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Diets of fishers (Pekania pennanti) and evidence of intraspecific consumption in Pennsylvania.


Re-introduction of extirpated wildlife populations is often a goal of wildlife agencies and land managers (Groom, 2006; Seddon et al, 2007). At a minimum understanding the diet of a re-introduced species is helpful for understanding resource needs, potential impacts of predation on resident species, and guiding adaptive management (Robbins, 1983; Breitenmoser and Haller, 1993; Stahler et al, 2006; Mee et al, 2007; Carlson et al, 2014). The fisher (Pekania pennanti) is a secretive mesocarnivore that has been the target of several re-introduction attempts over the past several decades (Lewis et al, 2012). The fisher's historic range once spanned throughout the majority of North America's northern forests (Powell, 1981). Habitat loss, unregulated trapping, and systematic elimination of predators resulted in a significant range contraction for fishers across the U.S. (Lewis et al, 2012). By the early 1900s, fishers-were extirpated from southern New England and the central and southern Appalachians (Kelly, 1977; Powell, 1981).

Throughout the 20th century, forest and wildlife management techniques improved and reforestation on many lands occurred via natural succession (Lorimer and White, 2003). Ultimately, the combined effect of habitat availability and increased societal acceptance shifted the landscape from a hostile, unaccepting place for fishers to a vacant landscape, far below carrying capacity (Kelly, 1977; Kellert, 1985; Klyza, 2001; Hardisky, 2001). These events spurred several fisher re-introduction efforts in the central and southern Appalachian Mountains (Lewis et al., 2012).

Few studies have focused on understanding the basic ecology of fishers in recently repopulated portions of the central and southern Appalachian Mountains (Dzialak et al., 2005; Gess et al., 2013). This knowledge gap may limit the ability of biologists and land managers to identify and manage high quality habitats for re-introduced fisher populations in Appalachia. Although previous studies have examined fisher diets, most have taken place in the northern portion of the species range in areas dominated by mixed and coniferous forests (Raine, 1987; Arthur et al., 1989; Golightly et al., 2006). It seems reasonable that, due to differences in potential prey communities, diets of fishers may differ substantially in the deciduous forests of the central Appalachians compared to more northerly portions of the species range. To help inform this aspect of fisher ecology, we examined the stomach contents of fishers from a well-established and expanding re-introduced population in Pennsylvania.



From 1994-1998, 190 fishers were translocated from northern New York and New Hampshire and released across northern Pennsylvania (Hardisky, 2001). These founding individuals and natural immigrants from neighboring states resulted in exponential fisher population growth and colonization of nearly all of Pennsylvania's forests (Lovallo, 2008). Forests are the dominant land cover type across Pennsylvania and comprise approximately 58% (670,000 [km.sup.2]) of the state (McWilliams et al., 2007). The majority of Pennsylvania's forests are deciduous with mixed-oak and northern hardwood communities comprising about 86% of the state's forests (McWilliams et al., 2007). Fishers in this study were collected from across much of Pennsylvania's forested regions (Fig. 1).


Fisher carcasses where found within 32 Pennsylvania counties, all within 230 km of 77.8031, 40.8752. We collected carcasses of road-killed, incidental trapper-killed, and legally harvested fishers from 2002 to 2014. We extracted the entire stomach from each carcass and placed it in a separate jar containing 10% formalin solution for 24 h. We then dissected each stomach and rinsed the contents of each over a wire sieve. The contents of each stomach were then teased apart and allowed to dry under a ventilation hood for [greater than or equal to] 24 h. Once dried, we sorted stomach contents into groups based on macroscopic classification of like parts: hard parts (bones, teeth, and claws), vegetative material, fur, feathers, and other materials (e.g., egg shells). If multiple tufts of hair appeared to be potentially from the same prey species, but the relationship was uncertain, we treated each tuft separately to help ensure we maximized prey detection probability. We first attempted to identify stomach remains macroscopically by comparing them to specimens from Indiana University of Pennsylvania's (IUP) museum collections.

