Population Densities Of Small Mammals Research Articles

Assessment of the Current State of the Hissar High-Mountain Natural Plague Focus in the Republic of Tajikistan

The review is devoted to discussing the results of survey in the Hissar high-mountain natural plague focus on the territory of the Republic of Tajikistan in 2015–2023. Regular examination of the focus was carried out in 1970–1991. According to its results, the main carrier of the plague pathogen was the juniper vole, in the population of which a strain of the non-main subspecies central asiatica (0.PE4) of Hissar biovar (0.PE4h) was steadily circulating, the strain considered avirulent for humans. During that period, 853 strains were isolated in the focus, of which 799 (93.7%) were from the juniper vole and its fleas. Secondary carriers – the silver vole, the pygmy wood mouse, and the gray hamster – were rarely involved in epizootics. The red marmot, which has epidemiological significance due to its hunting by the local population, occupies a special position in the focus. Socio-political phenomena at the end of the 20th – early 21st century caused a long break in the work of the Tajik Plague Control Station. In 2008, the Station was transformed into the Republican Center for Combating Quarantine Diseases under the Ministry of Health and Social Protection of the Population of the Republic of Tajikistan. In 2015, studies were resumed, but carried out on a small scale. In 2021–2023, epizootiological monitoring in the Hissar focus was performed jointly with Russian specialists with the testing of field material samples for plague and other natural-focal infections. Currently, against the background of low numbers and population density of small mammals and their blood-sucking ectoparasites – potential carriers and vectors of zoonoses – the circulation of agents of plague, tularemia, pseudotuberculosis, tick-borne viral encephalitis, Ixodidae tick-borne borreliosis, human granulocytic anaplasmosis, human monocytic ehrlichiosis has not been registered. Markers (DNA) of causative agents of leptospirosis and intestinal yersiniosis have been identified. Joint Tajik-Russian cooperation on issues of ensuring the epidemiological well-being of the population of the Republic of Tajikistan must be continued.

Community reestablishment and poor body conditions of small mammal assemblages in subtropical afforested ecosystems

Afforestation is one of the high-efficiency practices for the restoration of degraded forest ecosystems and has been increasingly used worldwide. It is crucial to determine the restoration status of wildlife in afforested forests to effectively monitor biodiversity conservation and the self-sustainability of forest ecosystems, as well as to effectively guide the implementation of afforestation projects in the future. China has the largest area of man-made forests around the world, but the effects of the restoration efforts on the faunal diversity in artificial woodlands are unclear. Here, we evaluated the effects of afforestation practices on small mammals in two types of planted forests: Japanese larch (Larix kaempferi, an introduced species) and China Armand pine (Pinus armandii, a native species) plantations, and two reference forests: young- and old-growth forests, at the community, population, and individual levels. The diversity of small mammals did not differ significantly among the four forest types and the population densities of small mammals in two plantations were between the young- and old-growth forests. Collectively, our findings indicated that small mammal assemblages had reestablished in afforested forests. Four body condition indices were measured in the Chinese white-bellied rat (Niviventer confucianus), a predominant species, and differences were found between afforested and reference forests. Chinese white-bellied rats had the lowest body masses and highest endoparasite loads in the Japanese larch planted forest, while body length or carcass mass did not differ among the four forest types. In comparison, study rats had slightly lower body masses and lower endoparasite loads in the China Armand pine planted forest than reference forests. These findings indicated that small mammals inhabiting both types of afforested forests had poor body conditions but these trends were worse in the Japanese larch planted forest, i.e., the introduced species. Given that afforestation is a widespread practice for global forest landscape restoration, we propose two suggestions to guide afforestation efforts and the subsequent management thereof: (1) native species rather than exotic species are preferable for use in afforestation projects, and (2) wildlife diversity in afforested forests should be considered when monitoring the restoration and management of degraded forest ecosystems.

