Recently I wrote a post (Do beavers prefer woodland edge trees?) in which I discussed why beavers might prefer trees growing in edge habitats (vs. mature forest interior within reach for the beaver).
I realized that there is a paucity of studies dedicated to which specific nutrients and what type of woody plant structures beaver seek out.
For example, any references to phloem being the actual resource beavers seek to obtain (vs. outer bark, cambium or xylem) are only found in half-scientific articles (not empirically tested).
I was curious as to what beavers search in their food as many species, for example, moose are highly selective and they seem to be able to diversify their diets so that all essential nutrients are obtained through foraging on a variety of plants each of which offer different nutritional possibilities.
In this post I will attempt to list some relevant information and I might add to the post if I find other articles (about beavers or about plants with regard to what is known/observed about beavers).
I am not going to focus on the tree species preference which has been discussed more extensively (unless the mentioning of the tree species is relevant to its particular nutritional profile).
- Food selection by beavers (Castor fiber albicus) in relation to plant chemicals and possible effects of flooding on food quality (Ganzhorn, J.U. & Harthun, M., 2006);
- Willow (Salix spp.) and aspen (Populus tremula) regrowth after felling by the Eurasian beaver (Castor fiber): Implications for riparian woodland conservation in Scotland (Jones, K. et al., 2009)
- Scaling of xylem and phloem transport capacity and resource usage with tree size (Hölttä, T. et al., 2013);
- References
Food selection by beavers (Castor fiber albicus) in relation to plant chemicals and possible effects of flooding on food quality (Ganzhorn, J.U. & Harthun, M., 2006)
In this study, nitrogen content as well as neutral detergent fiber (NDF) content (without sodium sulfate) and acid detergent fiber (ADF) content were analyzed in plants (woody plants but also grasses, sedges, forbs, aquatic plants etc. (see Table 1) foraged upon by beavers and not foraged upon by beavers (but found in the habitat where beavers foraged).
Soluble carbohydrates were also quantified as well as condensed tannins.
It was found that food items (species foraged upon by beavers) had higher nitrogen concentrations and lower NDF concentrations than non-food items (species growing nearby but not foraged upon by beavers).
This suggests that, like most herbivores, beavers optimize nitrogen (protein) intake.
Beavers did not attempt to choose plants with lower ADF which means that they did not mind the higher or lower cellulose and lignin content of different species.
However, beavers apparently attempted to avoid foraging on plants that offered bulk (NDF) resulting in slower food passage rate.
Beavers are hindgut fermenters which means that fibre (cellulose, hemicellulose, lignin etc.) becomes processed (fermented) after nitrogen has been extracted to produce proteins.
Accordingly, while beavers can process these fibrous structures, a great amount of them can reduce the rate at which proteins are produced because while these fibers are being fermented in the hindgut, new forage intake is halted and new proteins are not being produced (nitrogen is not being assimilated but also minerals are not being absorbed).
Beavers consume plenty of fibrous foods (many of which are woody plants with plenty of long-fermented lignins) which is one of the reasons why they engage in caecotrophy (eating their own faeces to redigest the plant matter and to extract more nutrients, including nitrogen, from this partly processed mass).
It appears that energy (soluble carbohydrates) is not a limiting factor relative to nitrogen uptake because hindgut fermentation allows to extract nutrients that allow to produce energy rather than proteins and beavers seem to reduce hindgut fermentation duration to speed up the passage rate.
On the other hand, it would seem that protein vs. energy compromise might be different when only woody plants are analyzed (as will be reviewed in another article).
(Personal note – there might be differences between nutritional selectivity in beaver age/sex groups as well as seasonal or life stage (e.g., lactation, active growth) differences as these estimates were derived from the overall foraging preferences in two colonies of 5 and 7 beavers.)
In this study (Table 1) it was also indicated which plants were taken by beavers (and even which plant organs):
- Alnus glutinosa – young bark (but not older bark of stems or twigs, nor leaves/twigs);
- Alnus incana – young bark and bark of young twigs;
- Anthryscus sylvestris – whole plant;
- Carex gracilis – whole plant;
- Carex paniculata – whole plant;
- Cirsium oleraceum – whole plant;
- Crataegus laeviata – leaves, twigs;
- Epilobium angustifolium – whole plant;
- Filipendula ulmaria – whole plant;
- Heracleum sphondylium – whole plant;
- Phalaris arundinacea – whole plant;
- Polygonum bistorta – whole plant;
- Ranunculus repens – whole plant;
- Rhinantus minor – whole plant;
- Rubus idaeus – whole plant;
- Salix cinerea – leaves, twigs;
- Salix triandra – small twigs, twigs;
- Salix x cericans – whole plant;
- Sanguisorba officinalis – whole plant;
- Urtica dioica – whole plant;
- Iris pseudacorus – whole plant;
- Juncus effusus – whole plant.
