Evolution of woodcutting behaviour in Early Pliocene beaver driven by consumption of woody plants

Dipoides sp. palaeoecology

The Bayesian mixing model indicates that Dipoides sp. consumed both woody plants and freshwater macrophytes in approximately equal proportions (Figs. 6 and 7), although it relied slightly more on freshwater macrophytes. This suggests that Dipoides sp. spent a greater proportion of time feeding in the water than on land.

The distribution of Dipoides sp. δ¹³C_col and δ¹⁵N_col is not entirely enclosed within the three primary producer functional groups analyzed (Fig. 6). This is likely the result of the relatively small plant macrofossil sample size. Submerged aquatic macrophytes, for example, are under-represented in the plant macrofossils available for stable isotope analysis. Macrophytes have highly variable δ¹³C and may have contributed more to Dipoides sp. diet than the mixing model suggests. Submerged macrophytes can be highly enriched in ¹³C because of physiological differences (primarily the use of ¹³C-enriched dissolved bicarbonate) or environmental conditions in the water column (i.e. boundary-layer effect)^54,55,56. In addition, tree bark is more enriched in ¹³C than tree foliage⁵⁷ and may have been a key resource for Dipoides sp.

The results of the dietary mixing model support the interpretation that woody plants were an important contributor to Dipoides sp. diet. It is likely that Dipoides sp. also used shrubs and trees as a source of construction material^10,11, but more evidence is needed to confirm this. Similar to extant Castor, Dipoides sp. may have also demonstrated regional differences in diet, where northern and southern populations utilized different resources according to their availability.

Nitrogen content and C/N as indicators of forage quality

Plant macrofossil nitrogen content (N wt%) and C/N are indicators of forage quality and may be used to interpret the relative nutrition of dietary inputs. Plants with high N (wt%) contain more protein and energy—likewise, low N (wt%) correlates with low plant digestibility, high fiber and high lignin compound content⁵⁸. Beaver Pond plant macrofossil N (wt%) and C/N are highly variable (Table 2, Fig. 5). Although there is considerable variability in C/N ratios depending upon which plant part was analyzed (i.e. seeds versus woody tissue), woody vegetation tends to have higher C/N ratios than macrophytes, and thus tends to be of lower food quality. However, the increased structural tissues in woody plants may have rendered them more effective winter cache foods.

In extremely seasonal environments such as the High Arctic, herbivores must use plant resources in a highly efficient manner. Herbivores must consume the highest quality forage possible during the brief growing season to maximize nutrient and energy gain. High quality forage typically includes young leaves with high nitrogen content, minimal structural (fibrous) tissues, and low defense compound content^59,60.

Within the Beaver Pond macrofossil assemblage, pod grass (an emergent macrophyte) and birch have the highest nitrogen content and lowest C/N (Fig. 5). A larger sample set is necessary to confirm this observation; however, current data supports the conclusion that emergent macrophytes and deciduous broadleaf trees were among the more nutritious types of forage available to Dipoides sp. at the Beaver Pond site. It should be noted that forage quality is not the only factor that governs herbivore feeding behaviour. Animals may preferentially target plants with higher biomass to minimize energy expenditure traveling between forage sites or select plants that grow in locations that minimize the risk of predation.

The C/N of high Arctic shrubs decreases over the course of the growing season⁵⁸. As there is no time-constraint on macrofossil deposition at the Beaver Pond site, variation in C/N may also be due to differences in plant phenological stage at time of incorporation into the peat layer. The incorporation of senescent plants into the peat deposit at the end of each growing season may in part account for the lower than expected macrofossil N (wt%) values reported from this site.

Beaver Pond site flora δ¹³C and δ¹⁵N

The Beaver Pond macrofossil assemblage contains a diverse range of terrestrial and freshwater plant species. The identified plant species in this study concur with previous interpretations that this was an open-forest landscape interspersed with shallow wetlands. Larch trees and cool-climate woody shrubs dominated the forest community. The wetlands supported both vascular macrophytes and dense assemblages of bryophytes.

The macrofossil δ¹³C are all within the range expected for primary producers utilizing the C₃ photosynthetic pathway and accessing either ambient or dissolved atmospheric CO₂ as their dominant carbon source. The δ¹⁵N of the macrofossils are also within the expected range for a riparian ecosystem in a cool climate biome.