Combining genetic non-invasive sampling with spatially explicit capture-recapture models for density estimation of a patchily distributed small mammal

Estimating the size of animal populations is essential for understanding the demography and conservation status of species. Genetic Non-Invasive Sampling (gNIS) combined with Spatially Explicit Capture-Recapture (SECR) modelling may provide a practical tool to obtain such estimates. Here, we evaluate for the first time the potential and limitations of this approach to estimate population densities for small mammals inhabiting patchily distributed habitats, focusing on the endemic Iberian Cabrera vole (Microtus cabrerae). Using 11 highly polymorphic microsatellites and two sex-linked introns, we compared population estimates in November/December 2011 based on live-trapping and gNIS and assessed the impact of distinct consensus criteria to differentiate unique genotypes. Live-trapping over 21 days captured 31 individuals, while gNIS over 5 days recorded 65–69 individuals. SECR models indicated that individual detectability was positively affected by live-trapping capture success on the previous occasion, while for gNIS, it was mainly affected by genotyping success rates and patch size. Live-trapping produced the lowest density estimates (mean ± SE) of 16.6 ± 3.2 individuals per hectare of suitable habitat (ind/ha). Estimates based on gNIS were higher and varied slightly between 25.2 ± 4.0 and 28.8 ± 4.5 ind/ha depending on assuming one or two genotyping errors, respectively, when differentiating individual genetic profiles. Results suggest that live-trapping underestimated the vole population, while the larger number of individuals detected through gNIS allowed better estimates with lower field effort. Overall, we suggest that gNIS combined with SECR models provides an effective tool to estimate small mammal population densities in fragmented habitats.

Evaluating camera traps as an alternative to live trapping for estimating the density of snowshoe hares (Lepus americanus) and red squirrels (Tamiasciurus hudsonicus)

Live trapping is one of the methods typically used to estimate population densities of small mammals, but this is labor-intensive and can be stressful to individuals. We assess the use of camera trap hit (detection) rates as a noninvasive alternative to live trapping for estimating population densities of snowshoe hares (Lepus americanus (Erxleben, 1777)) and red squirrels (Tamiasciurus hudsonicus (Erxleben, 1777))—two common small (≤1.5 kg) mammal species in the boreal forests of northern North America. We compared hit rates from camera trapping to live trapping mark-recapture density estimates and asked if the hit window—the length of time used to group consecutive videos together as single detections or “hits”—has an effect on the correlation between hit rates and live trapping density estimates. The relationship between hit rate and population density was sensitive to hit window duration for red squirrels with R 2 values ranging from 0.41 to 0.68, and a 5-min hit window generated the highest value. R 2 values for snowshoe hares ranged from 0.70 to 0.90, and a 10-min hit window generated the highest value, but hares were live trapped and filmed only at very low densities. Our results indicate that camera trapping is a robust means for estimating the density of red squirrels, but the appropriate hit window duration must be determined empirically if camera trapping data are to be used to monitor populations of this species. Additional live trapping and filming of snowshoe hares is required to better assess camera trapping of this species.

Can camera trapping provide accurate estimates of small mammal ( Myodes rutilus and Peromyscus maniculatus ) density in the boreal forest?

Abstract Estimating population densities of small mammals (< 100g) has typically been carried out by intensive livetrapping, but this technique may be stressful to animals and the effort required is considerable. Here, we used camera traps to detect small mammal presence and assessed if this provided a feasible alternative to livetrapping for density estimation. During 2010–2012, we used camera trapping in conjunction with mark–recapture livetrapping to estimate the density of northern red-backed voles ( Myodes rutilus ) and deer mice ( Peromyscus maniculatus ) in the boreal forest of Yukon, Canada. Densities for these 2 species ranged from 0.29 to 9.21 animals/ha and 0 to 5.90 animals/ha, respectively, over the course of this investigation. We determined if hit window—the length of time used to group consecutive videos together as single detections or “hits”—has an effect on the correlation between hit rate and population density. The relationship between hit rate and density was sensitive to hit window duration for Myodes with R2 values ranging from 0.45 to 0.59, with a 90-min hit window generating the highest value. This relationship was not sensitive to hit window duration for Peromyscus , with R2 values for the tested hit windows ranging from 0.81 to 0.84. Our results indicate that camera trapping may be a robust method for estimating density of small rodents in the boreal forest when the appropriate hit window duration is selected and that camera traps may be a useful tool for the study of small mammals in boreal forest habitat.

