The Taphonomic Effects of Eastern Gray Squirrels (Sciurus ... · Other known rodent bone-gnawers include Old World porcupines (Hystricidae) [14–23], North American porcupines (Erethizon

Journal of Forensic Identification66 (4), 2016 \ 349

1 Forensic Anthropology Program, Department of Anatomy and Neurobiology, Boston University School of Medicine, Boston, MA

2 Off ice of the Chief Medical Examiner, Boston, MA

Received February 5, 2016; accepted April 8, 2016

Article

The Taphonomic Effects of Eastern Gray Squirrels (Sciurus carolinensis) Gnawing on Bone

James T. Pokines 1, 2

Sierra A. Santana 1

Jessica D. Hellar 1

Patricia Bian 1

Alyssa Downs 1

Nora Wells 1

Meghan D. Price 1

Abstract: The eastern gray squirrel (Sciurus carolinensis) is known to gnaw on bone and thus has the potential to affect terrestrial surface remains in forensic scenes throughout its extensive geographic range in North America and other places in the world where it has been introduced. To determine the timing, extent, and characteristics of gnawing of this rodent species within an urban environment, an initial sample of 305 dry postcranial bones of white-tailed deer (Odocoileus virginianus) were wired to trees for a period of 8 weeks and observed every 2 weeks in multiple sites in Boston, Massachusetts. Squirrel gnawing damage included the typical parallel striations noted for rodents and the loss of epiphyses of long bones, marrow cavity exposure, and sculpting of bone margins, with a cumulative total of 58 out of the original sample of 305 bones (19.0%) having gnawing damage of some kind. When subtracting the bones lost during the experiment without previous gnawing, the cumulative total is 58 out of 271 bones (21.4%). Rodent gnawing can advance rapidly, potentially causing the loss of diagnostic bone features and obscuring previous trauma sites, and researchers should be aware of its effects on exposed skeletal remains.

Journal of Forensic Identification350 / 66 (4), 2016

IntroductionRodent gnawing is a common type of taphonomic alteration to

bone seen in multiple terrestrial contexts and frequently appears on human skeletal remains from forensic scenes [1–8]. Their gnawing damage to bone may reduce the identifiable features, limit the scope of metrical analyses, and be mistaken for other types of bone alteration, including gnawing by carnivores, other natural sources of bone surface striations, and some forms of perimortem trauma. The presence of rodent gnawing, however, also may help narrow the depositional environment for skeletal remains if they are later divorced from their original context. Rodent gnawing activities also may result in dispersal of bones away from where the bodies were initially deposited, thus reducing the amount of a skeleton recovered. Multiple members of the squir rel family (Sciur idae), including eastern gray squirrels (Sciurus carolinensis) [4, 7, 9, 10] and red squirrels (Tamiasciurus hudsonicus) [11], are known to gnaw on large ver tebrate bones and possibly scavenge soft tissue [12, 13]. Other known rodent bone-gnawers include Old World porcupines (Hystricidae) [14–23], North American porcupines (Erethizon dorsatum) [24–27], gerbils (Desmodillus spp.) [17], house mice (Mus musculus) [7], rats (Rattus spp.) [1, 4, 7], wood mice (Apodemus sylvaticus) [10], woodland voles (Microtus pine tor um) [28], and possibly deer mice (Peromyscus maniculatus) [29]. Some rodent species, especially Old World porcupines, are especial ly relevant to studies of human evolution, because they also will concentrate bones in dens for gnawing or gnaw on bones already accumulated in caves by hominins [14, 16, 17, 22, 30, 31] or other species [21, 23]. In forensic contexts, some rodents have even been found to nest within human remains, including the thorax [1] and cranium [28]. Forensic scenes may include indoor cases, where commensal rodent species have access to remains and may also cause soft tissue damage [32, 33], and former cemetery remains that were exposed on the surface [8]. Rodents also have been shown to disperse bone [1, 14] and to create pseudotrauma [18, 33–36]. Multiple types of taphonomic changes caused by rodents are pervasive among human remains from forensic scenes, and these require additional research regarding which rodent species have altered a given set of remains and their overall potential for altering them in a given environment [4].

Rodents may gnaw on bone for multiple reasons, including wearing down their ever-growing mandibular and maxillary incisors to maintain the length and sharpness of these teeth.