To further aid in the identification of stomach contents, we created hair scale slides for any hair samples that we could not identify macroscopically to species (Day et al., 1966). We used three identification keys and our reference slides to identify the mammalian hair found within fisher stomachs to the lowest possible taxa level (Mayer, 1952; Feretic et al., 2008). Although some prey could be individually counted within fisher stomachs as they were consumed whole (e.g., small rodents, etc.), larger prey could not be individually counted. Further, organ/muscle tissue consumed without associated hair is not discerned by this method. Therefore, we quantified dietary diversity (using percent occurrence) as a measure of presence-absence of each species within fisher stomachs (e.g., see Giuliano et al., 1989; Zielinski et al., 1999; Weir et al., 2005). The percent occurrence method is also limited in that multiple individuals of the same species found within a common stomach are considered as a single confirmation of occurrence. Ultimately, these methods provide a quantitative index for the relative importance of a wide range of potential food resources and are comparable to those used by previous fisher diet studies. Note that percent occurrence values may exceed 100% as individual stomachs may contain multiple food items.

We calculated the Shannon-Wiener Diversity Index (H) for all stomach contents of male, female, and all fishers combined (Shannon, 1948). We calculated H' as--[summation] [p.sub.i] In ([p.sub.i]) where [p.sub.i] is the proportion of the 2th species found within each stomach. Additionally, to quantify potential differences in prey size consumed by male and female fishers, we categorized prey into three size classes based on an average mass reported for a species: (1) small prey were <1 kg; (2) medium prey were 1-6 kg; and (3) large prey were >6 kg. We used a Multivariate Analysis of Variance (MANOVA) in program R to examine potential effects of sex on prey size class consumption frequencies (R Core Team, 2015). Finally, we estimated the amount of dietary overlap between males and females using a Pianka's Index (Pianka, 1973) where 0 represents no dietary overlap and 1 represents complete overlap. Overlap was estimated as ([summation][p.sub.ij] [p.sub.ik]) / [square root of ([sumamtion][p.sup.2.sub.ij] [summation] [p.sup.2.sub.ik])] where [p.sub.ij] and [p.sub.ik] represent the proportions of the ith species consumed by the jth and kth sex class.


We examined the stomach contents of 91 fishers (46 males: 42 females: three unknown) collected from 30 counties in Pennsylvania (Fig. 1). All fishers examined in our study were entire (e.g., no missing limbs or digits) upon their collection. Carcass collection was biased toward fall/winter months so we did not consider seasonal effects on diet. Of the fisher stomachs, 75 were from road-killed, 15 were harvested by trappers, and cause of death was not documented for one carcass. Six (6.7%; all from males) stomachs had no contents. Hair, feathers, teeth, and claws were the most useful "hard parts" for aiding in the identification of animal prey as bones were typically highly fragmented.

Evidence of mammalian and avian prey was found in 75 (83.3%) and 11 (12.2%) stomachs, respectively. Eleven (12.2%) stomachs contained fruit/seeds, four (4.4%) contained eggshells, and 11 (12.2%) contained foliage (Table 1). The most commonly detected orders were Rodentia (48.9% of stomachs), Carnivora (23.3%), and Artiodactyla (13.3%). Mesocarnivore remains (Carnivora and Didelphimorphia) were found in 29 (32.2%) stomachs. Small mammals (Rodentia: Cricetidae, Soricomorpha, Eastern chipmunk, and American red squirrel) were detected in 29 (32.2%) stomachs (Table 1). The most common individual species detected were the white-tailed deer (13.3%), fisher (12.2%), and eastern/Appalachian cottontail (Sylvilagas spp.; 12.2%).

