One of the founder animals of the beaver population at the river Hase.

Emsland Beavers

Zur deutschen Version Beaver biology

Systematics and evolution of the beaver


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Table of contents


Systematics of the beaver [Back to table of contents]

Within the beaver family (Castoridae) only two species, the Eurasian beaver (Castor fiber L. 1758) and the North-American beaver (Castor canadensis Kuhl 1820), are separated recently (Grzimek & Piechocki 1993).

Beaver systematics after Macdonald 2003
Beaver systematics after Macdonald 2003.
The taxonomical classification of the family Castoridae within the order Rodentia is discussed controversially. Because of morphological and anatomical discoveries, beavers are classified as members of the suborder of squirrels (Sciurognathi) in general (Macdonald 2003). Based on the same data, other authors classify the Castoridae as a separate suborder Castorimorpha (Thenius 1980).
However, by taking into account recent molecular biological data - namely different gene sequences of the core DNA - it appears that beavers have to be assigned to the New World heteromyids (family Heteromyidae) and pocket gophers (family Geomyidae) (Adkins et al. 2001; Huchon et al. 2002; Montgelard et al. 2002). The Heteromyidae and Geomyidae themselves seems to be closely related to the Myodonta (family Muridae - murids and family Dipodidae - jerboas and jumping mice) as well as the Anomaluromorpha (family Anomaluridae - scaly-tailed squirrels and family Pedetidae - springhares) (Huchon et al. 2002).

Beaver systematics after Huchon et al.  2002
Beaver systematics after Huchon et al. 2002.
However, even the molecular biological data are somewhat contradictory. For instance, when analysing the mitochondrial 12S rRNA the genus Castor has to be grouped clearly outside of all the other Rodentia (Adkins et al. 2001; Montgelard et al. 2002), whereas the examination of the von-Willebrand-factor (vWF) shows a closer relationship of the genus Castor towards the Sciuroidea (family Sciuridae and family Aplodontidae - mountain beavers) (Huchon et al. 2002), which corresponds to the genealogical tree derived by analysing the morphological-anatomical data.

Although the two extant species, the North-American and the Eurasian beaver, can not be distinguished by exterior features (Freye 1978), these two are distinct species because of karyotypical differences (C. canadensis: 2n = 40; C. fiber: 2n = 48) a hybridisation is not possible (Freye 1978). Besides, a RFLP-analysis of the cytochrome b-gene shows a clear separation of the two species (Kühn et al. 2000). Apart from that, there are obvious differences regarding the behaviour, as well as the physiology and the reproduction of both species (Djoshkin & Safonow 1972; Freye 1978): However, it is assumed that the North American Beaver has developed from the Eurasian Beaver. Both species do have a different chromosome set (see above) but the number of free chromosome arms is the same in both species (N.F. = 80), which suggests, that C. canadensis developed out of C. fiber after a Robertsonian chromosome fusion (fusion of two acrocentric chromosomes) of 8 chromosome pairs. Besides the chromosomal differences, there are further significant differences regarding the anatomy, morphology, population biology, parasitology, and behaviour, so that the division into two separate species seems to be justified (Heidecke 1986). Because of theses differences, discoveries made by observing C. canadensis can therefore not be generalised to C. fiber.

Evolution of the family of beavers (Castoridae) [Back to the top]

Scull of C. fiber as viewed from below.
Scull of C. fiber as viewed from below.
Although, due to their body size, the fossil record of beavers is comparably extensive (Korth 2001), the evolution of this family is still controversially discussed. This is partly because manly tooth have been found of this species, whereas the discoveries of bones (sculls or parts of the skeleton), which might be used for identification purpose, are relatively rare. For the division of fossil records into genera or species, specific features of the premolars and molars like the enamel pattern of the masticatory surface, the existence and the quality of plications at the buccal or lingual edge of the tooth crown or the quality of the root of the tooth.
Enamel pattern of the upper molars of C. fiber
Enamel pattern of the upper
molars of C. fiber
Apart from that, also features of the incisors have a diagnostic value, for instance the formation of grooves at the forefront of the incisors or the surface shape (arched or flattened) of the forefront of the incisors (Korth 2001, Recovets et al. 2009). However, the analysis of these features requires a high degree of experience because closely related species often look very similar. For example, the various species of Palaeocastor, an early beaver genus from the late oligocene to early miocene (approx. between 26.3 to 20.6 million years before present, see below), can hardly be distinguished by features of their teeth (Stefen 2010). Apart from that, even only distantly related representatives from different genera or subfamilies may have developed very similar characteristics in reaction to comparable environmental conditions (for example the type of diet; this is called convergent development; Recovets et. al. 2009).

