Color profile: Disabled Composite Default screen 2109 NOTE Killing technique of North American badgers preying on Richardson’s ground squirrels Gail R. Michener and Andrew N. Iwaniuk Abstract: Carcasses of 13 Richardson’s ground squirrels (Spermophilus richardsonii) cached during autumn by North American badgers (Taxidea taxus) in southern Alberta, Canada, were inspected to determine the capture and killing technique. Regardless of prey size (251–651 g) or torpor status (normothermic or torpid), badgers killed ground squir- rels with a single grasping bite directed dorsally or laterally to the thorax. The canines and third upper incisors of badgers generally bruised the skin without puncturing it, but caused extensive hematomas on the thoracic musculature and penetrated between the ribs, with associated breakage of ribs and hemorrhaging in the thoracic cavity. Internal organs and bones other than ribs were usually not damaged. Thoracic bites, rather than nape or throat bites, are used by several mustelids, including North American badgers, when capturing small prey (<10% of the predator’s mass). Résumé : Les carcasses de 13 Spermophiles de Richardson (Spermophilus richardsonii) trouvées dans des caches de Blaireaux d’Amérique (Taxidea taxus) dans le sud de l’Alberta, Canada, ont été examinées dans le but de déterminer les techniques de capture et de mise à mort utilisées par les blaireaux. Indépendamment de la taille des proies (251– 651 g) ou de leur état de torpeur (normothermie ou torpeur profonde), les blaireaux tuent les spermophiles d’une seule morsure dorsalement ou, latéralement, au thorax. Les canines et les troisièmes incisives supérieures du blaireau font généralement des ecchymoses sur la peau sans la perforer, mais causent des hématomes importants sur la musculature thoracique et pénètrent entre les côtes; les animaux ainsi blessés ont souvent des côtes cassées et des hémorragies dans la cage thoracique. Les organes internes et les os autres que les côtes ne sont ordinairement pas endommagés. Les morsures au thorax plutôt qu’à la nuque ou à la gorge sont utilisées par plusieurs espèces de mustélidés, y compris le Blaireau d’Amérique, au cours de leurs chasses aux petites proies (<10 % de la masse du prédateur). [Traduit par la Rédaction] 2113 Introduction Note typical musteline manner, with bites that penetrate the nape or throat, but kill small mammals and birds (<10% of the Relatively little is known about the killing technique of predator’s mass) by damaging the heart and major blood North American badgers (Taxidea taxus, family Mustelidae), vessels with a crushing bite to the thorax that often does not largely because they are nocturnal fossorial hunters that involve penetration of the skin. Given that North American capture, then immediately consume, most prey underground badgers are large (usually >6000 g) relative to the size (Messick 1987; Michener 2000). The most commonly of common mammalian prey such as ground squirrels reported prey-killing techniques of mustelids, particularly (Spermophilus spp., usually <500 g), pocket gophers mustelines, are dorsal bites directed to the nape of the neck (Geomys spp., usually <250 g), and mouse-sized murid ro- or the occiput of the skull and ventral bites to the throat dents (Microtus spp. and Peromyscus spp., usually <50 g) (Heidt 1972; Byrne et al. 1978; Rowe-Rowe 1978; Vargas (Lindzey 1982; Marti et al. 1993), badgers may likewise di- and Anderson 1998, 1999). The killing technique can, how- rect killing bites to the thorax rather than the nape or throat. ever, vary with prey size. For example, Ben-David et al. Occasional reports of aboveground captures of ground squir- (1991) observed that marbled polecats (Vormela peregusna) rels and pocket gophers by North American badgers, either kill large mammalian prey (>50% of predator’s mass) in the during daylight hours in natural habitats (Sawyer 1925; Schwab 1978) or at night in captivity (Lampe 1976), provide Received May 1, 2001. Accepted September 12, 2001. limited information on killing techniques. Only Lampe (1976) Published on the NRC Research Press Web site at reported the target of the killing bite, which he noted to be at http://cjz.nrc.ca on December 4, 2001. about the center of the body of a pocket gopher. As part of a long-term ecological study of Richardson’s ground squirrels 1 G.R. Michener. Department of Biological Sciences, (Spermophilus richardsonii) (Michener 1998, 2000), we had 4401 University Drive, University of Lethbridge, Lethbridge, a unique opportunity to inspect ground squirrels captured AB T1K 3M4, Canada. A.N. Iwaniuk. Department of Psychology and Neuroscience, and cached by badgers and thereby assess killing technique. 