How can the savannah elephant afford to produce feces richer than its food? part 2
Why has Loxodonta africana not evolved to digest its food thoroughly?
I suggest that the basic answer is: because large plants regenerate so rapidly, particularly when subjected to gross damage, that a niche exists for a gross forager that capitalises on quantity rather than depending on quality.
This answer may be hard to grasp initially. However, let us continue to improve our understanding by comparing L. africana to other mammals, before adding further layers of explanation. We may as well choose humans as our next example.
Homo sapiens has a similar gastrointestinal tract to Loxodonta africana. However, our species digests food far more thoroughly, partly because cooking promotes digestion (https://www.newscientist.com/article/mg23230980-600-every-human-culture-includes-cooking-this-is-how-it-began/).
Despite our simple gut, digestive thoroughness in humans rivals that in ruminants such as giraffes (https://www.journalofdairyscience.org/article/S0022-0302(68)87211-X/pdf and https://www.sciencedirect.com/science/article/pii/S0022030217310950#:~:text=On%20average%2C%20cows%20ate%2023,NDFD%2C%20and%20StarchD%2C%20respectively.). The average adult human eats about 2 kg of food per day and defecates at most a quarter of this mass (0.5 kg or less).
Human feces are certainly rich enough to attract large dung beetles, and under some circumstances are eaten by the domestic dog (Canis familiaris). However, this is partly because human feces:
- contain more water than those of giraffes, and
- can be protein-rich after animal matter is eaten beyond the point of satiety.
The digestibility coefficient (https://kb.wisc.edu/dairynutrient/414RN/page.php?id=56186#:~:text=A%20measure%20of%20the%20proportion,(intake%20%2D%20excreted)%2Fintake.) is more than 75% in Homo sapiens compared with only about 20% in L. africana. This means that digestion in humans is nearly fourfold more thorough than that in the savannah elephant, a difference unexplained by body sizes.
To understand the apparent wastefulness of digestion in L. africana, let us review the range of nutritional strategies in a broad spectrum of mammals. Some species make the least of what they eat, others make the most of what they eat, and many lie somewhere in between.
Plant-eating mammals vary greatly in thoroughness of digestion, elephants being at one extreme of the range. At the opposite extreme: the ruminant mode (https://en.wikipedia.org/wiki/Ruminant) is one of the two most thorough modes of digestion in mammals - the other being the caecotrophic mode (https://en.wikipedia.org/wiki/Cecotrope).
In both ruminant ungulates and caecotrophic lagomorphs, rodents and marsupials, the guts are long and complicated and food is repeatedly chewed and repeatedly fermented. The result is feces so depleted of nutrients that they can be nearly useless to feces-eating insects and other animals.
Bovines and hares, for example, process their food so extractively that the feces consist merely of finely mashed fibre in small quantity. Such matter resembles papier-mache (https://en.wikipedia.org/wiki/Papier-m%C3%A2ch%C3%A9) regardless of whether it is defecated wet (bovines) or dry (hares).
Mammals with thorough digestion have an advantage of being able to make the most of a limited quantity of food. Grazers maintaining natural lawns are an example. Relatively little is available each day, but this is made to suffice without necessarily slowing down metabolism, growth or reproduction.
At the other end of the spectrum are mammals with relatively short, simple guts that chew their food only once and only cursorily.
An extreme example of partial digestion, besides L. africana, is the giant panda (Ailuropoda melanoleuca, https://en.wikipedia.org/wiki/Giant_panda) which, like L. africana, digests and absorbs only 20% of the amount eaten. In some. cases, the feces remain greenish (e.g. https://es.123rf.com/photo_117579939_close-up-of-fresh-elephant-dung-excrement-on-ground-in-a-forest.html and https://www.gawker.com/5859434/worlds-most-expensive-tea-panda-poop).
The main advantage of partial digestion is that minimal time and energy are spent in processing a given particle of food. This can make economic sense where food is reliably abundant, and plentiful quantity compensates for indifferent quality.
to be continued...
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Very interesting, thank you. I look forward to more on this subject.
@doug263 Hi Doug, I'm glad to hear that. Please do keep reading as I'm improving the clarity as I go along...
The domestic pig (Sus scrofa) digests its food even more thoroughly (about 84%, https://www.sciencedirect.com/science/article/pii/S1871141317303414#:~:text=Digestibility%20of%20nutrients%20in%20pig,component%20of%20overall%20feed%20efficiency.&text=The%20repeatability%20estimates%20ranged%20from,variation%20between%20pigs%20in%20digestibility.) than does the human species (more than 75%). I infer that the domestic dog does not find the feces of the domestic pig to be worth eating, even though it does find human feces to be worth eating in certain circumstances.
