Mammals and Humans: Field Techniques for Studying Mammals
Mammals and humans: Field techniques for studying mammals
There are two general reasons for studying mammals in the field. The first is to provide numbers that are needed for biodiversity measures or population management; the second is to provide natural history information that is needed to better understand species' requirements or their roles within the natural community. This chapter provides broad guidelines that would assist someone in selecting appropriate field techniques. References provided at the end of the chapter provide more details on both study design and specific techniques.
Biodiversity measures are based on the ability to accurately count the number of species within a given area and usually some measure of their relative or absolute abundance. Population management of both common and rare species relies on accurate measures of population numbers or at least a way to measure population trends. Most mammal populations or communities are too complex for every individual to be counted, therefore a sample is often taken of the population and the number is estimated based on that sample. Unfortunately, obtaining these estimates is not an easy task. More than other vertebrate groups, mammals occupy a wide array of habitats and possess a broad range of body sizes. These factors make them difficult to survey as a group, and survey techniques have to be tailored for a specific species, or suite of species. Planning a biodiversity survey is a two-step process: the first step is to determine the level of information needed to meet objectives, and the second step is to tailor a survey to fit the attributes of the species.
Three levels of information that can be obtained from a survey include a species list, a relative index of abundance for each species, and an absolute density for each species. Generally, there is increasing cost and complexity as the level of information increases. For some mammal species, it is prohibitively expensive to estimate absolute density because of the habits of the mammal or the habitats it occupies. Solitary bats, which live in trees under strips of bark or in crevices, are a good example of a suite of species whose density estimate is logistically difficult to obtain. When planning a survey, the first consideration should be how necessary the increased information is to the management or research objectives. The initial survey of a park would not start with a density estimate of each mammal species, but rather a list of species found in the park. Often a mammal's relative density is adequate information to track changes in abundance within a park, and the saved money can be used for other conservation tasks such as patrolling. Within broad conservation plans for an area, mammal surveys should reflect a nested subset design. For example, following a complete species list for the area, some species from this list are monitored through an abundance index, and select animals from this group are targeted for detailed population and ecology studies that might include a density estimate.
If field technicians are working with a rare species or a harvested species, they might not be concerned with the higher levels of organization and start with a focal study on the target species. However, even under these circumstances, an index survey over a larger area may be more appropriate than a density estimate at one site. Project goals and financial logistics usually produce a compromise in how much information can be gathered. It is important the data collected are not stretched beyond their purpose, when compromises are made. Unfortunately, the scientific literature is full of indexes used to calculate densities and species lists used as indexes. Surveys are powerful tools in wildlife conservation and management, but when stretched beyond their ability they convey more confidence in the trends then the data warrant.
Species lists can be as complex as a complete mammal survey or as simple as presence/absence of a focal species. Species lists are composed through use of multiple means. Traditional field surveys that use traps, cameras, or transects can be supplemented by sociological techniques such as examination of local markets, discussions with local hunters, and inspecting kitchen remains in villages. When using traps or cameras, they are usually placed in a line to traverse as many habitat types as possible. The distance between the traps would be less than the smallest home range of the animal targeted. The larger the home range, the longer the traps should remain in one spot in order to account for the time it takes an animal to traverse its entire home range. For a rodent, traps might be in place for a few days; for a large predator, a few weeks would be more appropriate.
Sociological techniques offer their own challenges, as the barriers to an effective survey tend to be cultural rather than technological or biological. For example, who is interviewed, who does the interview, and even what dialect is used for the interview all affect the survey results. There are strong cultural pressures on the interview process that are difficult to comprehend or anticipate without extensive experience in the target community. Interview methods are effective for relatively large species that are regularly encountered or eaten by local people. Paying a bounty for local hunters to produce specific animals is tempting, but not recommended without consideration of the long-term impact of creating a market for wildlife.
Creating a species list is an open-ended activity; the longer researchers look, the more they find. A species accumulation curve, which graphs the number of new species detected for every additional unit of time or effort, can be used to gauge when a species list is complete. It should be noted that the method used to measure the number of species may only provide the number of species that can be detected.
Beyond the need to do a thorough survey, the most difficult component of a species list is comparability. If lists are to be compared between areas or to lists collected previously, there must be a way to measure the amount of effort used in the survey. Two biologists will not conduct surveys exactly the same way, and differences in lists are always subject to individual protocols.
