THE BURDEN OF CO-OCCUPANCY: INTRASPECIFIC RESOURCE COMPETITION AND SPACING PATTERNS IN AMERICAN MINK, MUSTELA VISON

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1 Jounal of Mammalogy, 84(4): , 2003 THE BURDEN OF CO-OCCUPANCY: INTRASPECIFIC RESOURCE COMPETITION AND SPACING PATTERNS IN AMERICAN MINK, MUSTELA VISON NOBUYUKI YAMAGUCHI AND DAVID W. MACDONALD* Wildlife Consevation Reseach Unit, Depatment of Zoology, South Paks Road, Oxfod OX1 3PS, United Kingdom Live-tapping and adiotacking of Ameican mink (Mustela vison) on 24 km of the Rive Thames, United Kingdom, evealed ange sizes (male: 6.8 km, female: 2.7 km), numbes of othe mink found in the ange of each male (0.98 males, 1.95 females) and each female (1.18 males, 0.32 females), and ange ovelaps between dyads of males (88.2%), a male and a female (male: 32.6%, female: 69.1%), and females (66.3%). We used these data to estimate the potential enegetic buden of conspecifics on each othe. As a monthly aveage, cohabiting males and othe females take an estimated 30% and 9%, espectively, of the total enegy consumption by mink within the ange of each female. Similaly, cohabiting males and females consume an estimated 21% and 40%, espectively, of the enegy in each male s ange. Intaspecific esouce depession may foce both sexes, especially males, to maintain lage anges. We suggest that, in the case of Ameican mink, intaspecific peyesouce competition affects the ange sizes of both sexes. Key wods: Ameican mink, home ange, intasexual teitoiality, intaspecific competition, lowland England, Mustela vison, esouce dispesion, spatial oganization The basic mustelid spacing patten is geneally thought to deive fom intasexual teitoiality, whee individuals maintain teitoies only with espect to membes of the same sex (eviewed in Macdonald 1992; Powell 1979). In pinciple, the consideable sexual dimophism in mustelids may allow 2 potentially competing individuals of diffeent sexes to have diffeent diets and thus to cohabit (McDonald 2002; Powell 1994). In pactice, few data demonstate clea esouce patitioning between the sexes. Two sexes compete though using the same pey species esouce depletion and, visiting pey patches in each othe s home anges, and theeby, affecting each othe s hunting success esouce depession (Chanov et al. 1976; Jedzejewski and Jedzejewski 1990; Powell 1994). The Ameican mink (Mustela vison) is a solitay, sexually di- * Coespondent: david.macdonald@zoo.ox.ac.uk mophic, semiaquatic mustelid (Dunstone 1993). Individuals of both sexes defend thei teitoies against othe conspecific individuals of the same sex, at least duing the nonbeeding season, and thee is evidence of intasexual teitoiality (Biks 1981; Dunstone 1993; Dunstone and Biks 1983; Geell 1970; Ieland 1988). While most females continue to maintain thei teitoies into the beeding season, many males abandon thei anges and oam ove lage aeas in seach of eceptive females (Biks 1981; Biks and Dunstone 1991; Dunstone 1993; Ieland 1988). Among polygynous mammals, it is widely held, although lagely untested, that the spatial distibutions of females ae detemined by food esouces and those of males ae stongly influenced not only by food but also by the dispesion of females (Clutton- Bock 1989; Ims 1988; Kebs and Davies 1341

2 1342 JOURNAL OF MAMMALOGY Vol. 84, No ; Macdonald 1983; Ostfeld 1986; Sandell 1989). In this context, a few studies have tackled the influences of habitat-elated poductivity on mink spacing pattens (Biks 1981; Biks and Dunstone 1991; Dunstone 1993; Halliwell and Macdonald 1996; Ieland 1988). Also a few epots have been published on esouce competition between mink and sympatic competitos (polecats, Mustela putoius Lodé 1993; Euopean mink, M. luteola Macdonald et al. in litt.; Maan et al. 1998; ottes, Luta luta Bueno 1996; Clode and Macdonald 1995; Canadian ottes, Lonta Canadensis Ben-David et al. 1995; Melquist et al. 1981; southen ive otte, L. povocax Medina 1997). Howeve, the ole of conspecific competition has emained lagely unquantified despite its theoetical impotance. In this aticle, we test whethe the spacing pattens of conspecifics affects the home-ange size in mink and the lage home-ange size of male mink eflects diffeences in cucial esouces between sexes, based on the demogaphy and spacing pattens of a fee-anging Ameican mink population in the Uppe Thames, United Kingdom. MATERIALS AND METHODS Study aea and tapping. The study aea consisted of appoximately 24 km of the Rive Thames (about N, 1 25 W), in a lowland, mixed agicultue landscape to the west of Oxfod, United Kingdom. The aea s mink population was not havested, and attempted contol was oppotunistic and not intense. Potential competitos such as ottes, polecats, stoats (M. eminea) and weasels (M. nivalis) wee eithe absent o uncommon. Mink wee tapped in commecial, single-enty aluminum mink and at cage taps of appoximately 14 by 14 by 76 cm (A. Fenn and Co., Redditch, Wocesteshie, United Kingdom). The study aea was divided into 4 stetches of ive, each consisting of 3 6 km between 2 neighboing locks. Tapping was conducted in each stetch fo 1 week, and a tap was set, on aveage, evey m of ivebank. We inceased tapping effot duing the beeding season by inceasing the numbe of taps set. The 4-week cycle of tapping was continued fo 28 months fom May 1995 and August A mink was classified as a kit if it was obseved o tapped with its mothe befoe dispesal, at appoximately 13 weeks old (ealy August); theeafte, it was classed as a juvenile until the onset of the fist beeding season (Januay), at appoximately 8 months old, and theeafte as an adult. Radiotacking. Captued animals wee fitted with watepoof adiotansmittes (fequency ange MHz) attached to collas with integal eed switches (Biotack Ltd., Waeham, Doset, United Kingdom) and monitoed though the use of eceives (M57, Maine Rada Ltd., Lowestoft, Suffolk, United Kingdom) connected to 3-element Yagi antennas (Biotack Ltd.). Radiocollas weighed appoximately 15 g, 3% of the lightest adult female mink caught in ou study aea (about 550 g). Duing adiotacking, the subject s location was ecoded evey 15 min; individuals wee geneally tacked between dusk and dawn. Fixes wee taken fom within m, and locations ecoded as coodinates on the map to the neaest 10 m. Pio to adiotacking, we evaluated tiangulation eo. The accuacy of tiangulation at a -m ange gave, fo antennas with igid elements, the fixation eo of m (SE n 9), and with flexible elements m (n 9). Howeve, tiangulation was aely necessay within the configuation of ou study aea, whee mink confined thei movements to a naow ibbon of ipaian habitat. In almost all cases, we could confim the subject s exact location. Analysis of tapping data. Individual mink wee identified using vental spot pattens (Chanin 1983; Dunstone 1993). Data wee analyzed on the basis of calenda month (facilitating diect compaison with, e.g., Ieland 1988; Smal 1991). Pesence of each individual in the study aea was assessed by the combination of monthly tapping, adiotacking, and occasional diect obsevations. Juveniles bon in the study aea wee geneally tapped fo the fist time in July, when they wee still closely associated with thei mothes. Juveniles wee included in analyses only afte August, when they stated to dispese. Following Hatle (1976) and Ieland (1988), females pesent fo 3 consecutive months wee classified as esidents. Because of the epoted

