Journal of Chemical Ecology, Vol. 8, No. 5, 1982
CASTOR SACS AND ANAL GLANDS OF THE NORTH AMERICAN BEAVER (Castor canadensis): Their Histology, Development, and Relationship to Scent Communication
JON M. WALRO
and GERALD
E. S V E N D S E N
Department of Zoology and Microbiology Ohio University Athens, Ohio 45701 (Received July 14, 1981; revised August 27, 1981)
Abstract--Both sexes of beavers possess a pair of castor sacs and a pair of anal glands located in paired subcutaneous cavities between the pelvis and the base of the tail. The castor sacs are not glandular in the histological sense, hence references to these structures as preputial glands or castor glands are misnomers. The wall of the castor sacs is plicate and comprised of three distinct zones: an outer layer of vascular connective tissue, a two- to five-cell-thick layer of mitotic epithelial cells, and several densely packed layers of cornified epithelium which grade into more widely separated sheets toward the lumen. Monocultures of a gram-positive facultatively anaerobic bacterium were present in the lumen of all castor sac preparations. Differences in the frequency of castoreum deposition were not attributable to differences in the structure of the castor sacs. The anal glands of beavers are holocrine sebaceous glands. These glands develop more rapidly than the castor sacs. Anal gland tissue from embryos exhibited cellular characteristics associated with the production of sebum. Secretory activity was evident in all preparations. The relationship of castoreum and anal gland secretion to scent communication among beavers is discussed. Key Words--Castor canadensis, castor sacs, anal glands, chemical communication. INTRODUCTION B o t h s e x e s o f b e a v e r s (Castor canadensis) p o s s e s s a p a i r o f c a s t o r s a c s a n d a p a i r o f a n a l g l a n d s l o c a t e d in s u b c u t a n e o u s c a v i t i e s b e t w e e n t h e p e l v i s a n d b a s e o f t h e tail ( S v e n d s e n , 1978). T h e c o n t e n t s o f t h e c a s t o r s a c s m i x w i t h urine to f o r m c a s t o r e u m . Beavers d e p o s i t this thin yellow liquid on m u d 809 0098-0331 / 82;;0500~809503.00/0 9 1982 Pleunum Publishing Corporation
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mounds constructed within the home range of the family (Aleksiuk, 1968; Svendsen, 1980; MOller-Schwarze and Heckman, 1980; Butler and Butler, 1979). The anal glands are located caudal to castor sacs (Svendsen, 1978). Ducts from the glands open on papillae lateral to the rectum and orifice leading to the vestibule of the castor sacs. Anal gland secretions vary in color from straw to light brown, are viscous and insoluble in water, and have a pungent odor. The variety of terminology reflects the paucity in data concerning the histology of these structures. "Castor glands" (Brady and Svendsen, 1981; Butler and Butler, 1979; Svendsen, 1978) and "preputial glands" (MfillerSchwarze and Heckman, 1980; Wilsson, 1971) have been used synonymously with "castor sacs" and "oil sacs" (Muller-Schwarze and Heckman, 1980) with "anal. glands." The purpose of this research was to describe the histology of the castor sacs and anal glands and test the following hypotheses: Both structures are true glands; both structures develop synchronously in beavers; and castor sacs and anal glands do not differ in structure or activity among beavers of different age or gender, or vary seasonally. These data will provide a baseline for assessing the role of castoreum and anal gland secretion in integrating the social behavior of beavers. M E T H O D S AND MATERIALS
The castor sacs and anal glands used for microscopic study were from beavers live-trapped in southeastern Ohio. Sexing and handling of these beavers followed the methods of Svendsen (1980). Animals were classified as adults, two-year-olds, yearlings, or kits. Beavers were anesthetized with Nembutal (pentobarbital sodium, Abbott Laboratories). Both pairs of anal glands and castor sacs were removed surgically from 11 animals of different age and gender. These animals were subsequently sacrificed. Portions of the excised glands were fixed immediately in glutaraldehyde, buffered with phosphate to pH 7.1, and rinsed with buffered sucrose. Pieces of tissues were postfixed in buffered 1% osmium tetroxide, dehydrated in an ethanol series, and embedded in an EponAraldite mixture. Tissues were sectioned on a Reichert OmU2 microtome. Thick sections were transferred to clean microscope slides and stained with toluidine blue. Thin sections were contrasted with uranyl acetate and lead citrate and examined on a Siemens Elmiskop I electron microscope. Paraffin sections of both anal glands and castor sacs from four beavers were prepared by fixing excised tissue in Carnoy's fixative for 20 hr at 4~ Tissues were dehydrated in an ethanol series, cleared with three changes of benzene, and embedded in paraffin under a vacuum. Sections.