There is a two-beaver-territory arrangement on our river which has been puzzling me because the beavers from either family do not seem to cross a certain natural boundary which is that of shallow rapids (formed by some rocks).
The rapids as such could be easily conquered by any beaver but I have never seen beavers from the downstream side of the rapids come upstream (even when the rapids were non-existent due to high water levels and, as a result, any avoidance behaviour which could have been the result of my presence, could be excluded; moreover, I am quote confident that the beavers were not aware of my presence). Nor I have seen the upstream family crossing over to the downstream range.
There are no scent-mounds, nor I have witnessed aggressive displays although beavers in both groups tend to spend considerable amounts of time in that specific area.
Essentially, I have been reluctant to assume that this boundary is not merely my own perception (as well as chance observations of beavers not crossing while, during my absence, they might be).
Would these neighbours not try to mark the boundary and would they not avoid hanging out there, especially, while both groups are present simultaneously?
I have discussed some of the peculiarities regarding beaver territoriality on our river in this post – Possible reasons behind the limited scent-marking activity in our beaver population.
However, the lack scent-marking does not explain the curious social arrangement whereby the two families are highly tolerant one of another and they even send alarm signals to one another (additional reasons behind my willingness to assume they are two distinct families but, of course, they could also be related, e.g., parent territory bordering the range occupied by their offspring and his/her mate).
I recently wrote about another individual (in another family way upstream) who is sort of living on the edge of its parents’ territory (A beaver den way above water level).
The range, in the case of the family discussed in this post, is suboptimal because one bank is highly disturbed and offers very little resource-wise. I also suspect that the range is very short and it has been supplemented with foraging activity on a wide and deep ditch that can be accessed by walking inland through the riparian forest; see – Beaver observations – an interesting family home arrangement.
If it is at all common for two-year-olds / three-year-olds (older siblings, subordinates) to reside on the outskirts of their parents’ range, it is possible that, on occasion these subordinates happened across a potential mate (who has perhaps been seeking to disperse through this territory), the parents might not object to rationing out a portion of their territory, especially, if it is suboptimal and if their remaining territory is large and bountiful enough.
Accordingly, these two families could be related, and the family downstream which is, indeed, living on a very limited, suboptimal range where they have to be rather creative in their resource acquisition, may have started out as a subordinate living on the least optimal part of its parents’ range until it was visited by a beaver of the opposite sex who then stayed and formed a pair with the offspring of the residential pair.
The parents would have been benevolent enough to allot the part of the range to their offspring, and the mutual tolerance (alarming one another, lack of scent-marking, last year’s kit hanging out frequently and alone right next to the neighbouring range, both groups foraging almost on the border, a potential den site on the border etc.) could have been the result of their relatedness.
However, the tolerance is apparently based on some unspoken (or rather, un-exuded and physically unenforced) rules because at least the adults do not cross over the rapids.
I will soon touch upon the issue of why this might not be the case with the subadults (yearling).
The downstream family has an important foraging sight right on the verge of the rapids where some willows grow close to the water-line on the highly disturbed side of the river.
On the night of May 19, we were quietly listening to an adult beaver from this group foraging on some branches but we could not see the beaver without giving away our own presence.
We were contented, however, because there was something utterly idyllic to the beaver foraging while the nightingales were singing, as well (perhaps having begun later than other nights).
Suddenly, we saw a smaller beaver (of the size of a yearling) swim from way downstream (perhaps some 100 – 150 metres) toward the rapids.
Now this was interesting because I had not yet observed a subadult (kit of 2023) in this particular group.
In fact, it had seemed to me that the family was composed of two adults and there were no smaller beavers at all.
I thought that perhaps I had missed out on a yearling or maybe one of the adults was relatively young (a two-year-old or a three-year-old) and while it could apparently make plenty of noise while splashing, it might have been not very large itself (but nifty with sound effects).
(Female beavers can be a bit larger than males and the size of both sexes is dependent on their age which is why I have not claimed specifically which sex would be the smaller one in the pair.)
The smaller beaver crossed the river and sort of swam toward the rapids, and I wondered if I was going to witness, for the first time, a beaver from the downstream family swimming into the upstream family’s range.
Close to where the adult had been foraging (but on the opposite side of the channel), the smaller beaver stopped and floated while the larger beaver ceased foraging and swam up to it.
