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Population and Movement Characteristics of Radio-Collared Striped Skunks in North Dakota During an Epizootic of Rabies

Results


We observed 31 adult striped skunks in 1991. Twelve females and 12 males were captured during April or May; one female died at capture and 23 were equipped with radio-collars. Six females and one male captured at feeding sites in late June were not equipped with radios. Radio-tracking was conducted from 17 April to 19 June. Two animals were monitored for only ≤ 4 days (one female was shot; one male dispersed). We obtained a mean of 74 (range, 64 to 91) locations of the remaining 21 animals, including a second male that dispersed 6.9 km between 0100 and 1800 on 20 May; it was collected 8.5 km from its capture location. One aggressive radio-equipped skunk was euthanized and found positive for rabies antigen by FA test.

We observed 62 adult striped skunks in 1992. Thirty-three females and 24 males were captured during March through June; one male died at capture and 56 were equipped with radio-collars. One female and three males captured at feeding sites in mid-July and one female found dead in June were not equipped with radios. Radio-tracking was conducted from 31 March to 15 July. Four radio-equipped animals (two males and two females) died before systematic monitoring began. We obtained a mean of 111 (range, 11 to 296) locations of the 52 radio-collared skunks; 11 of the 52 were located 215 to 296 times. Thirty-six (68%) of 53 skunks (some without radio-collars) were positive for rabies antigen by the FA test. Four additional skunks with lymphocytic meningitis were negative for rabies antigen by FA test. Those four also were negative for rabies by the mouse inoculation test and negative for canine distemper and other morbillivirus antigens by the immunohistochemical test.

We estimated ages of 29 skunks in 1991 and 41 skunks in 1992 (Table 1). Ages of 18 skunks (one with rabies in 1991 and the first 17 found dead or euthanized in 1992) were not estimated because of concern over risk of collecting teeth from potentially rabid animals. Among animals with ages estimated, we detected no year by sex interaction effect (chi-square = 2.71; df = 4; P = 0.61) or sex main effect (chi-square = 2.48; df = 2; P = 0.29); thus differences in proportions of skunks by age class among years appeared similar for males and females. We detected a significant main effect due to year (chi-squared = 53.61; df = 4; P < 0.0001). In 1992, a smaller proportion of skunks comprised age class 1 and larger proportions comprised age classes 2 and 3 than in the years 1979 through 1990 or 1991 (P < 0.05) (Table 2). We detected no differences in proportions of skunks between years 1979 through 1990 versus 1991 for age classes 2 or 3 (P > 0.05).

In 1991, all 18 females were pregnant or parous. In 1992, 21 (60%) of 35 females were pregnant or parous; nine (26%) had no evidence of recent pregnancy and the status of five (14%) was not determined. Of the 21 reproductively active females in 1992, 12 were positive for rabies and nine were negative; one of the 12 was about 30 days pregnant on 7 July. Of the nine females with no evidence of pregnancy, six were positive for rabies and three were negative. Two litters were born to the nine females that tested negative for rabies. Two other lactating females were positive for rabies, but litters were not observed before the females died. Based on behavior, another female positive for rabies also may have had a litter, but when found dead she was not lactating and apparently no longer had young.

In 1992, five skunks found in late clinical stage of rabies and carcasses of 21 others, including one skunk represented only by a chewed radio-collar, were found above ground (Table 3). Carcasses or remains of 11 skunks were in burrows; an additional three heads with attached radio-collars were found cached under 20 to 25 cm of soil.

We documented only two interactions during the study between radio-collared skunks and persons on the study area. On 1 June 1991, an aggressive rabid skunk attacked a farmer during daytime; had we not euthanized the skunk, he would have shot it. On 18 June 1992, another farmer shot a skunk at twilight; he said it was "normal acting". Both skunks were positive for rabies.

Based on bite marks on live skunks and carcasses, we believe that aggressive interactions occurred both years between radio-collared skunks and other carnivores, including other skunks. In 1991, two (6%) of 31 skunks had 3 to 4 mm diameter punctures on their chest or abdomen; both animals were alive and appeared healthy when euthanized, in spite of the punctures. Neither was tested for rabies. Based on the 35 to 40 mm spacing between punctures, we believe bites were inflicted by a large carnivore, possibly coyote (Canis latrans), red fox (Vulpes vulpes), American badger (Taxidea taxus), raccoon (Procyon lotor), or domestic dog (Canis familaris); all these species were present on the study area.

In 1992, 17 (47%) of 36 rabid skunks and five (29%) of 17 healthy skunks had bite marks or scars (Table 4). Based on 0.5 to 1 mm diameter punctures with spacing of 10 to 15 mm, we believe small bites were inflicted by skunks or domestic cats (Felis domesticus). It appeared that most small bites were not self-inflicted, because 12 (86%) of 14 animals were bitten on the muzzle, head, or attached radio-collar. We did not determine if cached heads were scavenged or severed from live animals.

Two radio-collared skunks died in 1991, one (not tested for rabies) by vehicle collision and one (rabid) attributed to shooting. We detected no difference in 1991 in survival rates between males and females (chi-square = 2.72, 1 df, P = 0.10) or between age classes 1 and 2 (chi-square = 0.72, 1 df, P = 0.40). The estimated survival rate of 23 radio-collared skunks between 18 April and 20 June 1991 was 0.849 ± 0.070 (Table 5).