Number of genera per era during the evolution of beavers (family Castoridae).
Number of genera per era during the evolution of beavers (family
Castoridae).
Currently the family of beavers (Castoridae) is only represented by a single genus (the genus Castor) with two species (Castor fiber in Europe and Castor canadensis in North America). However, at least temporarily this rodent group contains a significantly higher number of genera and species during their development (Korth 2001; Rybczynski et al. 2010).

Regarding the period of time since the first appearance of beavers in the late eocene, an increase in the number of genera and species can be observed from the eocene through the oligocene up to the miocene (from 23.03 to 5.333 million years before today), whereas subsequently the number of genera and species decreased again from the pliocene (from 5.333 to 2.588 million years before today) through the pleistocene (from 2.588 to 0.0117 million years before today) up to the holocene (from 0.0177 million years up to today). However, it should be noted that the various epochs or stages comprise a different time period (time period: eocene 22.1 million years, oligocene 10.9 million years, miocene 17.7 million years, pliocene 2.8 million years, pleistocene 2.6 million years, holocene 11700 years). So, among other reasons, during the miocene the most genera and species had been emerged, because it covers a much longer period of time (for the temporal classification of the individual epochs see the website of the "International Commission on Stratigraphy" at www.stratigraphy.org for example).

In North-America beavers appear in the fossil record during the late eocene (from 38.0 to 33.9 million years before today) for the first time, whereas the oldest fossil findings in Eurasia are dated to the beginning of the following oligocene (García-Alix et al. 2007; Korth 2001; Kurtén 2009; Rybczynski 2007; Rybczynski et al. 2010; Thenius 1980). So, the early development of beavers took place in North-America and from here Eurasia was colonised (probably via Beringia, the land bridge between North-America and Asia).

Subfamily Agnotocastorinae Korth & Emry, 1997 [Back to the top]

The oldest genera of the beaver family is probably Agnotocastor Stirton, 1935, whose representatives appeared during the late eocene or early oligocene (approx. 33,9 million years ago) in North-America (Korth 2001; Thenius 1980) and had become extinct again by the beginning of the miocene (approx. 23.03 million years ago; see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]). Closely related to Agnotocastor were the two genera Neatocastor Korth, 1996 and Hystricops Leidy, 1856, which were combined by Korth (2001) to the tribe Agnotocastorini Korth & Emry, 1997 (in contrast after Rybczynski [2007] Hystricops has to be assigned into the subfamily Castorinae). The tribe Agnotocastorini together with the tribe Anchitheriomyini Korth, 2001, to which the tree genera Anchitheriomys Roger, 1898, Propalaeocastor Borisoglebskaya, 1967, and Oligotheriomys Korth, 1998 are counted, are assigned to the subfamily Agnotocastorinae (Korth 2001). In 2004 Korth supplemented this tribe by the genus Miotheriomys Korth, 2004 (see Korth & Bailey 2006 as well). It should be noted, that in the older literature Anchitheriomys was attributed to the closely related family Eutypomyidae, which became extinct in the late oligocene (McKenna & Bell 1997). At least one species out of this genus growth as large or even larger than Trogontherium cuvieri (see below; Fostowicz-Frelik 2008).

All of these representatives - or at least Anchitheriomys (Rybczynski et al. 2010) - had probably adapted to an amphibian way of life (see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]), although, judged by different tooth and scull features, they still show some primitive characteristics (for instance, all members had two premolars in each tooth row of the lower jaw and additionally, members of the tribe Agnotocastorinae had also in each tooth row of the upper jaw a second premolar). The genera were mainly distributed in North-America, but single species also reached Europe and Asia. Most likely all modern beavers developed from the Agnotocastorinie and not from the Anchitheriomyini, although the latter showed more modern features than the Agnotocastorini (Korth 2001).

