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SEX RATIOS IN BOBCAT POPULATIONS

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									                    SEX RATIOS IN BOBCAT POPULATIONS
                                                                       RENN TUMLISON
                                                                     Department of Zoology
                                                                  Oklahoma State University
                                                                    Stillwater, OK 74078
                                                                               and
                                                                     V. RICK McDANIEL
                                                              Department of Biological Sciences
                                                                  Arkansas State University
                                                                State University, AR 72467


                                                                              ABSTRACT

                           Reported sex ratios in bobcat populations have suggested great variation ranging from strong male bias
                         to strong female bias. Explanations offered for these observations have included factors of mobility,
                         activity, behavior, inaccurate data collection, hunting pressure, and population density. Ratios are
                         probably most representative during the winter, when females are no longer under constraints of parental
                         care. Sex determination should be made by experienced observers and preferably through internal ex-
                         amination. The most productive direction for interpretation of validated ratios appears to be in terms of
                         population density, hunting pressure, and the timing of harvest.




                            INTRODUCTION                                              ment studies that consistently indicate that males move farther and oc-
                                                                                      cupy larger ranges than do females (Robinson and Grand, 1958; Bailey,
   Most published sex ratios for bobcats (Felis rufus) are based on rather            1972; Hall, 1973; Guenther, 1980; Hamilton, 1982). Ifmales are in-
static data accumulated over 1-3 years. These sex ratios (and explana-                deed more susceptible to trapping, then a female-dominated sex ratio
tions offered to explain them) have varied considerably, and some ex-                 in a sample reflects an even more female-dominated population in
planations are contradictory. Reported ratios have ranged from 0.40                   nature. McCord and Cardoza (1982), using the data of Donoho et al.
males/female (Foote, 1945) to 2.21/1 (Kohn, 1981a). Sex ratios may                    (1979), argued that the sex ratio of specimens taken before and during
change dramatically between years: the male-dominated ratio of Kohn                   the breeding season were similar, thus males were not more vulnerable
(1981a) dropped to 1.10/1 the next year (Kohn, 1981b). Alternatively,                due to reproductive activity. Interestingly, they stated that the seasonal
Erickson (1955) demonstrated a gradual change in sex ratios of 6496                  differences found by Erickson (1955) showed no real variation, but ex-
bountied bobcats from 0.97/1 in 1941 to 1.90/1 in 1952.                              amination of the original data indicated that males were collected in
   Explanations offered for observed sex ratios in bobcats include: 1)               stronger disproportion during the summer (i.e., that females were more
greater mobility, thus vulnerability, of males; 2) lesser mobility, thus             equally represented during the winter months).
vulnerability, of females; 3) increased activity of either sex during the               Interpretations of proportions are often conceptually biased. Chang-
breeding season; 4) seasonal differences due to maternal care of young;              ing proportions may mean that one sex has become more susceptible,
5) inaccurate sexing; 6) degree of hunting pressure; 7) density of the               or that the other sex has become less so. Some literature deals with the
population; and 8) differential attractiveness to baits. The purposes of             question of why male vulnerability reflected in the ratio has changed,