Gnawed objects can include rocks [37] and artificial items such as plastic electrical covers (pers. obs.). Gnawed bones also may provide nutrients such as calcium [5, 9, 19, 20, 34, 38]. Omnivorous rodents also may gnaw on wet bone, although this behavior leaves a different pattern than dry-bone gnawing [4, 5]. Wet-bone gnawing often targets the cancellous bone (and the soft tissue contained within, including fat) and may leave behind pedestaling, wherein the epiphyses are gnawed into with some surrounding bone left untouched [3, 5]. Dry-bone gnawing generally focuses on the cortical bone and concentrates on bone margins; in these cases, the cortical bone itself is targeted [5].

Apart from the effects of porcupines on bones recovered from potential archaeological or paleontological sites, lit tle research has been attempted to determine the effects of other individual rodent taxa on bone. Some information concerning rodent gnawing der ives f rom exper iments designed to determine a broader range of taphonomic changes in a given environment [39, 40]. For example, Young et al. [10] placed deer (Cervus nippon and Capreolus capreolus) carcasses in United Kingdom woodlands in order to study scavenger effects on surface remains. They found that during daylight hours, eastern gray squirrels gnawed skeletonized remains, which was just one taphonomic process observed. Other data come from case studies, such as Haglund [1] noting that rodents, including brown rats (Rattus norvegicus), gnawed both soft tissue and bone in three forensic cases. Pokines [28] identif ied a species of wild rodent (Microtus pinetorum) nesting inside a human cranium from a surface remains case, and the dry-bone gnawing on the skeletal elements was also consistent with this species. Klippel and Synstelien [4] introduced multiple samples into environments in Tennessee and Pennsylvania to determine the effects of brown rats and eastern gray squirrels on bone, both greasy and dry, and found multiple instances of gnawing attributable to the latter species on dry bone. In contrast, brown rats produced wet-bone gnawing patterns. Pokines et al. [7] also performed multiple dry-bone gnawing experiments with individual rodent species, including laboratory specimens of house mouse and brown rat, zoo specimens of African crested porcupine (Hystrix cristata), and wild specimens of eastern gray squirrels. Differences in the degree of gnawing and maximum striation width were noted.

The taphonomic behavior of rodents in urban areas is also of interest; many forensic cases are found in these or suburban settings. Eastern gray squirrels in particular have been noted by previous researchers to be a potential source of taphonomic


alteration [4, 7, 9, 10], and this species is ubiquitous in many urban settings. The present study examines the bone-gnawing behavior of eastern gray squirrels in an urban environment to determine the form and extent of gnawing of this species and to differentiate the taphonomic alterations of this species from other rodents.

Eastern Gray Squirrel Ecology and BehaviorThe eastern gray squir rel has been introduced to many

geographic areas outside of its original range [41], which was along the east coast of the United States, extending from Maine to Florida and reaching as far west as eastern Texas and as far north as Canada [42–44]. As of the early 20th century, the eastern gray squirrel had become one of the most ubiquitous wild mammals in urban America [45, 46]. Beginning in the early 1800s, the eastern gray squirrel was introduced to various locations throughout the United States and other areas in the world [42, 44, 47]. Introductions include California, Montana, Oregon, and Washington in the United States and Quebec, New Brunswick, British Columbia, Manitoba, Nova Scotia, Ontario, and Saskatchewan in Canada [42]. The eastern gray squirrel also was successfully introduced into the United Kingdom, Ireland, Italy, and South Africa [44, 47]. Within its established range, the eastern gray squirrel prefers to inhabit broad-leaved, mature deciduous and mixed woodlands, forested bottomland, towns, suburban woodlots, and city parks [47–51].

The home ranges of individual eastern gray squirrels can vary by location and depend on food availability and population density [44, 51, 52]. Home ranges can be as small as 0.4 ha and as large as 20.2 ha throughout the year, although less than 5 ha (approximately 12.4 acres) is more typical [44, 48, 49, 51, 53, 54]. Males were observed to occupy a slightly larger average range than females [53–55]. Home ranges of eastern gray squirrels often overlap with one another [42, 54]. Eastern gray squirrel population densities vary depending on habitat type, with denser populations present in urban versus non-urban settings [56, 57]. Densities as high as 21 squirrels per ha can occur in urban parks, whereas continuous woodlands typically suppor t less than 3 squirrels per ha [42, 56]. Eastern gray squirrels have the ability to make their home ranges more compact, and the overall density of squirrel populations may rise with increasingly fragmented habitats [57, 58]. Dense populations of eastern gray squirrels


within urban areas can be detrimental to surrounding plant and animal life [46, 50, 52, 58].

