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?
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.