Because fisher remains have not been reported previously in the literature, we carefully scrutinized the fisher remains we detected in order to exclude self-groomed hair. We detected fisher hair in 12 stomachs. Of these we determined the presence of fisher hair in one stomach may have been the result of self-grooming as it contained no fisher flesh, bones, or claws. The remaining 11 (four male and seven female) stomachs had fisher remains that were clearly the result of cannibalistic behavior. Of the 11 fisher stomachs containing conspecific remains, six were from road-killed animals and five from trapper-killed. Consumed fisher remains included copious amounts of hair, pieces of epidermis, and/or other hard parts that were from fishers (Fig. 2). In fact, eight of 11 (72.72%) stomachs containing fisher remains had at least a single fisher claw and many stomachs contained multiple claws. Most of these claws were still attached to the distal phalanx and many also included the digital toe pad as well as additional phalanx bones (Fig. 2). Ten of the 11 stomachs that contained fisher remains were from carcasses collected between NovFeb., which ruled out predation of natal fishers as a reason for consumption. The five stomachs from trapper-kills that contained fisher parts were from individuals that had all of their

feet intact and therefore ruled out self-mutilation as an explanation for consumption.

We identified 26 mutually exclusive dietary categories each for male and female fishers with a total number of prey categories of 32 for both sexes combined. Male (n = 46) and female (n = 42) fishers contained similar proportions of all three animal prey size classes (MANOVA, Wilke's [lambda] = 0.99, 3, 87 d.f., P = 0.98; fig. 3). Combined diversity of prey was high (H' = 2.92) with female diversity calculated higher (U = 2.38) than for males (H = 1.88). Our values for dietary overlap (O = 0.90) suggests there is a high degree of dietary overlap between male and female fishers.


Our study provides perspective into fisher diet in a portion of the species range for which information is limited (Dzialak et al., 2005). Our results suggest fishers are capitalizing on the diversity of prey species and vegetation found in deciduous forests of the central Appalachian Mountains. Much of our findings are in agreement with those from a number of previous studies which have described the fisher as a generalist carnivore that feeds on a suite of mammalian prey species supplemented with opportunistic exploitation of a variety of birds, squamate reptiles, plant materials, and fungi (De Vos, 1952; Giuliano et al., 1989; Powell, 1993; Martin, 1994; Bowman et al., 2006). Indeed, because prey specialization in North American Carnivora is believed to be a highly derived trait (Wesley-Hunt, 2005), it seems likely that generalist feeding behavior is the basal character for fishers and specialization on individual prey species (i.e., snowshoe hare and porcupine) is an example of generalist adaptability (Bowman et al., 2006). This generalist diet also makes intuitive sense as the fisher is found throughout much of North America, primarily limited by increased snow depth at the northern limits of its range (Krohn, 2012).

The central Appalachian Mountains provide the ideal setting for fishers to employ generalist feeding behavior because lower trophic levels are comprised of a very diverse faunal community. In fact, Pennsylvania alone hosts 66 species of terrestrial mammals, >190 breeding birds, and an abundance of squamate reptiles, amphibians, invertebrates, and plants that fishers can incorporate into their diets (Wherry et al., 1979; Hulse et al., 2001; PGC, 2011; Wilson et al., 2012). The diet of fishers in the central Appalachian Mountains is more diverse compared to most populations studied elsewhere (Martin, 1994). In fact, the diversity value (H') from our study is comparable to the highest value ever reported for the species (California; Zielinski et al., 1999). The high diet diversity reported for fishers in the Sierra Nevada Mountains of California was attributed to the absence of "normal" fisher prey, such as porcupines (Erethizon dorsatum) and lagomorphs in that region (Zielinski et al., 1999). Conversely, fishers in our study exhibited a high level of dietary diversity while simultaneously using porcupines and lagomorphs (cottontails) as common prey items. Because of this diverse prey base, fishers in the central Appalachians do not appear to be specialists on any individual species.

Fishers display a high degree of sexual dimorphism (most markedly in body size), and it has been suggested that their diets might also differ as a function of ideal prey size and foraging strategy (Weir et al., 2005). Whereas it seems reasonable to suggest that sexual dimorphism would result in prey resource partitioning in fishers, we found no differences between males and females with respect to either prey species or prey size-class composition. Females and males we sampled also had similar values for diet diversity. Our study is not the first to fail to detect dietary differences between male and female fishers (Giuliano et al., 1989, Dzialak et al., 2005). The diet similarities between sexes in our study could be influenced by the availability of road-killed carcasses. If road-killed carcasses of large species (i.e., ungulates, larger carnivores) are a major component of fisher diets, all size classes of prey would be accessible to all fishers, regardless of the consumer size/sex. However, even without the free availability of prey to both sexes, pronounced differences in fisher physiology are restricted primarily to body size (rather than jaw morphology and dentition; Powell, 1993) and differences in diet are not necessarily expected.

