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Wolves (Canis lupus) can be primary predators of beavers (Castor canadensis), but little is known about wolf-beaver dynamics. We identified kills from 1 wolf (V009) of the Ash River Pack in Voyageurs National Park from 1 April to 5 November 2015 to provide direct estimates of wolf pack kill and predation rates of beavers. We documented 12 beaver kills by V009 during the 2015 ice-free season and estimated V009 killed 22 beavers during this period. Based on the number of beavers killed by V009, we estimated the Ash River Pack removed 80–88 beavers (kill rate of 0.085–0.095 beavers/wolf/day), which was 38–42% of the beaver population in their home range during the ice-free season. Even with this substantial level of predation in 2015, the beaver population in the Ash River Pack home range increased by an estimated 43% in 2016, which suggested dispersal from more densely populated adjacent areas likely compensated for the effects of wolf predation. We have presented the first direct estimate of wolf kill and predation rates on beavers, but more research is necessary to understand how wolf predation affects beaver populations under a variety of conditions. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

Beaver ponds and beaver-impounded vegetation are indicators of past or present beaver activity that can be detected from aerial photography. A method to quantitatively relate these beaver works with the density of active beaver colonies could benefit beaver management, particularly in areas lacking beaver population data. We compared historical maps (1961–2006) of beaver works at Voyageurs National Park, Minnesota, USA with concurrent aerial surveys of beaver colonies. We tested 2 landscape-scale models of beaver colony density previously developed for a period of beaver population expansion (1940–1986), but they failed to predict colony density after 1986, a period of declining beaver population. We developed a new landscape-scale regression, calculating that 2.15% of the landscape would be flooded by every 100 additional beaver colonies (R2 = 0.53, P = 0.027). Classification tree analysis of individual pond sites showed that open water pond and impounded marsh area were the primary predictors of beaver colony presence or absence, but that the classification trees were far better at identifying inactive sites (>93% correct) than active sites (35–38% correct). The area of open water in beaver ponds is a good but not perfect indicator of beaver activity that can be used by wildlife managers as a landscape-scale indicator of beaver colony density. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Beaver (Castor canadensis) dam-site selection studies traditionally have relied on plant composition. To understand how habitat factors influence dam establishment, we compared 9 plant composition and size categories and their spatial distribution with 4 physical features at 15 active dams, 15 abandoned dams, and 12 no-dam sites. To establish pre-dam vegetation densities, plots downstream and upstream from impoundments were averaged. We found beaver relied on both physical (upstream watershed area and stream cross-sectional area) and vegetation (shoreline concentrations of woody plants with diameters 1.5-4.4 cm) factors in choosing dam sites. The model designed by McComb et al. (1990) was not effective in predicting dam sites in northern Ontario, therefore, we recommend that managers test the model's regional accuracy in determining site locations.

Controlling beaver (Castor canadensis) populations is a problematic issue for wildlife managers in North America. Management of beaver can be facilitated by determining sex ratio of a population, but this requires ready identification of gender for live and harvested beaver. We developed a technique using color and viscosity of anal gland secretion (AGS) to identify gender of beaver. Inexperienced volunteers inspected AGS from beaver in 3 regions of New York. Using AGS collected and stored in glass vials, 4 volunteers correctly identified sex of 24 beaver (16 M, 8 F) livetrapped during spring 1992; 2 additional pairs of volunteers correctly discriminated 45 beaver (21 M, 24 F) livetrapped in spring 1993 and 1994. Using fresh AGS viewed in the field, 1 volunteer correctly discerned 25 beaver (15 M, 10 F) caught in October 1993; and 2 volunteers distinguished sex of 5 beaver (3 M, 2 F) captured in November 1992 without error. Male AGS was Kraft brown (P55, Marker color system) to sepia (P56) and viscous (490-507 centipoise [cP] at 25.8 C and 60 rpm, Brookfield DV-11 + viscometer), while female secretion was pale to medium olive (P34, P39) or warm gray (P191), less viscous (4-28.7 cP, 23-25.4 C, 50 or 100 rpm) and flowed more (P < 0.001) than male AGS. Secretion color and viscosity were discriminating means of distinguishing the sexes in the field.

