Everyone reaches for a cold soda after a workout. We do not reason why; we just heed the urge. Our bodies direct us to what it needs most—water and sugar, in this case.
Animals behave in the same way, choosing foods to meet their needs. But some animals indulge in peculiar palate, eating dirt and toxins when more nutritious foods abound. Are these animals eating senselessly, or is there more to food than mere nutrition?
Eat, Digest, Excrete, Eat It Again
Hopping in the night, wild Japanese hares chew grasses and eat shrubs. By day, the hares rest and continue feeding but not on grasses.They bend down and swallow faeces from their anus.
We know faeces as the smelly remains of once delicious and nutritious foods. Yet a good number of small and medium-sized herbivores eat their own faeces. For these herbivores, faeces is precious.
Eating one’s own faeces, or auto-copraphagy, is best studied among the rabbits and hares, members of the Leporidae family, especially among the Domestic Rabbit and Japanese Hare. They like their faeces soft and warm, fresh from the anus, and gulp it down without chewing.
Food that leporids have ingested enters the large intestine between the caecum and colon. Here, the food mass—now called digesta—is pushed ahead through the colon and rectum to excretion. The larger, drier, harder to digest parts are discarded as hard faeces.
Periodically, the muscles of the large intestine push the fine particles, microbes and water of the digesta backward into the caecum. Within the caecum—practically a fermentation chamber—microbes work on the digestible fine food particles and extract more nutrients for their leporid hosts.
Eventually, the watery digesta in the caecum is excreted as soft faeces. Eating soft faeces allow leporids to further refine the digesta and extract as much nutrients as they could.
Some of the soft faeces are capsules of water, fine particles and microbes wrapped in a membrane “tough enough to be peeled off with forceps”. Rabbits swallow these soft capsule faeces whole. Scientists suspect that the membrane protects the microbes within the capsule faeces as they traverse the highly acidic stomach of rabbits.
Weight-for-weight, small mammalian herbivores burn more energy than large herbivores do. Hence they need to reap more energy from their food. All studies of faeces-eating herbivores found that the faeces they eat is more nutritious than those they ignore.
Scientists believe that the efficient use of one’s own faeces have enabled rabbits and hares to thrive on difficult diets like grass and woody plants, making rabbits and hares the most common medium-sized mammalian herbivores on land.
A host of mammalian herbivores join rabbits and hares in this underappreciated diet club. Guinea pigs, chinchilla, gophers, lemmings, voles and Kangaroo Rat all subscribe to the values of eating one’s faeces. In fact, guinea pigs have signed on for life membership: They die if deprived of eating faeces.
In Brazil, yellow-chevroned parakeets build nests in abandoned termite mounds. The birds scrape the mound walls with their hard bills, expanding the tight cavities to make space fit for a new parakeet family. Occasionally however, the parakeets stop scraping to eat bits of the mound walls.
Eating soil, called geophagy, isn’t uncommon among birds. Many birds eat soil from clay licks. But is eating soil from termite mounds different from clay lick?
Scientists took soil from termite mound walls and compared their components to soil from below the mounds. They found that termite mounds contain several times more organic matter and macronutrients (phosphorus, potassium, calcium etc.) but less micronutrients (iron, zinc, copper etc.) than ground soil.
Scientists propose that the yellow-chevroned parakeets might augment their nutrition with termite mound dirt. Nesting parakeets would require lots of macronutrients to meet their reproductive needs. Extra calcium and manganese from termite mound soil could help develop eggs and embryo.
Termite mound soil also has higher cation exchange capacity than ground soil. That means soil from termite mounds can better capture positively-charged nutrients or bind and neutralize plant toxins that often challenge the fruit-eating parakeets. By consuming termite mound soil, the birds may have an antidote for the plant toxins right in the walls of their home.
Elsewhere in Southeast Asia, orangutans eat leaves and fruits they carefully pick among the tree tops. When orangutans descend from the trees and travel across the jungle floor, they occasionally eat soil en route: They pause, smell the soil for a couple of minutes, and eat a pinch of it if suitable.
In the Sungai Wain Forest Preserve of Borneo, primatologist Anne Russon observed the orangutans ate clay. At those sites, clay could be easily obtained under the thin topsoil.
“It’s so easy to crack off a small lump of drier exposed clay or to scoop out a fingerful of damp exposed clay,” says Russon. “Which is what the orangutans did – simply picked up a little bit and popped in their mouth when the opportunity arose.”
We have long known that primates—chimpanzees, gorillas, lemurs—eat soil. Even among humans, soil has been part of diet and in some cultures, till this modern day.
Early humans could have imitated the animals to solve issues like iron or salt deficiency or diarrhea, says geologist William Mahaney who has been studying the behaviour in animals and humans for thirty years.
So it’s no surprise that orangutans eat soil too. But how do orangutans choose which soil to eat? Might they do it for an extra boost of macronutrients and detoxification of plant chemicals, as do the yellow-chevroned parakeets?
Russon collected the soils chosen by the orangutans and those immediately adjacent, and sent them Mahaney for further analysis.
Results show that orangutans pick soil that contains more clay and less organic matter. Salt (sodium chloride) and calcium—often touted to motivate soil-eating behaviour in animals—were in negligible amounts but iron content was higher.
Orangutans may prefer more clay for the very same reason we take kaolinite pills—to alleviate diarrhea. The unrestrained discharge of gastrointestinal waste is a terrible ailment common to many leaf-eating great apes, orangutans included. Clay minerals which Mahaney found in the orangutans’ soil pickings can tightly bind toxins in the gastrointestinal tract and neutralize them. Diarrhea—no more.