Studium synuzie drobných zemních savců (Insectivora, Rodentia) Kelečské pahorkatiny - Česká republika

In 2006 to 2007, the community of small terrestrial mammals was studied in a natural forest area of the Kelečská pahorkatina Upland situated on the boundary-line of the Oderské and Hostýnské vrchy Hills (district of Přerov, Czech Republic). Biotopes under study represented artificial plantings of beech, mature production forest stands and forest reserves. In total, 83 small terrestrial mammals of six species were caught by means of snap traps laid in lines. The majority of species colonized young plantings (n = 6, H´= 1.35), high forests showing lower diversity (n = 4, H´= 0.76). In 2006, population densities of small mammals were very low. In 2007, an increase in the population density of some species occurred, which was evidently affected by the in creased offer of food in the form of excessive seed crop (mainly of beech) in 2006. The most abundant species was Apodemus flavicollis, which represented more than half of all trapped small mammals. Arvicolidae and Insectivora showed very low populations. The high relative abundance of Apodemus agrarius in plantings of woody species was an interesting finding. These plantings are, with respect to low moisture and food offer, an atypical site for this species.

Abundance of larvae and nymphs of the taiga tick Ixodes persulcatus Schulze (Acarina, Ixodidae) on small mammals in deforested areas in the middle taiga subzone of Karelia

The data on long-term studies (1998–2004) of the population density of preimaginal stages of the taiga tick Ixodes persulcatus Schulze, 1930 collected from small mammals in deforested areas (foresting during 7, 12, and 25 years after forest cutting) in the middle taiga zone of Karelia are given. The lowest population density of I. persulcatus was observed in the most recently deforested area. The territory where forestation lasted for 25 years after deforestation was the most favorable biotope for the taiga tick development. Deforested areas, in comparison with secondary forests, are characterized by significantly higher population density of preimaginal stages of ixodid ticks; this phenomenon is determined first of all by the high population density of small mammals.

Impact of overgrazing on the transmission of Echinococcus multilocularis in Tibetan pastoral communities of Sichuan Province, China

Overgrazing was assumed to increase the population density of small mammals that are the intermediate hosts of Echinococcus multilocularis, the pathogen of alveolar echinococcosis in the Qinghai Tibet Plateau. This research tested the hypothesis that overgrazing might promote Echinococcus multilocularis transmission through increasing populations of small mammal, intermediate hosts in Tibetan pastoral communities. Grazing practices, small mammal indices and dog Echinococcus multilocularis infection data were collected to analyze the relation between overgrazing and Echinococcus multilocularis transmission using nonparametric tests and multiple stepwise logistic regression. In the investigated area, raising livestock was a key industry. The communal pastures existed and the available forage was deficient for grazing. Open (common) pastures were overgrazed and had higher burrow density of small mammals compared with neighboring fenced (private) pastures; this high overgrazing pressure on the open pastures measured by neighboring fenced area led to higher burrow density of small mammals in open pastures. The median burrow density of small mammals in open pastures was independently associated with nearby canine Echinococcus multilocularis infection (P = 0.003, OR = 1.048). Overgrazing may promote the transmission of Echinococcus multilocularis through increasing the population density of small mammals.

Small Mammal Responses to Thinning and Wildfire in Ponderosa Pine–Dominated Forests of the Southwestern United States

Abstract As part of a national experiment, the Fire and Fire Surrogate Project, we evaluated the effects of forest thinning on small mammal population densities and total small mammal biomass in ponderosa pine (Pinus ponderosa)–dominated forests at 2 study areas in northern Arizona and northern New Mexico, USA. We also evaluated the effects of wildfire on small mammal population densities and biomass after a wildfire burned a portion of one study area. Our statistical methods consisted of estimation of population densities in combined analyses across space and time, followed by a weighted regression analysis of treatment effects on densities. We hypothesized that habitat change postdisturbance would be the critical determinant of population responses to thinning and wildfire within 1 year of disturbances. Our results largely supported this hypothesis, as we documented predicted positive responses to thinning for deer mice (Peromyscus maniculatus), gray-collared chipmunks (Tamias cinereicollis), and least ...

Certain Behavioral Traits of Small Mammals in Abnormal Social Environment

In a social vacuum following a catastrophic decline in population density of small mammals, species-specific adaptive behavioral mechanisms optimizing reconstitution of the spatial and social structure are manifested to a different extent in different species. At the same time, the rate of structure and population recovery depends on specific social organization of a species. The field observation data on the behavior of three species, Pallas' pika as well as Mongolian and midday gerbils, in an experimentally depleted population are presented. Mongolian gerbil living in family groups demonstrated developed adaptive mechanisms of this kind in contrast to two other species with pronounced individual territorialism.