Subfamily Palaeocastorinae Martin, 1987 [Back to the top]

Fossile burrow of Palaeocastor fossor (after Peterson 1905).
Fossile burrow of
Palaeocastor fossor
(after Peterson
1905).
Subsumed under the rather primitive subfamily of beavers, the Palaeocastorinae, are members of Palaeocastor Leidy, 1869, Capacicala Macdonald, 1963, Pseudopalaeocastor Martin, 1987, Euhapsis Douglas, 1905, as well as Fossorcastor Martin, 1987 which were found from the late oligocene to the early miocene (approx. from 33.3 to 15.97 million years before today) in North-America only (Korth 2001). All representatives of this family showed distinct adaptations towards a mainly fossorial way of life: the strongly developed bones of the forelimb were shortened and broadened and show strong nails, the cervical vertebra were shortened and also the tail was quite short. Moreover, in some members the incisors pointed forwards out of the oral cavity, because they were used partly for digging (tooth digger). In comparison to modern beavers, all species of this family were relatively small reaching a body mass of less than 1.5 kg and a head-body length of not more than approximately 30 cm (Korth 2001; Rybczynski 2007). At least one member of this subfamily, the species Palaeocastor fossor Stirton, 1935 (nowadays termed Fossorcastor fossor Peterson, 1905), is known to build burrows, which showed an entrance tube that was spirally coiled and pointing up to 2.5 m vertically into the ground (some of theses burrows become fossilised and were termed Daemonelix). The nest chamber of the burrow, which pointed slightly upwards, was situated at the bottom of the coiled tube. Because usually several of theses burrows were found in close proximity, it appears that this animals were living in colonies similar to the North-American prairie dogs (Cynomys sp.; Peterson 1905).

Subfamily Castoroidinae Allen, 1877 [Back to the top]

Within the third subfamily described by Korth (2001), the Castoroidinae, subsumed in three groups (tribe Castoroidini Allen, 1877, tribe Nothodipoidini Korth, 2007, and tribe Trogontheriini Lyschev, 1973) are such genera, in which some members grow very large in comparison to other beaver species. Representatives of this subfamily first appeared at the beginning of the miocene in North-America (Korth 2001, 2008). Because of the body size of some species the members of this subfamily are sometimes termed "giant beavers", however, the body size is not the only criteria for the assignment to this subfamily (there are several species within the Castoroidinae, which remained relatively small).

After Korth (2001, 2008), within the tribe Castoroidini, whose representatives lived for a relatively long period from the beginning of the Miocene (about 23.03 million years before today) until about the end of the Pleistocene (about 0.012 million years before today), the genera Monosaulax Stirton, 1935, Prodipoides Korth, 2007, Dipoides Schlosser, 1902, Castoroides Foster, 1838 as well as Procastoroides Barbour & Schultz, 1937 are subsumed (according to some authors, Procastoroides is to be counted to the subfamily Castoroidinae but not to the tribe Castoroidini, see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]). Species of the genus Monosaulax appear to be the most primitive representatives of this group, whereas species of the genera Dipoides and Castoroides are classified as the most advanced representatives of this tribe. Mainly those species of the genus Castoroides living in North-America grew very large and reached a head-body length of up to 2.5 m and a body mass of 150 kg and above (Parmalee & Graham 2002), which matched the size of a black bear (Ursus americanus; Harrington 2007). It is assumed that representatives of Castoroides (at least two different species are distinguished: Castoroides ohioensis Foster, 1838 mainly distributed south of the Great Lakes and Castoroides leiseyorum Morgan & White, 1995 mainly distributed in the present-day Florida) were living near or in the water similar to the present-day beavers. However, because of the rounded endings of the lower incisors (within Castor these are flat) it is assumed, that the animals were not very effective in cutting wood like the modern beavers, but feed mainly on aquatic plants and non-woody vegetation growing near the bank, similar to the feeding habits of present-day muskrats (Parmalee & Graham 2002). Interestingly, the extant beaver representative in North-America (Castor canadensis) appears at the same time as these giant forms.