Response of beaver (Castor canadensis) to baited Conibear 330 traps set under the ice near lodges in Newfoundland indicated kits were as vulnerable at 1-3 m from the lodge as they were from 69 m. There was no difference (P > 0.05) between the percentages of kits and adults or males and females trapped. Of 46 colonies, 15% were single, 24% were pair, and 61% were family colonies. Colony size averaged 3.5 beaver for 26 colonies totally removed, of which 13 were family colonies averaging 5.3 beaver. There was no difference (P > 0.05) in the sex ratio. Of the family colonies, 42% contained 2.5-yearold beaver, and 24% contained 3 beaver 2.5 years old or older. Of the pair colonies, 42% contained a yearling. J. WILDL. MANAGE. 46(3):655-661 Because beaver are colonial and restricted in their movements during winter freeze-up, and because trapping is the usual method of harvest, knowledge of trap response, colony size, and age and sex structure is of importance to management. Most available data indicate that there are slightly more male than female beaver born, but the sex ratio of adults favors females (Shelton 1966). Some wildlife agencies restrict trapping near the lodge because kits allegedly are more susceptible there than are adults. Newfoundland regulations state that only 1 beaver per colony may be trapped. The extent to which trap response affects harvest under these regulations is unknown. Colony size has been reported in North America and Russia, but usually without comment concerning harvest history or their distribution or status as random or nuisance. This study was undertaken to increase the understanding of the effect of several colony parameters on response to winter trapping. This project was funded by the Newfoundland Wildlife Division. I thank wildlife technicians C. Finlay, S. Kelly, H. Abbott, D. Slade, D. Garrett, B. Porter, and L. Russell for field assistance; J. J. Spillett, Utah State University, for help with the data analysis; and J. Larson, J. Hickey, W. Rutherford, L. Krumholz, and referees for reviewing the manuscript.

Although breeding activity in a colony of beaver (Castor canadensis) usually is confined to an adult pair, sexually mature male and female progeny are present and may reproduce if 1 or both adults are removed. To assess behavior and reproduction in such progeny, 1 adult in each of 18 colonies was either surgically sterilized or sham-operated in summer 1975. Sterility was induced by either tubal ligation (56, 59 in 10 colonies) or castration (26, 29 in 4 colonies); sham operations were performed in 4 colonies (2d, 29). Behavioral and reproductive responses were normal in sham-operated controls, but 11 of 13 sterilized colonies had no reproduction by beaver. The 14th colony dispersed before parturition could have occurred. Reproduction occurred in 1 of 10 colonies with ligated adults, 1 of 4 with castrated adults, and 4 of 4 with sham-operated adults. Sterilization of either adult in a colony significantly reduces fecundity, yet permits maintenance of an established colony with its associated benefits. Breeding inhibition is discussed with respect to population density, dispersal of 2-year-olds, and beaver management. J. WILDL. MANAGE. 44(3):568-575 The reestablishment of beaver over much of their ancestral range has precipitated an array of man-beaver conflicts that will intensify as beaver increase their range and numbers. Fecundity control through induced sterility has been advocated for a variety of vertebrate pest species (Davis 1961, Knipling and McGuire 1972), including beaver (Arner 1964). Although there have been several attempts to develop this technique for beaver control (Blanchard 1964, Harper 1968, Nevers 1968, Gordon and Arner 1976), results were inconclusive and feasibility was untested. Theoretically, beaver should respond well to reproductive control because they are long-lived (Larson 1967), are believed to be monogamous (Novakowski 1965, Boyce 1974), and form discrete family units consisting of an adult pair and progeny of 2 successive years (Novak 1977). Yearlings and old r progeny, though sexually mature, usually do not breed while in the parent colony unless 1 or both parents are removed (Payne 1975). Our study (Brooks 1977, Fleming 1977) was designed to evaluate colony response when 1 member of the breeding pair is sterile. Technical assistance of C. E. Boardman and W. E. Dodge; and cooperation of the Metropolitan District Commission, Boston, administrators of the study area, are appreciated.

