Recently I came across the PhD Thesis publication by Mayer, M., 2017.
It consists of several scientific studies (5 papers).
In this post, I will be largely referring to the findings of Paper IV, ‘Territory size and age explain movement patterns in the Eurasian beaver’ (pp. 118 – 129) (although I might also discuss a few ideas touched upon in other parts of the thesis.
Beavers appear to face a trade-off regarding territory size between foraging resources and patrolling effort.
Smaller territories have reduced foraging access and, therefore, beavers have to forage further from the waterline because vegetation within the preferred 10 – 20 (av. 15) m becomes depleted.
This poses predation risk.
Smaller territory holders are also forced to more often engage in extraterritorial movements (intruding in neighbouring territories) possibly due to resource depletion in their own home ranges.
Meanwhile, large territory holders face other challenges.
They need to invest more time and energy in border patrolling (and territory patrolling in general, e.g., during spring when subadults tend to disperse and intrude far in other beavers’ territories).
This reduces the time that can be allocated to foraging.
Altogether, it would seem that intermediate territory size is optimal for beavers and, indeed, intermediate territory holders have the longest duration of occupancy (that might be viewed as success in tenancy and also reproduction because reproduction can be more successful in long-term territory holders.
These publications did not really discuss what determines the territory size – whether it is sometimes chosen by the beavers or ‘forced upon them’.
I believe it would be worth further studies.
Namely, territories can be acquired via different mechanisms and territory size (and quality) depends on external factors (but it might also depend on internal factors).
For example, in low densities individuals can select optimal territory size but they might also occupy larger than intermediate territories because the territorial tension is also low (patrolling is needed but it is less likely that neighbours would intrude with the purpose of foraging or claiming new areas).
As the population density increases, tension grows and new dispersers seek to settle.
As a result, territory size might shrink because the patrolling effort can be too great and in some places territories or parts of territories might be usurped by dispersers.
The territorial dynamic might become more heterogeneous with larger territories that still belong to the initial settlers or dispersers that have displaced the initial settlers etc. and with smaller territories occupied by more recent territory holders or less competitive territory holders.
Here, the quality of the individual might come into play.
For example, smaller individuals might not be able to outcompete others and they might accept suboptimal, small territories.
Meanwhile, as the population grows and individuals delay dispersal, they also reach greater body size and experience and they might not be daunted by competitors or larger territories that need greater patrolling effort.
They might be able to claim the optimal intermediate territories and also not opt out of larger than optimal territories.
The results in the same thesis demonstrate that older individuals spend more time on the shore and they also spend more time on territory boundaries (dominating individuals).
This might indicate that experienced, large, old enough individuals are not daunted by the need to patrol larger areas, nor they would have difficulties foraging further from the shore which means that older dispersers might occupy a greater range of territory sizes than younger dispersers.
Large beavers might also need larger territories due to greater energy demands with respect to body mass.
It would be interesting to test this hypothesis.
These (population density, age, experience and size of the disperser) would be external factors determining the future occupancy of small, intermediate or large territories.
There are other external determinants, e.g., habitat conditions.
For example, in river stretches that do not offer free movement over large distances (many shallows or other obstacles), patrolling large territories might be less feasible than in other river stretches that are deep enough and where the current and other factors do not impede movement.
Similarly, large territories could be obligate in stretches where the shore is not uniformly accessible or uniformly overgrown with palatable vegetation.
The beaver cannot exclude the suboptimal patches and therefore must extend its territory to cover also a sufficient amount of suitable patches.
Meanwhile, there might be internal attributes, as well.
Specifically, there could be genetic traits, life experience, behaviour patterns and individual personalities affecting the territory size that a beaver chooses if several options are available (which might rarely be the case in more densely populated areas and therefore should be studied in recently colonized areas).
For example, beavers that are more active or that have inherently superior metabolism could not mind settling in larger territories.
Beavers that have had bad experience with predators or that have dispersed from predator-dense area to a low predation risk area might still opt for larger territories that do not involve foraging far from the shore.
Previous experience (in the natal territory) might also be important.
Many species (e.g., roe deer) tend to choose new ranges that are similar to home ranges.
It is difficult to predict whether older vs. younger individuals would choose ranges more similar to their natal territory (acquired extensive experience of specific conditions) or more dissimilar (greater confidence in handling new situations).