Population indexes are usually species-specific and are based on the number of animals, or their sign, detected for each unit of effort. The effort can be measured in many ways, including number of miles/kilometers walked, number of traps set, number of trees examined, or number of hours observed. The index itself is usually tied to specific traits of the species: for example, claw marks on trees made by Asiatic black bears (Ursus thibetanus), calls by howler monkeys (Alouatta spp.), night nests of gorillas (Gorilla spp.), and latrines of rhinos are indexes that are not broadly applicable to other species. Indexes based on sign are conducted along transects of known length, with all sign within a set distance of transect recorded, or a predetermined area is searched for sign. When interpreting signs, one must have observed how such signs are actually created by the animal, as well as which activities do not leave any signs at all (i.e., a deer feeding on dead leaves shed by trees or small bits and pieces of conifer branches or lichen or small fruit leaves no trace at all). When the signs are not permanent, such as fecal pellets or tracks, and the area is to be resurveyed at a later date, the boundaries of the area can be marked and all sign cleared from the area at the end of each survey. When traps or cameras are used during a survey, the measure of effort is usually expressed as trap-nights (or camera-nights). This is the number of traps (or cameras) set each night multiplied by the number of nights. Using this criterion, a small number of traps used for a long period would be roughly equivalent to the effort of a large number of traps used for a short period.
It is the rare index for which the number of signs can be directly converted into the number of animals. This is because there are assumptions that must be used for each conversion. For instance, to convert the number of pellet groups detected to the number of deer, there must be estimates of how many pellet groups each deer produces daily and how rapidly the pellet groups degrade. Both these measures have a variance that is so large any resulting density estimate is meaningless. For most indexes, it is also difficult to determine if increased signs indicate increased numbers or increased activity. For example, would the detection of six pellet groups mean six deer used the area once, or that one deer used the area six times?
As it is tailored to a specific species, a good index can be quantified and it is likely that two biologists can compare results. The power of an index is to give relative comparisons between sites or periods for a minimal amount of work. Some effort must be made to verify that the index used reflects changes in density over the range measured. There is also the danger that an increased number of sign does not reflect more animals but rather shifts in habits such as diet or habitat. One must question whether seasonal increases in deer pellet groups in an old field represent an increase in the number of deer or a shift in habitat use by the same number of deer. If all habitats are being monitored simultaneously, it is possible to differentiate between shifts in habitat use and shifts in abundance.
No index can work under all circumstances, so a pilot study that measures both density and the index is preferable to making assumptions of correlation. Few indexes have a linear relationship with density over its entire range, as usually an index flattens out as density increases beyond a certain range. For example, an increase in the number of subadult or nonreproductive individuals may not be reflected in an index based on the number of morning calls by adult howler monkeys. It is important for the research or monitoring to demonstrate that the index is responsive to changes in density over the range. For many species, verification of an index has already been accomplished and a review of relevant literature
is recommended before undertaking an index survey. In short, good preparation by observing animals is required before collecting data and making inferences from them.
Density estimates have two components, and both cause difficulties to biologists: the amount of area surveyed and the number of animals. As opposed to an index, the density estimate relies on a measure of area surveyed and not on effort. If the survey area is a true island, then the measurement is straightforward. If the survey area has an arbitrary boundary between "inside" and "outside," assumptions have to be made as to how the animals move with respect to the boundary. If the area surveyed is a mosaic of favorable and unfavorable habitat for a species, sampling protocol must take this into account. Placing survey lines to estimate the number of animals within the favorable area while using both favorable and unfavorable habitat to estimate the survey area will overestimate the number of animals. Unfavorable habitat may not contain a large number of animals during the survey, but may be used at other times of the year or when environmental conditions change.
It is difficult to estimate density from a line of traps, as it is difficult to estimate the distance that animals are drawn into the traps. Removal trapping suffers from this handicap, as removal of animals creates a vacuum that will eventually be filled by immigrating animals. Most studies that rely solely on trapping to estimate density construct trapping grids or webs to estimate the size of the survey area. Without prior knowledge of an animal's home range size and movements, it is difficult to accurately calculate the area sampled using most capture techniques.