Ter r itor iality in the eastern gray squir rel occurs most frequently in the context of the breeding cycle [42, 54, 55, 59]. Females, especially during lactation, will defend their home ranges against other females, and male antagonistic behavior peaks in spring and autumn when young squirrels disperse from parental territories. The purpose of intraspecif ic aggression may be to remove young animals and immigrants from home ranges [59]. The number of individual squirrels affecting a given set of skeletal remains at any point in time therefore may be limited.

The proximity of humans to populations of eastern gray squirrels affects their behavior towards humans in general as well as their daily activity patterns. Eastern gray squirrels have also been noted to react differentially to predators, depending on the proximity of the predator and the associated risk [60–63]. Eastern gray squirrels that live in human-dense environments become accustomed to human proximity and do not react as strongly as their rural counterparts when approached [60, 63–65]. Furthermore, one constraint of existing in an urban environment is that eastern gray squirrels often rely at least partially on human refuse for food [45, 65, 66]. Eastern gray squirrels that inhabit urban environments therefore tend to be relatively active during the day. This pattern is different from their non-urban counterparts, which have a crepuscular activity pattern (active at dawn and dusk) [57].

The diet of most squirrel species is opportunistic, consisting of a variety of plant and animal material. The diet of the eastern gray squirrel primarily consists of tree seeds, f lowers, nuts, and fruits from a variety of plants [41, 65, 67–71]. They also will consume insects, fungi, bird eggs, and crops such as wheat and corn. Squirrels are known to hunt opportunistically and kill live prey such as frogs, nestlings, and occasionally adult birds [48, 56, 69, 72]. During the winter, they rely on food caches buried during the autumn as well as consuming fallback food items [41, 68, 70, 71]. As noted above, squirrels will gnaw on bone and antler [1, 4, 5, 44], possibly to obtain minerals including calcium. Callahan [72] noted that lactating female squirrels gnawed on bone more often than nonlactating squirrels, which supports the theory of rodents obtaining mineral nutrients from gnawed bone.


The gnawing mark patterns left by eastern gray squirrels are in part determined by their body and dentition size. Figure 1 shows the dentition of this species. All rodents have one incisor per quadrant, with different numbers of premolars and molars that play no direct role in bone gnawing. Wear on the inner margins of the incisors maintains their sharpness by producing an overall chisel shape, with the cutting occlusal surface perpendicular to the gnawed object. Gnawing often proceeds by holding the object in place with the maxillary incisors while dragging the mandibular incisors across the surface [2], but gnawing also can proceed where both sets of incisors are dragged across the surface in a converging motion [5, 28]. The narrowness of the mouth opening between the incisors often limits rodent species to gnawing on the margins of objects. Pokines et al. [7] examined the relationship between rodent maximum incisor width and the maximum striation width (MSW) left on experimental dry bone samples for multiple species whose ranges include Massachusetts or were available in a laboratory or zoo setting. In general, larger rodent species left wider striations on bones, although blurring together of striations was common, and much overlap in size was noted among species.

Figure 1Skull of an eastern gray squirrel (Sciurus carolinensis) showing dentition.

Note the single incisor per quadrant. Wear on the inner margins of the incisors (arrows) maintains their sharpness by producing an overall chisel shape with the cutting occlusal surface perpendicular to the gnawed object.


MethodsThe sample consisted of ver tebrae, r ibs, calcanei, tibiae,

metapodials (i.e., metacarpals and metatarsals combined), first phalanges, a scapula, femora, isolated radii, a fused radius-ulna, an isolated ulna, and pelves (Table 1) of white-tailed deer (Odocoileus virginianus). The majority of bones had been previously frozen and had been exposed outdoors for a minimum of 16 months to allow natural decomposition and degreasing. Other elements were obtained from potential forensic cases turned in to the Office of the Chief Medical Examiner, Boston, Massachusetts, and were fully degreased upon acquisition [73]. With the exception of all but one of the tibias, radii, and the radius-ulna that were previously machine-butchered at midshaft (with the distal portions remaining), completely undamaged elements were selected wherever possible in order to avoid any confusion between previous and squirrel-caused damage. Some elements retained minor amounts of adhering soft tissue, and no elements had reached weathering stage 1 [74]. Multiple elements were unfused.