3 Novembe 2003 YAMAGUCHI AND MACDONALD INTRASPECIFIC COMPETITION IN MINK 1343 seasonal change in thei spacing pattens (Biks 1981; Dunstone 1993; Ieland 1988), males wee classified as esidents only if they wee in the study aea fo 3 consecutive months in the nonbeeding season (May Decembe). Othe adults wee classified as tansients. Analysis of adiotacking data. Biks and Linn (1982) epoted that mink adiotacked at least twice a day evealed moe than 80% of thei total home anges in 5 days, and the entie home anges in 10 days. We included only individuals that wee adiotacked fo eithe 10 days o intensively (followed, on aveage, fo 20 h a day) fo 5 days duing the tacking peiods (citeia met by 13 animals out of a total of 24 to which adiotansmittes wee fitted). Analyses of the movements of semiaquatic mammals often assume that thei anges ae linea, aligned along the wate couse (mink Biks and Linn 1982; Bonesi 1996; Geell 1970; Ieland 1988; otte Dubin 1998; wate vole, Avicola teestis Macdonald and Stachan in litt.). Indeed, in ou study, all adiotacked mink geneally stayed 10 m fom the neaest wate souce (88% of fixes fo males and 95% fo females). Theefoe, the entie study aea was consideed as a ive coido and was divided into 200-m sections on opposite banks, to which each adiolocation was allocated. The home-ange size (expessed as length of wate couse) used by each adiotacked animal was calculated on the basis of the numbe of sections between the most upsteam and the most downsteam sections containing eithe adiolocations o captue points. Statistics. Mann Whitney U-tests, coected fo ties, wee used to assess the diffeence between 2 categoies unde test, and Kuskal Wallis tests wee used when thee wee 2 categoies. The binominal test was used to examine if the sex atio diffeed fom paity. Significance of coelation was tested by Kendall ank coelation tests. The sexual diffeences of monthly data wee tested using the Wilcoxon signed ank test. All tests wee pefomed using StatView 4.01 (Abacus Concepts, Inc., Bekeley, Califonia). Monthly data wee soted as follows fo seasonal analyses: ealy beeding season, Januay and Febuay; late beeding season, Mach and Apil; kit-eaing season, May, June, and July; kit-dispesal season, August and Septembe; and winte season, Octobe, Novembe, and Decembe. Geneal linea models (GLM) wee used to identify tends in the fine-scale distibution of adiolocations of 2 individuals whose anges ovelapped. Models wee of the fom (a pai b), whee a and b epesent the numbe of locations ecoded fo each membe of an ovelapping pai, and pai is the blocking facto epesenting the identity of each of the 13 pais used. Sepaate values fo a and b wee used fo each of the 200-m sections in which fixes of 1 individual wee ecoded. We an the analysis afte eliminating sections with 10 and then 20 locations fo at least 1 membe of the pai, to exclude sections not being intensively used by 1 animal. Location numbes wee log tansfomed if necessay to achieve nomality. Results ae pesented as mean SE. RESULTS Population. Fifty-one mink wee captued a total of 184 times (17 animals wee caught only once, 7 twice, 6 thee times, 7 fou times, 6 five times, 4 six times, and 1 animal each fo 10, 11, 14, and 18 times) duing 4,336 tap-nights between May 1995 and August 1997 an aveage of 23.6 tapnights fo each captue. The total numbe of captues in each month was positively coelated with the total numbe of diffeent individuals captued in the month (Kendall ank coelation test: n 23, 0.78, P ). The oveall monthly tapping success (numbe of captues pe tap-night) did not change seasonally, based on the 5 mink seasons, in both sexes (Kuskal Wallis tests: fo males, d.f. 4, H 3.2, P 0.53; fo females, d.f. 4, H 2.3, P 0.68). Tapping success did not diffe between sexes (Wilcoxon signed ank test, n 23, Z 1.07, P 0.28). The monthly likelihood of ecaptuing mink (total numbe of captues pe tapnight divided by the total numbe of diffeent individuals) did not change seasonally in eithe sex (Kuskal Wallis tests: fo males, d.f. 4, H 1.6, P 0.81; fo females, d.f. 4, H 1.8, P 0.77). Oveall, 27 diffeent males and 24 diffeent females wee ecoded in the study aea and male:female sex atio was 1:0.86