Subsumed as Nothodipoidini, the second tribe within the Castoroidinae, are the species Temperocastor Korth, 2008, Eucastor Allen, 1877, Nothodipoides Korth, 2002 as well as Microdipoides Korth & Stout, 2002, which were living in the time period from the middle miocene until the end of the pliocene (between 15.97 to 2.588 million years before present; Korth 2001, 2008). The genus Temperocastor represents the oldest member of this tribe and is followed by Eucastor. Whereas Eucastor is older than Nothodipoides (Korth 2008). Details on the age of the fossils of Microdipoides are not known. Representatives of the Nothodipoidini resembled those of members of the subfamily Palaeocastorinae in being of small size and adapted to a fossorial way of life. They also used their incisors for digging, which is expressed morphologically by the position of the incisors (the incisors are more or less inclined horizontally) and the stronger curvature of the palate in the region of the diastema (the gap between the incisors and the molars; Korth 2008).

After Korth (2001), the genera Trogontherium Fischer de Waldheim, 1809, Boreofiber Radulesco & Samson, 1972, Euroxenomys Samson & Radulesco, 1973, Youngofiber Cow & Li, 1978, and Asiacastor Lyschev, 1971 are subsumed to the third tribe Trogontheriini, which are known from early miocene until the end of the pliocene (from 20.444 to 2.588 million years) mainly of Eurasia (only the genus Euroxenomys was also found in North-America; Korth & Bailey 2006; McKenna & Bell 1997). Regarding their body size, the Eurasian members of the genera Trogontherium and Youngofiber (the latter only in Asia) are frequently treated as the Eurasian counterparts of the North-American genus Castoroides. However, these species did not grow quite as large as Castoroides ohioensis, for example. The species Trogontherium cuvieri Fischer, 1829, generally known as "Eurasian giant beaver", was probably only slightly larger than todays beavers and even smaller than the representatives of Youngofiber (Forstowicz-Frelik 2008; Kurtèn 2009). Regarding the scull size (scull length of Castor fiber: 160 - 170 mm; of Trogontherium cuvieri: approx. 210 mm; of Castoroides: 242 - 397 mm), Trogontherium cuvieri approximately reached a head-body length of 1.3 to 1.5 m, whereas the Eurasian beaver can reach a head-body length between 1.0 to 1.2 m and Castoroides ohioensis got near to 2.5 m (Fostowicz-Frelik 2008). Members of Youngofiber were approximately 5 - 10 % larger than Trogontherium cuvieri (Fostowicz-Frelik 2008), which amounts to a head-body length round about 1.4 to 1.7 m.
Again, in this tribe there are different opinions regarding the assignment of the individual genera. For instance, the status of Euroxenomys as a independant genus is not quite clear because the members of this genus show close similarity to Trogontherium. Therefore, some authors regard Euroxenomys as a subgenus of Trogontherium (Daxner-Höck & Bernor 2009; Giersch et al. 2010). Partly, Euroxenomys is not assigend to the Castoroidinae at all but simply subsumed under the family Castoridae (see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]).

Furthermore, even the delimitation of the tribe Trogontheriini as such and the assignment of these representatives to the subfamily Castoroidinae after Korth (2001) is discussed controversially (see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]). In the opinion of some authors a tribe Trogontheriini does not exist at all. Later on, Korth & Bailey (2006) also subsumed the newly described genus Priusaulax Korth & Bailey, 2006 under the subfamily Castoroidinae but without assigning this genus to one of the three tribes named above. Regarding scull and tooth features, Priusaulax mediates between the relatively primitive Agnotocastorinae and the more modern Castoroidinae, because there are similarities to Neatocastor and Monosaulax (the most primitive member of Castoroidinae) respectively (Korth & Bailey 2006).

It is assumed, that several Castoroidinae representatives were living semi-aquatic - similar to the modern beavers - namely Monosaulax, Prodipoides, and Dipoides from the tribe Castoroidini, Temperocastor, Eucastor, and Nothodipoides from the tribe Nothodipoidini, Euroxenomys from the tribe Trogontheriini (see above), as well as Priusaulax (Rybczynski et al. 2010). Apart from that, at least Trogontherium cuvieri is thought to lead a semi-aquatic way of life (Kurtén 2009), whereas this species inhabited mainly swampy areas and fed on herbaceous vegetation similar to today's muskrats or coypus. That the animals should have grazed plants floating on the water surface with their fleshy lips, was already classified by Mayhew in 1978 as a misinterpretation. Both, Dipoides and Trogontherium as well as Castoroides are assumed to have fed on the bark of woody plants just like modern beavers. However, the processing of wood by these species is thought to be less effective than in modern beavers, because of the convex edges of their lower incisors of this species, in contrast to the lower incisors with straight edges as in modern beavers, which makes the working on wood more economic (Fostowicz-Frelik 2008).