LANDSCAPE-LEVEL CORRELATES OF SMALL-MAMMAL ASSEMBLAGES IN FOREST FRAGMENTS OF FARMLAND

We studied effects of forest fragmentation on 15 species of small mammals, including 6 species of forest-dwelling granivorous rodents, in the Indian Pine watershed of west-central Indiana. Presence‐absence and population densities of small mammals were assessed in spring in 35 woodlots of various sizes (0.1‐150 ha) and 2 continuous forest sites ( .1,000 ha) using live traps in 1992‐1996. Presence‐absence and population density were related to landscape attributes using logistic and multiple linear regression models, respectively. Species richness of forest-dwelling small mammals increased with area and was highest in continuous forest sites. Nested subsets of the full complement of species were found in smaller woodlots. White-footed mice (Peromyscus leucopus) were ubiquitous, and eastern chipmunks (Tamias striatus) were nearly ubiquitous across the landscape; densities of both species were related inversely to forested area. Fox squirrels ( Sciurus niger) were found at 84% of study sites, and they did not respond negatively to isolation of forest patches. Red squirrels (Tamiasciurus hudsonicus) were unevenly distributed across the landscape and were found most often in woodlots with large core areas and simple shapes, possibly indicating sensitivity to edge. Southern flying squirrels ( Glaucomys volans) and gray squirrels (S. carolinensis) were restricted to continuous forest sites and .4.6-ha woodlots adjacent to other wooded habitat. Species of small mammals differ appreciably in their sensitivities to agriculturally induced fragmentation of forests. Interspecific differences within this assemblage were not due solely, or even primarily, to body size. Rather, differential responses of species to fragmentation likely resulted from variation in habitat breadth and ability to move through an agricultural matrix.

Predation of Gypsy Moth (Lepidoptera: Lymantriidae) Pupae by Invertebrates at Low Small Mammal Population Densities

A census of small mammal populations was taken, and freeze-dried gypsy moth, Lymantria dispar (L.), pupae were placed at 6 sites near the leading edge of moth spread into the southeastern United States. Pupal predation was monitored during 3 consecutive nights and a significant relationship between the number of live small mammals and predation was observed. There was a significant inverse relationship between vertebrate and invertebrate predation levels. Pupal predation by vertebrates increased as small mammal density increased, but invertebrate predation apparently decreased with increasing small mammal density. At all sites, pupae were preyed on beginning the 1st night of exposure, suggesting that the native generalist predator complex should exhibit no time lag in recognizing and accepting gypsy moth pupae as a new food item. The percentage of intact pupae preyed on by vertebrates increased from day 1 to day 2 but then declined on day 3. In contrast, invertebrate predation increased from day 1 to day 2 and remained at the higher rate on day 3. These observations are discussed in relation to behavioral characteristics of the predator populations.

Effects of Tall Fescue Endophyte Infestation on Relative Abundance of Small Mammals

AbstractSmall mammal populations were characterized nine times during six sampling periods on six plots in five different fields of tall fescue (Festuca arundinacea Schreb.) to determine differences in their abundance and species composition associated with the presence or absence of the endophytic fungus Acremonium coenophialum Morgan‐Jones & Gams. No differences were found in species richness (n = 4 for both types) between endophyte‐free (E−) plots and endophyte‐infested (E+) plots. Capture success, number of individuals, total captures, and recapture rates were more for E− plots than for E+ plots. All four species [eastern harvest mice (Reithrodontomys humulis Giglioli), short‐tailed shrews (Blarina brevicauda Gray), pine voles (Microtus pinetorum McMurtrie), and cotton rats (Sigmodon hispidus Say & Ord)] were captured more often in E− than in E+ plots. The diminished population densities of small mammals demonstrate the potential of E+ tall fescue as a permanent ground cover for waste disposal sites, orchards, tree farms, irrigation ditch banks, golf courses, and other residential, commercial, and industrial situations where burrowing activities of mammals may create human health hazards or financial burdens.

THE EFFECTS OF DIFFERENT GRAZING REGIMES ON THE POPULATION DYNAMICS OF SMALL MAMMALS IN THE EASTERN CAPE

SUMMARY Four permanent trapping grids on predominantly sweet grass-bush study plots and subjected to different grazing regimes, were monitored from June 1991 to July 1992. The diversity and population density of small mammals on these grids were affected by the pattern of animal stocking and grazing types. Only two species, Rhabdomys pumilio and Mastomys natalensis were trapped. Sites undergoing rotational resting, and those grazed by cattle, supported more rodents than constantly grazed ones and those grazed by sheep. Other environmental factors, excepting drought, had lesser impacts on the small mammal populations. The seasonal recruitment and breeding of the small mammals was affected by grazing whereas sex ratios appeared unaffected. The drought of 1991–92 was obviously a contributory factor to low small mammal numbers.