(10 #m) were cut and stained with either hematoxylin and eosin, azan, or trichrome stains
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according to Humason (1967). One of the aforementioned females was pregnant. Castor sacs from three of the fetuses (approximately 11-weeks-old) were embedded in paraffin, sectioned, and stained with hematoxylin and eosin. Structures from the remaining fetus were excised and immediately immersed in liquid nitrogen. Sections of 15-#m thickness were cut on an American Optical Cryo-cut microtome, transferred to clean cover slips, and allowed to thaw. Tissues were stained with toluidine blue, dehydrated in an ethanol series, cleared with xylene, and mounted on clean microscope slides for microscopic examination. RESULTS
Anatomy of Castor Sacs. The castor sacs have been reported to be diverticula of the urinary or reproductive tracts (Kacnelson and Orlova, 1954; Wilsson, 1971; Svendsen, 1978). Both the urinary and reproductive tracts of males and females terminate anterior to the vestibule that connects the castor sacs, thus the castor sacs cannot be derivatives of these tracts. Rather the castor sacs are epithelial pockets of the wall of the urogenital sinus. Sphincters are absent at the junction of the castor sacs and the sinus, thus the contents of the sacs can readily combine with urine to form castoreum. This arrangement necessitates the excretion of castoreum when beavers urinate. Histology and Development of Castor Sacs. Three distinct layers of tissue comprised the wall of the castor sacs: a peripheral layer of vascularized connective tissue, a layer of germinative epithelium two to five cells thick, and a zone of cornified layers of epithelium lining the lumen (Figure la). Projections of the connective tissue and the corresponding flexure of epithelial and keratinized epithelial cells form papillae which jutted into the lumen of the sac (Figure la). The distance between adjacent layers of cornified cells increased progressively toward the lumen (Figure lb). Ruptures in the innermost layers of cells were .common. One species of bacterium was observed in the lumen of all preparations of the sacs (Figure 2). Subsequent culture and characterization showed this bacterium to be a gram-positive, facultatively anaerobic, pleomorphic cocci. This bacterium imparted a granular appearance to the area adjacent to the layers of eornified cells lining the lumen of the castor sacs. Ducts leading from the wall to the lumen of the sac were conspicuously 'absent. In addition, no glands were present that emptied into either the castor sacs or the vestibule connecting the sacs. The castor sacs were poorly developed in prenatal beavers. Several layers of epithelial cells surrounded by connective tissue was the extent of development at 11-12 weeks gestation (Figure 3a). The walls of the sacs were beginning to convolute and a lumen was present, but the epithelial cells lining the lumen were not cornified (Figure 3b). The castor sacs of two stillborn
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FIG. 1. (a) Section through the wall of the castor sacs showing stratification of tissue layers and plicate structure, ct, connective tissue; ge, germinative epithelium; de, densely packed cornified epithelium. Granularity (arrow) results from sloughing of cells and bacteria in the lumen of the sac. Adult female. Trichrome, • 275. (b) Section through the wall of the castor sac. Lumen of the sac is at the left of the photograph. Dark layer of cells is germinating epithelium. Ruptures in cornified layers are visible at left (arrowheads). Bacteria are responsible for the observed granularity (arrow). Adult female. Azan, • 900.