The second component of the density estimate is the number of animals. To obtain a density estimate, a species usually needs to be observable or caught on a regular basis. It also helps if natural or added markings allow individuals to be identified. The two main techniques to estimate absolute density without changing the density are either mark/recapture or distance sampling. Mark/recapture uses an initial capture period that results in a known number of marked animals. A second capture period then looks at the ratio of marked to unmarked animals and estimates how many animals the population contains. Mark/recapture is based on the two assumptions that all animals can be captured and capture does not influence the probability of future captures and that animals can be marked and the marks will not fall off or influence the probability of the animal to be recaptured. There is also an assumption that no mortality, natality, or immigration occurs between capture sessions, but it is possible to relax this assumption and still estimate density.
For species that are readily captured, such as terrestrial rodents, it is a matter of trapping the animals in live-traps on a regular basis and estimating density. Commercial traps are available and techniques are well developed for small mammals. With all trapping, it is important to minimize trauma
to the animal not just because it is humane but also so that the initial capture does not influence the probability of recapturing the animal. When the intent is to recapture the animal, some period of pre-baiting (with the trap baited but locked open) will increase the probability of capture. By providing adequate food and bedding and checking traps regularly, most small mammals can be trapped repeatedly and their density estimated. If the first priority is to ensure that all species have been detected, then using a variety of traps, both live and kill, would result in a better survey.
There is no terrestrial mammal that cannot be captured and released in a humane manner. However, for many animals such as large predators, the first trapping is traumatic enough to preclude the animal being recaptured by the same method. This does not necessarily preclude density estimates, as there is no assumption in the capture/recapture model that the capture method is the same for each period. The animals can be captured with snares, affixed with a mark, and then recaptured by cameras at bait stations. The first capture can be as a neonatal in the nest, and the recapture in a trap. For instance, an animal's DNA can be obtained in a blood sample attained at the initial capture in a snare, and the DNA later recaptured in a hair sample snagged on barbed wire strung around a bait station. If the animal has unique markings, as do many large cats, both the initial capture and subsequent recaptures can be with trip-cameras.
Removal sampling allows the observer to estimate a population's density post-hoc. The number of animals removed for each unit of effort, or a change in the ratio of two classes of animals (i.e., antlered and antlerless deer), is the basis for most surveys of harvested large mammals. For both calculations, the advantage is that the researcher is not necessarily the one doing the removal. Large areas and large populations can be estimated using volunteer hunters that agree to follow simple rules such as harvesting only antlered animals. The disadvantage is that there are limited species that attract a large number of volunteer hunters and the removal is limited to populations that are robust enough to sustain a harvest. It is possible to use the catch-per-unit-effort technique without harvesting animals, but the observer must be able to identify animals that have been previously sighted. There are several statistical programs that are commonly used to analyze this type of data and details of the programs and their assumptions are available at <http://www.cnr.colostate.edu/~gwhite/software.html>.
For animals that are readily observable, or at least easier to observe than catch, strip transects or distance sampling techniques are used. In a strip sample, one counts all animals along a strip of known length and width. An assumption of this method is that all animals within the strip are counted. For most species, if the strip is wide enough to encounter a significant number of animals it is also wide enough to miss animals at the boundaries of the strip. There are ways to estimate the number of animals missed, such as double counting, but they do not solve the problem. Aerial surveys often involve strip samples where all animals within a strip along a side of the plane are counted by observers. These surveys are diurnal and usually occur in grassland or open habitats. Thermal imaging allows the heat signature of animals to be used to count animals at night. Use of thermal imaging for
density estimates is problematic because abiotic objects can radiate heat and be misidentified as animals. Also, canopy closure in forest settings is often dense enough to block the sensor's view of animals.
Distance sampling takes into account the assumption that animals can be missed along the transect, and the farther the animal is from the transect the more likely it is to be missed. If the researchers know how far they surveyed, the number of animals observed, and their perpendicular distance to the transect line, they can estimate the area that they surveyed and the number of animals that they did not see within that area. The number of animals seen plus the number of animals missed per unit area is a density estimate. The assumptions are that all animals along the line are observed; animals are randomly distributed relative to the line; and no animals are counted twice along the same line. This technique was originally developed to survey marine mammals, but has quickly been adapted for many large terrestrial mammals. There are many nuances to distance sampling, and it is difficult to cover them all in this space, but additional information is available <http://www.ruwpa.st-and.ac.uk/distance>.