Thirty-nine radio-collared skunks died in 1992, 24 of rabies, nine by predation, one by vehicle collision, one by shooting, and four of unknown causes. We detected no difference in 1992 in survival rates between males and females (chi-square = 1.27, 1 df, P = 0.26); however, skunks in age class 1 survived at a lower rate than did skunks in age class 2 (chi-square = 8.10, 1 df, P = 0.004). The estimated mean survival rate of 56 radio-collared skunks between 31 March and 15 July 1992 was 0.17 ± 0.04 (Table 5).

Highest rate of mortality overall in 1992 tended to be from rabies (0.67) (Table 5). Rabies tended to have a slightly greater influence on mortality of males (0.80) than females (0.61), but we observed little difference in effects of rabies between age class 1 (0.58) and age class 2 (0.57). Another source of mortality, predation by large carnivores, tended to have similar influence on mortality of males (0.28) and females (0.24). Skunks in age class 1, however, tended to suffer greater mortality from predation by large carnivores (0.58) than did older skunks (0.09). Seven of the nine skunks whose deaths we attributed to predation were rabid.

We observed considerable variation in mean nightly rates of travel of skunks by half-month period, with no consistent trends. We detected no differences in mean rates of travel due to main effects of sex, rabies condition, or their interaction (P ≥ 0.10). The mean rate of travel in a night between 19 April and 25 June for all skunks was 232 ± 14 m/hr.

Variation in rates of travel of both healthy and rabid skunks in 1992 was low (2 to 8%) due to differences between animals. However, rates of travel were nearly twice as variable between animals on different nights for rabid skunks (females, 23%; males, 33%) as for healthy skunks (females, 10%; males, 16%). Conversely, variation in rates of travel due to differences between locations within nights was less for rabid skunks (females, 75%; males, 65%) than for healthy skunks (females, 85%; males, 76%).

We detected considerable variation in mean nightly distance traveled by skunks by half-month period, with no consistent trend. We detected no differences in mean distances traveled in a night by skunks in 1992, due to main effects or their interaction (P > 0.23). Mean distance traveled in a night during 19 April to 25 June was 2047 ± 141 m.

We detected no differences in half-month home range estimates of skunks in 1992 due to main effects or their interaction (P > 0.23). However, there was a tendency for half-month home range sizes of all skunks to differ by half-month period (F = 2.31; df = 4,75; P = 0.07). Mean estimates of home range size tended to increase between late April (1.1 ± 0.5 km2) and late May (2.1 ± 0.5 km2). The overall mean home range size during 19 April to 25 June was 1.6 ± 0.4 km2.

Among skunks with rabies in 1992, mean rate of travel tended to decrease between the pre-clinical period (298 ± 48 m/hr) and clinical period (174 ± 48 m/hr) (F = 3.32; df = 1,16; P = 0.09) (Table 6). We detected no difference in mean rates of travel between males and females (F = 0.18; df = 1,16; P = 0.68), and no interaction between sex and period (F ≤ 0.01; df = 1,16; P = 0.98). The mean distance traveled in a night by skunks with rabies tended to be greater during the pre-clinical period (2318 ± 281 m) than during the clinical period (1497 ± 281 m) (F = 4.26; df = 1,16; P = 0.06) (Table 6). We detected no difference between males and females in mean distances traveled (F = 0.20; df = 1,16; P = 0.66) and no interaction between sex and period (F = 0.03; df = 1,16; P = 0.87).

The mean home range size of skunks with rabies varied by sex between pre-clinical and clinical periods (F = 4.67; df = 1,15; P = 0.05) (Table 6). We detected no difference in the home range size of males between the pre-clinical and clinical period (P = 0.10). During the pre-clinical period, home range size of males was greater than that of females (P = 0.01), but during the clinical period we detected no difference between home range sizes of males and females (P = 0.94). We detected no difference in home range size of females between pre-clinical and clinical periods (P = 0.22).

The pattern of deaths of rabid skunks tended to be more spatially clumped within the radio-collared population than if deaths were entirely random (P = 0.08). Much of the influence on spatial clumping was from deaths of the first nine skunks between 2 April and 7 May (Table 7); all nine were clustered within 3.2 km of the location of the first death (Fig. 1). Three of the nine were represented only by heads cached within 1.6 km of each other. After May 7, deaths from rabies tended to be more generally distributed throughout the population.

We did not detect a correlation between activity centers of skunks that died of rabies and dates of death (r = -0.02, P = 0.66). Although skunks that died of rabies between 2 April and 7 May tended to be located relatively close together in space and time, the relationship was less evident after 7 May.

We used information on 49 radio-collared skunks to examine use of feeding sites in 1992. Ten skunks were never detected at a feeding site. Thirty-nine animals visited feeding sites on 170 (6%) of 2946 occasions during night-tracking when located away from daytime retreats. Nine of the 39 animals were detected at a site only once and none was detected at more than one feeding site in an individual night. Only one skunk was detected at more than one feeding site during 1992; it was a 4-year-old rabid male that visited two adjacent sites on different nights. On 10 different occasions, more than two radio-collared skunks were detected at the same feeding site within 30 min of each other. Use of feeding sites varied between males and females depending on rabies condition (F = 9.16; df = 1,45; P = 0.004). Females with rabies were located at feeding sites more than twice as often (8%) as healthy females (3%) (P = 0.03). Conversely, males with rabies were located at feeding sites only half as often (4%) as healthy males (10%) (P = 0.04).


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