4 1344 JOURNAL OF MAMMALOGY Vol. 84, No. 4 FIG. 1. Seasonal changes of aveage monthly population of mink in the Uppe Thames study aea, United Kingdom. EB (ealy beeding season: Januay and Febuay), LB (late beeding season: Mach and Apil), KR (kit-eaing season: May July), KD (kit-dispesal season: August and Septembe), and W (winte season: Octobe Decembe). Numbes in paentheses indicate sample size. (1:0.64 among adults, n 41), which does not depat significantly fom 1:1 (binominal test, P 0.79 fo total population, P 0.21 fo adults). Among juveniles, the sex atio was 1:3 and also not significantly diffeent fom 1:1 (binominal test, P 0.08, n 12). The monthly aveage numbe of mink along the 24-km ive stetch was (1 mink pe 3.3 km) with males and females. Among males, the monthly population changed significantly thoughout the yea (Kuskal Wallis test: d.f. 4, H 19.66, P ) with 2 population peaks in ealy beeding season and kit-dispesal season (Fig. 1). Howeve, among females, thee was no significant diffeence in monthly population size between the 5 seasons (Kuskal Wallis test: d.f. 4, H 6.57, P 0.16) (Fig. 1). Recuitment, immigation, emigation, and mink biomass. The aveage ecoded litte size at about 2 months afte bith was (n 4), not significantly biased towad females (1:2.3, binominal test, P 0.34). Duing kit-dispesal and winte seasons, aveage monthly adult:juvenile atio was 1:0.46 fo males and 1:1.24 fo females, a significant diffeence between sexes (Wilcoxon signed ank test: n 11, Z 2.4, P 0.018). On aveage, each month, new individuals appeaed in the study aea, of which wee males and females. The numbe of newly ecoded adult males peaked in ealy beeding season (Kuskal Wallis test: d.f. 4, H 9.69, P 0.046) and newly ecoded juvenile males peaked in kit-dispesal season (Kuskal Wallis test: d.f. 4, H 14.86, P 0.005). Howeve, in females, only the numbe of newly ecoded juveniles peaked in the kit-dispesal season (Kuskal Wallis test: d.f. 4, H 11.16, P 0.025). On aveage, individuals disappeaed fom the study aea pe month ( males and females). Among males, the numbe of individuals that left the study aea changed thoughout the yea (Kuskal Wallis test: d.f. 4, H 10.32, P 0.035), and the disappeaance of adult males peaked in the ealy beeding season (Kuskal Wallis test: d.f. 4, H 13.84, P 0.008). Howeve, among females, thee was no significant seasonal change in numbes disappeaing (Kuskal Wallis test: d.f. 4, H 4.88, P 0.30). When the new appeaances and disappeaances wee combined, this mobile section of the population changed seasonally significantly in males (Kuskal Wallis test: d.f. 4, H 12.42, P 0.015), but not in females (Kuskal Wallis test: d.f. 4, H 5.43, P 0.25). A population instability index (numbe of appeaances and disappeaances each month summed, divided by

5 Novembe 2003 YAMAGUCHI AND MACDONALD INTRASPECIFIC COMPETITION IN MINK 1345 the total population of esident mink in that month) was high in ealy beeding and late beeding seasons and low in winte and kiteaing seasons in males (Kuskal Wallis test: d.f. 4, H 9.84, P 0.043), but again, did not vay seasonally in females (Kuskal Wallis test: d.f. 4, H 5.93, P 0.205). The aveage body weights of males and females wee 1, g (n 25) and g (n 16) espectively fo adults, those fo juveniles wee 1, g (n 3) and g (n 9), and fo nealy independent kits in July wee g (n 3) and g (n 6). Theefoe, fo the following modeling, the body weights of a male and a female duing kit-dispesal and winte seasons ae calculated on the bases of adult: juvenile atios as 1,298 g and 641 g, espectively, and fo the est of the yea they ae 1,374 g and 707 g. The total male and female mink biomass (monthly aveage) in the study aea is shown in Fig. 2. The male biomass changed significantly thoughout the season (Kuskal Wallis test: d.f. 3, H 15.25, P 0.002) although that of females did not (Kuskal Wallis test: d.f. 3, H 1.22, P these analyses excluded the kit-eaing season due to unknown biomass of kits). Home-ange size. The mean homeange size of males encompassed km of wate couse (Table 1). This was significantly longe than the km used by females (Mann Whitney U- test: n 5 and 8, U 0.5, P 0.004). Males 1 and 2 wee tansients duing the beeding season. Although male 3 tavesed the study aea fo 3 consecutive months, we lost his signal intemittently fom the 24- km study aea. Theefoe, the 4.5 km of the study aea he used may have been only a pat of his full ange. Futhemoe, although he was vey intensively adiotacked, male 4 had been followed only fo 5 days befoe he was found dead in a mooed boat. It may theefoe be appopiate to exclude these 2 individuals fom analysis. Then the mean FIG. 2. Seasonal changes of aveage monthly biomass of male and female mink in the study aea. Abbeviations as in Fig. 1. ange size of the emaining males was km, again significantly longe than that used by females (Mann Whitney U- test: n 3 and 8, U 0, P 0.013). Duing the beeding season, an aveage home ange of a female was km along the wate couse, not significantly longe than km duing the nonbeeding season (Mann Whitney U-test: n 6 and 4, U 11.5, P 0.91). Unlike females, all esident males disappeaed fom thei non beeding-season home ange by the end of Febuay and subsequently wee not found within the 24-km study aea. Radiotacking and tapping indicated that esident male 1, (which had stayed in his home ange fo the pevious 9 months) and male 5 (esident fo the pevious 11 months) used thei non beedingseason home anges until thei disappeaances in Febuay. Theefoe, fo the following analyses, the beeding-season home ange of male 1 until his disappeaance was

