For the love of food: the journey it takes for herbivores to eat their favorite foods

By Jackie Campbell
If you’re like me, then you probably follow the mantra food above all else. I think most people would agree that food is the most wonderful thing on the planet. It can be crunchy, juicy, sweet, salty, spicy, and tangy, you name it! However, for some herbivores, the plants that act as their primary source of food, could kill them if they ate too much of it in one sitting. It’s almost similar (though not with an outcome of death mainly just weight gain and regret) to those times when you say you’re going to eat just ten Cheetos from a bag and instead you end up eating the entire bag. However, unlike impulsive humans, herbivores have a way of shutting down their appetite and tell them to stop eating. In fact, this is essentially what herbivores that specialize eating plants that produce toxic secondary metabolites go through. They face the trade-off of having to eat their specialized plant without getting killed by it. How exactly do they do this?

 

Well for starters we should probably talk about plants. Like most living things plants do not want to be eaten. Crazy I know. In order to avoid being eaten, plants produce highly toxic secondary compounds (PSCs) or secondary metabolites (PSMs). What makes these compounds so toxic is that they contain a lot of phenolic compounds such as tanins. By producing these PSMs, plants can avoid being eaten. Yet some herbivores just can’t help feel tantalized by the forbidden fruit (pun intended) and must continue to eat the toxic plants. How could this phenomenon of continuing to eat plants with toxic secondary metabolites be evolutionarily favorable to these animals?

Evolutionarily speaking, if animals can specialize eating a specific food stuff that cannot be metabolized by another animal, this could effectively eliminate competition between animals for food. Or in the words of the great wildlife ecology professor Dr. Perrine “if you’re the only one that can eat the Eucalyptus and not die why wouldn’t you get high off Eucalyptus all day long?” However, everything in biology has a trade-off. By specializing in digesting a particular plant, they expend more energy having to detoxify toxic compounds and produce proteins that can accomplish this. Now that we know some animals like to get “high” off of these toxic secondary metabolites, we should figure out how they are able to manage eating them without dying?

Most animals can detect impending toxicosis after ingesting PSMs. This is accomplished through an activation of bitter receptors. Most of the compounds in PSMs are composed of quinones and tanins which are detected by these bitter receptors. When these receptors are activated, bouts of nausea typically follow. This is due to the PSMs damaging cells in the stomach and small intestine, which activates the release of serotonin and acts on the nausea center of the brain. Most herbivores would use this nausea as a learning experience, much like most of us would feel after eating an entire bag of Cheetos. They would then avoid eating these specific plants. However, for our specialist herbivores this nausea signaling causes them to act in a different way. They don't let a little nausea stop them from eating their favorite foods.

Specialist herbivores, such as Koalas, have evolved physiological mechanisms that allow them to keep eating the plants with PSMs in them. Much like I have developed the ability to eat an entire bag of Cheetos and not feel any shame afterwards. The key that these other herbivres are missing is the art of pacing. Or in the case of koalas, physiological mechanisms that regulate the rate of intake of the PSMs in order to avoid toxicosis or even death. So when a Koala is about to chomp into a delicious set of Eucalyptus leaves, 2 decisions will be made: 1) how much of the Eucalyptus leaves will the Koala ingest and 2) once the meal is over when can they eat again. Out of the two decisions the most important for me would be when I get to eat again hands down.  

In order to regulate how much PSMs are digested, specialist herbivores regulate how much they eat with each meal. This is to maintain the blood plasma concentration of the PSM below a particular physiological threshold. This threshold is determined by the herbivores rate and capacity to biotransform the toxins. In order to maintain levels below the specified threshold, this means that herbivores will consume less biomass per meal. There are certain physiological mechanisms that are utilized by the herbivore to keep the concentration of PSMs in the blood plasma below the threshold. One mechanism uses the bitter receptors located in the gut. The bitter receptors in the gut do not require a coordinating stimulus from the receptors in the mouth to generate a response to a bitter compound. Activation of the bitter receptors in the gut leads to early meal termination. Once the appropriate amount of toxin has been detected, 3 satiety signals are released. These signals are the cholecystokinin, peptide YY and glucagon-like 1 peptide. These signal to the animal to stop eating. Which is something I wish I had at a Mexican food restaurant that gives you endless chips and salsa before your meal. But if these signals are signaling the premature end of the meal how do herbivores compensate for the decreased amount of biomass taken in with each meal?

In order to compensate for the increased biomass with each meal, specialist herbivores eat more meals on average. They have to consume less with each meal in order to avoid absorbing too much of the PSM. However, it’s not enough to consume less with each meal it also means waiting an appropriate amount of time before eating again. This means that the animal will not eat again until the blood plasma concentrations of the PSM decrease enough to allow digestion of more of the plant material. This allows the animal more food intake for biomass without experiencing physiological damage. So how does this apply to the earlier statement “Koalas just get high all day long?”

Essentially, specialist herbivores such as koalas are required to conform to these digestive strategies in order to allow consumption of plant material such as Eucalyptus as their primary food stuff. However, koalas face a problem of digesting this plant material while avoiding toxicosis. Though they may have to spend more energy metabolizing their food and create enzymes that can detoxify the toxic compounds found within the leaves of the eucalyptus plant, they save a lot of energy not needing to forage or monopolize and compete for a general food stuff. Instead they can spend most of their days grazin on eucalyptus leaves all day long. Because what’s more awesome than being able to sit around all day eating food that no one else can eat?

References

Torregrossa, A., M. Denise. 2009. Nutritional toxicology of mammals: regulated intake of plant secondary compounds. Functional Ecology 23: 48-56.

Sorensen, J., J. McLister, M. Dearing. 2005. Novel plant secondary metabolites impact dietary specialists more than generalists (Neotoma spp.). Ecology 86: 140-154.

Foley, W., B. Moore. 2005. Plant secondary metabolites and vertebrate herbivores- from physiological regulation to ecosystem function. Current opinion in Plant Biology 8: 430-435.

Cork, S.J., I.D. Hume, T. J. Dawson. 1983. Digestion and Metabolism of a natural foliar diet (Eucalyptus punctate) by an arboreal marsuipial, the koala (Phascolarctos cinereus). Journal of Comparative Physiology 153: 181-190.