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FIG. 2. Bacterium inhabiting the castor sacs. Cornified epithelial layers are evident traversing obliquely in the micrograph, Adult female. • Inset: Dividing bacterium in lumen of the castor sac. Kit male. •
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Fro. 3. Development of the castor sacs in 11-week-old embryonic beavers. (a) Castor sacs appear as several layers of undifferentiated cells surrounded by connective tissue. Male. Toluidine blue, • (b) Lumen has formed but epithelial cells lining the lumen have not undergone keratinization. Male. Hematoxylin and eosin, •
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beavers were thin-walled and flaccid. The castor sacs of a 5-month-old kit were structurally indistinguishable from the sacs excised from adults. Histology and Development of Anal glands. The anal glands are compound tubuloacinar sebaceous glands (Figure 4a). Maturation of cells occurred centripetally within the acini. Growth of immature, uninucleate cells at the periphery pushed adjacent cells to the center of each acinus. Cells at the center of the acini exhibited characteristics associated with production of sebum: proliferation of lipid, enlargement, and nuclear degeneration (Figure 4b). Secretory activity was visible in all preparations of anal glands and was independent of age or gender of the beaver, or the season in which the sample was collected. The anal glands of beaver were well developed in embryonic beaver. Formation of lobes was well underway and mature sebaceous cells were visible in preparations (Figure 5a,b). The anal glands of kits at birth were relatively large and turgid with sebum. DISCUSSION
Castor Sacs. Reference to the castor sacs as "castor glands" or "preputial glands" is inappropriate. The preputial glands found in males (clitoral glands in females) of several species of rodents are true holocrine sebaceous glands (Brown and Williams, 1972). Castor sacs of beavers are pouches lined with cornified epithelial cells. These sacs produce no secretion, hence are not glands in the strict histological sense. However, sloughed epidermal ceils may contribute to the production of chemical signals in castoreum, and the castor sacs' function may be similar to that of a holocrine gland. No differences in the structure of the castor sacs were evident in kits > 5 months of age, yearlings, two-year olds, or adults. The absence of scentmound construction by kit beavers (Hodgdon, 1978; Svendsen, unpublished data) is not attributable to immaturity of the castor sacs. The frequency of scent-mound construction varies seasonally. Frequencies of scent-mounds constructed by different beaver families increased with increasing extralodge activity in late winter and early spring, peaked in May and early June, declined during July, and remained low throughout the remainder of the year (Svendsen, 1980). The castor sacs neither enlarge or regress on an annual cycle (Svendsen, 1978) nor do they vary seasonally in structure. In addition, exfoliation of cornified cells occurs at a constant rate (Goldschmidt and Klingman, 1974). The location of the castor sacs relative to the urethra makes it physically impossible for beavers to urinate without urine mixing with the contents of the castor sacs. We interpret this evidence to indicate that castoreum is being produced continually and is not correlated with the seasonal variation in scent-marking behavior. This system of scent
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FIG. 4. (a) General structure of anal glands. Uninucleate cells compose each acinus. Secretory activity is visible at arrow. Kit male. Azan, • (b) Maturation of sebaceous cells. Degeneration of nucleus, accumulation of lipid, and enlargement of cells are visible in center of photomicrograph. Adult female. Azan, •
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FIG, 5. Development of the anal glands in ll-week-otd embryonic beavers. (a) Branching to form compound structure is evident. Cells at center of photograph (arrow) show changes associated with production of sebum. Male. Toluidine blue, X250. (b) Mature sebaceous cells. Cells are anucleate and filled with lipid. Male, Hematoxylin and eosin, X500.
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production, which is continually "on", is modulated by the selection of the substrate for the deposition of castoreum. Castoreum that is produced during periods of low scent-marking activity is simply voided in the water. Urine extracted from the bladder of beavers and applied to artificial scent-mounds elicited no response from conspecifics whereas addition of castoreum did (Svendsen and Walro, unpublished data). These data indicate that the castor sacs are essential for the production of chemical signals in castoreum. Whether urine is necessary to interact with the contents of the castor sacs to produce a signal or is simply a mechanism for transport to the exterior is not known. This study is the first to demonstrate the presence of bacteria in the castor sacs. This monoculture of cocci appeared dark and crystalline in preparations stained with toluidine blue. This bacterium is likely the "calcium sediments" described by Kacnelson and Orlova (1954) in the castor sacs of river beavers (C. fiber). Bacteria are known to inhabit cavities and pouches of mammals, metabolize available substrate, and produce odoriferous compounds. These compounds are hypothesized to serve as chemical signals in several species of mammals (Albone et al., 1976). Whether the bacterium that inhabits the castor sacs is responsible for the chemical signals in castoreum or simply resides in the sac is not known. If the bacterium is responsible for the cues in castoreum, urine provides a more readily metabolizable substrate than the contents of keratinized cells lining the sacs. The development of the castor sacs in C. canadensis parallels the development of the sacs in C. fiber. Wilsson (1971) reported that the castor sacs of European beavers about one month of age were empty and flaccid, but sacs of three-month-old animals had castoreum in them. Anal Glands. The anal glands are true holocrine sebaceous glands. Differences in secretory activity of anal glands were not evident in histological preparations. All preparations of anal gland tissue exhibited secretory activity regardless of age, gender, or the time of year at which the sample was taken. Data from captive animals and marked animals in the field suggest that beavers anoint themselves with anal gland secretion during bouts of autogrooming (Svendsen and Walro, in preparation). The secretion has a dual function. Anal gland secretion supplements the sebaceous glands of the skin in maintaining the water-repellent quality of the fur. Excision of these structures reduces the ability of the pelage to repel water (Svendsen and Walro, in preparation). Prohibition of autogrooming in C.fiber produces similar results (Wilsson, 1971). Both the glands that produce a secretion which waterproofs and maintains the fur and the motor patterns associated with application of the secretion would be expected to develop early in semiaquatic animal. Such is the case in beavers. Anal glands synthesize sebum prior to parturition, and a kit, on its first evening outside the lodge, displayed well-developed motor patterns of autogrooming with the forepaws (Svendsen and Walro, in
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p r e p a r a t i o n ) . S i m i l a r results a r e r e p o r t e d f o r C. f i b e r . M o t o r p a t t e r n s o f a u t o g r o o m i n g d e v e l o p at f o u r d a y s p o s t p a r t u m in t h e E u r o p e a n b e a v e r - l o n g b e f o r e t h e kits b e g a n s p e n d i n g e v e n i n g s o u t s i d e t h e l o d g e ( W i l s s o n , 1971). P r e l i m i n a r y e x p e r i m e n t s i n d i c a t e t h a t a n a l g l a n d s e c r e t i o n is also a s o u r c e o f c h e m i c a l signals in b e a v e r s ( S v e n d s e n a n d W a l r o , in p r e p a r a t i o n ) . Thus beaver, when they groom themselves, impart an odor or disseminate scent to other parts of their body. T h i s s t u d y relates t h e h i s t o l o g y a n d d e v e l o p m e n t o f b o t h t h e c a s t o r sacs and anal glands to chemical communication among beavers. Isolation and p r o d u c t i o n o f c h e m i c a l cues in c a s t o r e u m a n d a n a l g l a n d s e c r e t i o n a r e subjects of future investigation. Acknowledgments--We thank Dr. Robert Hikida and Robert Staron for their technical assistance and the Ohio Department of Natural Resources, Division of Wildlife for their cooperation in obtaining beavers for this study. This research was funded in part by grants BNS76-06836 and BNS-76-06836 A01 from the National Science Foundation to G.E. Svendsen.
REFERENCES
ALBONE, E.S., GOSDEN,P.E., and WARE,G.C. 1976. Bacteria as a source of chemical signals in mammals, pp. 35-43, in D. M/iller-Schwarze and M. Mozell (eds.). Chemical Signals in Vertebrates. Plenum Press, New York, 609 pp. ALEKSIUK,M. 1968. Scent mound communication, territoriality, and population regulation in beavers (Castor canadensis Kuhl). J. Mammal. 49:759-762. BRADY, C.A., and SVENDSEN, G.E. 1981. Social behaviour in a family of beaver, Castor canadensis. Biol. Behav. 6:99-114. BROWN,J.C., and WILLIAMS,J.D. 1972. The rodent preputial gland. Mammal. Rev. 2:105-147. BUTLER,R.G., and BUTLER,L.A. 1979. Toward a functional interpretation of scent marking in the beaver (Castor canadensis). Behav. Neurol. Biol. 26:442-454. GOLDSCHMIDT,H., and KLINGMAN,A.M. 1964. Quantitative estimation of keratin production by the epidermis. Arch. DermatoL 88:709-712. HODGDON, H.E. 1978. Social dynamics and behavior within an unexploited beaver (Castor canadensis) population. Unpublished PhD dissertation, University of Massachusetts, Amherst. 292 pp. HUMASON,G.L. 1967. Animal Tissue Techniques. W.H. Freeman, San Francisco, 569 pp. KACNELSON,Z.S., and ORLOVA,I.I. 1954. The histological structure of the preputial glands of an adult beaver. Voronezj 5:58-63 (in Russian). MI~LLER-SCHWARZE,O., and HECKMAN,S. 1980. The social role of scent marking in beaver (Castor canadensis). J. Chem. Ecol. 6:81-95. SVENDSEN, G.E. 1978. Castor and anal glands of the beaver (Castor canadensis). J. Mammal. 59:618-620. SVENDSEN,G.E. 1980. Patterns of scent-mounding in a population of beaver (Castor canadensis). J. Chem. Ecol. 6:133-148. WILSSON,L. 1971. Observations and experiments on the ethology of the European beaver (Castor fiber L.). Viltrevy 8:116-261.
