Change Everything No 29: Slothly symbiosis: how moths, algae and mammals help each other out

What a life! Improving digestion with a spot of sunbathing

Book news

I’ll be back in Nottingham talking Change Everything on Monday October 14, in The People’s Bookshop, Durham on Tuesday, October 22 and at the Steam and Whistle in Cheltenham on Tuesday October 29. All welcome!

Slothly symbiosis

You might expect a little, richly illustrated book about sloths to cover the species’ fascinating biology, its curious place in cultural history (from children’s cuddly to murderous zombie), even to pick up the way in which the fossils of the extinct giant sloth were identified before the dinosaurs, so played for a while a major role in the public imagination. And Alan Bauch’s small but richly illustrated book, part of an increasingly extensive of “read in an evening” series from Reaktion books does all of those things.  If it has its frustrations – particularly a lack of indigenous perspectives, and a sense of being a literature review rather than a true engagement with the animal  – it is still worth picking up.

What I didn’t expect was quite how clearly it would present sloths as a model of symbiosis, a rich ecological niche all of their own, for the algae that grow on them, giving their coats a classically green tinge, and many invertebrates, particularly the “snout moth”, associated most closely with Bradypus, the three-toed sloths. It “lays its eggs in the faeces that the sloths deposit every week or so [more on that anon] on the ground. The larvae consume the excrement of the sloth and as they mature, each flies up to the canopy to find its own sloth…. It appears that the sloths that are populated with more moths tend to have more algae and nitrogen in their fur, which they can consume and consequent become more robust”. (p. 58) That’s something that’s been understood a while: Bauch found a frankly weird 1960s advert – some advertising man probably thought really hard about it – featuring a drawing of a sloth looking more like chimp swinging on a jungle vine with the immortal words “The Three Toes Sloth is protected by the camouflage of minute plants which grow on its fur. Let your protection be London and Manchester Assurance Company.”

That is in addition to the extensive microbiome that the sloths need to consume and detoxify their high cellulose, often apparently poisonous, diet. And the fact that recently brown bats have been observed snacking on the fur of three-toed sloths. Food for them, and likely a handy clean up of ticks, mites and triatomine (blood-sucking) bugs. (p. 158)

That advertisement suggests quite a major place in the public imagination, which is something sloths have seen rise and fall. Their place in the understanding of the ancient past started early. Darwin himself, travelling on the Beagle, in Patagonia found fossil remains of a giant animal he could not identify. He sent it back to anatomist Richard Owen, who identified it as a giant sloth, naming it Mylodon darwinii. The connection was clear, but the evolutionary paths took a like time to emerge, particularly the fact that the two-toed (Choloepus) and three-toed sloths are not very closely related. The next time you are looking for an example of convergent evolution, this fits the bill.

What we know now is that it is those ancient sloths (and probably their fellow megafauna the elephant-like gompotheres) that we have to thank for the avocado. Something to contemplate next time you mash and top with a dash of chilli. Ancient avocados had even larger seeds, and only these two species would have been large enough to eat a whole one. Their germination was triggered by passage through the digestive system, and given the slow digestion cycle of the sloths, deposited a fair distance from the parent tree. That the tree survive some 10,000 years after the giant sloth is yet more evidence that humans have been managing their landscapes for a very long time.

Choloepus Photo by Zorawar Bhangoo on Unsplash

From this period of colonisers and genocidists, there are the predictable stories of terrible behaviour towards living sloths. But there should be a special circle of hell for Edward Bancroft (1744-1822) who in his journal about his trip to Guyana, “tries to correct the ‘insuperable aversion to motion’ in sloths by ‘beating’ them, which results in some movement but only at the expense of ‘the most melancholy pitiful noise and grimaces’. Lest this be thought just of the age, in the same era zoologist George Shaw (1751-1813), while no fan, said in his Naturalist’s Miscellany (1789) “that the Sloth, not withstanding this appearance of wretchedness and deformity, is as well-fashioned for its proper modes and habits of life, and feels as much pleasure in its solitary and obscure retreats, as the rest of the animal world of greater locomotive powers, and superior external elegance.” (p. 88-89)