In addition to consuming a wide diversity of herbivore and lower trophic level prey, fishers in Pennsylvania seem to readily incorporate other mesocarnivores (6 spp.) into their diets. Fisher consumption of mesocarnivores has been documented in several other studies (Giuliano et al, 1989; Powell, 1993; Martin, 1994; Dzialak et al, 2005; Bowman et al, 2006). Mesocarnivores were found in nearly a third of the fisher stomachs we examined. This suggests mesocarnivores constitute a relatively important dietary component of Pennsylvania's fisher population.

Of the mesocarnivores we observed in fisher stomachs, by far the most novel and unexpected species was fisher. Cannibalism is not unusual among animals and it can influence population structure, life history, behaviors, and competition for mates and resources (Polis, 1981; Polis et al, 1984). A recent study by Gabriel et al (2015) described 58 predation events on fisher; while several predator species exploited fishers as prey (e.g., coyote, mountain lion Puma concolor, etc.), cannibalism was not observed. In contrast to our own work, Gabriel et al, (2015) worked within an imperiled population of fishers where the population size is small. Because Pennsylvania's fisher population is growing more rapidly than many northwestern subpopulations (Lovallo, 2008), different interaction dynamics would be expected among conspecifics within each region.

Although previous research on fisher diet reported the occurrence of conspecific hair in scat samples, all instances were attributed to self-grooming (Zielinski et al, 1999). We observed several instances of fisher hair in the stomachs we examined. While it is possible that the fisher remains we observed were the result of scavenging already dead fishers (e.g., road mortalities or fisher feet used as trapping bait), neither of these explanations seem likely; for road-killed conspecific scavenging, fishers would have had to heavily selected for roadside fisher carcasses, passing-up more common species like white-tailed deer, Virginia opossum, or raccoon. Moreover, most of the stomachs with evidence of cannibalism contained fisher toes, claws, and/or foot pads. While the paws of a fisher mark the only furred portion of the carcass which would typically not be sold with the pelt, the low fisher bag limit (one individual/season) would render fisher paws as trap bait a rare occurrence indeed. We find the observation of paws in stomachs to be rather perplexing for two reasons: (1) these body parts should have less nutritional value than fleshier body parts and (2) we rarely documented the consumption of feet/paws of other, equal-sized prey species. As such we hypothesize that fisher consumption of conspecifics may have been driven by a factor independent of nutrition.

We propose that the conspecific remains we observed in fisher stomachs may have been due to intraspecific strife that resulted in the death and subsequent consumed of particular body parts. Most instances of intraspecific predation and cannibalism reported in mammals are due to infanticide (Polis et al, 1984). However, this behavior does not explain the cannibalism we observed in fishers. Rather, it appears that adults were consuming other adults/subadults because the claw sizes we found in stomachs were comparable to the claw sizes of the consumers (Fig. 2). Additionally, 10 of 11 stomachs that contained fisher remains were from carcasses that were collected between Nov.-Feb., which would rule out predation of natal fishers. This timing also coincides with fisher dispersal; therefore, so perhaps the intraspecific predation we observed was the result of territorial disputes. Instances of adults killing and consuming other adults due to territorial disputes have been documented for at least eight mammalian species (Polis, 1981; Polis et al, 1984).