CASTOR SACS AND ANAL GLANDS OF THE NORTH AMERICAN BEAVER (Castor canadensis): Their Histology, Development, and Relationship to Scent Communication
JON M. WALRO
and GERALD
E. S V E N D S E N
Department of Zoology and Microbiology Ohio University Athens, Ohio 45701 (Received July 14, 1981; revised August 27, 1981)
Abstract--Both sexes of beavers possess a pair of castor sacs and a pair of anal glands located in paired subcutaneous cavities between the pelvis and the base of the tail. The castor sacs are not glandular in the histological sense, hence references to these structures as preputial glands or castor glands are misnomers. The wall of the castor sacs is plicate and comprised of three distinct zones: an outer layer of vascular connective tissue, a two- to five-cell-thick layer of mitotic epithelial cells, and several densely packed layers of cornified epithelium which grade into more widely separated sheets toward the lumen. Monocultures of a gram-positive facultatively anaerobic bacterium were present in the lumen of all castor sac preparations. Differences in the frequency of castoreum deposition were not attributable to differences in the structure of the castor sacs. The anal glands of beavers are holocrine sebaceous glands. These glands develop more rapidly than the castor sacs. Anal gland tissue from embryos exhibited cellular characteristics associated with the production of sebum. Secretory activity was evident in all preparations. The relationship of castoreum and anal gland secretion to scent communication among beavers is discussed. Key Words--Castor canadensis, castor sacs, anal glands, chemical communication. INTRODUCTION B o t h s e x e s o f b e a v e r s (Castor canadensis) p o s s e s s a p a i r o f c a s t o r s a c s a n d a p a i r o f a n a l g l a n d s l o c a t e d in s u b c u t a n e o u s c a v i t i e s b e t w e e n t h e p e l v i s a n d b a s e o f t h e tail ( S v e n d s e n , 1978). T h e c o n t e n t s o f t h e c a s t o r s a c s m i x w i t h urine to f o r m c a s t o r e u m . Beavers d e p o s i t this thin yellow liquid on m u d 809 0098-0331 / 82;;0500~809503.00/0 9 1982 Pleunum Publishing Corporation
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mounds constructed within the home range of the family (Aleksiuk, 1968; Svendsen, 1980; MOller-Schwarze and Heckman, 1980; Butler and Butler, 1979). The anal glands are located caudal to castor sacs (Svendsen, 1978). Ducts from the glands open on papillae lateral to the rectum and orifice leading to the vestibule of the castor sacs. Anal gland secretions vary in color from straw to light brown, are viscous and insoluble in water, and have a pungent odor. The variety of terminology reflects the paucity in data concerning the histology of these structures. "Castor glands" (Brady and Svendsen, 1981; Butler and Butler, 1979; Svendsen, 1978) and "preputial glands" (MfillerSchwarze and Heckman, 1980; Wilsson, 1971) have been used synonymously with "castor sacs" and "oil sacs" (Muller-Schwarze and Heckman, 1980) with "anal. glands." The purpose of this research was to describe the histology of the castor sacs and anal glands and test the following hypotheses: Both structures are true glands; both structures develop synchronously in beavers; and castor sacs and anal glands do not differ in structure or activity among beavers of different age or gender, or vary seasonally. These data will provide a baseline for assessing the role of castoreum and anal gland secretion in integrating the social behavior of beavers. M E T H O D S AND MATERIALS
The castor sacs and anal glands used for microscopic study were from beavers live-trapped in southeastern Ohio. Sexing and handling of these beavers followed the methods of Svendsen (1980). Animals were classified as adults, two-year-olds, yearlings, or kits. Beavers were anesthetized with Nembutal (pentobarbital sodium, Abbott Laboratories). Both pairs of anal glands and castor sacs were removed surgically from 11 animals of different age and gender. These animals were subsequently sacrificed. Portions of the excised glands were fixed immediately in glutaraldehyde, buffered with phosphate to pH 7.1, and rinsed with buffered sucrose. Pieces of tissues were postfixed in buffered 1% osmium tetroxide, dehydrated in an ethanol series, and embedded in an EponAraldite mixture. Tissues were sectioned on a Reichert OmU2 microtome. Thick sections were transferred to clean microscope slides and stained with toluidine blue. Thin sections were contrasted with uranyl acetate and lead citrate and examined on a Siemens Elmiskop I electron microscope. Paraffin sections of both anal glands and castor sacs from four beavers were prepared by fixing excised tissue in Carnoy's fixative for 20 hr at 4~ Tissues were dehydrated in an ethanol series, cleared with three changes of benzene, and embedded in paraffin under a vacuum. Sections.