Even more signs of sympathy – for the sloths and the indigenous humans in their environment who were being treated so horrifically, is evident in John Gabriel Stedman’s Narrative of a Five Years’ Expedition (1796). The author was part of the Dutch colonial forces, but no fan of the treatment of the people of Surinam. “That recognition manifests itself in his depiction of the ‘sheep sloth’ (ai, three-toed) and the ‘dog sloth’ (unau, two-toed) who take refuge on a blasted tree in a ravaged landscape.” (p. 123) And in Elizabeth Agassiz’s A Journey to Brazil (1868) she describes a ‘pet’ sloth on her boat up the Amazon (probably a maned sloth, with their wonderfully cuddly looking coat, so neatly divided down the midline to allow rain to run off while it hangs upside down):

“I never tired of watching him… he looks so deliciously lazy. His head sunk in his arms, his whole attitude lax and indifferent, he seems to ask only for rest. If you push him, of is, as so often happens, a passer-by gives him a smart tap to arouse him, he lifts his head and drops his arms so slowly, so deliberately, that they hardly seems to move, raises his heavy lids and lets his large eyes rest upon your face for a moment with appealing, hopeless indolence.” (p. 134-5)

Sadly his lifespan is likely to have been short. And no doubt miserable. And lest we be feeling two 21^st-century smug, I was astonished to read that in the US and the UK you can pay in zoos the have a “close encounter” with a sloth. “Selfie” culture has only added to their popularity. Rewarding for the human no doubt, but a study by World Animal Protection recently concluded that such interactions were more likely than not “compromising welfare”. When handled sloths display unusual behaviours  that “may be indicators of fear, stress and anxiety”. (p. 142)

Bradypoid Photo by Deb Dowd on Unsplash

And in 2013, locals in Panama had to spring to the defence of pygmy three-toed sloths, listed as Critically Endangered by the IUCN (and only described in 2001), when eight were due to be exported to the Dallas World Aquarium. Their release back into the wild was negotiated, but in the meantime two died. (p. 161) And, with sad inevitability, research just out indicates the major threat the climate emergency presents to the future of all sloths - “the energy limitations of these animals could make survival untenable by the end of the century, particularly for high-altitude populations”.

So finally, I promised to talk about digestion and defecation. Really – it is fascinating, with a rather cute behavioural twist. Being primarily leaf eaters (although two-toed sloths are more likely to add insects, fruit and hibiscus petals – which they love), they need to extract all possible nutrients, particularly protein, from their diet. Digestion starts with grinding teeth (which continue to grow through their life.) p. 48 and proceeds through four stomachs, in three of which microbes are key to breaking down the cellulose, notably the forestomach, where food can remain for 150 hours. Two-toed sloths can, like cows, chew their cud, returning swallowed leaves for extra mastication. But the really lovely bit is that when sloths lie with their stomach exposed on sunny days, they may actually be speeding fermentation in their gut (as well as presumably enjoying themselves).  At the end of all of this, sloths descend from the trees typically once a week to defecate, with the three-toed, like badgers, typically digging a neat hole for their waste (which may also suit their symbiotic moth). There are two theories about why they don’t just “go” in the trees – for the benefit of the moth, or because they also scrape up dirt to get minerals otherwise missing in their diet. (p. 51)

Picks of the week

Reading

My fellow member of the House of Lords, Professor Kathy Willis (also professor of biodiversity at the University of Oxford), has a book out that has definitely gone on my “to-read” list, Good Nature: The New Science of How Nature Improves Our Health reflected on here by the Guardian. Science is finally catching up with wisdom long embedded in folk culture.

Listening

If you hear the word “salons”, you probably think Paris, and ladies in extraordinarily fancy dresses and stylish chambers. Probably not Damascus: but the often enlightening Ottoman History podcast takes use there in the 16th-century, where elite men from Egypt and Syria in particularly sought to find a place in the Ottoman empire, as Arabophone and Turcophone cultures mingled and vied for influence. Gossip, sneaky social manoeuvring, showing off: it’s all there.

Thinking

Even that establishment organ The Economist is noting that the UK electoral system is becoming essentially a random number generator in terms of the results it produces. Democracy, it would be a good idea.

Researching

You might take it for a metaphor for this planet that we have thrown profoundly out-of-balance: the world’s largest organism, a giant stand of quaking aspen clones known as Pando (pictured below) that is thousands of years old, is threatened by overgrazing, by herbivores no longer kept in check by wolves and cougars, and by diseases linked to the climate emergency.

Almost the end

The “father of microplastics research” is reflecting on the 20 years since he coined the term. “"It's clear we are exposed to the particles in just the same way that the rest of nature is. We've shown with a range of animals that they can be harmfully affected in lab studies. Why would we imagine we're any different?"

What did you think?

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