On developpe et l'on teste deux modeles permettant de predire la densite maximum des colonies actives de Castor canadensis dans les cours d'eau (documentation portant sur 28 ans, dans le Massachusetts) et de classer les habitats

Feeding habits of wolves (Canis lupus) in central Ontario compared with results from the early 1960's indicated variation in use of prey species between nearby areas. A total of 1,943 scats was collected between May and September, 1963 to 1972, in 3 study areas in central Ontario. The occurrence of beaver (Castor canadensis) hair in scats increased and of deer (Odocoileus virginianus) hair in scats decreased as follows: Algonquin Park, beaver 7 to 55 percent, deer 76 to 33 percent; Pakesley, beaver 59 to 75 percent, deer 27 to 11 percent; Marten River, beaver 37 to 74 percent, deer 42 to 1 percent. Moose (Alces alces) hair in scats collected from all three areas occurred infrequently. Juvenile deer and moose generally were selected over adults. The change in foods of wolves apparently resulted from a decrease in availability of deer in all areas. A concurrent increase in beaver occurred on only one

An aerial cache survey was unreliable in indicating population size or trend of beaver (Castor canadensis) on 2 prairie rivers in Montana. Accuracy in locating caches was high (about 90%) and constant among years and areas, but colony size varied among areas and years. Data on age, sex, and reproduction are needed to determine colony size and properly evaluate aerial cache-survey results. J. WILDL. MANAGE. 47(3):697-703 Aerial censuses of beaver using sign observed have been described by Swank and Glover (1948), Fuller (1953), Townsend and Newby (1955), Hay (1958), and Payne (1981). Aerial surveys of caches (submerged piles of branches stored in fall for winter food) agreed well with ground surveys of active colonies in Colorado (Hay 1958) and in Newfoundland (Bergerud and Miller 1977, Payne 1981), where beaver built dams and lodges. The aerial cache-survey technique was inaccurate on streams in British Columbia (Slough and Sadleir 1977), but has not been evaluated on prairie rivers where beaver build dams rarely, live in bank dens, and often secure their caches near the bank. Even if aerial surveys accurately determine the number of colonies present, changes in colony counts may not necessarily reflect changes in population size, because colony size can change from year to year (Townsend 1953, Bergerud and Miller 1977). To properly evaluate the reliability of aerial cache surveys for estimating beaver populations, the accuracy of observing caches and the relationship between colony counts and population size must be examined. This paper evaluates the aerial beaver-cache survey on 2 prairie rivers in southeastern Montana and compares cache counts to population numbers. We thank the following trappers who provided jaws and reproductive tracts from beaver: D. Delcamp, R. Brauer, W. McConnachie, M. and M. Howry, and D. Schaff. We also thank D. Palmisciano for estimating the age of beaver jaws and H. E. Hodgdon, J. S. Larson, H. S. Hash, and R. P. Brooks for constructive reviews of the manuscript. This is a contribution from Montana Federal Aid to Wildlife Restoration Project W-130-R.