Natural history information
Before a species can be managed, it must be understood. A species has requirements for food, shelter, and habitat that directly impact management decisions. Understanding social structure, interspecific competition, predator pressures, movements, disease transmission, mating, and behavior help humans realize the impact of their actions on mammals. Most, though not all, of these factors cannot be determined from laboratory studies of captive individuals. Effective means for studying natural populations is a key ingredient to effective management of wild mammals.
The most straightforward means to derive natural history information is direct observation. Its value should not be underestimated, as direct observation is often the most effective way to place the trait in context of the animal's physical and social environment. The observer might be able to learn more about a species from a few hours of direct observation, than from a year of examining trip-camera photos or radio telemetry locations.
When designing a direct observation study, all terms must be understood and quantified, especially when more then one observer is used. The term "feeding" is readily understood at a basic level, but there are often many types and gradations of feeding in a natural setting. As with village interviews used to create a species list, the attitude and background of the observer sometimes influences how a behavior is recorded. An observer has to guard against anthropomorphic biases and also against interpreting events through preconceived theories. For example, which animal is considered dominant during an interaction should not be a qualitative measure, but based on quantifiable criteria.
As with indexes, direct observations often have a unit of effort. How many attempted matings, bark strippings, or social
grooming events observed is always a function of how much time was spent observing the animal. This is complicated when animals are part of a social group. Time spent monitoring the entire group is not the same as time spent observing one individual in the group. Usually too many activities are being conducted simultaneously to watch an entire group and researchers identify focal individuals that are monitored for a set period of time. This set period of individual observation might be punctuated by a sample of the behavior of each group member, referred to as a scan sample.
Direct observations are usually inexpensive; a good pair of binoculars or spotting scope and a watch are the main expenses. However, time is usually the limiting factor with direct observations. Many hours can be spent to obtain a few minutes of direct observation. With more cryptic or more diffuse animals, there is a point at which the observations are not worth the time spent to obtain them. Video cameras and recording equipment can be used to continuously monitor a location, with the tapes reviewed by the researcher at a later date; but
this would only be effective at feeding or watering sites that attract animals. In addition to time considerations, there is also the issue of how the observer's presence impacts the animal's behavior and movements. If the behavior being recorded is the result of the animal's movements away from the observer, or toward a concentrated food site, then the value of the observation is reduced. A period of habituation of an individual or group to the observer is standard practice. But this needs to be considered with some care. For instance, too much familiarity can trigger attacks, or habituated animals may be vulnerable after the study is terminated.
Many behaviors can be indirectly inferred from sign indexes and density estimates. Habitat selection is often measured through indexes that compare an animal's use between habitats or seasons. When indexes are used as a surrogate for direct observations, the closer the index is to the target behavior the less chance of error. For example, a browse index based on the number of buds clipped per tree is directly related to ungulate feeding, while pellet or track counts are more indirect indexes.
Radio telemetry, a means of indirect observation, is the most important advance in the last 50 years for the study of wild mammals. Before radio telemetry, researchers were limited by their ability to follow animals or detect their presence. Indirect indexes of activity and movement, such as sign counts, were the best means to measure habitat selection. Behavioral observations were often limited to sites near feeding or watering holes where animals could be observed from established blinds; when an animal disappeared it was often impossible to know if it had died, dispersed, or merely stopped coming to the observation site. Radio telemetry allows a researcher to locate a specific animal when it needs to be recaptured, observed, or its movements monitored. Radio telemetry allows a researcher to remotely track a specific animal's movements and survival with a minimal disturbance to its behavior. These abilities have opened observations into animal ecology that were unavailable to earlier workers.
Two components of radio telemetry are the receiver and transmitter. The receiver can pick up a range of frequencies and can be set to detect each unique transmitter. Traditionally, the animal carries the transmitter and the receiver is either a hand-held device or an orbiting satellite. The hand-held receiver can be moved on foot, by car, boat, or plane. To locate the animal, it can be approached directly, triangulated from a number of bearings from known locations, or by using the principles of the Doppler effect in the case of the satellite. More recent global positioning system (GPS) collars have the animal fitted with the receiver and the transmitters are aboard orbiting satellites. The receiver calculates its position based on the known position of the satellites and the time it takes a signal to travel between the satellite and receiver. The optimal arrangement is a package that contains a combination of units, so the animal's position can be determined through multiple means.