this paper are to consolidate thought on causes of observed sex ratios               and doesn't question whether changes in female vulnerability are respon-
in bobcat populations and provide some insight into the applicability                sible for observed changes. Sex ratios are often expressed as the number
of these explanations.                                                               ofmales per female, suggesting that we may conceive of females as con-
                                                                                     stants and males as variables. In contrast, Parker and Smith (1983) did
                                                                                     suggest that males become less prone to trapping than females that were
                   METHODS AND MATERIALS                                             hunting and caring for kittens during the winter. This supposes that
                                                                                     changes in female activity may result in changing sex ratios, that is,
   When researchers feel that the sex ratio of a species is equal at birth,           that male bias occurred because females were less likely to be trapped
they look for causes when equality is not found in a sample. Ifthe sample            rather than males more likely to be.
is of newborns, explanations usually invoke mechanisms for intrinsic                    Many trappers search for bobcat sign when locating trap sites. Because
population regulation such as sex-determining mechanisms and sex-                   male range is larger than that of females, a trapper would more likely
specific intrauterine mortality. Ifthe sample is from the general popula-            search for sign within a male bobcat's range but would more likely find
tion, explanations often include differential mortality due to factors              itin the more intensively utilized female's range (McCord and Cardoza,
such as behavior or hunting pressure. The recent proliferation of data               1982). Therefore, one might expect females to dominate in sex ratios
concerning sex ratios in bobcat populations has resulted in several ex-              due to an assumed greater chance of being located (Fredrickson and
planations for ratios that deviated from equality. We examined literature           Rice, 1979; Klepinger et al., 1979). Most of the sex ratios in Table 1
reporting sex ratios in bobcat populations to evaluate the logic of ex-              suggest equality or male dominance, thus it is unlikely that females have
planations and to consolidate thought on the causes of unequal ratios,               a greater chance of being located by trappers.
and to provide direction for future research. Most literature was located               Young (1958) described the success of a trapper who collected 321
using publications of abstracting services and indexes for particular jour-         bobcats in a 4-month period in Oregon. Females were not collected in
nals. In addition, letters were sent to several states to request informa-           substantial numbers untilthe latter part of October, when mothers and
tion on bobcat research completed in those states.                                  kittens began to travel. (Another interpretation may be that many of
                                                                                     the males had been removed, leaving primarily females to trap.) Erickson
                                                                                    (1955) and Gashwiler etal., (1961) found that female bobcats were pro-
                    RESULTS ANDDISCUSSION                                           portionately more often collected during the winter months. This period
                                                                                     represents the breeding season when parous females with previous lit-
    Several researchers have suggested that male-dominated ratios are               ters are dispersing their young as mating begins. Females, therefore,
due to greater mobility of males, leading to increased vulnerability to             become more mobile and effectively more trap-susceptible than in other
 trapping (Gashwiler et al., 1961; Fritts and Sealander, 1978) or shooting          seasons. Sex ratios observed during winter probably best reflect the ex-
(Knick et al., 1985). This argument is based on home range and move-                isting ratios in most populations.