Or perhaps the higher clay contents help orangutans digest their plant diets better. Orangutans get their energy and nutrients from fermentation of plant foods which happen in their microbe-rich caecum. The fine particles of clay could improve fermentation by keeping the foods longer in the caecum and retaining moisture.
The soils that orangutan chose also had higher levels of rare earth elements. But Mahaney is not sure what to make of it.
“We do not really how much could be taken up [by the orangutans] or what the effects would be,” says Mahaney. “Studying this would require millions” which such fundamental research projects would struggle to win from current funding agencies that gravitate towards applied research.
Unpalatable Toxins, Unpalatable Me.
No doubt faeces and soil would stand out on a menu, but at least they help animals reap more nutrients and detoxify. Yet a good number of insects eat toxic plants that offer little nutrition.
One such insects are the Osmia mason bees in Colorado, USA. Black coloured with iridescent hues of blue and green, these solitary bees feed their larvae with pollen, like those from legumes which make a nutritious pack for the developing larvae.
However, some Osmia bees, like Osmia montana and Osmia subaustralis, collect only pollen of the plant family Asteraceae that contain less proteins. Studies show that bees grow slower on Asteraceae diet. Bees that have not adapted to Asteraceae would even die on a pure diet.
“At our study site, Osmia bee larvae that eat only Asteraceae pollen always take two years from egg to adult,” says insect ecologist Jessica Forrest who researches the bees at the Rocky Mountain Biological Laboratory in Colorado.
“Other Osmia bees that do not eat Asteraceae pollen can emerge within the following year.”
So why would O. montana and O. subaustralis raise their young on a poor and arguably toxic diet? The motivation, or at least part of it, seems to be to deter parasites from invading the nest.
Female Osmia bees build nests in cavities—holes in dead wood, burrows in the ground, or even empty snail shells. They erect walls of mud and leaves to form cells, deposit an egg in each, provision with pollen and seal the cells. The larvae hatch, eat the pollen and grow, eventually chewing their way out of the cells.
Fresh mud walls are vulnerable to attacks by parasitic Sapyga wasps. Before the mud walls dry and harden, the wasps sneak their eggs into the Osmia nest cells; when the wasp larvae hatch, they consume everything edible within the cell, including the Osmia egg. In the field, almost one-third of Osmia nests are thus parasitized by Sapyga wasps.
Forrest and her colleagues noticed however that Sapyga wasps examined and turned away from Osmia nests that had orange greasy pollen on them—telltale marks of Asteraceae pollen. Forrest wondered if Asteraceae pollen deterred Sapyga parasitism.
A survey of about 630 Osmia nests in the field suggests that Asteraceae pollen provision reduces Sapyga parasitism. Sapyga wasps infiltrated a third of nests that did not use Asteraceae pollen but none of the 72 nests that did.
Forrest tried to rear Sapyga larvae on different pollen diets in the lab. Eight of 38 Sapyga larvae on non-Asteraceae pollen survived to the last stage while only one of 30 on pure Asteraceae pollen did.
Scientists do not yet know how Asteraceae pollen undermines insect development. It might be what’s inside the pollen—toxins, or lack of proteins and crucial amino acids—or outside—the pollen sprouts spikes that might hurt insect gut.
Hence, some Osmia bees might use Asteraceae pollen, despite its less-than-rewarding nutrition, because it makes their nests less appealing to parasitic Sapyga wasps. But Forrest thinks that Sapyga wasps cannot be the sole force driving Osmia bees to choose Asteraceae pollen.
“Osmia bees that specialize on Asteraceae pollen do so even in places where Sapyga wasps are not present,” says Forrest. Osmia bees likely eat Asteraceae pollen for reasons beyond nutrition and Sapyga parasites, “but we do not know yet.”
While some strict specialists like Osmia montana and Osmia subaustralis eat from a narrow set of nutritionally sub-par foods, there are insects, like the wood tiger moth (Parasemia plantaginis), that would add toxic plants to their general diet.
Sporting orang patches on a black body, the wood tiger moth caterpillar poses a conspicuous target against green leaves. But the caterpillar does not hide because it is unpalatable for many predators. The orang patches scream: Remember my foul taste! And the larger the orang patches, the faster predators like birds learn to avoid the caterpillars.
The wood tiger moth’s nasty flavour comes partially from its diet. According to a 2015 study, the caterpillar eats various plants including plantain which contains high amounts of defensive chemicals called iridoid glycosides. The caterpillar acquires just small amounts of these chemicals from plantain leaves, but enough to discourage enemies.
Ants love a sugary drink. But add bits of (mashed) plantain-fed wood tiger moth caterpillar to the sugar, and ants turn away. “It is often found that what is unpalatable for ants is also unpalatable for birds,” says insect ecologist Carita Lindstedt, co-author of the study.
Some wasps hijack the caterpillar’s body and grow within. But the wasps parasitize fewer of plantain-fed caterpillars than those fed dandelions (a natural food of the caterpillars). Even fewer new wasps eventually emerge from plantain-fed caterpillar hosts.
But wood tiger moths pay a price for using plantain. Caterpillars on plantain grow slower and gain less weight compared to those on dandelion, another item on the caterpillar’s extensive diet. More interestingly, caterpillars that have incorporated higher levels of plant defensive chemicals also sport smaller orang patches, suggesting some physiological trade-off between the two traits.
Although animals do eat some strange foods, those are usually parts of a wider diet. A successful individual must match its complex needs with various nutrients and constituents that inevitably come from diverse sources.
The wood tiger moth makes a good example. “Wood tiger moths grow very fast on nitrogen-rich lettuce but survival and immunity is weak,” says Lindstedt. Plants like plantain improve survival against parasites but slow growth. “Lettuce is fast food, but the caterpillars would need some healthy food or else they would die.”