6 1346 JOURNAL OF MAMMALOGY Vol. 84, No. 4 TABLE 1. Home-ange size of adiotacked individuals eithe duing the beeding season (Januay Apil) o the nonbeeding season (May Decembe). Female 4 was studied fist as a juvenile, late as an adult. Status efes to esident () o tansient (t). Individual Age Status Tacked peiod (numbe of fixes) Range (km) Nonbeeding season Male 1 Male 2 Female 1 a Female 2 a Female 4 a Female 5 b Beeding season Male 3 Male 4 c Male 5 Female 1 Female 3 Female 4 Female 6 c Female 7 Female 8 Juv adult a Tacked peiod does not include beeding season (Januay Apil). b Lived outside main study aea. c Died duing the tacking peiod. t t 23 May 08 Decembe 1995 (382) 06 July 05 Septembe 1995 (124) 24 July July 1996 (79) 15 June August 1996 (297) 16 Octobe May 1996 (196) 02 Septembe 06 Octobe 1995 (137) 30 Januay 19 Mach 1996 (971) 21 Febuay 25 Febuay 1996 (313) 30 Januay 22 Febuay 1997 (344) 19 Mach 26 Apil 1996 (504) 11 Apil 02 May 1996 (68) 27 May 30 Apil 1996 ( Apil 23 Apil 1996 (221) 04 Febuay 17 Apil 1997 (903) 02 Apil 17 Apil 1997 (312) substituted by his non beeding-season ange and male 5 s non beeding-season ange by his beeding-season ange (see Table 1). Home-ange ovelap. A mink home ange was not completely exclusive, and othe mink of both sexes wee pesent within it. As a monthly aveage, within the home ange of each of the 5 adiotacked males, othe males and females wee found. Within the home ange of each adiotacked female (n 7; female 5 excluded because she lived outside the main study aea), males and othe females wee found. Females had a lage numbe of males in thei home anges duing the beeding season compaed with the est of the yea, and the seasonal diffeence was statistically significant (Kuskal Wallis test; d.f. 4, H 14.81, P Fig. 3). Although females may also tend to have moe females in thei home anges duing the beeding season (Fig. 3), any such tend was not statistically significant (Kuskal Wallis test: d.f. 4, H 8.13, P 0.09) no was it fo males (Kuskal Wallis tests: othe males, d.f. 4, H 4.22, P 0.38; females, d.f. 4, H 4.32, P 0.36 Fig. 3). The extent of ange ovelap was analyzed on the bases of adiotacked individuals (Table 2). When 2 home anges ovelapped, on aveage 66.3% 15.6% (n 4) of the home ange of a female was ovelapped by that of the othe female, and 69.1% 9.4% (n 10) of the home ange of a female was ovelapped by that of the ovelapping male. Similaly, on aveage 88.2% 7.4% (n 2) of the home ange of a male was ovelapped by that of the othe male and 32.6% 5.1% (n 10) of the home ange of a male was ovelapped by that of the ovelapping female. On the bases of adiolocations, on aveage, 78.1% 10.8% (n 4) of adiolocations of a female wee found in the aea ovelapped by the home ange of the othe female and 68.8% 10.5% (n 10) of the locations of a female wee found in the aea used by the ovelapping male. Similaly, on

7 Novembe 2003 YAMAGUCHI AND MACDONALD INTRASPECIFIC COMPETITION IN MINK 1347 aveage, 84.6% 3.7% (n 2) of the locations of a male wee in the aea ovelapped by the home ange of the othe male and 36.2% 7.9% (n 10) of the locations of a male wee in the aea used by the ovelapping female. The ovelapped aea and the numbes of adiolocations in it wee significantly positively coelated (Kendall ank coelation tests: n 26, 0.838, P ). Spacing pattens inside home anges. A male home ange contained, on aveage, sections with no adiolocations, significantly moe than the sections of a female (Mann Whitney U-test: n 5 and 8, U 3, P 0.012). Howeve, the aveage male home ange did not contain significantly moe sections with adiolocations of eithe 0, 10, o 20 than did a female home ange (Mann Whitney U-tests: n 5 and 8; fo 0, U 12, P 0.24; fo 10, U 18.5, P 0.83; fo 20, U 17.5, P 0.71). Using only sections with 10 adiolocations fom 1 animal, the GLM detected a significantly negative elationship between the numbe of locations of each membe of the pai ecoded in sections (n 13 pais, 99 sections: F 4.84, d.f. 1, 85, P 0.035). Including sections with 20 locations fo 1 of the pai impoved the elationship consideably (n 13, 75: F 40.57, d.f. 1, 61, P ). Post hoc analysis evealed that this elationship held within male male (n 1, 8: F 7.91, d.f. 1, 6, P 0.031) and male female (n 10, 52: F 35.3, d.f. 1, 41, P 0.001) pais, but not female female pais (n 2, 15: F 2.20, d.f. 1, 12, P 0.163). Biomass within an adult female s home ange. If B is the mink biomass to be suppoted within the home ange of an adult female that successfully epoduces in the study aea, then FIG. 3. Seasonal changes of aveage monthly numbe of othe females and males accessing the home anges of adiotacked females and males. Abbeviations as in Fig. 1. B x y, (1) whee x epesents the body weight of the home-ange occupant and he offsping and