Perhaps this is also related to family relationships because I expect that natal dens would not be located in the periphery and the necessity to attend to the young (and to spend time with the mated female who is nursing the young and thereby restricted in her movements) could influence the territory sizes, for example, in males resulting in more paternal vs. less paternal individual territory selection.
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Finally, I was wondering whether territory size affects, for example, aspects of mortality.
If small territory holders are necessitated to forage further from the shore, they might be subject to greater predation risk than intermediate or large territory owners.
Meanwhile, large territory owners could suffer more frequent injuries due to territorial conflicts.
However, the latter conditions might be difficult to test because large territory owners patrol more and possibly have to defend their territory often.
The studies by Mayer, M. do not claim that their intraspecific conflict rates are also consequently higher because intruders are met in all sizes of territories and small territory holders tend to intrude in neighbouring territories leading to different type of intraspecific conflict where the small territory holder is the trespasser and becomes chased out.
Still, I presume that larger territory owners might have a greater number of intrusions merely to the probability of trespassing per area.
While both small and large territory holders only have two boundaries (at the end of their river stretch), trespassing (e.g.. during dispersal) occurs also within the territories and small territories can be travelled through in a shorter time with less frequent foraging efforts (especially, if the foraging must occur far from the shore).
Thus, large territory holders might have increased rates of encounters with trespassers.
While trespassers themselves tend to avoid being noticed, territory holders must be assertive if the conflict is unavoidable and it is difficult to predict who would suffer more often – a frequent trespasser or a frequent defender.
One of the papers included in the thesis by Martin, M. quoted a publication by Rosell and Bjørkøyli, 2002 (which I have not read yet) that concluded – neighbours tend to be more tolerant toward trespassing neighbours than toward trespassing strangers.
Thus, small territory holders that trespass into neighbouring territories could be tolerated to a greater degree than strangers travelling through large territory holder territories leading to differential animosity levels.
Accordingly, large territory owners might suffer more frequent injuries due to intraspecific conflicts.
Mortality might be the lowest in intermediate territory owners resulting in greater longevity.
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I would also be interested to discover how small vs. intermediate vs. large territories affect family relationships and routines.
As mentioned before, kits in large territories might receive reduced attendance rates at least by the father who has to patrol the boundaries and scourge out intruders.
This could also affect foraging distance from the partner and the ability to teach important skills to the offspring if the offspring cannot follow on the patrolling duties (it was demonstrated that dominating individuals patrolled more than subordinates).
The time allocated to family interactions might be reduced.
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Large territory holders might also attest to different health consequences than small territory holders.
Small territory holders might suffer nutrient deficiencies and heightened stress levels due to predation.
Meanwhile, large territory holders might suffer health effects due to overexertion of physical capacities (swimming is more energy demanding than foraging on the shore).
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The study by Havens, R.P. (Master Thesis), Beaver home ranges and movement patterns on the Embarras River Watershed in east central Illinois (2006) indicated that in the studied population (North American beavers) home range size did not differ pre- and post-dispersal.
This could indicate that beavers tend to choose ranges similar to those they dispersed from.
It could also be relative to the population density (availability of a variety of ranges) which was moderate to high in the studied population (ca. 0.4 colonies / km) if I understand correctly.
Thus, the population area was probably settled and ranges were taken over or inherited ‘as they were’ without great selectivity.
As to the external factors, in this study food availability (preferred foraging species and proximity to maize and soy) determined home range size.
Home range size was also correlated with water level fluctuations (it became smaller during drought when beavers were restricted to the stretches that still had high enough water level).
Sex determined home range size in summer when females lactated keeping close to the den.
The same publication (Havens, R.P., 2006) also stated, ‘Studies in Mississippi and Montana suggested that juveniles sometimes pass through available, high quality sites only to settle in apparently less suitable habitat (Weaver, 1986; Harris, 1988) suggesting that genetic or other factors unrelated to habitat quality may influence the distance traveled or the settlement location.’
This could indicate that related individuals might make similar settlement choices, including the selectivity for territory size.
It would be difficult to disentangle decisions that arise from genetics vs. natal range experience factors.
It would interesting, however, to test what factors those are because they could indicate what considerations determine beaver selectivity for home sites – and whether any of them indicate at a decision between tolerating greater intraspecific strife potential vs. greater predation risk.
Notes by Ieva 