We hypothesize fishers may exhibit a behavior whereby territorial disputes can result in an individual killing a conspecific and then consuming its feet in order to remove further chemical signaling from plantar glands (Buskirk et al, 1986). The frequency of this behavior would be intensified, and therefore more likely to be observed, in high-density fisher populations. Indeed, a review of studies on intraspecific predation found 65 intances of increased predation rates due to overcrowding or high population densities (Polis, 1981), and one explanation for this observation was that at higher population densities there is an increased frequency in which conspecifics violate minimum distances for intraspecific space use (Polis, 1981). The fisher population we studied in Pennsylvania has experienced a dramatic increase over the past two decades (Lovallo, 2008). Observations of intraspecific predation might be harder to observed for populations that occur at lower densities (Polis, 1981), such as most fisher populations. This may explain why no fisher diet studies conducted previously reported evidence of cannibalism. That said, because our observations of conspecific remains in the stomachs of fishers were opportunistic rather than the outcome of experiment-based research, we are limited in our ability to explain the cause of the frequent evidence of cannibalism among fishers in our study population. Future research that provides insight regarding causal mechanisms of cannibalistic behaviors in fishers would be worthy. Specifically, future research should attempt to investigate the frequency at which intraspecific predation occurs in other fisher populations, the demographic conditions (i.e., densities) that influence its frequency and the extent to which this behavior may regulate populations.

Acknowledgments.--A portion of this work was funded by the Pennsylvania Game Commission (PGC) State Wildlife Grant Program. The Indiana University of Pennsylvania-Research Institute and School of Graduate Studies and Research provided additional funding. We would like to thank the many PGC regional staff who provided carcasses for this study. Finally, we would like to thank the thoughtful in-depth comments by two anonymous reviewers who strengthened this manuscript considerably.


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Department of Biology, Indiana University of Pennsylvania, Weyandt Hall, Room 114, 975 Oakland Avenue, Indiana 15701



Pennsylvania Game Commission, 2001 Elmerton Avenue, Harrisburg 17110

(1) Corresponding author present address: Cornell University, Natural Resources, 11 IE Fernow Hall, 226 Mann Dr., Ithaca, New York 14853: e-mail:

(2) Present address: Northwest Mississippi Community College, 4975 Highway 51 North, Senatobia, Mississippi 38668-1701

Caption: FIG. 1.--Pennsylvania counties from which fisher (Pekania pennanti; n = 91) carcasses were collected from 2002-2014. Numbers within each county boundary represent the number of carcasses from each county from which we had reliable spatial data (n = 79)

Caption: Fig. 2.--Stomach contents from a road-killed male fisher (Pekania pennanti) in Pennsylvania. Among the prey items detected in this stomach were the claws and toes of another fisher. The forepaw from the consumer is also shown for scale. In many cases, the conspecific claws extracted from fisher stomachs were comparable in size to those of the consumer

Caption: Fig. 3.--The size classes of prey contained within fisher (Pekania pennanti) stomachs collected as road-kill and trapper harvests in 2002-2014, Pennsylvania. Small prey (<1 kg), medium prey (1-6 kg), and large prey (>6 kg) were quantified within fisher stomachs. Male (light bars; n = 46) and female (dark bars; n =41) fishers did not differ in the proportions of their diet comprised of any size class
TABLE 1.--The frequency of dietary components for fishers (Pekania
pennanti) collected in Pennsylvania, 2002-2014. Fisher stomach
contents were identified to the species level by a combination of
microscopic hair inspection and macroscopic examination of bones,
teeth, claws and other hard parts. We classified each prey into size
classes based on mass with <1 kg as "small" (S), 1-6 kg as "medium"
(M), and >6 kg considered "large" (L)

