Sick Fruit Bats Also Self-isolate, Tel Aviv Scientists Discover

Egyptian fruit bats are the latest among the social animals shown to hunker down and perch on their lonesome when ill, potentially reducing infection rates in their community

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Egyptian fruit bat
Egyptian fruit batCredit: Yuval Barkai, from Instagram account
Ruth Schuster
Ruth Schuster

Fruit bats love company. Like us, they like to live in colonies and they never stop talking. Now it turns out that we have another thing in common: slinking off and isolating when we feel sick.

“Social roosting imposes a major risk of pathogen transmission,” points out the team led by Tel Aviv University’s Prof. Yossi Yovel in a new paper published in the Annals of the New York Academy of Science. Fruit bats don’t nurse one another when they fall ill so there is little upside in remaining with the group, and a sick bat is more likely to infect its peers if crowded with them in the cave, as opposed to coughing by itself somewhere else.

So that infection risk is mitigated if sick bats flap off and perch on their lonesome. It’s the fruit bat equivalent of social distancing to prevent coronavirus infection.

In fact it’s common in social animals to self-isolate when sick – from the lowly ant to the vampire bat to monkeys.

In the case of the vampire bat, sick animals also abandon the communal roost, as was proven by making some sick and giving others a placebo, and tracking them with teeny backpack transmitters. House mice also form social groups but hunker down alone when ill, according to a 2016 study from the University of Zurich. That study found that mice can identify sick peers, and don’t necessarily avoid them –”it was the sick mouse that removed itself from the group,” the researchers wrote at the time.

An Egyptian fruit bat in Tel Aviv, waterskiingCredit: Yuval Barkai, from Instagram account
An Egyptian fruit bat in Tel Aviv, eating fruitCredit: Yuval Barkai, from Instagram account

Now this tendency has been shown also in Egyptian fruit bats, which aren’t just social, they’re hyper-social, coauthor Maya Weinberg explains to Haaretz. How hyper-social are they? They’re so hyper-social that if you check their intestinal bacteria (microbiome), as one does, a bat’s microbiome will be more like those of its cluster-mates than like its own microbiome tested at a previous date. “There’s nothing like that in other mammals,” she says.

They are also hyper-communicative, the same Tel Aviv University research group showed in a previous study, and apparently complain a lot. Hence it was something of a surprise that the super-social Egyptian fruit bat also skedaddles when sick – it’s so contrary to their basic nature.

“The hypothesis had been that the group would leave the sick one – but we found the sick one was the one that actively left,” she says.

They knew the isolating bats felt sick because they made it so.

The team worked on both captive bats and free-range bats living in a “semi-open” colony at Tel Aviv University: The little things are free to come and go and forage as they wish. Bats were injected with a “false alarm” protein that made them feel sick and maybe also nauseous for a few days.

Rare photo of a mother fruit bat flying with her baby clinging to her tummyCredit: Yuval Barkai, from Instagram account
An Egyptian fruit bat in Tel AvivCredit: Yuval Barkai, from Instagram account

The team stresses that no bats were hurt in this research. Fruit bats have extremely rapid metabolisms and basically no fat – they chew, swallow, digest and make, and can’t go two days without sustenance. So the subject bats were treated to mango juice, their favorite, to keep them from dehydrating to death while suffering the effects of the protein shot, Weinberg explains, emphasizing that they did not actually make the bats sick, they just made them feel sick. Got it.

Along with their fiendishly fast metabolisms, their inflammatory reaction is extremely fast too, she says.

How exactly do we know when a free-range bat feels feverish? We don’t. But they checked the temperature of captive bats who received protein shots, not with oral or anal or armpit thermometers thank heavens, but by using a sensor pressed against their fur.

Fruit bat fur is incredibly soft, by the way. Like baby rabbit soft.

And thus we know that the Egyptian fruit bat, similarly to the vampire bat and several other social animals, isolates when feeling feverish.

Fruit bat eating datesCredit: Yuval Barkai, from Instagram account

We add that given the speediness of their metabolism, it’s all the more surprising that they can live for decades – the Egyptian fruit bat can flap on for some 30 years.

Asked if that’s unusual in bats, Weinberg notes that there are 1,436 bat species known so far and they are highly diverse, so who knows. She adds that an insectivorous bat in Russia was recaptured 41 years after it was tagged, and even then it had been tagged as an adult.

Bottom line: When you have a headache and fever, do you go to a party? You do not, especially when everybody else hanging upside down at the party is grousing about everybody else. You isolate. So do they. “Sickness behavior is a relatively new area – behavior not derived from the disease but that serves a purpose,” Weinberg says. “The bat’s own behavior serves the physiology of contending with the disease.”

Finally, she stresses that fruit bats are not a reservoir of disease, as is often charged.

Humans have “excessive” inflammatory reactions; theirs is superior and low-key, contending with and eradicating a wider range of pathogens. “They are not constant shedders!” she says. When they get infected with a pathogen, say, a type of coronavirus for instance, their immune systems react and they can therefore test positive for antibodies against the pathogen – which doesn’t mean they’re carrying the disease. “They overcome it; they’re not constant shedders,” Weinberg says. “Give them the credit for that: the ability not to die, and not to be constant shedders.”

Fruit batsCredit: Yuval Barkai, from Instagram account