The Effects of Loblolly Pine Plantations on Small Mammal Populations

Small mammal populations were analyzed over a yearly cycle in four contrasting but similar-aged habitats of the Georgia Piedmont. An abandoned pasture and a broomsedge field were sampled monthly in conjunction with two similar habitats containing loblolly pines which were planted at the time of abandonment. Cotton rat {Sigmodon hispidus) was the only mammal species regularly captured. Cotton rat densities were significantly greater and their population composition remained more constant in the natural vs. planted habitats. Discriminant function analysis indicated that foliage height diversity, cover, and plant and litter biomass were all significant habitat variables accounting for the population differences. The planting of pines in monoculture significantly altered the structural nature of the understory vegetation and thus the small mammal microhabitat. The pines increase the rate of succession and the system rapidly becomes a marginal small mammal habitat. Sigmodon populations present during the early stages of pine growth were smaller, had larger home ranges, and contained a remarkably high percentage of transient males. As the pine canopy closes, a complete shift occurs in the understory vegetation composition and structure. Small mammal population densities are extremely low following pine canopy closure.

Metoda określania zagęszczenia w populacjach małych ssaków przy użyciu działki odłownej i transektów

Swift D. M. & Steinhorst R. K., 1976: A technique for estimating small mammal population densities using a grid and assessment lines. Acta theriol., 21, 32: 471—480 [With 2 Tables & 1 Fig.]. A technique, for estimating densities of small mammal populations utilizing live trapping on a grid and assessment lines is presented. Information from the assessment lines permits estimation of the area of effect of the grid and of the proportion of the population marked within that area. These, in conjunction with the number of animals known to have marked on the grid, yield a Lincoln Index-type of estimate of population density. Results of a field validation study of the technique are presented.

The structure of tick-borne encephalitis (TBE) foci in Central Europe.

The investigations on the structure of TBE foci were carried out in the localities Jívová (northern Moravia), Topolčianky and Žírany (Tribeč Mountains), and environs of Debreczén (Pannonian lowland, Hungary). The biphasic activity curve of Ixodes ricinus, characteristic for the central European conditions was confirmed in all three types.In some localities two or three tick species as vectors come into consideration and the unequal seasonal occurrence of individual species results in a permanent presence of virus-harbouring ticks in nature throughout the year.The average population density of ticks per hectare in a focus near topolčianky in mid-April, 1969, reached 46,000 nymphae and 50,000 adults. About 1% of the tick population were found to be viruliferous.Great differences were found in the composition of plant communities, mammalian synusies and in the population density of small mammals and ticks within the regions studied.Each elementary focus consists of many "microfoci" where the territories of individual host species overlap in space and time. Nests in tree holes and underground probably also belong to these microfoci. The maintenance hosts and indicators in each type of focus are mentioned. Free-living Carnivora can be considered as incidental hosts in each type of focus.

Population Densities of Small Mammals in Open Field Habitats in South Sweden in 1964-1967

The abundance of small mammals was investigated in an abandoned field and in a pasture in southern Sweden in 1964-67 by removal trapping with trap lines. Microtus agrestis was the most common small mammal in both habitats. It was most abundant in the abandoned field where it increased after a crash during spring 1964 with less than 5 animals per ha to a peak in autumn 1966 with about 60 animals per ha. During summer 1967, after a new crash, there was a very rapid increase to about 50 animals per ha. Apodemus sylvaticus was most abundant in the pasture with a peak density of 5 animals per ha. Sorex araneus showed greatest density in the abandoned field, varying between approx. 3 and 10 animals per ha. In the rodent species there were pronounced peaks in October while the lowest density was normally found in March-April. S. araneus showed peaks either in October or in the middle of the summer. Pe3loMe H4ccxIegOBanacb 'mCJeHHOCTb MeJIKHX MjieKOIIHTaioIUIHX Ha 3a6poLiieHHbIx IIOJI5X H Ha naCT6Hiuax B IONCHOIR IJIBeIHH B 1964-67 rogax MeTOAOM UOBTOPHOrO OTJIOBa. Microtus agrestis HaH6oJIee o6bIHbIEA BHA B o6oix MeCToo6HTaHHIfX, OH HaH6OJIee MHOrO'IcJIeHeH Ha 3aneKH. fnocnie BCnaIIaIUK BeCHOA 1964 roga IIJIOTHOCTb XCHBOTHbIX CTana HH)Ie 5 3K3/ra, oceHbWO 1966 r. oHa so3pacJIa xro 60 3K3/ra. JIeTOM 1967 r. IIJOTHOCTb 6bICTpO yBenJiaWIHnCb np1MepHO Ao 50 3K3./ra. Apodemus sylvaticus HaH6onIee o6HIneH Ha naCT6HTie c MaKCHMaJIbHOil IJIOTHOCTbO 5 3K3.fra. Sorex araneus HMeeT HaHBbICMYIO HIIOTHOCTh Ha 3anJ1ew 3-10 3K3/ra. Y rpMl3yHOB 3aperHctpHposaH 3aMeTHM161 HH1X 'IHCJIeHHOCTH B OKTI6pe H MHHHMajibH6 H qHCjieHHOCTh B MapTe-aInpeJie. S. araneUs HMeeT 1IHK qiCJceHHOCTH JIH60 B OKTI6pe, irn6o B cepeARHe JIeTa. Manuscript accepted October 1967.