This is a fascinating thread - thank you for posting!
@markdeeble Hi Mark, You are most welcome and thanks to you and Victoria for educating the world about elephants, with regards from Antoni
Thank you @milewski - it is great to have found this thread and such an interesting discussion. Thanks @zarek for making me aware of it.
http://zoonewsdigest.blogspot.com.au/2013/04/zoo-news-digest-1st-6th-april-2013.html
The giant panda, odd in being a herbivorous member of the Carnivora, is even odder in lacking any fermentation chambers in its gut, and it is one of extremely few mammals that eats mainly greens without being able to ferment cellulose by means of lactobacilli as is done by ruminants and hind-gut fermenters. So, beyond the paradox of a carnivoran turned grass-eating, we have the larger paradox of a specialised herbivore that lacks the main digestive strategy of herbivorous mammals, namely fermentation of cellulose to produce digestible microbial biomass and volatile fatty acids. The main tactic of the giant panda is just to compensate quantity for quality, pushing through enormous quantities of bamboo in compensation for its inability to digest the fibrous part of the bamboo, which is most of the bamboo. I can’t think of any other mammal that approaches a specialised diet of grass with such a specialised lack of fermentation chambers, although this possibly also applies to some degree to the red panda.
My own paper on iodine has added an aspect to this assessment of the trophic ecology of the giant panda, in pointing out that owing to an iodine-poor environment and the goitrogens in bamboo the giant panda may find iodine even scarcer than other food components, i.e. iodine may be the most controlling nutrient in its diet.But now that we know that butyric acid may be a vitamin for humans, I see a potential role for butyric acid as a ‘limiting’ factor in the breeding of the giant panda in captivity.Could butyric acid supplements boost the libido, fertility, birth rate, and survivorship of breeding populations of the giant panda in zoos.?
Not just the usual resources, of protein and energy, but also butyric acid, May be forfeited in an animal that lacks a capacity to digest cellulose symbiotically. So, even if the giant panda is adapted to just ‘skimming’ the most easily digestible proteins and sugars from its largely cellulose diet, and adjusting by means of a slow pace of life, it could still be that butyric acid (our vitamin M for humans and giant panda alike) is the most ‘limiting’ factor in the ecology of the giant panda – perhaps even more so than iodine.
So I noted with interest in http://www.pnas.org/content/early/2011/10/11/1017956108
that an investigation of the microbial biota in the gut of the giant panda, which lacks the lactobacilli etc. normally associated with the guts of herbivorous mammals, retains Clostridium. I infer that, as in humans, clostridia – although unsuited to digesting a lot of true cellulose in the way lactobacilli do – can use the small part of bamboo that consists of ‘soft fibre’ (equivalent to hemicellulose or resistant starch) to make butyric acid, which is then absorbed through the colon wall by the giant panda.
http://www.pnas.org/content/early/2011/10/11/1017956108
It is possible for a carnivore like the giant panda to bend the rules in the way it has, approaching a bulk-and-roughage diet of ‘woody grass’ (bamboo) by just shoving vast quantities through and forsaking any attempt to ferment the cellulose in the way virtually all other mammalian herbivores do. But this creates an anomaly, because no other herbivore has proven capable of surviving with such a strategy. Even with the slow pace of life of the giant panda, i.e. a lowering of demand in keeping with the reduced digestion of a grass diet, there is still a puzzle in how the giant panda can survive on its diet with the digestive strategy it has adopted, because the cellulose-fermentation system generates not just protein and energy, but also vitamins. I infer from http://www.pnas.org/content/early/2011/10/11/1017956108
that the giant panda has kept an ability to generate sufficient vitamin M after all, by means of clostridia which ferment the small proportion of its bamboo diet that consists of what Jaminet would call ‘resistant starch’.
A mammalian herbivore can perhaps forgo its cellulose-digesting system and just shove fibre through its gut at a great rate instead, skimming off the tiny proportion of the bamboo that consists of protein and sugar, as long as it adopts slow reproduction, growth, and metabolism. But it cannot forgo its vitamin M-generating system unless it can find a food-supplement for vitamin M. No food eaten by the giant panda supplies butyric acid as such: it neither drinks ruminant milk nor practises technical fermentation in the way that human populations do. But the diet and gut anatomy of the giant panda do seem to retain the capacity to ferment a ‘soft fibre’ component by means of clostridia, which may generate negligible protein or energy but generates the vital factor that is vitamin M. If so, supplementing the giant panda in captive breeding programmes with butyric acid could, possibly, make a difference to breeding success.