The larger beaver, before approaching the smaller beaver, made a roll diving in and emerging shortly thereafter.
Then the larger beaver swam to the distance of about half a metre from the smaller beaver.
They did not touch, there were no vocalizations but, almost as if according to some silent, mutual understanding, the beavers turned around and the large beaver swam back to its foraging site (where it resumed munching on the willows) while the smaller beaver swam back downstream where, some 200 metres from the rapids, it dived in and vanished.
I cannot imagine why a female, presumably in her current nursing state, would expend energy swimming against the current to exchange some communication with her mate, granted that forage was not given to her, nor there was any alarm.
Certainly, I assume that the female would be eager to check in with her mate, but, at the time, the costs might simply be too high because the kits are dependent on her milk alone.
Meanwhile, a male might have checked in with the female foraging due to mate guarding behaviour but the interaction itself did not resemble a relationship between someone who is in charge (with an intent) arriving to assess the security status of their mate.
It looked the other way around, i.e., that the beaver who had been foraging was in charge and it even seemed that this beaver sent the other beaver back with some sort of task or message.
Also, if the theory of the subordinate inheriting a part of their parents’ range is true, both mates would probably have been young and of about the same size but the little beaver was considerably smaller and resembled a yearling.
Altogether, I concluded that a yearling had swum up to the adult (possibly, a male) and the adult had perhaps sent it back to the den site with some kind of message (on the grounds that the yearling had emerged from about the same spot where it disappeared but the yearling never climbed out on the shore which suggests there could have been a den it visited).
It is curious to wonder what the specific task was and how it had been conveyed (e.g., why did the larger beaver dive under water before approaching the subadult?).
What seems equally curious to me is – where did the yearling come from?
Piecing these stories together, I have come to a suspicion that the yearling was a sibling and not an offspring to one of the mates in this pair.
There was a yearling beaver hanging out almost every night right across the rapids in the upstream range (see Little Scholar in – Two beaver youth personality profiles).
Then, for about a week, I did not observe this yearling in the usual spot at all.
This was not unusual because beavers tend to disappear from a part of their territory for a week or so and I assumed that the yearling might have developed new interests (in its new siblings who are located in a den quite far from the rapids or in exploring new foraging sites).
The beavers downstream seemed to send alarm signals for this yearling, and perhaps now the yearling has taken initiative in visiting its nieces and nephews in its brother’s/sister’s family.
It would also explain why this subadult was headed toward the rapids before it was ‘intercepted’ by the larger beaver.
However, I believe that the subadult was checking in with the larger beaver because it slowed down and stopped near where the larger beaver was foraging as if waiting whether the adult would spare a moment to swim up to it.
Perhaps, if the larger beaver had not, the subadult would have crossed the rapids back into its parents’ territory.
It is known that beavers can recognize their own kin by anal gland secretions.
Moreover, while relatives can recognize one another (e.g., a sibling is able to recognize its sibling which it has never even met), it is also possible for a mate to recognize unfamiliar (and, of course, familiar) siblings of their mate (e.g., a female might recognize her mated male’s siblings even if she has not met them before) (Sun, L. & Müller-Schwarze, D., 1997).
It is possible that the yearling has only recently begun to take such great interest in its related neighbours and their kits.
The unrelated mate might yet be a bit unaccustomed to these ‘trespasses’ and the diving in could have been a behaviour aimed at smelling the yearling in order to determine whether the yearling is not an intruder.
It would suggest that perhaps the female (who is maybe in the den) is related to the yearling (and the upstream family) while her mate (the male foraging) is not, and he might be wary to admit the yearling to its territory but, at the same time, the presence of the yearling could be useful if the yearling helped in guarding the kits or performed other chores, or provided psychological support for the first-time mother.
The unrelatedness between the larger and the smaller beaver (the male and the yearling) might also explain the overall lack of vocal and tactile communication.
It is still, nevertheless, curious what the task was entrusted with the yearling and how it was relayed because it truly appeared that the yearling, upon having received some communication from the male, turned around and headed back to the den.
Maybe the mother wished to leave in order to forage and she had sent the yearling to fetch the male so that the male came to stay close to the kits while the female was out.
Perhaps the male trusted the yearling to babysit the kits instead.