  92
                                         Proceedings Arkansas Academy of Science, Vol. 42, 1988
                                                                      Renn Tumlison and V. Rick McDaniel



              .
    Table 1 Literature data          on sex ratios of bobcats in North America.                 and Cardoza (1982) alluded to Foote's (1945) female-dominated ratio
                                                                                                when they provided information discounting the credibility of early sex
    State    or Region        N      Ratio (otf/°)          Source                             ratios from Vermont.
                                                                                                   Logically, a female-dominated population would optimally effect a
    Vermont              351             0.40*          Foote, 1945
                                                                                               population increase in a polygynous species. Mech (1975) found that
    South Dakota
    Wisconsin
    Virginia
                          76
                       1381
                        144
                                         0.43*
                                         0.64*
                                         0.87
                                                        Fredrickson
                                                                          .
                                                                     & Rice, 1979
                                                        Klepinger et_ a\_ , 1979
                                                        Progulske, 1952
                                                                                                sex ratios of wolf (Canis lupus) pups varied according to population
                                                                                                density, with male domination significant in litters from high density
    Oklahoma            411              0.87           Rolley, 1983
    California            60             0.94           Lembeck, 1978                          populations. Conversely, an equal sex ratio or a preponderance of
    Kansas               725             0.95           Johnson, 1979                           females was produced where the population had been exploited and
                                                        Crowe, 1975
    Wyoming
    Alabama
                        161
                        213
                                         1.01
                                         1.01           Miller, 1980                           had existed at a lower density. In a resource-limited environment, a
    Arkansas             471             1.01           Tutnlison, 1983                        preponderance ofmales would tend to stabilize the growth rate of the
    Washington         1238              1.01           Knick e_t aK , 1985                    population by decreasing the percentage of breedable females and
    Northeastern U.S.   180              1.05           Pollack, 1950
    Washington          176              1.07           Sweeney, 1978                           thereby prevent over-utilization of limited resources. Assuming bob-
    New Mexico          150              1.08           Young, 1958
                                                        Parker & Smith, 1983
                                                                                                cat reproduction is a density-dependent function, then, male-dominated
    Nova Scotia          580             1.08
    North Dakota          42             1.10           Kohn, 1981b                             sex ratios may indicate saturated or unexploited populations. Lembeck
    Utah                 792             1.1l a         Gashwiler e^ al^. , 1961                and Gould (1979) found a preponderance ofmales in high-density bobcat
                                                        Erickson, 1955 (his study)
    Michigan
    Texas
                        103
                      14256
                                         1.19
                                         1.20*          Blankenship & Swank, 1979
                                                                                               populations and females in low-density populations.
    North Carolina       505             1.21*          King et_    , 1983                         Recent estimates of the sex ratio for Arkansas bobcats (Tumlison,
    Minnesota           169              1.22*          Berg, 1979                             1983) indicated a 1:1 distribution, although Fritts and Sealander (1978)
    Arizona            8703              1.26*          Young, 1958
    Utah              28432              1.29*          Gashwiler e± aj.. , 1961               determined a ratio of 1.69/1 in Arkansas a decade earlier. Their data
    Idaho                316             1.32*          Bailey, 1979                           represented an unexploited and probably high-density population,
    Colorado           2443              1.37*          Donohoet^a^., 1979                     whereas the present population is exploited. Ifthe findings of Mech
    Texas             13737              1.40*          Roberson, 1981
    Michigan           6496              1.54*          Erickson, 1955 (bountied)              (1975) for low-density versus high-density wolf populations are ap-
    Arkansas            180              1.69*          Fritts & Sealander, 1978               plicable to bobcats, exploitation may partially account for the dispari-
    North Dakota          93             2.21*          Kohn, 1981a
                                                                                               ty between previous and present bobcat sex ratios in Arkansas. Alter-
    * indicates      a significant difference    from an even ratio, Chi-square     (p<0.05)   natively, hunting pressure itself may be responsible for the change in
                                                                                               sex ratios from Arkansas. Gilbert (1979) suggested hunting pressure
      the authors      did not provide a reason    for distinguishing a subgroup     (N=792)
                                                                                               might be responsible for skewed ratios in bobcats based on similar
            of the   total sample  (N=28432)                                                   research with black bears (Ursus americanus). As hunting pressure in-
                                                                                               creases, the effect ofdifferent home range size decreases and sex ratios
                                                                                               in the kill approach equality. Sex ratio, then, may reflect harvest