Impact energy values required to cause death of anesthetized furbearers by blows delivered to the neck or chest region were determined as an aid to the development of "humane" traps. Thresh- old values for neck blows were 575 cm kg for raccoors (Procyon lotor), 430 cm kg for mink (Mustela vison) 58-63 cm kg for muskrats (Ondatra zibetTicus), and 805 cm kg for beavers (Castor canadensis). The threshold values for chest blows were 1,150, 520, 155, and 780 cm kg for raccoons, mink, muskrats, and beavers, respectively. Autopsy indicated severe internal damage occurred in all species well below threshold levels. EEGs and ECGs were taken. ECGs were not good indicators of terminal damage for beavers and muskrats, because the heart continued to beat beyond 10 minutes for most animals that could be considered clinically dead. Pelt damage occurred in muskrats at energy levels only 30 percent above threshold. J. WILDL. MANAGE. 40(4):669-676 Controversy regarding the humaneness of current trapping methods for furbearing animals has prompted government agencies, trapper associations, and private agencies to seek alternatives to existing methodology. The Humane Trap Development Commit- tee (HTDC) of the Canadian Federation of Humane Societies, in 1970, undertook a pro- gram designed to "alleviate unnecessary suffering of furbearers." The program was to achieve this objective "through the de- velopment of modern and humane traps to serve as efficient and preferred replace- ments to present inhumane methods." One portion of this study concerned the mechanical aspects of various spring impact traps. Energy output, speed of closure, and clamping force values already have been established for a variety of traps (Tschoepe

Recent beaver (Castor canadensis) activity in a large-tooth aspen (Populus grandidentata) woodland had an indirect and undesirable influence on underplanted white pine (Pinus strobus). The pine plantings, made in 1955, had grown well in the partial shade and had been free from the white-pine weevil (Pissodes strobi), which had severely attacked white pine in an adjacent open-grown plantation. In the fall of 1962, beavers moved into the area, felling the aspen, and opening up the woodland. The ecological conditions created were ideal for the weevil, which appeared during 1963 and attacked 75 percent of the pine. Beavers were active during the fall of 1962 in certain areas of Isabella County, located in the center of Michigan's Lower Peninsula. One woodland they selected was composed predominantly of 30-year-old large-tooth aspen which had been under861 been carried by these beavers for 17-22 months. Four plastic-marked beavers have been recaptured alive on the beaver study area in Manitoba. These beavers had gone through a winter with their plastic markers, and seven of the eight markers were still firmly fixed to the metal ear tags. Plastic-marked beavers have been readily identified while they were swimming freely on the surface of the water, and they have also been easily identified from as much as 30 feet away while swimming below the surface in clear water. There have been hundreds of positive observations made of individually known beavers in Newfoundland and Manitoba. Many of these observations were made at active colony sites, but some were made of transients traveling outside their normal territory. Identification of free swimming, individually known beavers has resulted in the accumulation of numerous data that was not possible with other tagging methods. This same plastic-marking technique was used to tag marten (Martes americana) in Manitoba during 1961, but a smaller marker is recommended for this animal. The same technique could probably be employed to mark other mammals such as fishers (Martes pennanti), otters (Lutra canadensis), squirrels, and groundhogs (Marmota monax).

Information on the early history of the beaver (Castor canadensis missouriensis Bailey) in the Black Hills of South Dakota is meager, but it is well known that at the time of Custer's expedition in 1874 beaver dams were so numerous that the troops often had trouble in finding places to cross the streams. Old residents recall hearing their grandfathers or fathers tell of the numerous beaver and dams; nearly every stream had beaver. The remains of many of these dams can still be found in the valleys of the region. Ransom (1912)1 says, "By 1855 the fur trading days were about over in the Dakota field." The big rush of settlers into the Northern Black Hills did not occur until 1876 when gold, not fur, was the motivating force. As further evidence of the decline in

From 1951 to 1957, 2,200 beavers (Castor canadensis) were livetrapped in north and central Wisconsin, ear-tagged, and moved to new release sites. Recaptures totaled 472. Mean movement distances of recaptured beavers transplanted to streams were more than twice as long as for beavers transplanted to potholes and lakes. This relationship held true for both sexes. Landlocked waters are recommended as the best release sites for transplants. For several decades, Wisconsin statutes have authorized the Wisconsin Conservation Department to remove beavers that disrupt various human activities by flooding roads and farm fields, raising lake levels, or interfering with fisheries. State-employed trappers have livetrapped many such beavers and transplanted them to areas where further damage seemed unlikely. These animals have provided an opportunity to study several aspects of beaver life history, especially movement patterns. So many people were involved in this project that it is difficult to single out individuals to acknowledge. We are especially grateful to D. R. Thompson for data analyses, to the livetrappers and other personnel of the Wisconsin Conservation Department, and to the dozens of private trappers who gave us information on their trapping efforts.