8 1348 JOURNAL OF MAMMALOGY Vol. 84, No. 4 TABLE 2. Home-ange ovelap between adiotacked individuals calculated both as ovelapped home-ange length divided by total home-ange length of the animal (aea) and as popotion of adio locations within the aea of ovelap (adiofixes). Fo example, 38.5% of the home ange of male 1 is ovelapped by that of female 2, and the ovelapped aea contains 32.7% of all adiofixes of male 1. Juv- Studied 1st as juvenile, then as adult. No. 1st individual Age Ovelap (%) Aea Radiofixes No. Age 2nd individual Aea Ovelap (%) Radiofixes Male male ovelap Male Male Male female ovelap Male 1 Male 1 Male 1 Male 2 Male 3 Male 3 Male 4 Male 4 Male 5 Male 5 Female female ovelap Female 2 Female Female 2 Female 4 Female 6 Female 1 Female 1 Female 3 Female 2 Female 4 Female 1 Female 3 Female 6 Female 8 Juv Juv y is the aggegated body mass of othe mink using he home ange. Fo simplicity, we assume i y wp (0 i n), (2) i whee n is the numbe of mink fo which the home anges ovelap with the female s home ange, w i is the body weight of animal i, and p i is the popotion of the numbe of animal i s adiolocations within the occupant s home ange compaed with the total numbe of adiolocations of animal i. Substituting fom ou esults, equation 2 can be witten as y 0.362n w 0.781n w m m f f (nm nf n), (3) whee n m and n f ae, espectively, the numbe of males and females fo which home anges ovelap with the female s home ange, and w m and w f ae the aveage body weights of males and females, espectively. Now, by substituting equation 3 into equation 1, we have B x m f 0.362n w m 0.781n w, (4) f Fom Dunstone (1993) and ou esults, we assume that the female beas 6 kits (3 males and 3 females) at the beginning of May, and 3 of them (1 male and 2 females) suvive until July. We assume 4 kits (2 males and 2 females) still suvive in June. Fom Dunstone (1993), we fix the aveage body weight of kits in May at about 10% of July s body weight fo males (79 g) and about 13% of July s body weight fo females (55 g). Similaly, the aveage body weight of kits in June is about 45% of July s body weight fo both males (356 g) and females (247 g). This gives the estimated monthly aveage biomass of the pimay occupant s kits duing the kit-eaing

9 Novembe 2003 YAMAGUCHI AND MACDONALD INTRASPECIFIC COMPETITION IN MINK 1349 FIG. 4. Seasonal changes of the estimated aveage monthly biomass of mink suppoted by a female s home ange. Males means males whose home anges ae at least patially ovelapped with he home ange, Females means othe females whose home anges ae at least patially ovelapped with hes, and Kits means the home-ange owne and he offsping. season as 1,165 g. The aveage numbes of males and females found in a female s home ange in each season is given above. The estimated mink biomass to be suppoted within the home ange of an adult female peaked in ealy beeding and kit-eaing seasons (Fig. 4). The weight-specific enegy equiement of mink is estimated to decease popotionally to (weight 1 /weight 2 ) 0.25 as weight inceases (Dunstone 1993). Theefoe, the enegy equiements pe unit body weight of an aveage adult male (1,374 g), an aveage male duing kit-dispesal and winte seasons (1,298 g), and an aveage female duing kit dispesal and winte seasons (641 g) ae estimated about 85%, 86%, and 102% of that of an aveage adult female (707 g), espectively. Theefoe, based on enegy equiements, they ae 1,168-, 1,116-, and 654-g equivalents of an aveage adult female, espectively. A simila calculation gives the monthly aveage enegy equiements of the pimay occupant s kits duing the kit-eaing season as the same as that of an adult female of 1,383.5 g body weight. On this basis, as a monthly aveage, male mink take about 30% of the enegy consumed by all mink accessing the pimay female s home ange. Whee both sexes hunt the same pey species and the pey ae homogeneously distibuted, in the absence of males, a female might have needed a smalle home ange. Similaly, othe females in he home ange make up a monthly aveage of about 9% of the enegy consumed by all mink accessing the home ange. On an annual basis, it is estimated that dependent kits consume about 33% of all enegy taken by a female in he home ange. Consideing this, a 707-g female, eaing kits hypothesized as above, needs to obtain, by hunting, about 90% of all enegy consumed by a 1,374-g male annually. Resouce depession within an adult female s home ange. In addition to the foegoing biomass effect, the cost to a female of fulfilling he enegy equiements is expected to incease as the numbe of othe individuals (eithe males o females) using he home ange inceases. This is because such exta individuals cause esouce depession (Chanov et al. 1976), focing he to spend moe enegy to hunt fo he minimum enegy equiement (Powell 1994). Fom Powell (1994), pey availability (A) to a home-ange owne can be witten as A a n (0 a 1), (5) whee a is the possibility with which the owne can access an undistubed food patch when she shaes the home ange with anothe individual and n is the numbe of othe individuals accessing he home ange. Powell (1994) poposed

