Recently I began considering digestive processes in relation to sociality.
Different food items require varied periods of digestion.
I have not really found that many publications addressing how easy or difficult it is for a species to digest something although these findings are implied in some rumination studies as well as grazing/browsing studies as well as dietary studies where digestion rates have been indexed in order to quantify biomass consumption from remains found in scat.
I am afraid to make any assumptions because, for example, I would esteem that lignin is harder to digest than cellulose (based on fungal studies) but, at the same time, e.g., roe deer prefer browse to graminoids suggesting that for this species (and other browsers) lignin might be an easier feat than cellulose while ruminants with longer retention rates in stomachs apparently are better at this.
However, fibre, generally, seems to be slowing the digestion rate wherefore it is ingested to retain other resources longer when nutrients are scarce and when any food item should be processed to extract all value (e.g., one of the reasons behind bark consumption in winter).
As far as I understand, microbiota can also vary throughout seasons (Deutsch, A. et al., 1998).
The digestion rates impact, for example, seed dispersal distances by large herbivores or large omnivores that can transport seeds and propagules even over tens of kilometres although values < 10 km are more common (e.g., García-Rodríguez, A. et al., 2021).
My concern, on this occasion, however, does not lie with digestive health, nor seed dispersal.
I have been wondering how the digestibility of the dominant food resources used by a species (or a local population of a species) affect their territorial (and consequently also social) behaviour.
For example, if the species forages on resources that are relatively quickly digested, it would alter the potential of scent-marking territories (in species where faeces are used in territorial scent-marking behaviour).
Namely, if the individual has foraged on food items that are not retained long in the gut, the scat will have to be deposited closer to the feeding patch (here it is important to note the foraging strategy, as well, because some species spend longer periods foraging in the same place while others might feed at a rate that could be described rather as ‘on the move’).
The period between ingestion and depositing would determine how far the deposit can be made and if the deposit (or some of the deposits) have to be utilized in territory marking activities, the individual would have to make certain decisions, e.g.,:
- Strategically feeding closer to the spot which should be marked that day in order to keep the scent-marks fresh if the territory is large;
- Maintaining a smaller territory if most food resources are easily digested and the foraging is central around, e.g., the den site;
- Evolving a territorial scent-marking strategy that does not involve faeces but urine or gland secretion;
- Evolving a social system where the territory is maintained by several individuals who can visit the necessary boundary marking spots but who, accordingly, do not forage collectively because their foraging has to be scattered to cover the patrolling duties etc.
What comes to mind, is the interesting case of badger sociality in the UK.
In the UK, badgers have relatively small territories abundant in earthworms.
These territories are inhabited and maintained by an extraordinary number of social group (clan) members.
The UK badgers also appear to have the so-called main sett which is not used exclusively (outlier setts can be utilized for shorter periods) but which appears to be focal and defended (some individuals should probably inhabit the main sett at all times to protect it from a takeover or to maintain it in inhabitable order).
I cannot find much information regarding the digestibility of the earthworms apart from the fact that they contain very high levels of protein (which is why, for example, wild boar piglets consume them in large amounts – see Baubet, E. et al., 2003).
Animal protein is supposedly not easily digested, however, badgers are adapted to an omnivorous (including carnivorous) diet and animal proteins might not hinder their digestive capacities.
Earthworms, meanwhile, are usually foraged within the same relatively restricted patch for long periods.
If earthworms are not too difficult to process but they have to be hunted for a long time in the same feeding patch, the individual badger might be restricted in its abilities to deposit the respective scat resulting from the earthworm consumption in a system of borderline latrines within a territory that is large enough to cover the individual’s foraging needs through the seasons.
Additionally, the individual would be restricted in the roaming potential due to the necessity to maintain and defend the main sett (which can be also physically central to the badger territory).
As a result, the individual might not be able to invest enough time in feeding if the borders had to be patrolled while still returning to the main sett for the daytime rest.
Such constraints might facilitate sociality because other group members might forage closer to other borderline latrines and not all group members would be obliged to return to the main sett at all times because a proportion of the clan would always be there.
This could also explain the solitary feeding pattern of the badgers.
It is a behaviour that results not necessarily from reduced sociality or competition but rather from the necessity to patrol the territories which is a shared duty in populations that feed in a very localized manner and that perhaps cannot retain the digested matter in the gut for long enough to deposit it very far from the feeding patch.
Another aspect is that perhaps enough biomass needs to be accumulated in order to produce a depositable scat.
Which means that the foraging bout needs to be long enough before the dropping can be produced in the nearby borderline latrine and thus the individual is restricted in how much time can be spared to returning to the main sett (choosing to use an outlier sett instead).
Some foraging items might not influence territorial (and social) behaviour due to the time of retention but rather due to the time accumulation in order to form a deposit.
For example, while earthworms are high in protein, they also contain lots of water which might make it necessary for an adult badger to consume a lot of them in order to produce fecal matter of any noteworthy substance (that is not too runny and remains in the latrine to signal its purpose for the period that is needed until the following patrol of the respective border post).
However, the gut retention time is not necessarily of importance in this scenario and simple time constraints might prove enough to determine the planning of the feeding with respect to sett and territorial defense.
Personally, I suspect that the retention is of importance because otherwise a larger territory could be maintained by these large groups and in some places, as far as I understand, badgers even tend to defend parts of territories that are of seasonal significance while leaving some other parts rather unprotected because they do not contain crucial resources outside of a limited time period.
Normally, if the group was capable, they would probably include the suboptimal patches strictly within their territory, as well.
It might be interesting to study food resource digestion rates and scent-marking behaviour in species that use fecal matter to determine the boundaries between individual home ranges or larger social groups.
It would be also interesting to study it from the perspective of social group development, structure and behaviour.
Another curious aspect might be related to, for example, digestion interacting with movement activity.
Some species might be better equipped to digest food while remaining relatively stationary but others might need some ‘exercise’ to ‘get the poop out’.
This could affect the lifestyle evolving sedentary and short-ranged vs. mobile and larger-ranged species and the respective social/territorial patterns.
Notes by Ieva 