Gnawed

Element Initial n = Ungnawed Gnawed Gnawed Then Lost

Lost Without Gnawing

Long BonesFemur 2 1 1 0 0Metapodial 62 34 23 0 5

Radius* 3 3 0 0 0

Radius-ulna* 1 0 1 0 0Ulna 1 0 1 0 0Tibia** 24 12 4 1 7Compact Bones

Calcaneus 50 41 3 4 2First phalanx 109 81 9 3 16

OtherVertebra 30 25 3 1 1Rib 19 14 2 0 3Pelvis 3 1 2 0 0

Scapula 1 1 0 0 0

Total 305 21349 9

3458

* Previously butchered at midshaft.** All but one tibia previously butchered at midshaft.

Table 1Gnawing activity by element, all bones (n = 305).


The total sample size for this study initially was 305 bones. A maximum of five bones per tree were placed in seven different neighborhood locations within the city of Boston, Massachusetts. The bones were labeled then wired to the tree branches a minimum height of ~1 m and a maximum height of ~6 m above ground level, with the majority clustered at ~1 to 2 m above ground level. Tree types were chosen for accessibility from the ground, and no particular species or sizes were chosen. Eastern gray squirrels were noted to be active in all areas. Multiple bone types were placed in each location so that squirrels in each area had access to multiple bone types at all times. Placing the bones in trees largely excluded bone gnawing from terrestrial commensal species such as rats (Rattus spp.) or house mice also found in this environment. This urban setting also lacked other arboreal rodents, so any gnawing damage found on the bones reasonably could be ascribed to eastern gray squirrels alone. The bones were placed on 13 September 2015 and subsequently checked every two weeks for signs of gnawing for a duration of eight weeks. All evidence of taphonomic alterations was documented and photographed. Two motion capture cameras were available to record the gnawing activity in one protected location for one week to obtain direct confirmation regarding eastern gray squirrel interaction with the bones.

The present study examined multiple aspects of bone gnawing: rate of gnawing, overall percentage of bones affected, preferences for types of bone, preferences for areas on a bone, and maximum striation width. The rate of gnawing was extrapolated from the progression of observations recorded every two weeks. The gnawing was scored as light (few striations), medium (areas of margins gone), or heavy (sculpting of bone, including exposure of the marrow cavity). Locations of the gnawed areas on each element were recorded to determine whether there were any preferred areas for gnawing (shafts, epiphyses, etc.).

ResultsThe study began with a total of 305 bones. Throughout the

study, 34 bones were lost without any previously recorded gnawing. A total of 58 bones were gnawed (21.4% of the remaining bones), including 9 bones that were lost after gnawing was noted previously (Table 1). During the experiment, multiple eastern gray squirrels were observed gnawing on bone directly by the authors, and many squirrels were noted in the general vicinities of the sample locations on most days. As noted above, the motion capture cameras also frequently recorded eastern


Figure 2Eastern gray squirrel (Sciurus carolinensis) gnawing on a white-tailed deer

(Odocoileus virginianus) vertebra, with image captured by an automatic game camera.

gray squirrels near the bones in two locations (Figure 2), and no other rodent species were detected near the bones. Some bone loss is known to have been from human action in these densely populated urban areas (i.e., heavy wires had been untwisted). Other bone loss can be explained most logically as a result of squirrel action: because of the morphology of various bones (e.g., calcanei), wires could be slipped off, especially after some gnawing had occured. More elaborate wiring systems may have interfered with squirrel gnawing patterns and so were avoided.


Rate of GnawingThe advancement of bone gnawing is shown in Figure 3. By

the end of week 2, 29 of the available bones were gnawed (9.5% of the original 305 bones), and 5 bones were lost. A total of 30 out of the remaining 300 bones available after week 2 experienced squirrel gnawing activity by the end of week 4 (10.0%); 12 bones were gnawed that had been previously ungnawed, and 18 bones that were previously gnawed had additional gnawing. The cumulative bone gnawing activity for the sample at the end of week 4 was 41 (13.4% of the original 305 bones). Only 293 bones remained by the end of week 4; 1 bone was lost after previously recorded gnawing, and 6 more bones were lost without previously recorded gnawing.