Interestingly, Salix caprea was not taken while other Salix spp. were foraged upon (leaves, twigs or even whole plant) and it is also curious to determine why only young bark was consumed in Alnus glutinosa and Alnus incana (where also bark of young twigs was consumed).
Additionally, what constitutes the difference between forbs taken and not taken?
Table 1 provides the chemical composition measurements.
With respect to Alnus glutinosa, young bark had a far higher nitrogen content while its NFD was average.
However, nitrogen content was also high enough in barks of small twigs and NFD was not that different (even lower).
Significant differences (not statistically significant because I have no access to statistic analysis tools) were found between tannins in both food items (tannin content 4.5 times higher in the bark of small twigs compared to young bark) as well as between soluble sugars (2.5 times higher in young bark compared to small twigs).
Perhaps beavers attempt to avoid tannins or they might select soluble sugars in these woody plants.
However, in Alnus incana bark of young twigs was foraged upon despite its relatively low protein content (average NDF) and much higher tannin concentration (1.6 times higher than in small twigs of Alnus glutinosa where supposedly tannins were avoided).
Thus, tannins as such might not have been of importance in avoiding Alnus glutinosa twigs and perhaps there were some other secondary compounds that A. glutinosa produces but A. incana does not.
Overall, it seems that soluble sugars might be selected in woody plants even if they do not come with high nitrogen content as long as the rate of passage through gut is not impeded (i.e., soluble sugars are not combined with high NDF).
It should be noted that bark of young twigs was analyzed in A. incana while bark of small (but not necessarily young) twigs was analyzed in A. glutinosa. Beavers did not forage on bark of young twigs in A. glutinosa.
Bark of young twigs in A. incana, despite its overall poor performance (but also not too high NDF) had high soluble sugar concentrations (2.2 x higher than in young bark of A. incana and also among the top values in the organs of both Alnus spp. overall.
It is not known when precisely the young twigs were consumed but if it was in spring, perhaps the soluble sugars were selected for (even over nitrogen) in order to boost energy levels to recover from the long winter and fat loss.
This might be especially true of last year kits and yearlings as these two social groups tend to suffer the greatest body mass losses during winter that can result in higher mortality rates.
Higher soluble sugar values were discovered in many non-foraged organs of A. glutinosa (bark of medium-sized twigs and bark of young twigs.
Apparently, these sugars came in a combination with higher NDF and therefore were not foraged on while perhaps beavers cashed in on sugars in A. incana bark of young twigs.
I believe there are some other (perhaps not directly nutrition-related) chemicals involved in A. glutinosa bark which are not present in A. incana and which determine the foraging choices.
It is curious that Salix caprea was not foraged on while other Salix spp. were.
Typically, Salix spp. are considered highly preferred forage (alongside aspen) and it is not discerned which Salix spp. beavers consume.
Looking at the constitutional differences between Salix caprea and other willows, it is seen that their nitrogen content was lower in all parts but small twigs (in which it was a bit higher than in twigs of S. triandra that were foraged upon (but there were differences in S. triandra twig nitrogen between sites, the values being much higher in one site).
However, also S. x sericans was consumed (whole plant) despite lower nitrogen content than in S. caprea small twigs.
NFD did not seem to differ significantly although they were lower in S. cinerea.
But, for example, while nitrogen was higher in small twigs of S. caprea and NDF was also lower than in twigs of S. triandra and whole plant of S. x sericans, the small twigs of S. caprea were still not consumed.
Condensed tannins were also lower in small twigs S. caprea than in other two items of lower protein content that were foraged upon.
Soluble sugars do not offer any insight into these foraging choices, either.
Thus, it can be assumed that there were also some other chemicals involved or that these plants were not uniformly accessible (e.g., growing in different distances to water or requiring different foraging effort).
However, I believe that beavers did distinguish between high nitrogen Salix spp. and the lower nitrogen S. caprea and that the foraging choice was based not only on the tree species but also on the specific conditions in which it grew (nutrient assimilation).
On the other hand, it is curious that S. caprea had low nitrogen but high soluble sugars which suggests it was not nitrogen-limited to the point that carbohydrate production was inhibited.
I find it astounding that beavers did not forage on Populus tremula but these trees (only in one of the sites) had very low nitrogen and soluble sugars while NDF was typical of woody plants.
Perhaps these particular trees were growing in nutritionally limiting conditions.
It is worth noting that on occasion P. tremula and S. caprea were, indeed, unselected for due to their specific growing conditions (low nutritional value), despite aspens and willows being highly preferred forage by beavers, it would suggest that beavers can avoid species they are ‘hard-wired’ to consume if these species are nutritionally deficient in the particular site (especially, long-term vs. seasonally).