Small Mammal Populations on Islands

Small mammal populations on three islands in Adirondack lakes were studied by trap removal, and results compared with trap removal data from three similar mainland areas. Number of species present on islands was less than on the mainland, but island size did not affect faunal diversity. Microtus were much more prevalent on islands and occurred in forested habitats. Total population density of small mammals on the islands was determined by extended removal trapping. Total density and density by species are given. Short periods of removal trapping do not indicate species composition of the community because some abundant species are seldom captured in the early part of the trapping period. Species differences in initial attraction to the traps or changes in behavior following extensive removal of "social dominants" may be involved. Probability of capture is variable, and not consistently related to population density. Several census methods are evaluated and shown to provide an inaccurate estimate of population. Comparison of capture rates on mainland areas with that on islands suggests that invasion may not be a major factor until after 10 to 15 days of trapping. Long—continued trapping on mainland areas produces periods of high catch which may exceed capture rates of the initial period. The reasons for these periods of high catch are not known.

Responses of Small Mammals to Live-traps and Weather Conditions

A comparison was made between the responses to livetraps of marked and unmarked individuals of Microtus pennsylvanicus and Peromyscus leucopus. Previously captured individuals of M. pennsylvanicus were more readily captured than were the unmarked ones, even after an interval of a month. There was no difference in the rates of capture of marked and unmarked P. leucopus. Comparisons of a given live-trapping technique utilized in two different habitats (marsh and old field) indicated no significant difference in its effectiveness after the second day of trapping. M. pennsylvanicus, in an old field, was found to abandon diurnal activity in favor of nocturnal and/or crepuscular activity when the temperature rose above 200 C. During the winter, when the temperature dropped below 00 C, activity was much less at all times. Except for less activity during the days the temperature rose above 200 C, such responses were not observed in the marsh. Blarina brevicauda was found to be more active on cloudy days than on those that were sunny or rainy. No correlations between activity and temperature variations was observed for this species. Estimates of population densities of small mammals are normally derived from trapping data. The results depend upon a variety of factors in addition to the actual abundance of the mammals. Variations in responses to traps, positioning of the traps, home range size, amount of movement within the population, and weather conditions influence the estimates of population densities (Sealander and James, 1958; Goodnight and Koestner, 1942; Hayne, 1950; Calhoun and Casby, 1958; Getz, 1961a). Responses to traps are particularly important. A method widely used in determining population densities from trapping data is the proportional method (Lincoln Index or modifications thereof; Hayne, 1949). A basic assumption involved in this method is that the probability of capture of the marked animals is the same as that of the unmarked animals. Morris (1955) and Chitty and Kempson (1949) found that marked individuals entered traps more readily than did those that had not been captured previously. Tanaka (1956) found the same to be true of certain species, while in others the probability of capture decreased after the initial capture. He further found that some may be isoresponsive. Leslie (1952) assumed the marked and unmarked individuals to be captured with equal facility, although prebaiting was utilized to overcome any initial reluctance of individuals to enter traps (this initial reluctance has been discussed by Sealander, et al., 1958). Responses to weather conditions also influence estimates of population densities, particularly those sampled by means of snap-back traps. Burt (1940), Gentry and Odum (1957), Pearson (1959, 1960)