A total of 35 out of the remaining 293 bones available after week 4 experienced squirrel gnawing activity by the end of week 6 (11.9%); 9 bones were gnawed that were previously ungnawed, and 26 bones that were previously gnawed had additional gnawing. The total cumulative bone-gnawing activity for the sample at the end of week 6 was 50 (16.4% of the original 305 bones). Only 288 bones remained by the end of week 6; 4 additional bones were lost after previously recorded gnawing, and 1 more bone was lost without previously recorded gnawing.

A total of 32 out of the remaining 288 bones available after week 6 experienced squirrel gnawing activity by the end of week 8 (11.1%); 8 bones were gnawed that were previously ungnawed, and 24 bones that were previously gnawed had additional gnawing. The total cumulative bone-gnawing activity for the sample was 58 (19.0% of the original 305 bones), with a total of 9 previously gnawed bones lost. The cumulative total for bone gnawing is 58 out of 271 bones (21.4%), if the bones that were lost during the experiment without previous gnawing are subtracted. Only 262 bones remained by the end of week 8; 4 more bones were lost after previously recorded gnawing, and 22 more bones were lost without previously recorded gnawing.


Figure 3Cumulative gnawing totals by week, tallying bones that were lost without previous evidence of gnawing, ungnawed, lost after previous evidence of

gnawing, or gnawed.


Examples of gnawing advancing over multiple weeks on individual bones are illustrated on a white-tailed deer ulna (Figure 4), calcaneus (Figure 5), metapodial (Figure 6), and f irst phalanx (Figure 7). The degree of gnawing advanced either by squirrels returning repeatedly to the same location or by extending their gnawing to other portions of the bone, and margins generally were targeted. Progressive gnawing from week 2 to week 6 removed the entire proximal end of this ulna and affected other margins to a lesser degree (Figure 4); this bone experienced no additional gnawing by week 8. The calcaneus (Figure 5) by week 2 had three small areas of gnawing that had removed portions of the proximal, middle, and distal margins and by week 4 had continued gnawing in all three areas, with the proximal end largely removed. This calcaneus was lost prior to subsequent observation times, possibly because of the wire being easier to slip off once the proximal end was gone. In the case of the metapodial (Figure 6), new gnawing was recorded at the end of each 2-week interval. Gnawing completely removed the distal end after week 2. By week 4, gnawing had continued in this location until the marrow cavity was exposed and then continued down the shaft by week 6 and week 8. The first phalanx (Figure 7) f irst showed alteration by week 6, with gnawing to both ends, and by week 8 had expanded gnawing to both ends, with minor additional gnawing damage. A longer duration of exposure likely would have caused additional gnawing to many of the elements in the sample, because in each case substantial bone remained.


(b) (c)Figure 4

Eastern gray squirrel (Sciurus carolinensis) gnawing on a white-tailed deer (Odocoileus virginianus) unfused proximal ulna: (a) week 2: the proximal end

(right) has been extensively gnawed; (b) week 4: more of the proximal end has been removed, and gnawing now includes the articular surface; (c) week 6: gnawing has

continued in all previous areas, and the proximal end has largely been removed. No additional gnawing occurred week 8. Note the wiring system necessary for

attachment; the wire presence may sometimes interfere with gnawing access. Note also the paler bone exposed underneath the stained outer bone.

(a)

(a) (b)Figure 5

Eastern gray squirrel (Sciurus carolinensis) gnawing on a white-tailed deer (Odocoileus virginianus) unfused calcaneus: (a) week 2: three small areas of gnawing, removing portions of proximal, middle, and distal margins; (b) week 4: gnawing continued in all three areas, with the proximal end (left)

largely removed. This bone was lost prior to subsequent observation times.


(c) (d)Figure 6

Eastern gray squirrel (Sciurus carolinensis) gnawing on a white-tailed deer (Odocoileus virginianus) unfused distal metatarsal: (a) week 2; (b) week 4; (c) week 6; (d) week 8. Note the gnawing progressively removing the distal

end, exposing the marrow cavity, and continuing down the shaft.

(a) (b)

(a) (b)Figure 7

Eastern gray squirrel (Sciurus carolinensis) gnawing on a white-tailed deer (Odocoileus virginianus) fused first phalanx: (a) week 6: gnawing to both ends, showing dorsal surface; (b) week 8: gnawing to both ends has

expanded, showing ventral surface. Minor additional gnawing damage was noted. Note also the paler bone exposed underneath the stained outer bone.