4401 University Drive, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada. Methods 1Corresponding author (e-mail: [email protected]). The presence of predators was noted on a near daily basis Can. J. Zool. 79: 2109–2113 (2001) DOI: 10.1139/cjz-79-11-2109 © 2001 NRC Canada J:\cjz\cjz79\cjz-11\Z01-169a.vp Tuesday, December 04, 2001 11:10:17 AM Color profile: Disabled Composite Default screen 2110 Can. J. Zool. Vol. 79, 2001 Fig. 1. Representative examples of trauma to hibernating Richardson’s ground squirrels (Spermophilus richardsonii) captured by North American badgers (Taxidea taxus). (a) A pair of bruises, inflicted by upper canines without skin penetration, on the inner dermal sur- face of the left thorax. The distance between the midpoints of the 2 bruises is 35 mm. (b) Hematomas (darkly pigmented areas) on the left and right thoracic cage of the same individual (mass = 444 g) as in a.(c) Dermal hematoma (darkly pigmented area located dor- sally on reflexed skin) and 3 puncture wounds in the left thoracic musculature (indicated by toothpicks penetrating these wounds) of another individual (mass = 421 g). during a 14-year study (1987–2000) of a population of Richardson’s likely inflicted damage, skulls of 2 North American badgers (adult ground squirrels. The study population was located 5 km east and female PMA75.17.2 and adult male PMA86.8.11) obtained from 1 km south of Picture Butte in southern Alberta, Canada (49°52′N, the Provincial Museum of Alberta were matched against visible 112°43′W, elevation 870 m asl), on a 1.4-ha area surrounded by damage. cultivated fields and a farmyard into which both ground squirrels and predators intruded (Michener 1996). All ground squirrels were individually identified with a numbered tag in each ear. From 1988 to 1999, some ground squirrels (<10% annually) were individually Results fitted with a temperature-sensitive radio transmitter attached to a 3–5 mm wide collar, then located twice daily in their sleeping Of Richardson’s ground squirrels cached by North Ameri- chambers or hibernacula to monitor body temperature (Michener can badgers, a minority (31% of 16 radio-collared animals 1992). and 27% of 11 non-radio-collared animals) exhibited exter- Although direct observations of hunting were infrequent, over- nal signs of trauma, in all cases limited to inconspicuous night hunting by badgers was evident the following morning from blood stains either beside 1 nostril (n = 3), on the thorax the appearance of freshly disturbed soil beside large (≥20 cm diame- (n = 4), or on the throat (n = 1). Necropsies confirmed that ter) tunnels. All aboveground mounds of soil created by badgers puncture wounds penetrating the skin were uncommon (found were searched for carcasses of ground squirrels. Additionally, when in 2 of 13 animals). In contrast, bruises and subcutaneous the signal from a radio-collared ground squirrel was displaced to a hematomas were visible on the inner dermal surface of 11 new underground location coincident with badger excavation near the squirrel’s last location, the radio collar was disinterred. From ground squirrels (Fig. 1). Dermal bruising corresponded in 1990 to 1999, 27 Richardson’s ground squirrels were found cached location to trauma of underlying thoracic musculature. singly either underground (n = 11 radio-collared animals) or above Large hematomas were present on the thoracic muscula- ground (n = 5 radio-collared and 11 non-radio-collared animals; ture of all 13 necropsied ground squirrels, usually (n = 10) Michener 2000). Carcasses cached underground by badgers were on both the left and right sides (Fig. 1b). Additionally, all retrieved for examination 0–11 days (mean = 2 days) after caching. carcasses exhibited discrete, roughly circular wounds to the Most carcasses (24 of 27) were cached in September–November, thoracic musculature, with at least 1 wound penetrating into when the majority of ground squirrels were hibernating (Michener the thoracic cavity through the intercostal muscles (Fig. 1c). 2000). All 27 carcasses were examined for external signs of badger- Ten ground squirrels had broken ribs. Numbers of thoracic inflicted injury. Four carcasses cached in autumn 1992 (1 non- radio-collared and 3 radio-collared) and 9 cached in autumn 1998 wounds and numbers of broken ribs did not differ signifi- (all radio-collared) were stored at –20°C for subsequent necropsy. cantly between male and female ground squirrels, between In July 1999, carcasses were thawed to room temperature, weighed torpid and nontorpid ground squirrels, or between ground to the nearest 1 g, then inspected for trauma to the skin, thoracic squirrels killed by the badger hunting in 1992 and the badger and peritoneal walls, internal organs, and skeleton. We recorded hunting in 1998 (Table 1). Neither the number of thoracic the identity of broken bones and the number and location of bruises, wounds nor the number of broken ribs was correlated with hematomas, and muscle wounds. To assess which teeth had most mass (range 251–651 g) of the 13 cached carcasses (Spearman’s © 2001 NRC Canada J:\cjz\cjz79\cjz-11\Z01-169a.vp Tuesday, December 04, 2001 11:10:26 AM Color profile: Disabled Composite Default screen Note 2111 Table 1.