This paper is an evaluation of numerous beaver-census techniques as they apply to the Rocky Mountain region. In designing plans for management of the beaver (Castor canadensis), primary consideration must be given to enumeration and appraisal of extensive populations. Reports on surveys of this nature (Atwater, 1940; Tappe, 1942; Bradt, 1947; Swank and Glover, 1948; Fuller, 1953; and Knudsen, 1951, 1954), varying widely in approach and results, have been relatively frequent. In these studies, the census techniques mentioned have dealt almost exclusively with the use of lodges, dams, slides, food caches, and scent mounds as indices of beaver numbers. Verification of the accuracy of such population measurements was a necessary requisite for sound management plans in mountain terrain.

The food habits of the lynx (Lynx canadensis) on the Island of Newfoundland were investigated during the period 1956-61. Results are based upon analyses of the contents of 206 digestive tracts, 116 identified lynx scats, and 104 presumed lynx scats. Snowshoe hares (Lepus americanus) occurred in 73 percent of the total tracts and scats in all seasons. Birds, especially smaller species, occurred in 21 percent of the material but were taken most in spring and summer. Mice occurred in 14 percent of the material during snow-free periods of years when they were abundant. Carrion resulting from big game hunting occurred in 20 percent of the tracts and scats, principally during fall and winter. Little evidence of predation upon domestic animals was found. Information on hunting habits of lynx, obtained from tracking studies, is presented. A large population of lynxes in western Newfoundland between 1953 and 1956 prompted the investigation of the status of the lynx. This report presents information on food habits, which was obtained between 1956 and 1961. The paucity of terrestrial vertebrate prey species on the Island of Newfoundland is worthy of note. Amphibians and reptiles are not native to the Island. The green frog (Rana clamitans) has been successfully introduced and is locally abundant in the southeastern part of the Island. The extant terrestrial mammalian fauna consists of the following: masked shrew (Sorex cinereus), little brown bat (Myotis lucifugus), brown bat (Myotis keenii), arctic hare (Lepus arcticus), snowshoe hare, beaver (Castor canadensis), meadow vole (Microtus pennsylvanicus), muskrat (Ondatra zibethicus), Norway rat (Rattus norvegicus), house mouse (Mus musculus), red fox (Vulpes fulva), black bear (Ursus americanus), marten (Martes americana), short-tailed weasel (Mustela erminea), mink (Mustela vison), otter (Lutra canadensis), lynx (Lynx canadensis), moose (Alces alces), woodland caribou (Rangifer caribou), and domestic dogs, cats, cattle, sheep, and goats. All are widely distributed and common, except the shrew, arctic hare, and the marten. The shrew was introduced recently near the village of St. Georges. The marten is restricted to timbered areas, and the animal and its sign are seldom seen. The arctic hare is restricted to barren areas and mountaintops not readily accessible to man. The arctic fox (Alopex lagopus) and the polar bear (Thalarctos maritimus) are occasional