While Dipoides sp. most likely consumed leafy tree branches and woody tissues (cambium), it is worth noting that plant seeds and cone bracts were analyzed in this study due to ease of macrofossil taxonomic identification. Leaf δ¹³C is typically lower than that of other plant parts⁶¹, although there is no clear pattern in intra-plant variation of δ¹⁵N.

Moss

Samples of the dominant Beaver Pond site bryophyte, Scorpidium (hooked scorpion moss), have very low δ¹³C for a primary producer (− 36.6 to − 34.6‰). This pattern is consistent with modern mosses collected from freshwater habitats in Subarctic and Arctic regions^13,62,63,64.

Environmental conditions dictate moss δ¹³C rather than species-specific physiological differences. Peat mosses can grow partially or fully submerged in water. Given that mid-Pliocene atmospheric CO₂ concentration levels were similar to modern (~ 400 ppm)^15,65,66, moss exposed to the atmosphere would preferentially have used the abundant ¹²CO₂, resulting in low δ¹³C. Alternatively, low δ¹³C in peat mosses can also indicate an underwater growing environment rich in ¹³C-depleted respired CO₂ from surrounding plants⁶².

Moss macrofossil δ¹⁵N is relatively high for a photosynthetic organism (mean =  + 4.8‰). This is indicative of either the presence of ¹⁵N-enriched sources of bioavailable N (i.e. dissolved nitrates, organic proteins such as urea or amino acids), or increased nutrient availability^45,61. Unlike vascular plants, mosses do not uptake compounds through their roots. Rather, they obtain nutrients from wet or dry deposition through their leaves^67,68. Today, beaver ponds are considered to be N sinks, with elevated rates of bacterially mediated denitrification⁶⁹. These bacterial processes result in ¹⁵N-enriched products that are readily dissolved and used by plants (including moss) living in an aqueous environment. Decomposition processes also increase plant δ¹⁵N over time⁴⁷ and remineralized organic debris decomposing in wetlands may be particularly ¹⁵N-enriched.

Beaver Pond bulk peat samples and moss macrofossils show a similar isotopic pattern (low δ¹³C and high δ¹⁵N), which suggests that hooked scorpion moss contributed substantially to peat biomass accumulation at the Beaver Pond site.

Macrophytes

Beaver Pond macrophyte δ¹³C fall well within the albeit very wide known range for modern freshwater plants (− 50 to − 11‰, see Osmond et al.⁷⁰, Keeley and Sandquist⁵⁴, Mendonça et al.⁵⁵, and Chappuis et al.⁵⁶). It is reasonable to assume, however, that the very small sample size in this study hides the potential extent of the carbon isotope variability of macrophytes at the site.

Pod grass and bogbean are classified as emergent macrophytes (they grow rooted in water-logged substrates, but their leaves are exposed to the atmosphere), while pondweed grows entirely submerged. Submerged macrophytes become more enriched in ¹³C as the dissolved CO₂ pool (the dominant carbon source) becomes increasingly limited⁵⁴.

The Beaver Pond site pondweed δ¹³C is relatively low (− 26.5‰) for a submerged macrophyte. This indicates that it grew in an aquatic environment with adequate dissolved CO₂. This is in keeping with the interpretation that the Beaver Pond was a fen (near neutral pH, cool water temperature) during the Pliocene. A low δ¹³C may also indicate high influx of terrestrial organic biomass or mosses (with low δ¹³C) into the water that subsequently remineralized and contributed to the dissolved inorganic carbon pool.

Environmental conditions strongly influence aquatic plant δ¹⁵N. Beaver Pond macrophyte δ¹⁵N (range =  + 0.2 to + 2.7‰) indicate interspecific access and use of a variety of different sources of bioavailable N within the water column and substrate. The most likely N sources are microbial-fixed atmospheric N₂ (which ranges from –2 to + 2‰), the products of nitrification/denitrification processes (¹⁵N-enriched NH₄⁺ or NO_x), and remineralized ¹⁵N-enriched organic material (either terrestrial or aquatic)^71,72,73.

Larch

Larch (the extinct species Larix groenlandii) is the most common vascular plant species in this macrofossil assemblage.

There is an offset of ~ 2‰ between the δ¹³C of (i) larch shoots/buds (which bear the needles) and cone bracts, and (ii) larch seeds. Larch shoots and cones (δ¹³C range = ‒25.4 to − 25.1‰; mean = − 25.3‰) are more depleted of ¹³C than larch seeds (δ¹³C range = − 23.3 to − 22.7‰, mean = − 23.1‰). This could be indicative of seasonal physiological or environmental conditions experienced by larch trees at the Beaver Pond site. The cones and shoots of extant larch trees begin growing in the early spring and have lower δ¹³C, whereas their seeds (higher δ¹³C) do not develop and mature until mid to late summer⁷⁴.

