As I have been reading about the many different microorganisms (but also enzymes) that are involved in the digestion of food in mammals, I have come to understand that different organisms and compounds are involved in digesting materials in which either sugars, proteins, starch, cellulose, lignin etc. dominate.
Many mammals have evolved symbiotic associations with the types of bacteria, for example, that assist in the processing of their dietary type (e.g., grass-eaters would have microbiota specialized in cellulose while browsers would have microbiota specialized in lignin, and some quasi-obligatory or obligatory carnivores are not efficient at digesting starches etc.).
While it all makes sense, I was considering the apparently wondrous microbiota in the guts of omnivores where many different types of food would enter demanding to be processed.
For example, brown bears eat almost anything.
Moreover, their diet can be seasonal leading to perhaps seasonal changes in the composition of their gut microbiota as well as to seasonal changes in enzyme production to digest the dominant resource of the specific period (e.g., grasses and forbs in spring, fruit and seeds in autumn, meat during winter and early spring etc.).
Do bears, accordingly host all the necessary microorganism in their digestive system at all times (and some of them simply are activated and encouraged to multiply under specific dietary conditions) or is there some type of re-acquisition of symbiotic microorganisms from the bears’ environment in order to re-establish the seasonal microbiota community?
Where do these organisms come from? Are they already found near on the food that is being consumed is that how they enter the digestive tract? Or do animals sometimes need to, for example, consume particular types of food in order for the microorganisms associated with these foods to assist in the digestion of other types of food? (For example, it is thought that fermented milk and fermented products in general can assist in the digestion of other foods in humans.)
And what about the enzymes?
While microorganisms might, indeed, remain alive, albeit at lower activity and reproductive rates inside of one’s body even during seasons and periods when they are not the most needed helpers, the production of enzymes is costly in terms of energy, nutrients invested in their production etc.
Do omnivores have multifunctional enzymes that strikingly differ from specialized, one-function enzymes observed in less generalist eaters? Namely, can the enzymes produced by omnivores carry out several functions adjusting their activity to the current demand? Or is there some type of gene expression mechanism whereby omnivores halt the production of some enzymes during some periods?
And if multifunctionality does not exist, is it costly, from evolutionary perspective, to have invested in the development of many different pathways some of which are not needed at all times but only during specific, yet crucial periods (hyperphagia or energy levels recovery during spring)?
Are we limited in the number of functions and agents we can evolve?
Certainly, we are limited in the amount of energy and nutrients we can invest in producing these enzymes.
One might argue – if some enzymes are not active all the time, they do not cost, either.
However, when the species was still ‘on the evolutionary move’ (which, in fact, never ceases), a choice probably had to be made whether to invest in a new pathway which is enzymatic or, for example, to change behaviour.
Is there a difference between species that have evolved a large number of enzymatic pathways and species the enzymes of which are more limited (but perhaps multifunctional) in their numbers/types with regard to the behaviour of these species?
Such differences might also have come about with respect to the environment in which the species finds itself.
As enzymatic synthesis and operation requests for nutrients. perhaps species that have had to adjust to nutrient poor systems, have evolved behavioural adaptations rather than an abundance of enzymatic mechanisms to cope with their lifestyle choices.
However, in this post I, in fact, wished to address another aspect of omnivory.
Regardless of the mechanisms through which omnivory, especially, seasonal omnivory is ensured, it is probably quite true that during specific seasons, the gut microbiota in the omnivores becomes altered to process the seasonally abundant foods.
Thusly, the animals, such as bears, might ‘carry’ different and dynamically changing communities of microorganisms in their tummies.
(Enzymes are also worth exploring because different enzymes probably request certain nutrients for their production that become redirected from other life processes and, in turn, these enzymes might cause higher concentrations of certain elements and combinations of elements in the organism that has biochemical implications.)
It is known that gut microbiota can have an impact on our mental state (memory, mood, emotional regulation) but also muscle maintenance, neuromuscular activity and other physiological processes.
Diet rich in starches, sugars, proteins etc. can influence the composition of our microbiota.
But if our microbiota can influence our mental and physical performance, it might follow that a diet seasonally dominated by a specific dietary macronutrients, might – through the effects of microbiota on our organism – determine how we are inclined to feel, act, move etc.
Thus, does it not follow that animals such as brown bear who can rather drastically alter their diet based on the seasonally abundant resources, also rather drastically alter their microbiota and, consequently, their outlook and performance.
(It is interesting to consider whether bears, as omnivores, have adapted their diet not only to the seasonally abundant resources but also to the resources that do not interfere with the bears’ seasonal activity patterns.
Namely, if bears have to increase their movement rates or decrease them, or if bears have to be more alert during some seasons etc., they might choose or avoid certain dominant food groups if those food groups are or are not compatible with their life cycle demands.