Subfamily Castorinae Hemprich, 1820 [Back to the top]

Within the fourth subfamily Castorinae the seven genera Steneofiber Geoffroy Saint-Hilaire, 1833, Chalicomys Kaup, 1832 (North-American authors often use the name Palaeomys Kaup, 1832 for this genus), Sinocastor Young, 1934, Schreuderia Aldana Carrasco, 1992, Zamolxifiber Radulesco & Samson, 1967, Romanofiber (partly termed Romanocastor) Radulesco & Samson, 1967 as well as Castor Linnaeus, 1758 after Korth (2001) und Rybczynski et al. (2010). The representatives of this subfamily had a relatively long evolutionary history because the oldest genus Steneofiber is already known from the early oligocene (33.9 million years before today) and the genus Castor has survived until today (McKenna & Bell 1997). The Steneofiber species from the early oligocene were probably the first representatives of the Castoridae in Europe (Wu et al. 2004). Possibly the names Zamolxifiber and Romanofiber are synonyms because the members of these two genera show many similarities regarding the enamel pattern (Korth 2001). Although both of these genera only emerged by the end of the pliocene (and disappeared again shortly after), they show close resemblance to the genus Stenofiber, which members already appeared in the early oligocene and became extinct - according to present knowledge - in the middle miocene (so a long time before Zamolxifiber and Romanofiber appeared; Korth 2001; McKenna & Bell 1997).

Some authors also group the genus Hystricops (which is seen as a member of the tribe Agnotocastorini of the subfamily Agnotocastorinae by Korth [2001], see above) under the Castorinae (see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]). The genus Sinocastor was declares as a synonym of the genus Castor by Xu (1994). Later, Rybczynski et al. (2010) came to the conclusion, that the status of Sinocastor as a separate genus should be retained, because Sinocastor can be clearly separated from Castor by certain scull characteristics.

Also within the Castorinae, several genera seemed to be adapted to a life within and near the water for instance the genera Steneofiber, Chalicomys, and Castor of course (Rybczynski et al. 2010).

Genus Castor Linnaeus, 1758
Because of the fossil findings and the similarity to the Eurasian Steneofiber, the representatives of the genus Castor Linnaeus, 1758 - including the extant species C. fiber and C. canadensis - are thought to be of European origin some 9.0 to 4.9 million years ago and have subsequently spread to North-America from here (Xu 1994; Rybczynski et al. 2010; Horn et al. 2011). In East Asia the first members of Castor only appeared relatively late during the middle pleistocene about 0.781 million yeas ago, which was probably due to a competition between Castor and the representatives of the genus Sinocastor, which occurred until the middle pleistocene in China (Rybczynski et al. 2010). However, there is some doubt about the European origin of the genus Castor, because the first fossils of North-American Castor representatives can be dated back to the same period of time (after genetic analyses between 6.6 to 4.9 million years before present) at which the first fossils in Europe can be dated (see above). Therefore, it is possible as well, that this genus had developed in North-America and subsequently spread to Europe (Horn et al. 2011). After the opinion of still other authors the oldest Castor species from Europe (C. neglectus Schlosser, 1902) from the middle to late miocene with an age of 10 to 12 million years is significantly older than the oldest North-American fossil discoveries from the late miocene about 7 million years ago (Rybczynski et al. 2010), which in turn would speak for a European origin of the genus Castor. As shown in the chapter Sytematics of the beaver, the chromosome structure implies, that the North-American beaver (C. canadensis) has developed from the Eurasian beaver (C. fiber) and not the other way round, which suggests a European or Eurasian origin of the genus Castor as well.

The partial contradictions regarding the time and place of origin of the genus Castor and others are mainly due to the weaknesses of today's methods of age verification of fossils, which might not be solved in the near future.