However, these are very complex messages and it is astounding to imagine how they were gotten across.
There could have been other types of messages, e.g., the female could have run out of snacks (if such are stored in the den) and she could have sent the yearling to fetch some snacks (which the yearling would have been more comfortable to achieve in its own territory) but, having been intercepted by the male, the yearling could have somehow let know that the female was hungry and the male might have resolved to carry branches to the den.
The female might have simply wished to see the male, and the yearling’s task could have been that of letting the male know that his presence was needed in the den (with or without snacks).
I left very briefly thereafter, and I do not know for how long the male was foraging and what happened next (whether the female exited the den or whether the male swam down to the den).
I will keep observing in order to determine whether I might be correct but, altogether, this might be a potentially curious situation whereby the female has inherited range from her parents where she has started her family with a male who has dispersed into her home.
Thereafter, they might have kept out of the parental range but, at times, they might have still communicated across its border (e.g., the female might have yet been protective of her younger sibling sending alarm signals to warn it while the parents might have sent alarm signals for their elder daughter).
Once the female gave birth to the kits, the yearling might have taken interest in the den (perhaps also spurred by the fact that these kits are only partly related to it while the parents’ own offspring are quite much like the yearling itself; and also because the sister’s den is close to the yearling’s favourite hang-out; and also because the yearling’s own parents are probably very preoccupied and not in as much need of its help as the younger, less experienced couple).
The female and her mate might have admitted the yearling to become an involved uncle/aunt and the yearling might be now living on both ranges.
The yearling’s presence could be of even more crucial importance if, as suggested by my observation, an otter that has moved in to dwell in the area (on May 22, I believe I observed an otter hunting right where the yearling had been swimming the night before on the downstream pair’s range and where the natal den might be located; see – Otter observation (May 22, 2024) – an otter hunting ducklings?).
Otters probably could pose a threat to small beaver kits.
The interactions themselves are fascinating.
Perhaps the yearling had picked up some olfactory cues also by the female in the den, and the male could have done ‘readings’ from the scent carried on the yearling’s fur in order to find out the wishes by its mate.
This could explain why the yearling swam all the 100 – 150 metres upstream with its head and some of the back raised above the water (not entirely submerged).
On one hand, perhaps it is better to be visible to the unrelated male so that no misunderstandings occurred (sneaking past might be dangerous if the male detected it and reckoned it was an intruder).
Also, certainly, beavers do swim considerable distances only partly submerged.
But, in the deeper parts (especially, so close to the disturbed area where there are human dwellings), beavers seem to prefer to cover distances of > 100 metres in a dive.
Perhaps the yearling tried to avoid washing off the scent cues (messages) that it was carrying on its fur.
The yearling may have stopped for the male to simply read the messages distantly (from the opposite bank which was some 5 metres away) but the male might have decided to engage in closer communication with the intent to send a message (the yearling itself) back to the female.
However, if the female truly sent some olfactory residue message to the male in the form of its younger sibling, the male did not anyhow mark the yearling.
Maybe the male had to finish foraging for some reason (this family is rather limited in its diel activity because it is subject to high disturbance and once the humans awaken, the beavers vanish off to dens), and the message that the male sent was the yearling itself (if the yearling was to babysit on behalf of the male).
I do not suppose that the male would not have recognized the yearling without sniffing underwater at its anal region.
Perhaps this was the first time the male admitted the yearling to the den without the supervision by the two adults, and the determination of relatedness could have been done not with the purpose of recognizing the individual but with the purpose of assessing whether the yearling could be trusted with the kits.
Namely, yearlings probably do babysit the kits of their parents (their younger siblings).
But this yearling was not a sibling to the kits it had been visiting.
Accordingly, the male might have taken a sniff in order to evaluate not the degree of similarity between the yearling and the female (the male’s mate, the mother of the kits) but the degree of similarity between the yearling and the kits (the male’s offspring).
If the yearling smelled not only similar to the female (the mother) but also to the kits, the male might have felt assured the yearling was almost like a sibling to the kits (a social category allowed to babysit).
References
Sun, L. & Müller-Schwarze, D. Sibling recognition in the beaver: a field test for phenotype matching. Anim Behav. 1997 Sep;54(3):493-502. doi: 10.1006/anbe.1996.0440. PMID: 9299035.
Notes by Ieva 