10 1350 JOURNAL OF MAMMALOGY Vol. 84, No. 4 a 1 dc, (6) whee d is a coefficient descibing patch depession afte it is visited by anothe individual and c is the popotion of all the patches in an individual s home ange that is visited by an individual pe unit time. As d inceases, it becomes moe difficult to catch the pey afte the patch has been distubed. If a mink visits 10 patches a day and thee ae 20 patches in its home ange, then c is 0.5. Adapting equation 6 to the spacing pattens of female mink, we get a 1 dcp. i (7) With this equation, and incopoating ou esults, equation 5 can be witten as A b(1 dc p ) nm(1 dc p) nf 1 1 m1 m 1 f1 f b(1 dc p ) nm(1 dc p) nf 2 2 m2 m 2 f2 f, (8) whee b 1 is the popotion of pey species 1 in mink diet in tems of captue. If a mink captues 200 animals pe unit time including 50 of species 1, then b 1 is The coefficient concening patch depession with efeence to pey species 1 is d 1,c m1 and p m ae male-specific values fo c concening pey species 1, and p concening home ange ovelap; similaly c f1 and p f ae fo females. Fo the sake of simplicity, we fix all values fo c m and c f as C. Then, fom the esults, equation 8 can be witten as A b( d C) nm( d C) nf b( d C) nm( d C) nf (9) This equation descibes the availability (monthly aveage) of pey, on the basis of the numbe of individual pey animals, to a home-ange owne that patially shaes he home ange with n m individuals of males and n f individuals of females. Feeas and Macdonald (1999) epoted the most impotant pey fo mink in the Uppe Thames egion is abbits, which fom an estimated 43% of ingested enegy, much highe than the 2nd most impotant pey, which is fish (27%). Also, Yamaguchi et al. (2003) suggested that abbit waens ae the most impotant single habitat featue concening the habitat pefeences of mink of both sexes in the Uppe Thames. These findings suggest that the availability of abbits is cucial fo mink suvival, and both sexes ae likely competing fo this pey. If we concentate on abbit availability only, then equation 9 is witten as A b( d C) nm( d C) nf (10) When the pimay occupant does not shae he home ange with any othe mink, then A A 0 b 1. Now we fix d 1 C as 0.05 i.e., if a mink visits 25% ( 0.25) of all waens in its home ange pe unit time (say 24 h) and anothe hunt would be ineffective in the waens fo the following 4.8 h afte each visit ( , then ) 0.1 and 0.2. Based on equation 9, the availability of abbit A, elative to A 0 ( b 1 1), to a female in the Uppe Thames egion is estimated smalle duing the ealy beeding season (Fig. 5). As a monthly aveage, othe male mink and female mink (with thei kits) espectively claim 25% and 38% of the enegy consumed by all mink in a male s home ange. Howeve, ou only instance of 84.6% (88.2% in tems of aea ovelapped) ovelap between 2 males duing the beeding season depats fom geneal findings of male teitoiality. Intasexual home-ange ovelap among males is geneally minimal duing most of the nonbeeding season, with the possible exception of kit-dispesal season when dispesing male juveniles may be found in esident males home anges (Biks 1981; Dunstone 1993; Dunstone and Biks 1983; Geell 1970; Ieland 1988). Now, if we fix the ovelap between males

11 Novembe 2003 YAMAGUCHI AND MACDONALD INTRASPECIFIC COMPETITION IN MINK 1351 as 20% duing kit-eaing and winte seasons, then, as a monthly aveage, othe male and female mink (and dependent kits), espectively, use 21% and 40% of the enegy consumed within his home ange by all mink. The estimated effect of esouce (abbit) depession to a male, elative to that whee thee is no othe mink in the home ange, with fixed d 1 C values of 0.05, 0.1, and 0.2, is lage in the ealy beeding season (Fig. 5). FIG. 5. Seasonal changes of the estimated elative availability of abbits (monthly aveage of A defined by equation 9) to a female and a male with thee fixed values of d 1 C of 0.05, 0.1, and 0.2. DISCUSSION Tapping success and demogaphy. Live tapping is likely to undeestimate population densities. Some mink evade tapping; Hatle (1976) estimated that untapped mink compised 20% of the total population along an 8.5-km shoeline whee he made 137 captues with 3,911 tapnights (28.5 tap-nights pe captue) ove 5 yeas. In compaison, we tapped 24 km of ive, accumulating 184 captues in 4,336 tap-nights (23.6 tap-nights pe captue) in 28 months. The aveage ange of adiotacked adult females in ou study aea was about 2.8 km along the watecouse, and within this ange we set, on aveage, 13 o 14 taps, all yea ound. In addition to the consideable intensity of ou tapping, we have no evidence of biases esulting fom seasonal diffeences in the ate of tapping success, seasonal diffeences of ecaptue ate, o sexual diffeences of tappability. We judge, theefoe, that while ou population estimates must be consideed as minima, they ae pobably close to eality. Geell (1971) epoted that sex atio in an exploited mink population was female biased (1:1.5), and that, in anothe, initially exploited, population, it swung fom 1:1.2 to 1:0.71 following a potection fom havesting. Pevious studies also indicate a highe popotion of juveniles to adults in havested (4.5:1) vesus unhavested (0.3: 1) populations (Dunstone 1993). In ou study, the oveall juvenile:adult atio was 0.4:1, and the sex atio was male-biased (oveall 1:0.86, in adults 1:0.64) paame-

