I have been observing beavers for some time now and as I am in no possession of any equipment that might assist during the dark hours, I often listen in on beaver activities.
I believe that beavers make different sounds while:
- foraging on woody items that they have already harvested vs. felling woody items of smaller of larger sizes;
- foraging on woody items of different species (e.g., osiers vs. ashes or hazels).
These differences are subtle but they might be useful for someone who enjoys spending time by the river during hours when visual observation is impaired due to lack of light.
I would like to hone my skills of detecting what the beaver might be processing according to the sound that the beaver produces.
For example, when beavers are foraging on plants (regardless of their diameter and form of attachment, e.g., stem vs. branch) which are not yet felled or removed, the sound, in my opinion, is different from when the item has been cut and when it can be handled by the beaver at angles that are probably more comfortable for the beaver and that the beaver can adjust to a greater degree.
The sound perhaps differs by its length.
It seems to me that beavers are taking quicker, shorter bites out of forage items that are on the ground or that the beavers can get a grip on (hold).
‘Shorter bites’ are not shorted in terms of the size of the bite taken but rather in terms of the duration of the bite.
When handling such items, beavers seem to apply a ‘woodchipping method’.
Meanwhile, during the process of detaching the branch or stem from the base, the sounds have greater duration and they are less frequent, also with longer intervals between them at times (when the beaver probably has to readjust its body position to access the item the access angles of which are ‘controlled’ by the woody plant).
Beavers might be more careful while foraging on attached woody organs – deliberating their moves and applying more of active cognition.
Meanwhile, once the item has been selected, tested (for its nutritional value, palatability, accessibility etc.) and procured and it does not have to be worked on basis of its growth pattern (it can be manipulated or it lies in a characteristic, typical position), the beaver begins to gnaw speedily without paying detailed attention to each bite.
The intervals between bites become short and regular (without the little ‘breaks’ that the beaver might take to figure out access points of perhaps to overcome some structural peculiarities).
The bites themselves are of short duration.
The sound of beavers foraging on felled trees might be more similar to the sound of beavers foraging on cut branches not because the items and handled in the same manner but because after the tree has been felled, it no longer requires as great of a cognitive approach and many trees fall in a characteristic position which the beaver is probably very familiar with.
Thereby, beavers might also forage more speedily and ‘mechanically’ on trunks of felled trees despite the fact that access is more restricted in these cases and the beaver cannot manipulate the object as freely.
I believe that there are also differences between foraging sounds depending on the tree species (its bark and wood structure) and, of course, based on the structural component (organ) that the beaver is foraging on.
I have noticed that the sound of foraging on osiers/willows (when they have already been cut) resembles what could be described as munching on a carrot.
I have not been very successful at determining differences between other species and discerning peculiar sound patterns that could lead to assumptions over the taxonomic identity of the beaver’s meal.
Some plants do not sound as crunchy as osiers but I am not certain which plants those are because I have not been able to see what the beaver is foraging on and I have not been able to later visit these specific foraging sites to try and find the cut stems.
Recently I heard a very strange sound which, initially, I doubted came from a foraging beaver but later I had to revise my assumptions.
The sound was reminiscent of someone playing the castanets close by the river (on a shore where not only nobody would be playing castanets but where humans do not venture, nor there are any man-made objects).
It was too loud and forceful to believe readily the sound was produced by a beaver.
If I did not know any better, I would think that some sea otters had moved in and that they were perhaps opening molluscs with rocks (I recommend this nice review of stone handling behaviour in different otter species by Bandini, E. (2021)).
However, Eurasian otters (our only otter species) do not live in this particular area (for reasons only known to them because they should according to their ecology unless there are some serious barriers to their distribution from upstream and downstream on this very same river).
Eurasian otters have not been observed to use stones as tools for opening invertebrate prey but they have been observed to manipulate rocks for other (unknown, perhaps play) purposes (dropping rocks in water and retrieving them, juggling rocks, rolling a rock on the surface of standing stones; see Table 2 in the review Bandini, E. (2021)).
I listened in on the sound and it did not exhibit the the mechanical regularity of artificial objects or objects impacted by wave action (there could have been some debris partly washed out on the shore or stuck against some fallen log and being bumped into it by the current).
The sound seemed haphazard enough to have been produced by someone living who subjects the sound production to their own will which follows internal stimuli rather than external, abiotic stimuli.
The only living being in the vicinity that I was aware of was a beaver who had recently swum precisely into that direction (it is also a favourite beaver foraging spot where they can find a nice sandy patch between the water and the riparian slope).
I could not imagine what the beaver could have been foraging on in order to produce such sound and I dismissed this hypothesis.