(10 #m) were cut and stained with either hematoxylin and eosin, azan, or trichrome stains
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according to Humason (1967). One of the aforementioned females was pregnant. Castor sacs from three of the fetuses (approximately 11-weeks-old) were embedded in paraffin, sectioned, and stained with hematoxylin and eosin. Structures from the remaining fetus were excised and immediately immersed in liquid nitrogen. Sections of 15-#m thickness were cut on an American Optical Cryo-cut microtome, transferred to clean cover slips, and allowed to thaw. Tissues were stained with toluidine blue, dehydrated in an ethanol series, cleared with xylene, and mounted on clean microscope slides for microscopic examination. RESULTS
Anatomy of Castor Sacs. The castor sacs have been reported to be diverticula of the urinary or reproductive tracts (Kacnelson and Orlova, 1954; Wilsson, 1971; Svendsen, 1978). Both the urinary and reproductive tracts of males and females terminate anterior to the vestibule that connects the castor sacs, thus the castor sacs cannot be derivatives of these tracts. Rather the castor sacs are epithelial pockets of the wall of the urogenital sinus. Sphincters are absent at the junction of the castor sacs and the sinus, thus the contents of the sacs can readily combine with urine to form castoreum. This arrangement necessitates the excretion of castoreum when beavers urinate. Histology and Development of Castor Sacs. Three distinct layers of tissue comprised the wall of the castor sacs: a peripheral layer of vascularized connective tissue, a layer of germinative epithelium two to five cells thick, and a zone of cornified layers of epithelium lining the lumen (Figure la). Projections of the connective tissue and the corresponding flexure of epithelial and keratinized epithelial cells form papillae which jutted into the lumen of the sac (Figure la). The distance between adjacent layers of cornified cells increased progressively toward the lumen (Figure lb). Ruptures in the innermost layers of cells were .common. One species of bacterium was observed in the lumen of all preparations of the sacs (Figure 2). Subsequent culture and characterization showed this bacterium to be a gram-positive, facultatively anaerobic, pleomorphic cocci. This bacterium imparted a granular appearance to the area adjacent to the layers of eornified cells lining the lumen of the castor sacs. Ducts leading from the wall to the lumen of the sac were conspicuously 'absent. In addition, no glands were present that emptied into either the castor sacs or the vestibule connecting the sacs. The castor sacs were poorly developed in prenatal beavers. Several layers of epithelial cells surrounded by connective tissue was the extent of development at 11-12 weeks gestation (Figure 3a). The walls of the sacs were beginning to convolute and a lumen was present, but the epithelial cells lining the lumen were not cornified (Figure 3b). The castor sacs of two stillborn
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FIG. 1. (a) Section through the wall of the castor sacs showing stratification of tissue layers and plicate structure, ct, connective tissue; ge, germinative epithelium; de, densely packed cornified epithelium. Granularity (arrow) results from sloughing of cells and bacteria in the lumen of the sac. Adult female. Trichrome, • 275. (b) Section through the wall of the castor sac. Lumen of the sac is at the left of the photograph. Dark layer of cells is germinating epithelium. Ruptures in cornified layers are visible at left (arrowheads). Bacteria are responsible for the observed granularity (arrow). Adult female. Azan, • 900.