12 1352 JOURNAL OF MAMMALOGY Vol. 84, No. 4 te values chaacteistic of an undistubed, stable population. Futhemoe, Ameican mink in the Thames catchment may have eached caying capacity, having settled thee 25 yeas peviously (Macdonald and Stachan in litt.). In summay, we assume that ou study population is a satuated mink population along a lowland watecouse with little diect human-caused motality. Oveall, ou esults ae in accod with ealie evidence that mink spatial oganization is based on intasexual teitoiality. The local mink population inceased twice annually. Duing the beeding season (late winte ealy sping), oaming tansient males caused one incease, and in autumn, dispesing juveniles caused anothe. Although esident males may abandon thei home anges and stat to oam duing the beeding season, females tend to emain in thei home anges thoughout the yea. Biomass within an adult female s home ange. The esults suggest that the home ange of a epoductively successful female may need to suppot, as a monthly aveage, 3 times he own enegy equiements duing the kit-eaing season. She needs to suppot almost as much duing the ealy beeding season, howeve, because of the impact of tansient males. As a monthly aveage, males take almost 50% of the enegy estimated to be consumed by all the mink accessing an aveage female s home ange duing the ealy beeding season. Males move longe distances duing the beeding season (Biks 1981; Ieland 1988); thus, they may use food esouces less intensively and thei impact pe unit aea is likely to be diminished. On the othe hand, the aised activity level of those males duing the beeding season (Ieland 1988) may be intepeted that males need moe enegy, compensating fo the lowe intensity at which they use the aea. Unde the assumptions of ou model, had neithe males no othe females cohabited in he home ange, a female in ou study aea might have needed only about 60% of he obseved ange. Resouce depession within an adult female s home ange. The esults suggest that a female has access to a lowe monthly availability of abbits duing the ealy beeding season (Fig. 5) as a esult of the lage numbes of tansient males accessing he home ange. Inteestingly, the pimay occupant of a female s home ange enjoys the highest abbit availability duing the kiteaing season. This is not only because the model assumes that dependent kits ae unlikely to hunt fo themselves but also because, at this time, thee is a elatively low biomass of othe cohabiting mink in he home ange (Fig. 3). Indeed, the esults indicate thee will be no othe females in he home ange duing the entie kit-eaing season (Fig. 3). Beeding females in the Uppe Thames egion ae vey stongly associated with abbit waens (Yamaguchi et al. 2003), and excluding competing females fom these esouces may be enegetically wothwhile. By compaison, it may be physically impossible fo a female to exclude a much lage male fom he home ange (Macdonald 1992; Powell 1994). Indeed, males ae pesent in he home ange even duing the kit-eaing season (Fig. 3). On the basis of the numbe of adiolocations in each 200-m section, howeve, the dyads within 10 ovelapping male female pais showed a significantly negative elationship, suggesting that males and females may avoid each othe within aeas of ovelap. Males may hunt abbits moe than do females (Biks 1981; Ieland 1988). Howeve, possible sexual diffeences in favoed abbit size (McDonald 2002) may have little impact on the extent of esouce depession. As Macdonald (1992) suggested, whee small odents ae a cucial food fo mink, the female s capacity to fit into buows too naow fo males may offe oppotunities fo niche patitioning. In the Uppe Thames egion, howeve, small mammals fom only about 10% of ingested enegy of mink (Feeas and Macdonald 1999). Theefoe, the combination of inceased pedato biomass

13 Novembe 2003 YAMAGUCHI AND MACDONALD INTRASPECIFIC COMPETITION IN MINK 1353 and esouce (abbit) depession due to cohabitation with males may foce a female to maintain a lage home ange in the beeding season. These pessues ae not diminished duing the subsequent, enegyconsumptive, kit-eaing season o duing the kit-dispesal season when he ange must accommodate dispesing juveniles. Thus, intaspecific foces exet inflationay pessues on female home-ange size all yea ound. We estimated the abbit availability (A) assuming, fo simplicity, that the esouce level (numbe of abbits) does not change thoughout the yea. Howeve, in England geneally, the numbe of abbits is lowest in ealy sping (Mach Apil, coinciding with the late beeding season) and highest in late summe (Septembe Octobe, coinciding with the kit-dispesal season Thompson and King 1994). The extent of these seasonal vaiations is wildly vaiable and hence had to genealize, but the peaks may be twice as high as the toughs (Thompson and King 1994). Theefoe, duing the beeding season, home-ange occupants may suffe even lowe abbit availability than we estimate. On the othe hand, the abbit availability duing the kit-dispesal season may not be highe than ou estimate. Male home ange. Assuming that male home-ange sizes wee detemined by food abundance, Sandell (1989) pedicted male home-ange size on the basis of size and enegy allomety as male home ange size [female home ange size (male weight) 0.75] (female weight) 0.75 (11) This assumes that esouces used by females and males have the same patten of availability and that the sexes ae not cohabiting o, if they ae, that they ae not depleting each othe s esouces. The fist assumption may not be plausible eveywhee and the second is definitely not. Also, this allomety does not take into consideation dependent kits, which consume one-thid of all enegy annually obtained by a mothe. Consideing the enegy equiements of kits, following equation 11, the pedicted size of a male s home ange would be only 1.1 times as long as a female s ange. This would pedict male home anges of 3.03 km along the wate couse of ou study aea, less than half the aveage of 6.80 km evealed by adiotacking. To cicumvent the second assumption above, we popose the following modification of Sandell s allomety: male home ange size [female home ange size E f (male weight) 0.75] [E (modified female weight) 0.75 m ], (12) whee E f and E m ae the popotions of enegy consumption by female and male ange occupants, espectively, compaed with the total enegy consumption by all mink (in geneal, all competing pedatos) accessing the home ange. Fo example, if a female home-ange occupant takes only 30% of the total consumption, E f is 0.3. Modified female weight is obtained as a combined aveage weight of a female and he dependent kits, standadized fo enegy equiements pe unit body weight. Fom the esults, equation 12 pedicts the size of a male s home ange as 1.73 times longe than that of a female (about 4.7 km). This still ignoes esouce depession and assumes that the esouces used by females and males have the same patten of availability. Fig. 5 suggests that males ae moe susceptible to esouce depession than ae females. With a value of d 1 C of 0.2, a female still has nealy 90% of abbit availability compaed with that without esouce depession. Howeve, a male had only about