Bone PreferenceThe most gnawed bones were the metapodials (39.7%),

followed by f i rst phalanges (20.7%) and calcanei (12.1%) (Table 2). Chi-squared tests show that there was a significant difference in preference between the metapodials and all non-metapodials (χ2 = 15.41, p<0.001). There was also a significant preference difference between the phalanx and all non-phalanx bones (χ2 =6.08, p=0.014). However, there was no signif icant preference difference between the calcaneus and all non-calcaneus bones (χ2 =1.61, p=0.204). There was also a significant preference difference between the metapodials and phalanges (χ2 =14.88, p<0.001).

Element/Bone Type Gnawed Ungnawed % of Gnawed

% of Gnawed Within Group

Long bones (femur, metapodial, radius/ulna,

and tibia)31 50 53.4 38.3

Compact bones (calcaneus and first phalanx) 19 122 32.8 13.5

Vertebra 4 25 6.9 13.8Pelvis 2 1 3.4 66.7

Rib 2 14 3.4 12.5Scapula 0 1 0.0 0.0

Total 58 213

Table 2Gnawing activity by bone type, bones present throughout entire experiment or

showing gnawing prior to loss (n = 271).

When classified by bone type, the strongest preference was for the long bones (metapodial, femur, radius, radius-ulna, ulna, and tibia), forming 53.4% of the total gnawed bone. The compact bones (calcaneus and first phalanx) formed 32.8% of the total gnawed bone. Perhaps because of their small sample size, all other types of bones (vertebra, pelvis, rib, and scapula) seemed to be less affected by gnawing. Chi-squared tests were run to determine whether significant differences were present between bone type groups. The result showed that there was a significant gnawing preference difference between the long bones and all non-long bones (calcaneus, first phalanx, vertebrae, pelvis, rib, and scapula) (χ2 =19.54, p<0.001). There was also a significant gnawing preference difference between the compact bones and non-compact bones (metapodial, femur, radius, radius-ulna, ulna, tibia, vertebra, pelvis, rib, and scapula) (χ2 =10.98, p<0.001).


Soft Tissue PreferenceAnalyses were carried out to determine whether a gnawing

preference was present for bones with traces of adhering soft tissue. Twenty-four of the 271 bones in the f inal sample had minor amounts of adhering soft tissue. A total of 20.8% of the bones with adhering soft tissue were gnawed, and 21.5% of the bones without adhering soft tissue were gnawed, indicating no demonstrated gnawing preference for elements with traces of soft tissue (χ2 =0.04, p=0.850).

Fusion Status PreferenceThe frequency of gnawing between fused and unfused bones

was examined. Within the sample, 83 of the 271 bones in the f inal sample were unfused and 188 were fused; 28 of the 83 unfused bones (28.6%) were gnawed, and 30 of the 188 (16.0%) fused bones were gnawed. A chi-squared test showed that there was a signif icant gnawing preference difference between the selection of fused and unfused bones (χ2 =4.69, p=0.030). This difference, however, may have been due to a large number of the metapodials being unfused (61.4% of the analyzed metapodials were unfused).

Preference for Location on Bone To analyze preference for location on bone, metapodials and

the f irst phalanges were examined, because sample sizes for other elements were too small to be considered for individual evaluation. Some gnawing location preferences were observed on the metapodials. Medium to heavy gnawing was observed on the epiphyseal ends (Figure 8). Of the gnawed sample of metapodials, a total of 95.7% showed gnawing marks at the epiphyseal ends, suggesting a preference for this location. When the metapodial shaft was gnawed, the squirrels chose to gnaw the posterior shaft more than the anterior shaft (60.9%), suggesting a preference for the posterior shaft.

The bones were observed to test preference for sharp edges or margins as opposed to broader surfaces as gnawing sites. For all gnawed bones, 100% had gnawing of the margins. Only 34.7% of the bones had broad surfaces that were gnawed in addition to the margins. A chi-squared test showed that there was a significant gnawing preference difference between areas of the bone that were margins and areas of the bone that were smooth/f lat (χ2 = 47.51, p<0.001).