Traditional tradeoffs in radio telemetry are between power output and battery weight. Attempts to increase the range or duration of the signal are matched by increases in unit weight. Combining types of radio telemetry units into one collar also increases package weight. A general rule is that a package weight's should be less than 5% of the animal's body weight, but there are enough exceptions to this rule to warrant a pilot study before attaching packages to wild animals. Advances in battery technology and computer software have made weight considerations less important, especially for larger mammals. When the battery power cannot be sustained for the length of the project, microchips can regulate when the unit is active. Weight limitations are still serious concerns for species weighing less than 2.2 lb (1 kg).
A limitation of radio telemetry is that the observer must capture the animal to attach the telemetry unit. Most animals can be captured, but the time and labor involved can consume a large part of a project's budget. Once a telemetry unit is attached, the logistics of recovery are simpler. The telemetry unit can lead to the animal for application of anesthesia, or a remotely triggered tranquilizer dart can be inserted in the collar. When the study has terminated, the unit can be released,
either by providing a weak link or a remote-release magnetic mechanism in the collar.
Once locations have been collected for an animal, it is possible to determine its home range, habitat use, and multiple other natural history parameters. An additional benefit of radio-collared animals is the ability to construct life history tables. It is difficult to determine if wild animals have died or migrated when they stop being detected by other means. However, which fate occurred is very important for most modeling of animal populations. With radio-collared animals, it is possible to differentiate between mortality and migration, and to determine the timing and cause of mortality.
Of the two considerations when designing a field study, the level of information needed and the limitations imposed by the animal itself, it is the latter that usually dictates what is possible. When deciding the proper field technique, consideration has to be given to the size of the animal, its niche (i.e., arboreal, volant, terrestrial, subterranean, aquatic), and its behavior (i.e., cryptic, nocturnal, social, vocal). General guidelines can be given to the limitations imposed by each type of mammal.
Large terrestrial mammals
Visible, large mammals can have density estimates derived from distance sampling techniques. When large mammals are not easily visible due to dense habitat, nocturnal activity, or low density, they all leave sign that is observable and can be used as an index. Bears leave claw marks in trees, gorillas create nightly nests, elephants deposit conspicuous dung, and ungulates leave tracks in most soils. Removal techniques are suitable for large mammals that are harvested. Large animals are also most easily captured with trip-cameras. Indeed, camera surveys of large predators are the norm in most forest habitats. It is difficult to convert these camera indexes into density estimates, with the exception of animals with unique markings that are surveyed using trip-cameras. Most large mammals have large home ranges, which make it difficult to estimate the area being sampled by any technique. Direct observations are most frequently used with large mammals, and telemetry units can be large enough to contain most features needed. A limitation might be that the capture of the animal for attachment of telemetry unit will take special skills and equipment.
All volant mammals are relatively small and most are nocturnal. The most readily surveyed are the communal species that can be found in caves for all or part of their annual cycle. Bats can be counted in the roost, or individuals captured with nets, or counted visually when the bats enter or exit the cave. For bats with solitary roosts or to sample foraging sites of communal species, nets that span natural passageways or watering holes are the norm. However, it is difficult to erect mist-nets and harp-nets to match the space being used by the
bats, particularly bats that forage above the canopy or within complex foliage.
Direct observation is usually not possible, especially during foraging. An exception is small light tags that can be placed on bats to observe short-distance foraging. It is possible to observe maternal behavior in communal nesting species either directly or with video equipment. Radio telemetry units are just reaching the size where movements can be monitored for longer than a day, but weight is still a serious concern.
A technique with potential is using the bats' ultrasonic call to identify species and possibly individuals. Devices can record and catalog the calls to a computer where a number of call parameters can be measured. At this time, there are quantifiable means to differentiate some, but not all bat species based on call parameters. Usually the calls can be used to predict which suite of species generated the call, but not the exact species. There is individual variability in the calls and it may be possible in the future to capture calls, as one would pictures of animals with unique markings. At this time, not enough call parameters can be measured to recommend use.
All are relatively large and most are cryptic, in that a good portion of their time is spent below the surface of the water. Most marine mammals are too large for extensive trapping and are best surveyed with distance sampling, though trapping can be conducted on some dolphin, manatee, or otter species that inhabit coastal waters. Direct observations are limited to the animal's time above the surface of the water or using scuba equipment. Radio telemetry can provide important movement data, but difficulties include the loss of signals when the animal is below the surface of the water. Sonar can be used to track movements and sounds of larger whales.
