stronger penchant for exploration.Yuval Barkai

Separate research has found that in difficult times, mommy rats groom their pups less, creating a generation of vigilant, easily stressed rats that are better able to deal with their hard environment, Harten explains. Urban bats are likely more exposed to stressors than their counterparts in nature. The Tel Aviv team is certainly not saying that cortisol is responsible for creating daring batlets, but it could be a factor.

Harten adds that the bat experiments were held in rather small colonies. The mothers mostly perch near other bats or flap down to the bowl of fruit to collect a meal, so the baby bats have little opportunity to learn. It’s a very small space, leaving the bats with few opportunities to show if they are bold or not bold. Their range of behavior in the cell is limited, Yovel says.

“The urban environment presents animals with more challenges and a greater variety of situations. It is therefore plausible that bats and other animals living in the city require more boldness and higher learning skills,” Yovel says. For bats, it’s complicated, of course. But the boldness seems to be passed down with the mother’s milk.