14 1354 JOURNAL OF MAMMALOGY Vol. 84, No. 4 60% of such availability. This alone may foce males to maintain lage home anges solely on the gounds of thei enegy equiements. If we assume that ange size inceases invesely to these availability values, then a male s home ange is pedicted as 5.09 km when the value of d 1 C is Similaly, home-ange size of 5.55 km and 6.72 km would coespond to the values of d 1 C of 0.1 and 0.2, espectively. All these figues ae much lage than estimates based on the taditional fomula fo calculating enegy equiements (equation 11). These esults may suggest that enegy equiements alone explain why male Ameican mink home anges ae much lage than those of females although the total annual enegy equiements of an adult male and a epoductively successful adult female do not diffe geatly. Indeed, thee is evidence that male mink ae not linked epoductively to the females whose home anges they ovelap (Yamaguchi et al. 2003), and this casts futhe doubt on the poposition that male Ameican mink maintain lage home anges fo epoduction. Cuently, it is widely believed that, in many species of Canivoa, and paticulaly in the Mustelidae, the diffeential in size between the home anges of males and females is explicable by the geate enegetic demands esulting fom the male s lage body size as well as the sexual diffeences of the spacing pattens concening epoduction (Sandell 1989). This may indeed be pat of the explanation, but ou analyses show that such calculations should also take account of the extent of home-ange ovelap with othe conspecifics and, often, othe sympatic canivoes. ACKNOWLEDGMENTS We thank the Envionment Agency and the People s Tust fo Endangeed Species fo financial suppot. This wok was also funded in pat by the Oveseas Reseach Student Awad and the New Centuy Scholaship to N. Yamaguchi. We thank J. Gittleman, C. King, R. Powell, S. Rushton, and M. Thom fo thei helpful comments, P. Johnson fo his statistical advice, and M. Thom fo pefoming geneal linea models. LITERATURE CITED BEN-DAVID, M., R. T. BOWYER, AND J. B. FARO Niche sepaation by mink and ive ottes: coexistence in a maine envionment. Oikos 75: BIRKS, J. D. S Home ange and teitoial behaviou of the feal mink (Mustela vison Schebe) in Devon. Ph.D. dissetation, Exete Univesity, Exete, United Kingdom. BIRKS, J. D. S., AND N. DUNSTONE Mink. Pp in The handbook of Bitish mammals. 3d ed. (G. B. Cobet and S. Hais, eds.). Blackwell Scientific Publications, Oxfod, United Kingdom. BIRKS, J. D. S., AND I. J. LINN Studies on the home ange of feal mink (Mustela vison). Symposia of Zoological Society of London 49: BONESI, L Spatial oganisation and feeding ecology of the Ameican mink (Mustela vison) in a coastal habitat. M.Sc. thesis, Univesity of Duham, Duham, United Kingdom. BUENO, F Competition between Ameican mink Mustela vison and otte Luta luta duing winte. Acta Theiologica 41: CHANIN, P. R. F Obsevations on two populations of feal mink in Devon. Mammalia 47: CHARNOV, E. L., G. H. ORIANS, AND K. HYATT Ecological implications of esouce depession. Ameican Natualist 110: CLODE, D., AND D. W. MACDONALD Evidence fo food competition between mink (Mustela vison) and otte (Luta luta) on Scottish island. Jounal of Zoology London 237: CLUTTON-BROCK, T. H Mammalian mating systems. Poceedings fo the Royal Society London Seies B. Biological Sciences 236: DUNSTONE, N The mink. T. and A. D. Poyse, Ltd., London, United Kingdom. DUNSTONE, N., AND J. D. S. BIRKS Activity budget and habitat usage by coast-living mink (Mustela vison). Acta Zoologici Fennica 174: DURBIN, L. S Habitat selection by five ottes Luta luta in ives of nothen Scotland. Jounal of Zoology London 245: FERRERAS, P., AND D. W. MACDONALD The impact of Ameican mink Mustela vison on wate bids in the uppe Thames. Jounal of Applied Ecology 36: GERELL, R Home anges and movements of the mink Mustela vison Schebe in southen Sweden. Oikos 21: GERELL, R Population studies on mink Mustela vison Schebe in southen Sweden. Viltevy 8: HALLIWELL, E. C., AND D. W. MACDONALD Ameican mink Mustela vison in the Uppe Thames catchment: elationship with selected pey species and den availability. Biological Consevation 76: HATLER, D. F The coastal mink on Vancouve Island, Bitish Columbia. Ph.D. dissetation, Uni-

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