A number of physiological and environmental conditions could be responsible for this offset between needle/-bearing structures and seeds. Atmospheric vapor pressure deficit (aridity) induces stomatal closure in vascular plants⁷⁵. This restricts not only the rate of water leaving the needle/tree, but also that of atmospheric CO₂ entering it. Stomatal closure reduces CO₂ entry and results in less discrimination against ¹³CO₂. High levels of solar irradiance in the summer increase the rate of CO₂ assimilation. Plants growing at very high latitudes experience 24-h of daylight during the summer. This creates a greater demand for CO₂ to maintain photosynthesis and less discrimination against ¹³CO₂. Both aridity and increased light levels could contribute to why Beaver Pond larch tissues grown late in the summer are more ¹³C-enriched than those grown in the early spring/the previous fall.

Alternatively, trees can use water and carbon (in the form of sugars) stored during the previous year to promote new growth during the early spring when leaves are absent and light levels are low. Tissues that develop early in the growing season (i.e. needle-bearing buds and shoots) can therefore reflect the δ¹³C of photosynthetic conditions from the previous growing season^76,77. In addition, differences in the macromolecular (lipid, protein, sugar) composition of larch buds/needles versus seeds could account for their offset in δ¹³C (i.e. lipids are typically more ¹³C-depleted than proteins).

Larch δ¹⁵N (mean =  + 2.7‰) indicate that these conifer trees had access to N sources other than “light” fixed atmospheric N₂. Given the proximity of wetlands, the root systems of larch trees may have had access to ¹⁵N-enriched dissolved nitrates in the surrounding water-logged soils. Increasing foliar N concentration due to atmospheric N deposition also drives up plant δ¹⁵N^78,79.

Aridity may also have influenced terrestrial plants growing at the Beaver Pond site. Higher rainfall is inversely correlated with δ¹⁵N, where rainier ecosystems tend to produce more ¹⁵N-depleted plants⁸⁰.

Similar to δ¹³C, there is an offset in δ¹⁵N (and N wt%) between larch needle-bearing structures (mean =  + 3.8‰; 0.9%) and larch seeds (mean =  + 2.1‰; 0.3%). This could indicate differences in the macromolecular composition of these different tissue types (where high N content typically indicates higher tissue protein content).

Comparison of Dipoides within Castoridae

The composition of Dipoides sp. diet differs from that of other members of Castoridae that lived in North America during the late Cenozoic. Pliocene High Arctic Dipoides sp. (n = 5), modern subarctic Castor canadensis (n = 4) (Table 1), and late Pleistocene Castoroides ohioensis (n = 11) (Table 1) δ¹³C_col and δ¹⁵N_col are compared in Fig. 3. A correction for the Suess effect was first necessary render the δ¹³C of all three genera comparable. The carbon isotope composition of atmospheric CO₂ has changed over time with global climatic conditions. More recently, anthropogenic burning of fossil fuels that has rapidly released CO₂ enriched in ¹²C into the atmosphere^46,81. Hence, a correction is needed when comparing δ¹³C of organic samples from different time periods to account for this isotopic variation in the primary carbon source of photosynthetic organisms at the base of the food web.

Suess effect corrections of + 2.02‰ and − 0.1‰ were applied to the δ¹³C_col of modern C. canadensis (collected in 2013 and 2014) and Castoroides (late Pleistocene in age), respectively. These corrections were based on the average δ¹³C of atmospheric CO₂ (δ¹³C_CO2) calculated from Pliocene dual-benthic and planktonic foraminifera proxy records, spanning from ~ 4.1 to 3.8 Ma (average δ¹³C_CO2 = − 6.55‰)⁷⁶. These foraminifera proxy records are approximately contemporary with the Beaver Pond site. Average δ¹³C_CO2 for 2014 (− 8.57‰) was compiled from the Scripps CO₂ monitoring program. Average δ¹³C_CO2 for the late Pleistocene (− 6.45‰) was compiled using ice core data from Schmitt et al.⁸².