Freshwater mammals are generally smaller and more readily trapped. These mammals also usually spend some portion of their time on land and leave obvious sign such as gnawing on trees or construction of homes, nests, and dams. Radio telemetry and direct observations are more effective with this group because of their time out of water. Small semiaquatic insectivores are cryptic, difficult to capture, and leave no observable signs. With this subgroup, neither direct observations or radio telemetry have proven effective. They can best be sampled with pitfalls or some form of kill trap.
Small terrestrial mammals
Most of these species are too cryptic to be observed reliably, and live or kill trapping are the most common survey means. Traps can be obtained from several commercial sources and can be scaled to the size of the animal. Food baits are used to lure animals into traps. If done properly, animals can be captured repeatedly and all segments of the population can be sampled. Direct observation has not been effective, with the exception of those species at the larger end of the range, such as ground squirrels. Radio telemetry units are small enough for most species, but the range of the small units is below the dispersal distance of most rodents and limits their utility in some studies. For species less than 0.4 oz (12 g), most commercial traps are ineffective and radio transmitters are short-lasting due to weight considerations. Pitfall arrays are often used to record a species presence; pitfalls are buckets that are either deep enough that the animal cannot jump out or are partially filled with liquid to kill the animal. Animals are not lured into the buckets with bait, but rather barriers funnel all passing mammals into the buckets.
As with terrestrial small mammals, subterranean mammals are cryptic, but unlike terrestrial small mammals they are usually difficult to capture alive. Commercial killing traps can be obtained and modified to capture most species. Evidence of digging activity can be used to indicate a species' presence, but these signs of activity are not often correlated with density and are a poor index. Some subterranean mammals have a portion of the year, or day, that they spend aboveground and this is the period during which they are most easily captured and surveyed. Direct observations and radio telemetry are ineffective because the soil blocks both sight and radio signal transmission.
Most can be captured in traps, but only a few can be captured repeatedly, either due to large home range or behavioral wariness. Traps are either large holding traps or snares and leg-holds. Wariness prevents some species from readily entering box traps. With proper training, snares with springs and leg-holds with padding can humanely capture animals for release. When animals are not captured, most are large enough to be readily detected through tracks, sign, or cameras. For predators, animals can be attracted to specific sites with food baits or scent lures, which, derived from glands or urine, are commercially available and can be used to bring animals over a site prepared for tracks or monitored with a trip-camera. These lures were originally developed for snare or leg-hold trapping and are still effectively used for that purpose. Radio telemetry is used extensively with this group; the unit is attached during the first capture.
It is difficult to capture arboreal mammals because of the logistical complications in working in the canopy. As with subterranean mammals, if arboreal species have a period of the day or year that they use the ground or come close to the ground, they can most readily be sampled at that time. Sometimes species can be anesthetized with drugs delivered in a dart, but still there is the problem of the animal falling from the canopy when the drug takes affect. Diurnal species can be surveyed with distance sampling, or mark/recapture techniques when there are unique markings. Nocturnal species are more difficult to survey visually because the darkness increases their cryptic ability and there is difficulty in estimating distance; indexes can be derived from calls and visual detections along transects. For nocturnal species, use of a spotlight to detect "eye-shine" makes transect surveys feasible. Many species do have unique calls that can be used for index surveys. Radio telemetry can be used if the animal can be captured. For animals that forage high in the canopy, the utility of radio telemetry is diminished because it is difficult to estimate the third dimension in an animal's space use.
Clearly, one technique cannot be used to study all mammals. A study that attempts to record the density and behavior of all mammal species within an area has a daunting task that will take time and money to accomplish. A population study for a single species or suite of species must be tailored to match the habits and attributes of the animal. An advantage to mammal studies is that there is a rich literature of mammal field research conducted over the past 100 years that can provide guidelines. With increasing human densities, the affairs of mammals and humans can no longer be separated, and understanding wild mammals is the first step to ensuring their survival.
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"Mammals and Humans: Field Techniques for Studying Mammals." Grzimek's Animal Life Encyclopedia. . Encyclopedia.com. 18 Apr. 2019 <https://www.encyclopedia.com>.
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