Thusly, the adaptation by omnivores might have evolved also in compliance of the dominant macronutrient with regard to the seasonally dominant activity and its implications on behaviour.)
In the studies dedicated to human microbiota, quite frequently, a polarization is achieved through determining which foods (at what concentrations) are good or bad for us.
To some degree, we are attempting to explore who we are as a food-consuming species, what our limits are and what is most profitable to us.
In this post, I would like to largely avoid the polarization of the macronutrients into their good/bad extremes.
In some ways, it could be perceived as ‘worldviews’ that certain macronutrients produce whereby some gut microbiota might induce more active states while other might bring about relaxation and ‘idleness’ etc.
Those are not ‘good’ or ‘bad’ states but they create a biochemical setting within which the animal has to make its daily lifestyle choices.
Mostly, in the human dietary studies, a link has been made between, e.g., sugars, fats, starches and microbiota as well as between, e.g., sugars, facts, starches and out mental/physical state.
But it has not been studied much how these effects might be mediated through the very changes in microbiota (a review of research can be found in Berding, K. et al., 2021).
It is also precarious to extrapolate, for example, carbohydrate effects on humans onto other species.
Thus, I cannot offer much hard data to explore how the seasonal changes in diet might bring out different predominant mindstates and activity patterns in the bears.
However, I believe it would be most curious to explore these interactions.
Also, it would interesting to study how omnivores manage seasonal diet rich in specific macronutrients.
Mostly, adverse effects are observed when the consumption of a certain macronutrient exceeds certain thresholds.
For example, lack of fats and carbohydrates in diet can be as detrimental as excess thereof.
Moderation is the rule frequently in humans.
But in omnivores with a seasonal pattern of macronutrient consumption, excess becomes the norm.
Are brown bears better adapted than we are to tolerate what might be described as ‘mood swings’ or overpowering mental/physical states caused by their seasonally dynamic diet?
Are they as adept at regulating their conduct on a physical level as they are on a mental level?
Have they learned to utilize the chemical reactions produced through digestion to their benefit rather than allowing themselves to be manipulated by these ‘medicating effects’?
Is there a change in bear behaviour when one macronutrient group becomes seasonally replaced by another and the bear has to adapt to this new shift?
Or are these shifts, as stated before, already integrated into the bear’s annual lifestyle so that the changes from some states into others occur naturally, smoothly, not causing disruption to their activity and their seasonal needs and goals?
It is also interesting that some microbiota might have evolved the ability to induce certain states in the consumer of the food that the microbiota is processing because these states benefit the microbiota.
For example, many herbivores have to feed on a relatively small patch staying relatively inactive and, on some occasions, also foraging selectively (on tiny parts of the entire plant such as fruit or seed).
In order to consume these types of foods (and to deliver these foods into their digestive tract where the microbiota has access to them and can thereby enjoy the resources as well as multiply), the animal has to adhere to specific types of behaviours (low activity, low movement rates, patience, perhaps attentiveness to detail etc.).
Thusly, the microbiota might wish to encourage these types of mental and physical states in the forager because, that way, the microbiota would come by the desired resource in greater amounts and/or more frequently.
Meanwhile, carnivores have to exert strength and to travel long distances.
It is an entirely different behavioural setting through which a carnivore feeds its microbiota.
Thereby, certain microbiota might attempt to chemically alter the mental and physical states of its host in order to promote the desired foraging behaviour.
Perhaps as the consumption of certain macronutrients increase, microbiota, too, activate their chemical signals commanding to intensify these beneficial behaviours.
There could be a ‘tug of war’ over the mental/physical states in omnivores during periods of dietary change as the contrasting microbiota with its contrasting objectives attempt to exert influence over the consumer’s behaviour.
Interestingly, in temperate regions, where seasonality often results in many different species (more or less omnivorous) to adapt to the new resource availability conditions, the mindset might become somewhat attuned to a similar outlook in the species that begin utilizing the seasonally abundant resource (and thereby activating the corresponding microbiota with its chemical influence).
Perhaps there are cognitive, physiological and psychological conditions shared among species as a prevalent seasonal mood due to the shared dominant resource and its chemical/structural composition.
I hope someday to have learned enough about microbiota to make some more specific factual statements but currently I can only wonder how bears can utilize these extremes without falling victim to what, in humans, is such a frailty of mental and physical dependency on our body chemistry.
References
Berding, K. et al. Diet and the Microbiota-Gut-Brain Axis: Sowing the Seeds of Good Mental Health. Adv Nutr. 2021 Jul 30;12(4):1239-1285. doi: 10.1093/advances/nmaa181. PMID: 33693453; PMCID: PMC8321864
Notes by Ieva 