Besides the already mentioned C. neglectus, from the genus Castor are known several other species, that, however, became extinct in the past and only the two species (C. fiber and C. canadensis) survived until today. The view that C. neglectus is the oldest member of the genus Castor (Rybczynski et al. 2010) is not entirely uncontroversial. Recovets et al. (2009) for instance, are of the opinion that the species name C. neglectus is only an invalid synonym of Trogontherium (Euroxenomys) minutum (Trogontherium minutum was a growth-restricted relative of the Eurasian "giant beaver" Trogontherium cuvieri, see above). Somewhat younger than C. neglectus is the species C. californicus Kellog, 1911 (a invalid Synonyme is C. accessor Hay, 1927; Repenning et al. 1995), which lived from the late miocene to the end of the pliocene (approx. 11.62 to 2.588 million years before present) and was discovered as the first member of the genus Castor in North-America (Rybczynski et al. 2010) Regarding the time of occurrence the sources are somewhat contradictory, dating the first appearance of the genus Castor in Europe as well as North-America much later (see above). At nearly the same period of time than C. californicus the species C. zdanskyi Young, 1927 appears, which was found however, not in North-America but in China (Xu 1994). From the early pliocene (between 5.4 to 4.4 million years), that is much later than C. californicus and C. zdanskyi, the Eurasian species C. praefiber Deperet, 1890 is known (Rekovets et al. 2009). Similar to the somewhat younger European species C. plicidens Major, 1874 from the late pliocene to the middle pleistocene (3.4 to 1.8 million years; Kurtén 2009), it might be possible that both species names are but invalid synonyms of C. fiber (see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]). Even more recent than C. praefiber and C. plicidens is the species C. tamanensis Verestchagin, 1951 (different notation: C. tamanseis; Rybczynski et al. 2010) from the middle pleistocene (1.7 to 1.0 million years before today) of eastern Europe, which might be the predecessor of C. fiber (Rekovets et al. 2009). However, based on fossil findings the species C. fiber seems to occur as early as the middle miocene (approx. since 11.6 million years before today; (see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]) and therefore the species C. tamanensis known since the middle pleistocene could not be the predecessor of C. fiber (unless C. tamanensis appeared much earlier). At about the same time period (middle to late pleistocene; 1.8 to 0.0117 million years), the species C. anderssoni (Schlosser, 1924) is dated, which fossils have been found in China (Xu 1994). Invalid synonyms of C. anderssoni are Chalicomys anderssoni Schlosser, 1924 as well as Sinocastor anderssoni Young, 1934 (see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]).

The separation of the two extant species C. fiber and C. canadensis also took place a long time ago. As the analysis of mitochondrial DNA reveals, both species were already separated about 8.0 to 7.6 million years ago (Horn et al. 2011). However, such dating attempts based on genetic data has some problems as well, because the calibration of a &qout;molecular clock" is not that easy.

After the opinion of some authors, the segregation of the Eurasian beaver into the different subspecies (see the chapter on the Subspecies of the Eurasian beaver) took place about 150 000 to 110 000 years ago (analysis of the mitochondrial cytochrom b gene; Durka et al. 2005), whereas, after the opinion of some other authors, this event already happened some 210 000 years ago (analysis of the entire mitochondrial DNA; Horn et al. 2011). This also reveals that dating of evolutionary events based on molecular data can be just as problematic as the dating of fossils under certain circumstances.

Other members of the Castoridae [Back to the top]

Besides the already mentioned genera, which can be assigned to different tribes and subfamilies, there is at least one additional genus, Migmacastor Korth & Rybczynski 2003, which can not be assigned to one of these higher taxa so far. It is agreed that the members of this genus belong to the Castoridae, but it is not clear, to which of the four subfamilies this genus has to be assigned (Rybczynski 2007; Rybczynski et al. 2010). Fossil findings of this genus are from the time period between the late oligocene up to the early miocene (between 24.80 to 20.43 million years before today; see the "Paleobiology Database" at http://paleodb.org [accessed on 12. March 2013]). Similar to representatives of the subfamily Palaeocastorinae, Migmacastor was adapted to a fossorial lifestyle and the members of this genus used their incisors for digging as well (tooth digger; Rybczynski 2007).