Waterfowl utilization of 41 beaver ponds in south-central New BIunswick was studied during 1969. Ponds occupied by beaver (Castor canadensis) temporarily or during the entire field season, were defined as active. There were lS active and 26 inactive ponds. Black ducks (Anas rubripes) comprised 27 of the 47 broods observed. In addition> green-winged teal (Anss carolinensis), hooded merganser ( Lophodytes cocullz8;us ), ring-necked duck ( Aythys collsvis ), common goldeneye ( Bucephala clangula ), and mallard (Anas platyrhynchos) broods were seen. Thirty-eight broods were sighted on active ponds and only nine on inactive ponds Active ponds contained 0.75 brood per acre and inactive ponds 0.11 brood per acre. The seasonal drop of the water level was less in active ponds than in inactive ponds Hence, brood cover was more abundant in active pondsX and the active ponds were better brood-rearing areas. Few studies have been conducted to assess the significance of beaver ponds to waterfowl populations. Major contributions to waterfowl ecology on beaver ponds have been limited to studies conducted in the United States by Beard (1953)> Spencer ( 1957 ), Stanton ( 1965 ), and Nevers ( 1968 ) . Additional infoimation has been provided by Speake (1956), Lacaillade (1959), and Knudsen (1962). Although eastern Canada is the heart of the black duck's breeding range, the contribution of its beaver ponds as breeding habitat for the black duck has not been assessed. The lack of information on waterfowl use of beaver ponds, particularly use by the black duck in eastern Ganada, prompted the present study. Itis study, conducted in 1968 and 1969> was designed to assess the utilization7 by breeding waterfowl and by broods, of beaver ponds in the vicinity of FrederictonJ New Brunswick. A secondary objective was to investigate the physical characteristics influencing brood utilization of beaver ponds. Ihe author is indebted to B. S. Wrightr Dlrector, Northeastern Wildlife Station, for 1 The study was supported, in part, by a Wildlife Management Fellowship from Canadian Industries, Limited. 2 Present address: Fredericton,, New Brunswick, Canada. 740 advice and guidance throughout the study. I thank Dr. L. A. Dionne, University of New Brunswick, and Dr. Patricia RobertsPichette for identifying plant specimens; and J. Bidlake, New Brunswick Department of Natural Resources, for providing aerial photographs of the study area.

One hundred and ninety-two beavers (Castor canadensis) were livetrapped and tagged in order to determine the origin of troublesome beavers on private lands. Data from 87 retrapped animals showed that 19 had moved at least 1 mile in 7to 28-month periods and the 19 moves averaged 5.3 airline miles. An adult male moved 11.3 miles, the longest recorded. Of the limited sample, the yearling age-class, and the males of all age groups, showed the greatest migration tendencies. Evidence from one colony indicated that some beavers have an innate tendency to leave their home colony when about 2 years old, rather than being driven off by their parents. The common migration pattern was from high elevation public land to private holdings downstream where beaver structures caused flooding of crops, pastures, and roads, and the interruption of irrigation and stock-watering systems. Beavers perennially cause problems for ranchers, farmers, and highway crews in Bear Lake and Caribou counties in southeastern Idaho. Long trapping seasons and vigilant conservation officers assure that the majority of beavers in complaint areas are annually eliminated. However, beavers soon move in to fill the vacancies. In 1962 a study was initiated to determine the productivity and origin of these migrants (Leege 1964). The productivity data have already been reported (Leege and Williams 1967). Thanks are extended to Dr. Paul D. Dalke, Idaho Cooperative Wildlife Research Unit, and Roger M. Williams, Idaho Fish and Game Department, for their counsel throughout the study and for reviewing this manuscript. John Kindall assisted with the livetrapping and provided most of the tag returns through his fur trapping activities.

The writer believes the observations here recorded will be of interest to those whose knowledge of the beaver rests upon familiarity with the animal in its more northern haunts. There aspen is by far the most important food species, followed by birch, alder, wild cherry and willow, and occasionally oak and a few other species. The population of the native beaver (Castor canadensis missouriensis Bailey) by 1927 had become so depleted that the animal was thought by many to be extinct. Bennitt and Nagel (1), however, reported a few colonies of native beaver surviving in northwestern Missouri as late as 1934. In 1928 two pairs of Michigan beavers, obtained by the Game and Fish Department, were released in Indian Trail State Park on a tributary of the Meramec River near its headwaters in the Ozark portion of the State. Soon afterward beaver cuttings were found along the deeper water behind an old masonry dam in the Meramec River a few miles below the point of release. In 1929 four additional pairs of Michigan beavers were obtained and penned in a spring branch in Indian Trail Park from which they escaped downstream to the Meramec River. Shortly afterward a small colony was reported also about four miles upstream from the original colony at a point in the river known as Dent's Ford.