Later I listened in for the sound because if it was repeated on the following nights in the same spot, it could have come from some fixed source (an object or an abiotic interaction between objects and natural forces).
The sound was not there.
The next time I heard it (in about the same spot) was weeks later and, once more, I heard it after a beaver had swum in that direction and it came from the beavers’ favourite foraging spot.
I had to revise my denial of beavers playing castanets.
While I do not think that the beaver was playing with any objects (I suppose the beaver was earnestly foraging), the beaver must have been handling a foraging item that produced the same loud, rhythmical sound at least two times (which suggests it could be a characteristic sound to that particular foraging item or perhaps in that particular foraging location).
I still hope to resolve the mystery but, for now, I have no answers as to what foraging choice by the beaver results in the ‘castanet effect’.
These ‘castanet observations’, however, led me to wonder about several issues.
Firstly, I realized how loud the foraging on woody plants can actually be.
This made me recall the many instances when I had clearly witnessed a beaver reacting to environmental noises (potential threat) while foraging on some items busily.
I have tried to watch TV shows (without headphones) while having lunch and I am aware how difficult it can be when the meal consists of crunchy items.
And I was not under threat of predation.
I wonder if animals whose daily activities (e.g., foraging) consist of behaviours that result in loud noises (which could possibly impair vigilance), might have evolved an ability to tune out the frequency of the typical foraging sound that they make.
That is to say, perhaps such species (including beavers) have become expert at listening in on the environmental cues while ignoring the very loud noises they are making themselves.
Perhaps the tail slapping behaviour by beavers is so conspicuous both because beavers must warn one another over great distances but also because the alarm has to be heard by the fellow beaver even if the fellow beaver is ‘playing castanets’ at the moment or carroting down an osier.
The current of the river is also at times quite loud.
Beavers have rather soft, quite voices (during personal communication) and it would be impossible for them to produce a squeal or other type of vocalization which could reach another beaver, e.g., 500 m upstream during spring flood while the other beaver was in some shrub cover munching down on crunchy plants.
It is also worth mentioning that sound travels faster in water than it does in air and, therefore, the loud splash might be designed partly to send the sound across the water.
However, a timely warning is more needed by a beaver on land than in water which is why the intent is probably to use the water medium as an enhancer in order to disperse the sound far and quick but the objective is not to warn only those who are in the water.
Perhaps beavers do not suffer the same auditory impacts while foraging on crunchy plants as we do (due to some differences in their physiology).
Or perhaps they have learned to zone out from their foraging noise and zone in on the environmental signals.
Some species do surprise me.
For example, tawny owls can hear rodents under snow but their own hooting is one of the loudest sounds in the forest at night.
One would assume that a species which has such acute hearing, would twinge at loud noises but it appears that some animals perhaps can apply a type of bimodal hearing whereby they differentiate in how they perceive familiar sounds that are often repeated by themselves or in their vicinity vs. sounds that must at all times become processed because they contain signals that are crucial for survival and proper response at the given time.
Another aspect which I began contemplating was that of sound enjoyment.
For example, nobody knows why otters sometimes play with rocks and there could be many reasons but what if they also enjoy the sound the rocks make splashing into water or clashing one against another?
What if some species are, in fact, making music?
For example, beavers are exposed to these wood-working sounds every night.
They must have developed some preference for certain rhythms and perhaps when they hear certain sounds in the environment (that are not food-related but that are similar to their own foraging sounds), they begin mentally ‘rubbing their paws’ in anticipation of a meal (much like we salivate while passing by a bakery with its door open).
Sounds can also be manipulated.
For example, the regularity of the foraging pace upon easily manipulated items could be observed not simply because the activity has become automatic but also because such regular sounds are less notable for the conscious mind, and the beaver might be able to ignore them (they turn into a background noise) instead focusing on more important sounds (where their family are, whether there are not any predators around etc.).
I also wonder if beavers sometimes ‘play their forage’ by investing their own psychological and physiological state into the sound of the item they are handling.
For example, some foraging items might ‘sound differently’ because they are highly palatable (the beaver is very eager to forage) or because they are frustrating, or because they grown in habitats where the beaver is more anxious and watchful (and therefore either observes longer intervals between foraging bouts to keep vigilant or forages more quickly than usual).
I suppose that it would be a high level of mastery if I could determine not only what type of material/what species the beaver is foraging on (based on the sound the beaver makes) but also how the beaver is feeling, whether the beaver is healthy or not, whether the beaver is enjoying its meal or not enjoying its current dining room etc.
References
Bandini, E. (2021). A Short Report on the Extent of Stone Handling Behavior Across Otter Species. Animal Behavior and Cognition. 8. 15-22. 10.26451/abc.08.01.02.2021.
Notes by Ieva 