Moreover, the division of the different genera mentioned above is not uncontroversial. For instance, in the database of the website "Paleobiology Database" (see http://paleodb.org) the following genera are not assigned to a certain subfamily: Agnotocastor, Anchitheriomys, Boreofiber, Capatanka, Chalicomys, Euroxenomys, Neatocastor, Oligotheriomys, Palaeomys, Priusaulax (termed "Priusalax" in the database, which might be a case of mistyping), Propalaeocastor, Romanocastor, Trogontherium (the database also mentioned a genus "Trigontherium", which might be a mistyping as well), Youngofiber, as well as Zamolxifiber. Furthermore, the subfamily Agnotocastorinae, as well as the tribes Agnotocastorini, Anchitheriomyini, and Trogontheriini are not mentioned in the database.

Table 1: Division of the family Castoridae in genera, time of occurrence, distribution, and number of species per genus. For some genera, maps on the fossil findings are available (please click on the genus name). (Note: On very small devices the table might not be displayed correctly.)

Sub-
family

Tribe

Genus

time period
[million years]

Distribution

Number of
Species

Agnoto-
castorinae

Agnoto-
castorini

Agnotocastor

37.200 - 26.300

North-America

5

Neatocastor

28.400 - 20.430

North-America

1

Hystricops

20.430 - 5.332

North-America

2

Anchi-
theriomyini

Anchitheriomys
(Syn. Amblycastor)

20.430 - 2.588

Holarctic

3

Propalaeocastor

33.900 - 28.100

Eurasia

2

Oligotheriomys

33.900 - 15.970

North-America

3

Miotheriomys

-

North-America

1

Palaeo-
castorinae

Palaeocastor
(Syn. Capatanka)

33.300 - 23.030

North-America

5 (6)

Capacikala

28.400 - 23.030

North-America

2

Pseudopalaeocastor
(Syn. Nannasfiber)

24.080 - 15.970

North-America

2

Euhapsis

26.300 - 20.430

North-America

4

Fossorcastor

26.300 - 15.970

North-America

3

Castoro-
idinae

Castoro-
idini

Monosaulax

23.030 - 5.332

Holarctic

6

Prodipoides

15.970 - 10.300

North-America

2

Dipoides
(Syn. Paradipoides)

13.600 - 0.300

Holarctic

8

Castoroides

1.800 - 0.012

North-America

2

Procastoroides
(Syn. Eucastoroides)

4.900 - 3.600

North-America

3

Notho-
dipoidini

Temperocastor

15.970 - 13.600

North-America

1

Eucastor
(Syn. Sigmogomphius)

15.970 - 2.588

Holarctic

4

Nothodipoides

15.970 - 10.300

North-America

2

Microdipoides

13.600 - 10.300

North-America

1

Trogon-
theriini

Trogontherium

15.970 - 0.126

Eurasia

3

Boreofiber

5.332 - 3.600

Eurasia

1

Euroxenomys

20.430 - 15.970

Holarctic

3

Youngofiber

20.430 - 15.970

Eurasia

1

Asiacastor

28.100 - 11.620

Eurasia

3 (4)

Priusaulax

23.030 - 20.440

North-America

1

Castorinae

Steneofiber

37.200 - 3.600

Eurasia

4 (7)

Chalicomys
(= Palaeomys)

23.030 - 0.781

Eurasia

2

Sinocastor

15.970 - 2.588

Eurasia

1 (3)

Schreuderia

11.620 - 5.332

Eurasia

?

Zamolxifiber

5.332 - 2.588

Eurasia

1

Romanofiber
(= Romanocastor)

5.332 - 2.588

Eurasia

1

Castor

8.700 - 0.000

Holarctic

4 (6)

Migmacastor

24.800 - 20.430

North-America

1

Subspecies of the Eurasian beaver [Back to the top]

The extant Eurasian beaver Castor fiber is divided into eight subspecies, whereas the division was based mainly on the geographical separation and not on genetic differences (Heidecke 1984; see the map below). The eight subspecies of the Eurasian beaver are:

These eight subspecies can be subsumed into three groups, which are taxonomically more closely related. After Heidecke (1986) the subspecies galliae and albicus form the group of the "western beavers", fiber and osteuropaeus the group of the "Scandinavian and eastern European beavers", and the subspecies belarusicus, pohlei, tuvinicus, and birulai can be grouped as the "Asian beavers". There are pronounced differences between these three groups with regards to the morphology and the type of parasites. For instance, within the eastern European beaver there is a high proportion of animals with a black fur colour, whereas the black fur colour is much rarer in the other subspecies. Despite these differences all subspecies seem to be able to crossbreed without problems (Heidecke 1986).