Wet-Bone GnawingA pattern of rodent wet-bone gnawing was observed on one

metapodial only (Figure 9), which had deep gouging into the cancellous bone of the proximal end beyond what occurs as the incidental damage to cancellous bone typical of dry-bone gnawing. Squirrels in this one location may have been attracted to any residual contained soft tissue and gnawed preferentially into the cancellous bone [5].

Figure 8Eastern gray squirrel (Sciurus carolinensis) gnawing on a white-tailed deer (Odocoileus virginianus) unfused metatarsal, showing typical damage to the proximal end (right) and shaft margin (bottom). Note also the finer striations along the ventral shaft margin (top) and the paler bone exposed underneath

the stained outer bone.

Figure 9Eastern gray squirrel (Sciurus carolinensis) gnawing on a white-tailed deer

(Odocoileus virginianus) unfused distal metatarsal that followed the wet-bone pattern. Note the preferential scooping of the cancellous bone in the center,

leaving the cortical bone behind.


Maximum Striation Width (MSW)Maximum striation width could be measured for 39 of the

sample elements. Even where measureable, most rodent damage consisted of heavily blurred together individual striations that could not be measured. Five elements had an MSW of 0.4 mm, 19 elements had an MSW of 0.5 mm, and 15 elements had an MSW of 0.6 mm. The maximum incisor width for a sample of 10 eastern gray squirrels was measured previously and was determined to be 1.56 ± 0.14 mm, range 1.25–1.82 mm (maxillary) and 1.40 ± 0.11 mm, range 1.25–1.64 mm (mandibular) [7]. Thus, their incisors tended to leave striations that were narrower than their maximum width, which is consistent with the narrowing of these teeth toward the extremity of their occlusal surfaces. The present findings are in keeping with previous gnawing research for this species, where a sample of nine bones gnawed by eastern gray squirrels had an MSW of 0.6 mm [7].

DiscussionRodent gnawing is commonly found among forensic cases

in Massachusetts, including recent terrestrial human [6, 28] and nonhuman [73] cases and former cemetery cases where the bones were exposed on the ground surface [8]. At least 23 rodent species, endemic and introduced, are currently found within this state, and most of these are found in the surrounding New England region or more broadly [7]. Multiple rodent species are comparable in size to eastern gray squir rels, including other members of Sciuridae, with many smaller species and some larger species present [7]. In addition, multiple species of Lagomorpha are present, but their potential as bone-gnawers is generally unknown. The rodent dry-bone gnawing on the present experimental sample is consistent with much of that encountered on outdoor terrestrial forensic cases from Massachusetts [6, 28, 73], on both human and nonhuman bones turned in for analysis or recovered from forensic scenes. Eastern gray squirrels may be responsible for much of this taphonomic alteration, especially outdoors in urbanized areas where other rodents are kept away by human activity.

The rodent species in this region and other regions in North America have been little researched regarding their individual potential taphonomic effects on forensic scenes. Their effects may include the slow obliteration of portions of bones, which may reduce their locatability and identifiability and the extent of biological profile information that can be obtained from them.


Currently, data are lacking regarding the comparative features of individual rodent species’ bone gnawing and detailed analyses of their dispersal behavior for bones encountered among typical forensic ter restr ial scenes, and the authors hope that other researchers will carry out research similar to the present study in different regions with different rodent species.

Rodent gnawing is just one type of taphonomic alteration found on bones in this terrestrial New England environment. Other common alterations include subaerial weathering [74], green algae formation, and soil staining [6]. Any kind of color change or bleaching that occurs pr ior to rodent gnawing, however, tends to make subsequent rodent gnawing more visible as unbleached or unstained bone underneath is exposed (Figures 4, 7, 8). Such color alteration is typical in terrestrial environments and was present throughout the present sample, although none of the bones were substantially sun-bleached.

The only common natural taphonomic alteration that likely could be mistaken for rodent gnawing is carnivore gnawing. Tooth marks left by carnivores tend to occur earlier in the taphonomic sequence, because they are usually attracted to fresher bone with substantial associated soft t issue [5]. Carnivores may continue to be attracted to exposed skeletal remains for months or longer, and as the bones become dryer, rodent dry-bone gnawing activity may increase [4]. The exception to this pattern is rodent wet-bone gnawing, where some species are attracted earlier in the decomposition sequence to still-wet bones and tend to gnaw through thinner cortical areas to access contained soft tissue. Wet-bone rodent gnawing damage can look similar to carnivore gnawing in that epiphyses are often targeted, but the areas surrounding the missing bone lack the associated pits, punctures, striations, and other markers of carnivore gnawing activity. The missing portions of bone also tend to be smaller and narrower when caused by rodents [5].