This might mean that foraging for trees and shrubs is a highly cognitive process in beavers rather than a process during which they identify the best foraging species and harvest them indiscriminately.
It is not surprising because beavers must make active decisions regarding species/diameter with regard to the tree/shrub distance from the water.
Felling angle might also be an active consideration.
If beavers base their selective choices (of woody plants) not merely on species but also on the nutritional value of the specific individual, this could be of relevance to cafeteria-type feeding experiments.
Interestingly, Crataegus laevigata had comparatively low NDF for a tree/shrub.
The comparison among herbaceous species is more difficult to achieve without statistical analysis tools.
It would appear that beavers prefer plants that grow close to the river channel but also that are large enough (mostly, but see e.g., Ranunculus repens) and that grown in dense clusters.
Some of these considerations could be traded for others, e.g., the aforementioned Ranunculus repens can be found in bunches and often without venturing far from the water (or staying in the water as R. repens can sometimes grow as an emergent plant) which could compensate for its relatively small size; besides, under some conditions it is not such a small plant.
Ranunculus repens has a moderately high nitrogen count but it also has plenty of soluble sugars and some of the lowest NDF values.
It might be interesting, once more, to compare why the beavers foraged on Carex gracilis and C. paniculata but not on C. brizoides, nor on C. nigra.
C. gracilis and C. paniculata are taller plants than C. brizoides and C. nigra.
It is curious to wonder whether beavers might have adjusted their ‘forage value’ assessments looking up (to shrubs and trees) as well as down (to aquatic plants) and thus those medium-sized plants in the middle that are not tall, nor they creep close to ground are somewhat dismissed.
However, beavers did not select, for example, Phragmites australis which is a tall plant with plenty of nitrogen and sufficient sugars but also high NDF.
Regarding nutritional values, the Carex spp. foraged upon had higher nitrogen values except for C. gracilis which had high enough values in June but low values in August but it was still selected in August.
Of course, beavers might not be at all times aware that the nutritional quality has seasonally changed and, also, C. gracilis had very high soluble sugar content in August which perhaps compensated for low nitrogen.
It is interesting that C. gracilis had extremely high NFD values (and so did sedges in general) both in June and August.
Perhaps foraging choices in August are already rather limited as many plants are ’rounding up their growth season’ (the study was conducted in Central Germany) and the sugar content was of significance (there could be altered energy needs in August as everybody is trying to fatten up before the cold sets in, especially, the cubs).
The non-foraged sedges had low nitrogen, high NDF and while soluble sugar content in C. brizoides and C. nigra (both June) was high enough, apparently it could not compensate for high NDF and low nitrogen.
I would say that beavers can opt for soluble sugars as long as NDF is low, and such plants might be eaten even if they do not offer much nitrogen.
But the combination of low nitrogen and high NDF would not be mitigated by high soluble sugar content.
C. gracilis had high enough protein, average NDF and high enough sugar.
It is curious why C. gracilis had higher sugar content in June compared to August while a dramatic opposite scenario was observed in C. paniculata.
It overall appears that some species (e.g., Heracleym spondilium) had high nitrogen and sugar values in some location but not in another while some species (e.g., Sanguisorba officinalis, especially with regard to sugars) had high values in June but low values in August.
It seems that beavers had adapted to utilizing these species without perhaps discerning the changes in their quality between sites and seasons (another reason could be the aforementioned decreasing availability of herbaceous forage in August).
Maybe beavers are not as thorough in their assessments of herbaceous species because these plants do not demand such high energy investment as trees do.
In woody plants, beavers can apparently tell differences between the nutritional content of even different organs but herbaceous species might be viewed more as ‘snack-food’ and the high sugar content in some might suggest that beavers could use the sweetness as a general indicator of their quality (similarly as we assess the ‘quality’ of our snacks).
Anthryscus sylvestris was clearly selected for soluble sugars, not nitrogen but it also had low NDF values.
Filipendula ulmaria was a very profitable plant in both respects (nitrogen and sugars) with low NDF to top the attraction.
Phalaris arundinacea is an interesting case because it had low protein, high NDF and soluble sugars that could hardly compensate for the high NDF.
It is not really clear why P. arundinacea was foraged upon.
P. bistorta had high enough nitrogen and very high sugars in June but not in August. It also had very low NDF.
Ranunculus repens was likely chosen for sugar content although its nitrogen was not low, either.
Sanguisorba officinalis was one of those plants that would have been chosen for soluble sugars in June (some of the highest values among all plants) because nitrogen content was low but it was still foraged on in August when sugar level had dropped 4.5 times while NDF had risen 1.7 times (clearly, beavers knew that S. officinalis was a sweet choice but they had not figured out it was not a sweet choice close to autumn, besides it had grown heavier on stomach).
However, it should be noted that some of these herbaceous species might not have been foraged upon extensively and if they were taken in small amounts, the NDF load might not have proven as aggravating and limiting.