Taxa                              Common name              Size

    Odocoileus virginianus        White-tailed deer         L
    Sorex cinereus                Masked shrew              S
    Blarina brevicauda            Short-tailed shrew        S
    Sylvilagus spp.               Cottontail spp.           M
      Erithrizon dorsatum         N. American porcupine     M
      Castor canadensis           American beaver           L
    Neotoma magister              Allegheny woodrat         S
    Microtus spp.                 Vole spp.                 S
    Microtus pinetorum            Woodland vole             s
    Microtus pennsylvanicus       Meadow vole               s
    Myodes gapperi                Red-backed vole           s
    Peromyscus spp.               Deer mouse spp.           s
      Marmota monax               Woodchuck                 M
      Sciuris carolinensis        E. gray squirrel          s
      Tamias striatus             E. chipmunk               s
       Tamiasciurus hudsonicus    Red squirrel              s
      Canis latrans               Coyote                    L
      Vulpes vulpes               Red fox                   M
      Pekania pennanti            Fisher                    M
      Procyon lotor               Raccoon                   M
      Mephitis mephitis           Striped skunk             M
      Lasionycteris noctivagans   Silver-haired bat         S
    Didelphis virginiana          Virginia opossum          M
    Avian egg                               --              --
      Phasianus colchicus         Ring-necked pheasant      S
        Poecite atricapillus      Black-capped chickadee    S
        Picoides pubescens        Downy woodpecker          S
        Elaphe obsolete           Black rat snake           S
       Lithobates                 True frog species         S
  Actinopterygii                  Bony fish
        Orthosoma brunneum        Brown prionid beetle      S
      Foliage                               --              --
      Fruit/seeds                           --              --

Taxa                              Total (%)   [male] (%)   [female] (%)

Mammalia                          75 (83)     37 (80)      36 (88)
    Odocoileus virginianus        12 (13)      7 (15)       5 (12)
  Soricomorpha                     7 (8)       3 (7)        4 (10)
    Sorex cinereus                 3 (3)       2 (4)        1 (2)
    Blarina brevicauda             1 (1)       0            1 (2)
    Sylvilagus spp.               11 (12)      6 (13)       5 (12)
  Rodentia                        44 (49)     24 (52)      19 (46)
      Erithrizon dorsatum         10 (11)      6 (13)       3 (7)
      Castor canadensis            2 (2)       1 (2)        1 (2)
  Cricetidae                      22 (24)     13 (28)       9 (22)
    Neotoma magister               1 (1)       1 (2)        0
    Microtus spp.                 11 (12)      6 (13)       5 (12)
    Microtus pinetorum             1 (1)       0            1 (2)
    Microtus pennsylvanicus        5 (6)       3 (7)        2 (5)
    Myodes gapperi                 3 (3)       2 (4)        1 (2)
    Peromyscus spp.               11 (12)      6 (13)       5 (12)
    Sciuridae                     17 (19)     10 (22)       7 (17)
      Marmota monax                4 (4)       3 (7)        1 (2)
      Sciuris carolinensis         8 (9)       4 (9)        4 (10)
      Tamias striatus              3 (3)       2 (4)        1 (2)
       Tamiasciurus hudsonicus     3 (3)       2 (4)        1 (2)
  Carnivora                       21 (23)      9 (20)      11 (27)
    Canidae                        5 (6)       2 (2)        2 (5)
      Canis latrans                2 (2)       1 (2)        1 (2)
      Vulpes vulpes                3 (3)       1 (2)        1 (2)
      Pekania pennanti            11 (12)      4 (9)        7 (17)
      Procyon lotor                6 (7)       3 (7)        3 (7)
      Mephitis mephitis            1 (1)       1 (2)        0
      Lasionycteris noctivagans    1 (1)       1 (2)        0
    Didelphis virginiana           8 (9)       4 (9)        3 (7)
Aves                              11 (12)      6 (13)       4 (10)
    Avian egg                      3 (3)       3 (7)        0
    Galliformes                    1 (1)       1 (2)        0
      Phasianus colchicus          1 (1)       1 (2)        0
        Poecite atricapillus       1 (1)       0            1 (2)
        Picoides pubescens         1 (1)       0            1 (2)
        Elaphe obsolete            1 (1)       1 (2)        0
       Lithobates                  1 (1)       0            1 (2)
  Actinopterygii                   2 (2)       0            2 (5)
        Orthosoma brunneum         1 (1)       1 (2)
      Foliage                     10 (11)      4 (9)        6 (15)
      Fruit/seeds                 11 (12)      6 (13)       4 (10)
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Article Details
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Author:McNeil, Darin J., Jr.; Nicks, Courtney A.; Wester, Jennifer C.; Larkin, Jeffery L.; Lovallo, Mathew
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1U2PA
Date:Apr 1, 2017
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