The bones in the present sample were all in an advanced dry stage, with only minor amounts of desiccated soft tissue adhering to some of them. In all but one sample, the overall gnawing followed the dry-bone pattern; one distal metatarsal displayed a gouged cancellous portion more consistent with the wet-bone pattern. Because of their omnivorous diet, eastern gray squirrels may be attracted to fresher bone as well as dry bone and may in some cases produce a wet-bone gnawing pattern. Additional experiments directly comparing dry bones with


wetter bones having different degrees of internal soft tissue retention (primarily fat) may answer these questions.

The present research also may introduce biases into eastern gray squirrel gnawing behavior that are currently unknown. The present research utilized restrained bones, which may tend to receive more intense gnawing in the same locations, because they cannot be manipulated and accessed to the same degree that bones lying unrestrained on the ground surface can. The wires used to secure the bones of course present a localized obstruction to gnawing wherever these were placed. The secure setting generally away from humans and most wild predators also may allow longer bouts of uninterrupted gnawing to bones which, for the most part, could not be dispersed (any dispersed bones were not recovered).

ConclusionEastern gray squir rels gnaw on bone in urban and rural

settings, and the gnawing patterns found in the present research match other cases of rodent gnawing from forensic cases in this environment [6] and previous experimentation [7]. Margins of bones were favored, apparently because these can be more easily f it between the maxillary and mandibular incisors. A statistically significant preference for long bones was noted in the present sample. It is unknown whether there is a nutritional reason for this preference or whether these relatively large bones are easier for the squirrels to locate.

The present research demonstrates that bone gnawing by eastern gray squirrels can be rapid and invasive, with much sculpting of bone and loss of features over time. Signif icant damage to mult iple elements accr ued over the 8-week observation period, and additional observation time likely would have increased the degree of gnawing damage. In the case of human remains, the loss of bone features could hinder element identification and also eliminate anatomical landmarks necessary for bone measurement, thus decreasing the ability to develop a biological profile from a set of remains. In particular, the loss of long bone epiphyses greatly reduces the available metrical data that could be collected in order to derive estimates of living stature, sex, and ancestry or directly observe fusion states. Rodent gnawing of exposed margins can obscure bone modifications caused by perimortem trauma, including blunt force fracture sites, gunshot wounds, and sharp force marks from dismemberment. The rodent striations themselves also


have the potential to be mistaken for sharp force trauma, and all forensic anthropological practitioners must be aware of the forms that this type of taphonomic alteration may take based on the rodent species found in their jurisdiction.

The present research could not examine the role of eastern gray squirrels in bone dispersal [1, 75], because this would have introduced the variable of other rodent species gnawing on bone and made it impossible to document the types and advancement of gnawing damage. Because some missing elements from the present sample can most logically be ascribed to squirrel instead of human actions, this and other small rodent species may be the source of some dispersal of remains left exposed in terrestrial settings. Future experimentation also may detect seasonal variations in gnawing activity and address the overlapping roles of multiple scavenging species exploiting the same set of vertebrate remains. The role of eastern gray squirrels in wet-bone gnawing damage to bone also needs to be examined; the present study primarily examined the dry-bone pattern. It is clear that this rodent species and likely others with similar behavior patterns can have a profound effect on exposed terrestrial human remains, and the effects of these species must be taken into account in any forensic analysis of human recovery patterns and osseous surface alteration.

AcknowledgmentThe authors thank the Boston University School of Medicine,

Forensic Anthropology Program and the Off ice of the Chief Medical Examiner, Boston, for use of the experimental bones utilized in this research. The authors also thank Dr. Joan Baker, Dr. Ericka L’Abbé, and the three anonymous peer reviewers for their valuable comments on earlier versions of this manuscript.

For further information, please contact:James T. PokinesForensic Anthropology Program, Dept. of Anatomy and NeurobiologyBoston University School of Medicine72 East Concord Street, L1004Boston, MA 02118 [email protected]


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The Taphonomic Effects of Eastern Gray Squirrels (Sciurus ... · Other known rodent bone-gnawers include Old World porcupines (Hystricidae) [14–23], North American porcupines (Erethizon