Urtica dioica is, of course, high nitrogen plant which, for the most part (with the exception of month of June in one study site but not the other) also offered plenty of soluble sugars and moderate NDF.
Iris pseudocarus has plenty of sugars, high nitrogen in June and average NDF.
Thus, I cannot really agree with the authors of the study who concluded that beavers were not looking for soluble sugars when herbaceous and woody species were analyzed together.
It is probable that nitrogen is problematic and where it is low, it must be compensated for by soluble sugars. Also, sugar foods should not reduce the uptake rates (NDF must be low or moderate).
Willow (Salix spp.) and aspen (Populus tremula) regrowth after felling by the Eurasian beaver (Castor fiber): Implications for riparian woodland conservation in Scotland (Jones, K. et al., 2009)
The study by Jones, K. et al., 2009 was not conducted with the objective of determining beaver foraging preferences and requirements but with the objective of assessing regrowth of felled or partly-felled willows and aspen.
However, I though the results were of significance because I have been attempting to evaluate the extend to which beavers are cultivating their food resources.
For example, if some regrowth statistics were higher than others and beavers demonstrated the behaviour of favouring these particular forms of felling (e.g., partly-felled to fully-felled), it might suggest beavers are aware of the lusher regrowth after their selected activity compared to the non-selected activity (however, there might be other reasons why beavers fell the tree completely vs. non-completely).
The study results are the following:
- After complete felling, 86% of willows showed regrowth from stumps in river habitat (drier, no roe deer access) and 77% showed regrowth in lake habitat (inundated, little roe deer foraging);
- Regrowth from completely severed logs showed values of 39% and 34%, respectively;
- Incompletely-felled willows constituted 8.7% and 0.7% of all beaver-cut willows in both sites, respectively;
- Of incompletely-felled willows, in the river habitat, 86% showed regrowth from the stump and 96% showed regrowth from the trunk while, in the lake habitat, 100% showed regrowth both from stump and trunk;
- In the first 8 months of their residency in the river habitat, beavers had completely felled all 11 mature aspen trees but, over the two following years, beavers had not touched any of the 85 suckers produced by the surviving root system (phenolic resin in young aspens).
Scaling of xylem and phloem transport capacity and resource usage with tree size (Hölttä, T. et al., 2013)
This study is not dedicated to beavers and their foraging preferences, nor their nutritional requirements.
However, the study, among other research topics, addresses nitrogen content in the woody tissues which might be of importance when assessing beaver selection of the said tissues or plant organs (organ segments).
I will list some of the relevant findings.
Nitrogen content increased in the living bark (primary and secondary phloem + vascular cambium) and in the whole bark with decreasing tree diameter, i.e., smaller trees had greater N content in bark and the living tissues than larger trees.
(Nitrogen content in xylem remained unchanged with respect to tree diameter although it is not clear whether this refers to heartwood or heartwood + vascular xylem.)
The greatest nitrogen concentration was found in aspen whole bark and the least in pine whole bark (of the same diameter).
Phloem nitrogen in small twigs was comparable to that in foliage (in aspen and pine).
Aspen had a larger amount of phloem and higher phloem to xylem ratio than pine.
In small trees (< 10 m in height), the highest nitrogen concentrations were found in leaves.
The taller the tree, the greater the proportion of N in phloem and xylem compared to foliage.
Heartwood determined phloem nitrogen content in larger trees, i.e., absence of heartwood or high heartwood N concentrations in such trees decreased the role of phloem as nitrogen sink (while in low N heartwood trees phloem was the more important N sink relative to xylem).
Within a 10 m tree, the axial distribution of phloem was concentrated toward the apex (volume, cross section) while xylem was distributed more evenly.
Xylem and phloem N was more concentrated in the apex area (especially, phloem and, especially, in aspen.
Xylem conductivity was greater at the base but phloem conductivity was greater toward the apex (under conditions of limited heartwood formation).
References
Ganzhorn, J.U. and Harthun, M. (2000), Food selection by beavers (Castor fiber albicus) in relation to plant chemicals and possible effects of flooding on food quality. Journal of Zoology, 251: 391-398. https://doi.org/10.1111/j.1469-7998.2000.tb01090.x
Hölttä, Teemu et al. (2013) Scaling of xylem and phloem transport capacity and resource usage with tree size. Frontiers in Plant Science, Vol. 4, https://doi.org/10.3389/fpls.2013.00496
Jones, K. et al. (2009). Willow (Salix spp.) and aspen (Populus tremula) regrowth after felling by the Eurasian beaver (Castor fiber): Implications for riparian woodland conservation in Scotland. Aquatic Conservation: Marine and Freshwater Ecosystems. 19. 75 – 87. 10.1002/aqc.981.
Notes by Ieva 