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FIG. 2. Bacterium inhabiting the castor sacs. Cornified epithelial layers are evident traversing obliquely in the micrograph, Adult female. • Inset: Dividing bacterium in lumen of the castor sac. Kit male. •
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Fro. 3. Development of the castor sacs in 11-week-old embryonic beavers. (a) Castor sacs appear as several layers of undifferentiated cells surrounded by connective tissue. Male. Toluidine blue, • (b) Lumen has formed but epithelial cells lining the lumen have not undergone keratinization. Male. Hematoxylin and eosin, •
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beavers were thin-walled and flaccid. The castor sacs of a 5-month-old kit were structurally indistinguishable from the sacs excised from adults. Histology and Development of Anal glands. The anal glands are compound tubuloacinar sebaceous glands (Figure 4a). Maturation of cells occurred centripetally within the acini. Growth of immature, uninucleate cells at the periphery pushed adjacent cells to the center of each acinus. Cells at the center of the acini exhibited characteristics associated with production of sebum: proliferation of lipid, enlargement, and nuclear degeneration (Figure 4b). Secretory activity was visible in all preparations of anal glands and was independent of age or gender of the beaver, or the season in which the sample was collected. The anal glands of beaver were well developed in embryonic beaver. Formation of lobes was well underway and mature sebaceous cells were visible in preparations (Figure 5a,b). The anal glands of kits at birth were relatively large and turgid with sebum. DISCUSSION
Castor Sacs. Reference to the castor sacs as "castor glands" or "preputial glands" is inappropriate. The preputial glands found in males (clitoral glands in females) of several species of rodents are true holocrine sebaceous glands (Brown and Williams, 1972). Castor sacs of beavers are pouches lined with cornified epithelial cells. These sacs produce no secretion, hence are not glands in the strict histological sense. However, sloughed epidermal ceils may contribute to the production of chemical signals in castoreum, and the castor sacs' function may be similar to that of a holocrine gland. No differences in the structure of the castor sacs were evident in kits > 5 months of age, yearlings, two-year olds, or adults. The absence of scentmound construction by kit beavers (Hodgdon, 1978; Svendsen, unpublished data) is not attributable to immaturity of the castor sacs. The frequency of scent-mound construction varies seasonally. Frequencies of scent-mounds constructed by different beaver families increased with increasing extralodge activity in late winter and early spring, peaked in May and early June, declined during July, and remained low throughout the remainder of the year (Svendsen, 1980). The castor sacs neither enlarge or regress on an annual cycle (Svendsen, 1978) nor do they vary seasonally in structure. In addition, exfoliation of cornified cells occurs at a constant rate (Goldschmidt and Klingman, 1974). The location of the castor sacs relative to the urethra makes it physically impossible for beavers to urinate without urine mixing with the contents of the castor sacs. We interpret this evidence to indicate that castoreum is being produced continually and is not correlated with the seasonal variation in scent-marking behavior. This system of scent
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FIG. 4. (a) General structure of anal glands. Uninucleate cells compose each acinus. Secretory activity is visible at arrow. Kit male. Azan, • (b) Maturation of sebaceous cells. Degeneration of nucleus, accumulation of lipid, and enlargement of cells are visible in center of photomicrograph. Adult female. Azan, •
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FIG, 5. Development of the anal glands in ll-week-otd embryonic beavers. (a) Branching to form compound structure is evident. Cells at center of photograph (arrow) show changes associated with production of sebum. Male. Toluidine blue, X250. (b) Mature sebaceous cells. Cells are anucleate and filled with lipid. Male, Hematoxylin and eosin, X500.
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production, which is continually "on", is modulated by the selection of the substrate for the deposition of castoreum. Castoreum that is produced during periods of low scent-marking activity is simply voided in the water. Urine extracted from the bladder of beavers and applied to artificial scent-mounds elicited no response from conspecifics whereas addition of castoreum did (Svendsen and Walro, unpublished data). These data indicate that the castor sacs are essential for the production of chemical signals in castoreum. Whether urine is necessary to interact with the contents of the castor sacs to produce a signal or is simply a mechanism for transport to the exterior is not known. This study is the first to demonstrate the presence of bacteria in the castor sacs. This monoculture of cocci appeared dark and crystalline in preparations stained with toluidine blue. This bacterium is likely the "calcium sediments" described by Kacnelson and Orlova (1954) in the castor sacs of river beavers (C. fiber). Bacteria are known to inhabit cavities and pouches of mammals, metabolize available substrate, and produce odoriferous compounds. These compounds are hypothesized to serve as chemical signals in several species of mammals (Albone et al., 1976). Whether the bacterium that inhabits the castor sacs is responsible for the chemical signals in castoreum or simply resides in the sac is not known. If the bacterium is responsible for the cues in castoreum, urine provides a more readily metabolizable substrate than the contents of keratinized cells lining the sacs. The development of the castor sacs in C. canadensis parallels the development of the sacs in C. fiber. Wilsson (1971) reported that the castor sacs of European beavers about one month of age were empty and flaccid, but sacs of three-month-old animals had castoreum in them. Anal Glands. The anal glands are true holocrine sebaceous glands. Differences in secretory activity of anal glands were not evident in histological preparations. All preparations of anal gland tissue exhibited secretory activity regardless of age, gender, or the time of year at which the sample was taken. Data from captive animals and marked animals in the field suggest that beavers anoint themselves with anal gland secretion during bouts of autogrooming (Svendsen and Walro, in preparation). The secretion has a dual function. Anal gland secretion supplements the sebaceous glands of the skin in maintaining the water-repellent quality of the fur. Excision of these structures reduces the ability of the pelage to repel water (Svendsen and Walro, in preparation). Prohibition of autogrooming in C.fiber produces similar results (Wilsson, 1971). Both the glands that produce a secretion which waterproofs and maintains the fur and the motor patterns associated with application of the secretion would be expected to develop early in semiaquatic animal. Such is the case in beavers. Anal glands synthesize sebum prior to parturition, and a kit, on its first evening outside the lodge, displayed well-developed motor patterns of autogrooming with the forepaws (Svendsen and Walro, in
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p r e p a r a t i o n ) . S i m i l a r results a r e r e p o r t e d f o r C. f i b e r . M o t o r p a t t e r n s o f a u t o g r o o m i n g d e v e l o p at f o u r d a y s p o s t p a r t u m in t h e E u r o p e a n b e a v e r - l o n g b e f o r e t h e kits b e g a n s p e n d i n g e v e n i n g s o u t s i d e t h e l o d g e ( W i l s s o n , 1971). P r e l i m i n a r y e x p e r i m e n t s i n d i c a t e t h a t a n a l g l a n d s e c r e t i o n is also a s o u r c e o f c h e m i c a l signals in b e a v e r s ( S v e n d s e n a n d W a l r o , in p r e p a r a t i o n ) . Thus beaver, when they groom themselves, impart an odor or disseminate scent to other parts of their body. T h i s s t u d y relates t h e h i s t o l o g y a n d d e v e l o p m e n t o f b o t h t h e c a s t o r sacs and anal glands to chemical communication among beavers. Isolation and p r o d u c t i o n o f c h e m i c a l cues in c a s t o r e u m a n d a n a l g l a n d s e c r e t i o n a r e subjects of future investigation. Acknowledgments--We thank Dr. Robert Hikida and Robert Staron for their technical assistance and the Ohio Department of Natural Resources, Division of Wildlife for their cooperation in obtaining beavers for this study. This research was funded in part by grants BNS76-06836 and BNS-76-06836 A01 from the National Science Foundation to G.E. Svendsen.
REFERENCES
ALBONE, E.S., GOSDEN,P.E., and WARE,G.C. 1976. Bacteria as a source of chemical signals in mammals, pp. 35-43, in D. M/iller-Schwarze and M. Mozell (eds.). Chemical Signals in Vertebrates. Plenum Press, New York, 609 pp. ALEKSIUK,M. 1968. Scent mound communication, territoriality, and population regulation in beavers (Castor canadensis Kuhl). J. Mammal. 49:759-762. BRADY, C.A., and SVENDSEN, G.E. 1981. Social behaviour in a family of beaver, Castor canadensis. Biol. Behav. 6:99-114. BROWN,J.C., and WILLIAMS,J.D. 1972. The rodent preputial gland. Mammal. Rev. 2:105-147. BUTLER,R.G., and BUTLER,L.A. 1979. Toward a functional interpretation of scent marking in the beaver (Castor canadensis). Behav. Neurol. Biol. 26:442-454. GOLDSCHMIDT,H., and KLINGMAN,A.M. 1964. Quantitative estimation of keratin production by the epidermis. Arch. DermatoL 88:709-712. HODGDON, H.E. 1978. Social dynamics and behavior within an unexploited beaver (Castor canadensis) population. Unpublished PhD dissertation, University of Massachusetts, Amherst. 292 pp. HUMASON,G.L. 1967. Animal Tissue Techniques. W.H. Freeman, San Francisco, 569 pp. KACNELSON,Z.S., and ORLOVA,I.I. 1954. The histological structure of the preputial glands of an adult beaver. Voronezj 5:58-63 (in Russian). MI~LLER-SCHWARZE,O., and HECKMAN,S. 1980. The social role of scent marking in beaver (Castor canadensis). J. Chem. Ecol. 6:81-95. SVENDSEN, G.E. 1978. Castor and anal glands of the beaver (Castor canadensis). J. Mammal. 59:618-620. SVENDSEN,G.E. 1980. Patterns of scent-mounding in a population of beaver (Castor canadensis). J. Chem. Ecol. 6:133-148. WILSSON,L. 1971. Observations and experiments on the ethology of the European beaver (Castor fiber L.). Viltrevy 8:116-261.