Plants growing during these three different time periods (Pliocene, late Pleistocene, and modern/2014) would reflect the δ¹³C of contemporary atmospheric CO₂. Therefore, changes in δ¹³C_CO2 help explain differences in δ¹³C between Dipoides sp. and modern C. canadensis. Additional factors, however, are important in explaining the wide range of δ¹³C and large enrichment in ¹³C measured for Castoroides.

Dipoides sp. diet composition differs from that of Castoroides (the Pleistocene giant beaver) (Figs. 3 and 7). Castoroides’ high δ¹³C_col and δ¹⁵N_col (mean δ¹³C_col = − 17.6‰ and mean δ¹⁵N_col =  + 5.8‰) indicate a diet composed predominantly of aquatic (particularly submerged) macrophytes and minimal woody plant material (Table 1)¹³.

In comparison with Castoroides, both Dipoides sp. and C. canadensis have a relatively small range of δ¹³C_col and δ¹⁵N_col (Table 1) (Fig. 3). Dipoides sp. mean δ¹³C_col and δ¹⁵N_col are higher than those of modern C. canadensis (Fig. 3). This is attributable to either variation in diet between the two species, or changes in global C and N baselines over geologic time.

Previous mixing model studies predict that extant C. canadensis diet is composed of approximately equal proportions of woody terrestrial plants and aquatic macrophytes¹³. However, this can vary by latitude and season. For example, extant C. canadensis in the Canadian subarctic vary their winter diet significantly depending on habitat⁸³. It is worth noting that extant C. canadensis does not occur north of 70° latitude and High Arctic Dipoides sp. living at 78° latitude may have employed different dietary strategies.

Dipoides sp. may have relied more heavily than C. canadensis on underwater stores of tree branches to survive the long, dark polar winter. Tree bark is more ¹³C-enriched than leafy vegetation⁵⁷ and increased consumption could account for the higher δ¹³C_col seen in Dipoides sp. Variation in the quantity and type of macrophytes consumed by each beaver species could also account for this difference (i.e. emergent and floating macrophytes are, on average more ¹⁵N-enriched than submerged macrophytes).

Changes in the isotopic composition of the C and N baseline between the Pliocene and the present could also account for the isotopic offset between beaver species. Further investigation of possible changes in the δ¹⁵N baseline of flora in terrestrial high latitude environments during the Pliocene would be a valuable avenue of future research.

Dipoides sp. behaviour and evolutionary implications

Evidence from the Beaver Pond site has implications for our understanding of Dipoides sp. ecology. These data also contribute to our understanding of the evolution of behavioural transitions within Castoridae. In particular, how Castor’s distinctive complex of behavioural traits (tree harvesting, underwater food caching, and construction behaviour) may have evolved. A new hypothesis of behavioural evolution in castorids based on evidence from the fossil record (i.e. fossil burrows, cut wood, and stable isotope measurements) and skeletal-dental morphology is mapped onto a simplified phylogenetic tree in Fig. 8^{42,84,85,86,87}.

Castoridae is a group of herbivorous rodents comprising roughly two dozen genera. Most fossil castorids fall within two major groups: a clade of fossorial specialists (Palaeocastorinae) and a semiaquatic clade^{42,84,86,88,89,90}. The latter includes Castor and Dipoides. Members of the fossorial clade (~ 7 genera) possess striking specializations such large digging claws, extremely reduced tails, and broad, procumbent incisors for digging. In some cases, specimens have been found within fossil burrows (i.e. Palaeocastor, or “The devil’s corkscrew” burrows discovered in the plains of North America⁸⁸). The semiaquatic group comprises two subfamilies, Castorinae (~ 6 genera, including Steneofiber and the extant Castor), and Castoroidinae (~ 7 genera, including Dipoides and the giant beaver, Castoroides). The oldest definitive Castorinae in the fossil record is Steneofiber eseri from the early Miocene (France, MN2, ~ 23 Ma). S. eseri shows evidence of living in family groups and swimming specializations⁹¹. This, in combination with aDNA evidence¹², suggests Castorinae and Castoroidinae are derived from a semiaquatic ancestor in the early Miocene.

Digging behaviour was not just characteristic of the fossorial group and appears within the semiaquatic clade as well. Castor, though not morphologically highly specialized for the task, digs bank burrows and creates extensive canal systems⁹². In addition, the extinct semiaquatic beaver Steneofiber eseri was found within a burrow⁹¹. Considering the phylogenetic distribution of burrowing behaviour within the Castorid tree (Fig. 8), it is likely that the common ancestor of the fossorial and semiaquatic clades also burrowed. Thus, the appearance of burrowing behaviour within Castor and Steneofiber are seen as a retention of a primitive trait⁹.