Field trials in central Alabama demonstrated that beaver (Castor canadensis) ponds could be economically managed for high yields of Japanese millet (Echinochloa crusgalli var. frumentacea. The methods used over a 3-year period, beginning in 1958, for 30 field trials consisted of: (1) breaking beaver dams late in spring or early summer, sowing Japanese millet, and maintaining drainage until mil- let matured, by repeated breaking if needed; and (2) breaking beaver dams, using log drains to maintain drainage, sowing Japanese millet, and removing drains after millet matured. Log drains proved to be the more economical method, $4.15 per acre. Yields of Japanese millet seed were 6 to 10 times those of the native plants of value for duck food that became established in drained beaver ponds. Landowners had no difficulty in leasing duck-shooting rights on the managed beaver ponds. The low cost per acre of establishing duck food plants indicates that this type of development is practical for beaver ponds in central Alabama.

Data on the productivity of beavers (Castor canadensis), including sex and age ratios, litter size, and rate of pregnancy, were gathered from livetrapping and fur-trapping operations in 1953-56 and 1962-64. Standard methods were used for determining sex and age of both live beavers and carcasses. Uteri hardened and discolored by preservatives were cut longitudinally to expose breaks on the internal ridge of tissue which pointed out sites of placentation. Counts of placental scars and fetuses indicated average litter size to be 3.1 and 3.4, respectively. The percentage of yearling and older females with placental scars dropped from 46.2 in the mid-1950's to 32.3 in the 1960's in southeastern Idaho, possibly owing to increased disturbance of colonies from accelerated fur trapping. Males consistently outnumbered females in kit and yearling age-classes while females were more abundant among the adults. The sex ratio of 352 beavers examined was 113 males per 100 females. Twenty-one percent of these animals were kits, 23 percent yearlings, and 56 percent adults. A disturbed (trapped) beaver population had a lower percentage of kit and yearling beavers than did an undisturbed population (35 percent vs. 49 percent). A highly migratory population consisted of 38 percent late yearlings, supporting the belief that beavers leave their home colony when almost 2 years old. This paper summarizes data on beaver productivity collected in Idaho in two separate periods, 1953-56 and 1962-64, and adds comparable data to the beaver literature from many other states. A method is also described for delineating placental scars on uteri that have been hardened and discolored by preservative solutions. Gratitude is expressed to the many beaver trappers who collected uteri and counted fetuses and to conservation officers and other personnel of the Idaho Fish and Game Department who distributed preservatives and solicited trappers' cooperation. Thanks are extended to Dr. Paul D. Dalke for his counsel during the portion of the work done by the senior author and for reviewing this manuscript. Dr. Edson Fichter and Levi L. Mohler also reviewed the manuscript and contributed editorial suggestions. Statistical assistance was provided by Drs. Robert Giles and Dale Everson. MATERIALS AND METHODS Collection of Materials and Data State trappers collected uteri of beavers during the fall trapping seasons of 1953 through 1956. They counted fetuses during the spring trapping season in 1956. The materials and data were collected throughout the state and sent to the junior author for analysis. During the 1962-63 fall through spring trapping season, two beaver trappers in southeastern Idaho, John Kindall and Warren Moore, collected uteri and counted fetuses preliminary to a study of productivity and movements by the senior author (Leege 1964). In the 1963-64 season, additional reproductive tracts, fetus counts, and data on sex and age ratios were gathered by Leege with the assistance of these two trappers. Beavers were livetrapped during the summer months in 1962 and 1963 in portions of Bear Lake and Caribou counties in southeastern Idaho. One hundred ninety1Contribution from the Idaho Cooperative Wildlife Research Unit; The College of Forestry, Wildlife and Range Sciences, University of Idaho; the Idaho Fish and Game Department; the Wildlife Management Institute; and the U. S. Fish and Wildlife Service, cooperating; and Idaho Fish and Game Department P.-R. Project W-108-R. 2 Present address: Idaho Fish and Game De