Coronavirus: the batty back story

“RaTG13 is the name, rank and serial number of an individual horseshoe bat of the species Rhinolophus affinis, or rather of a sample of its feces collected in 2013 in a cave in Yunnan, China. The sample was collected by scientists from the Institute of Virology in Wuhan that year. Stored away and forgotten until January this year, the sample from the horseshoe bat contains the virus that causes Covid-19.”


This is quoted from an article by Matt Ridley at his “Rational Optimist” blog site, on the bat origins of SARS-covid19. Remarkably it was found in its present, fully infective form in a Chinese bat cave in 2013. Seven years ago.

Scientists aren’t yet sure if the bat virus from Rhinolophus was transmitted first to the pangolin (a type of anteater heavily traded in bush markets and at extinction risk) as an intermediate host, or directly to humans. In either case, the virus originated with Rhinolophus.

So bats find themselves uncomfortably in the spotlight as originators of covid19. Revenge for all those bat-chopping wind turbines? Let’s hope that humans don’t wreak revenge on our flying mammal cousains as Chairman Mao did in his cull of sparrows to divert the blame for bad harvests.

So why the bat-virus connection? Bats are the source of many viral zoonoses. Rabies, Ebola, Marburg virus, SARS (sudden acute respiratory syndrome) and MERS (Middle Eastern respiratory syndrome) all originate in these furry flying mammals. How so?

It’s a difficult feat for mammals to fly. We mammals have “tidal” lungs where oxygen rich air goes in and oxygen depleted air goes out by the same bronchial route, getting mixed up in the process. By the time inspired air reaches the alveoli where gas exchange occurs, its oxygen concentration is ten times less than that of atmospheric air. A bad design feature.

How else could a lung be designed? – you might ask. Well, take a look at birds. (And other dinosaurs.) In the avian (bird) lung the air transport is unidirectional, not tidal, with the crucially important consequence that air arrives at the alveoli with the full undiminished atmospheric concentration of oxygen (20% or so).

OK so how is this achieved? Do they only breathe in all their lives, with their lungs getting bigger and bigger till they die? No. That would be poor design. Instead the bodies of birds and other dinosaurs are permeated with a system of air sacs. They even fill some inner bone cavities, conferring the additional advantage of lightness. In-breathed air goes not to the lungs directly but instead it goes first to all the distributed air sacs. From those air sacs the air (still fully oxygenated) goes into the lungs, and via the lungs out of the body. A round trip in which the flow of air is unidirectional and oxygen reaches the alveoli with undiminished atmospheric concentration of oxygen.

Therefore the efficiency of the avian lung is ten times higher than that of the tidal mammalian lung (and also the hepatic pump tidal lung of crocodiles). Both Saurischian and Ornithischian dinosaurs also had this avian type lung.

There are enormous implications of this massively more efficient avian lung. The bird lung can be much smaller and still achieve the needed gas exchange for prolonged energetic flying. The lung does not need to expand and contract nearly as much. You don’t see a bird’s body expanding and contracting with breathing. In fact, it can’t – a bird’s rib cage can and does extend from the thorax all the way to the pelvic region. No need for a six-pack on a seagull. No need for a soft unprotected abdominal region to allow panting. There is only a small posterior membrane at the birds rear end where breathing related expansion and contraction takes place, but it’s small and hard to notice.

That’s why a bird can take to the air in Scotland and land in Africa. Or remain aerobatically airborne for days on end like swallows or swifts. Mammals are incapable of anything remotely approaching this metabolic feat.

What has any of this to do with coronavirus? There is the bat connection. Bats are mammals that do successfully fly, although not as far or fast as birds. The majority of a bat’s body is filled by a big pair of tidal lungs that compensate for their inefficiency by just working very hard. Now due to the extreme metabolic-energetic demands of flying with the wrong kind of lung, bats’ general metabolic rate is very high. This high metabolic rate of all a bat’s tissues has an important implication regarding viruses. The faster metabolising bat cells and tissues are able to tolerate a higher viral load than their more sluggish land-bound fellow mammals. This is why bats carry round so many viruses. Viruses like bats as hosts because more of them can cosy up together in the animal and they fly all over the place giving opportunities for travel and socialising. And also if jumping to other species if it’s really their lucky day.

So it’s the high metabolic rate of bats necessitated by their flying that accounts for their high viral load and their role in communicating viruses. Add to that their evolutionary proximity. Yes – rodents and bats are closer evolutionary and genetic cousains to us humans and primates than most other mammals such as the large Laurasiothere group containing cats and dogs, horses, deer, pigs etc.

The Chixilub meteor has a lot to answer for.

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