Until today I would have hypothesised that if the giant panda is controlled dietarily by something, then that something is iodine. Now I see that butyric acid (vitamin M) is another contender.
The digestion of the giant panda is far more superficial, again, than that of elephants, because a) the panda is much smaller than elephants, and so the size of the gut contents is less, which means more rapid transit in panda than in elephants, and b) elephants have considerable chambers in the form of its colon and caecum (more or less in line with those of humans), whereas the giant panda (if memory serves) has a gut of extremely simple design, more or less just a tube. The giant panda could theoretically have developed its colon and caecum to converge with elephants, but (if memory serves) instead it has kept gut chambers to a minimum and just concentrated on shoving food through as rapidly as it can (which makes sense given that it has whole mountainsides of bamboo more or less to itself). Whereas elephants derive ?70% of their respiration from volatile fatty acids, the corresponding figure for the giant panda might be maybe 10%; and pandas have an even slower pace of life (metabolism, growth, and reproduction) than elephants relative to their body mass. Pandas are fairly unspecialised mammals apart from their colouration and their ‘false thumb’, whereas elephants are extremely specialised in many aspects of their anatomy (with the exception of their gut). Part of this comparison should be the ape Gigantopithecus, but there’s so little fossil material of the latter that it would be hard to get information on it. Gigantopithecus is thought also to have been a bamboo-eater. Considering that bamboo did not grow on the Mediterranean and probably the Indonesian islands inhabited by tiny forms of elephants, and considering that the Asian elephant always had access to the habitat of the giant panda (and could theoretically have been the main bamboo-eater in China, usurping the niche of the giant panda), it is a good question indeed why a niche for the giant panda arose in the first place.It would be interesting to know whether elephant milk contains butyric acid.In elephant digestion, butyrate (which is the conjugate base of butyric acid i.e. with the H+ removed) is formed through very superficial digestion. However, panda’s digestion may be considerably less intense than an elephant’s.I assume that the giant panda cannot synthesise its own butyric acid in its own cells (as opposed to bacterial cells in its gut). If the giant panda can in fact perform this synthesis for itself (in much the same way as many animals, perhaps including pandas, can synthesise their own vitamin C), then the whole argument falls apart and the question of vitamin M in pandas becomes irrelevant. That is, on reflection, the more probably scenario, not so?I don’t know if the Chinese use the faeces of the giant panda medicinally, but see http://www.sbs.com.au/news/article/2012/01/09/entrepreneur-quits-job-panda-poo for a whacky take on this.
Panda faeces are extremely different from human faeces, in both quantity and quality. A panda can defecate dozens of times per day (one source says up to 40 times), and the faeces are so superficially digested that they are still green.
The butyric acid (vitamin M) economy of the giant panda may differ from that of elephants. The intestines of the giant panda are extremely simple, even more so than those of other bears. There is no colon to speak of, let alone any caecum. So the whole digestive strategy is just to skim the easily digestible proteins and sugars from a largely indigestible diet of grass material so fibrous that it is actually woody. It is hard to imagine a simpler gut in a herbivore.
The gut of elephants, although poorly developed compared to that of rhinos or hippos, is far more differentiated than that of the giant panda.
The giant panda has done the opposite of what we would expect, with its gut, for what is by far the most herbivorous of all Carnivora and one of the few mammals that specialises on a diet of woody grasses.
With such poor prospects for bacterial synthesis of butyric acid in its gut, the giant panda can synthesise butyric acid in its own cells, much as it can probably synthesise its own ascorbic acid. However, this needs to be tested. If in fact the giant panda cannot perform such synthesis, then the supplementation of this species in captivity with butyric acid, as vitamin M, might be a breakthrough.
The paper Stevens & Hume (1998) http://physrev.physiology.org/content/78/2/393 is informative.
http://wren.aps.uoguelph.ca/apsblog/blittley/
The following shows the longer intestine of a bear other than the giant panda:
http://wren.aps.uoguelph.ca/apsblog/blittley/
The gut of the African elephant is far more complex and capacious than that of the giant panda. When one also considers how much bigger elephants are than pandas, with correspondingly slower throughput from mouth to anus, there is far, far more opportunity for microbial synthesis of vitamins, as well as microbial digestion of cellulose, in the gut of elephants than in the gut of the giant panda.
http://physrev.physiology.org/content/78/2/393
https://www.pnas.org/doi/10.1073/pnas.1017956108
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