       Some researchers have compared sex ratios based on method of col-                       pressure. Gilbert (1979) cited Lembeck and Gould (1979) as support,
    lection to gain insight on or to validate ratios. Erickson (1955) and Fritts               however Erickson (1955) found male-bias to increase with increasing
    and Sealander (1978) found no significant difference between sex ratios                    harvests.
    in trapped versus hunted specimens. However, Klepinger et al. (1979)                          Gilbert (1979) tried to explain equal sex ratios as being the result of
    suggested that hunters sometimes bias sex ratios by selecting only larger                  an excess ofavailable females in older age classes. He cited Crowe and
    cats (males) due to trophy value. Brittell et al. (1979), Sweeney (1978),                  Strickland (1975) and Fritts and Sealander (1978) to support the con-
    and Knick et al. (1985) noted that hound-hunted samples were male-                         cept of greater vulnerability of males, stating that young males had a
    biased while trapped samples had an even ratio. Therefore, comparisons
                                                                                               greater chance of being caught, leaving more females available in older
    of bobcat sex ratios calculated for hunted versus trapped specimens can-                   age classes. However, the number of "surplus" females in older age
    not be used for ratio validation. Still, distinction by method ofcollec-                   classes found by Fritts and Sealander (1978) is insufficient to balance
    tion does provide insight to the nature of bias in the samples.                            the loss ofmales in younger age classes, thus the sex ratio was not equal
       Male-dominated sex ratios have been reported from Arizona, Arkan-                       (1.69/1). The sex ratio reported by Crowe and Strickland (1975) was
    sas, Colorado, Michigan, Minnesota, North Carolina, North Dakota,                          equal, and males did dominate younger and females older age classes.
    Texas, and Utah (Table 1). Because sexes are sometimes dispropor-                          However, almost one third of their total sample was not sexed, thus
    tionately collected during different seasons, the male dominance in some                   Gilbert (1979) assumed the sex ratio in that part of the total sample
    of these studies may be partially attributed to year-long collections                      ws proportionate to that of the sexed samples. If this assumption is
    (although Fritts and Sealander [1978] felt that this had not affected the                  false, the inferences made could also be false.
    Arkansas sex ratio). Bailey (1972) found sex ratios to vary among age                         The timing of the harvest season is another important factor in the
    groups: 1.0/1 (kittens), 3.0/1 (transients), 0.6/1 (adults). Parker and                    interpretation of sex ratios. With extended seasons, an increase in the
    Smith (1983) found males dominated the first age class, the ratio was                      proportion ofkittens often occurs in exploited populations (Parker and
    even in young adults, and females dominated in older adults. Thus,                         Smith, 1983; Knick et al., 1985). Because sex ratios in younger age classes
    different sizes ofclasses may result in sex ratios biased in favor of males                often favor males (Bailey, 1972; Parker and Smith, 1983; Knick et al.,
    (males dominate in younger, much larger classes). Additionally, male                       1985), later or extended seasons will likely produce more males and
    dominance in Texas (Roberson, 1981) may be partly explained by in-                         earlier seasons relatively fewer females.
    accurate sexing because ratios were based on furbuyer reports. Arkan-                         Other explanations forbias in observed sex ratios of bobcats are of
    sas bobcat sex ratios determined from buyer reports favor males (1.5:1;                    interest but too few data are available to do more than note them here.
    L. Johnston, pers. comm.) but the ratio determined from carcasses col-                     Young (1958) thought differential attractiveness to baits may cause biases
    lected during the same period approximated a 1 :distribution (Tumlison,
                                                       1                                       in trapping data. Fritts and Sealander (1978) suggested that higher
    1983), possibly because bobcats are more difficult to sex than most                        postnatal mortality of female kittens might occur in the event of in-
    furbearers. McCord and Cardoza (1982) reported bobcat sex ratios deter-                    tralitter competition for food, and that smaller females might have lower
>   mined by wildlife personnel in Vermont heavily favored females until                       survivorship after maternal care was withdrawn.
    verification by internal examination revealed an almost equal sex ratio.
    A postscript in Fredrickson and Rice (1979) indicated that the female-
    biased ratio in South Dakota approached a 1:1 distribution based upon
    internal examination of carcasses collected during the succeeding year.                                                CONCLUSION
       Females were strongly dominant in Vermont, Wisconsin, and South
    Dakota samples (Table 1). More harsh northern climates might require                          Itis evident that explanations given for observed sex ratios inbob-
    a female-dominated population for higher reproduction to offset higher                     cat populations are not always consistent or biologically defensible.
    mortality due to greater winter stress. Sex ratios from North Dakota                       Greater mobility of males as an explanation of male-dominated ratios
                                                                                               versus lesser mobility of females as an explanation of female-dominated
    and Washington do not support this interpretation. Further, McCord


                                                Proceedings Arkansas Academy off Science, Vol. 42, 1988                                                                  93
                                                          Sex Ratios in Bobcat Populations



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  94                                    Proceedings Arkansas Academy off Science, Vol. 42, 1988
                                                 Renn Tumlison and V.Rick McDaniel



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   sas. 158 pp.




                                  Proceedings Arkansas Academy off Science, Vol. 42, 1988                                               95

								
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