Remaining populations of the Eurasian beaver Castor fiber at the beginning of the 19th century.
Remaining populations of the Eurasian beaver Castor fiber at the beginning of the 19th century (after Babik et al. 2005;
Durka et al. 2005; Halley & Rosell 2002, 2003; changed).

Also the North-American beaver is divided into several subspecies. Similar to C. fiber, the separation of the different ecotypes of this species is mainly based on geographical criteria and not on genetic differences. Because of the large range of the North-American beaver, which spreads from Alaska to Mexico, this species is divided into 24 subspecies, which can not be addressed in detail at this point (Baker & Hill 2003).

The separation of the Eurasian beaver into geographical subspecies was somewhat "disturbed" in the past few years, because unfortunately some re-introduction programs did not pay attention to which subspecies the released animals belonged. So, for example, during the re-introduction program at the Eifel eastern European beavers were released, although - from a geographical point of view - Rhône or Elbe beavers should have been released at this locality. In Bavaria, the Switzerland, as well as in Russia no care was taken of the origin of the animals during the different re-introduction programs and as a consequence, several subspecies even got mixed. Because of the close taxonomic relationship it can not be ruled out that during an encounter of these subspecies a hybridisation and - in the long term - an extinction of this subspecies will take place. This contradicts the idea of the conservation of a certain species, which should also include the protection of locally adapted populations or ecotypes as a basic principle.
Moreover, in some regions (e.g. Finland, Poland, the former Soviet Union, as well as France) even the North-American beaver Castor canadensis has been released. Also this has to regarded as negative, because canadensis can replace the Eurasian beaver due to differences in social behaviour and the higher reproduction rate in C. canadensis. However, a hybridisation of fiber and canadensis is not possible, because this two species can not produce fertile cross-breeds due to differences in their karyotype (see above).

Despite this somewhat unfortunate development, the conservation efforts of the last decades have saved most subspecies of the Eurasian beaver from the thread of an immediate extinction. One exception are the subspecies pohlei, tuvinicus and birulai, which only have a limited range and which respective populations only have a limited number of animals.

Similar Species [Back to the top]

Coypu - notice the bright nasal area (Photo: A. Linow)
Coypu - notice the bright
nasal area (Photo: A. Linow).
Regarding the anatomy, beavers resemble the South-American coypu Myocastor coypus, as well as the North-American muskrat Ondatra zibethicus, which can both be found in many European waters nowadays. However, both these semi-aquatic rodents did not reach the size of a fully grown beaver.

The coypu, belonging to the family of nutrias and coypus (Myocastoridae), can reach a head-body length up to 65 cm and a body mass up to 9 kg, whereas the muskrat, belonging to the family of voles (Arvicolidae), reaches a head-body length up to 40 cm and a body mass of up to 2.4 kg. Although theses species are quite different in body size, they can Swimming coypu - notice the large part of the back above the water line.
Swimming coypu - notice
the large part of the back
above the water line.
easily be confused especially when swimming in the water.

In addition to the body size, the most obvious distinguishing feature between the three types is the "trowel", the dorsoventrally flattened, scaly tail of the beaver. In comparison, the tail of the coypu is sparsely haired and round in diameter, Swimming muskrat (Photo: M. Homuth)
Swimming muskrat (Photo:
M. Homuth).
whereas the tail of the muskrat is laterally flattened and almost naked. Moreover, the coypu has long and noticeably white vibrissae, which are lacking in the beaver as well as the muskrat.

Another distinctive feature between the three species is the posture of the animals when swimming. Regarding the beaver, during active swimming only the head and sometimes a small part of the pelvis is kept above the water line, whereas in swimming coypu and especially muskrat larger part of the back are always kept above the waterline. In the beaver, larger parts of the back are only seen above the waterline when the animals lay motionless in the water not generating any propulsion.

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