If burrowing behaviour in semiaquatic castorids is the primitive condition, it is likely Dipoides burrowed as well, as seen in other semiaquatic rodents today such as Castor, but also Crossomys (earless water rat), Myocastor (nutria), and Ondatra (muskrat)⁹². It is also possible that Dipoides constructed lodges. Extant Castor and Ondatra are known to construct burrows and lodges, depending on the characteristics of the habitat. Bank burrows are associated with stream environments, whereas lodges are better suited to calmer waters⁹². Unlike Castor, extant Ondatra construct their push-up lodges using cattails and other fibrous vegetation rather than wood. The abundance of cut wood at the Beaver Pond site¹¹ suggests that Dipoides sp. had the option to incorporate wood into their nesting structures, and possibly built lodges.

Given the occurrence of woodcutting and woody plant consumption within both subfamilies of semiaquatic castorids (represented by Castor and Dipoides in Fig. 8), it seems likely these behaviours appeared in the common ancestor of the semiaquatic group. Woody plant consumption may have preadapted castorids to exploit colder environments that arose during and after the late Miocene. Castor canadensis does not hibernate, but builds and sink rafts of branches and foliage to use as a source of fresh food during the winter months^1,93. Dipoides sp. may have also engaged in this behaviour and used underwater caches of branches as a primary food source to survive the consecutive months of darkness during the high latitude winter when plants become dormant. The use of woody plants in this way may have been key to allowing beavers to disperse between North American and Eurasia, which required crossing the Bering Isthmus⁹⁴, a high latitude landmass. Curiously, given that a diet rich in woody plants appears to be the primitive condition of semiaquatic castorids, the absence of woody plant consumption seen in the Pleistocene giant beaver Castoroides¹³ must be interpreted here as an evolutionary loss and potentially a leading factor in their extinction (Fig. 8).

Among living mammals, Castor’s dam construction is a unique and highly derived behaviour – an evolutionary puzzle, associated with a set of innate behavioural specializations⁹⁵. For example, dam construction is well known to be triggered by the sound of running water alone⁹⁵. The presence of such “hard-wired” behaviours may be associated with the ancient origins of this behaviour. Molecular and fossil occurrence records indicate that the split between Eurasian and North American Castor arose around 7.5 Ma ago^96,97, implying that dam building behaviour itself is at least as old.

Definitive fossil evidence for dam building by an extinct beaver is currently lacking. Consequently, dam building behaviour is shown as possibly arising only on the lineage leading to Castor. Hypothetically, dam building may have arisen from beavers collecting branches near their burrow/lodge for feeding purposes and the accumulations of sticks could have dammed streams by happenstance. The effects may have been multifold. A deeper pond is an effective defense mechanism and provides a safe refuge from predators. Raised water levels also create more favourable conditions for underwater food caching of branches in sub-freezing winter conditions because the deeper water would prevent an underwater food cache from being locked in ice. As such, natural selection would have favoured animals that maintained the dam, presumably as an extension of their pre-existing nesting behaviour such as lodge building. In this scenario, the climate cooling that started around 15 Ma ago and continued into the Pleistocene would have provided an interval where behaviours promoting over-wintering survival, such as underwater food caching branches and dam building, would have been increasingly reinforced by natural selection.

It seems unlikely that the common ancestor of all semiaquatic beavers was a dam-builder. Extant Castor is a large powerful rodent weighing 12–25 kg, with some individuals as large as 40 kg⁹². Its body size is one factor that allows the animal to harvest branches and whole trees to build lodges and maintain dams over multiple years. The Beaver Pond site Dipoides sp. was also a large rodent and was roughly two-thirds the size of an average extant Castor. In contrast, the less-derived semiaquatic beavers, such as the Miocene Eucastor tortus (Castoroidinae) and Steneofiber eseri (Castorinae) were small (~ 1 kg, or less), suggesting that the common ancestor of the semiaquatic lineage was also small bodied. Although the common ancestor of the semiaquatic beaver lineage is inferred to have consumed woody plants (this study), and may have used branches in creating food piles and wood for lodge construction, it would have been too small to have had the capacity to build and maintain dams. As such, if Dipoides sp. did exhibit dam building behaviour, it would be the result of parallel evolution within the Castoroidinae and Castorinae lineages.

Source: Ecology - nature.com