Using claw marks to study lion predation on giraffes of the Serengeti

Authors


  • Editor: Virginia Hayssen

Correspondence

Megan K. L. Strauss, 100 Ecology Bldg., 1987 Upper Buford Circle, Saint Paul, MN 55108, USA.

Email: strau102@umn.edu

Abstract

Although lions Panthera leo are the main predators of the giraffe Giraffa camelopardalis, interactions between these species are rarely observed directly. As a consequence, little is known about the effects of lions on giraffe mortality and behavior. We test patterns of lion predation on Masai giraffes Giraffa camelopardalis tippelskirchi using a new methodology: lion claw marks observable on the skin of live giraffes. We studied 702 individually known giraffes in 3 non-neighboring areas of Serengeti National Park, Tanzania between August 2008 and November 2010. Lion claw marks were observed on 13% of giraffes older than 1 year. Claw marks were most frequently detected on giraffe hindquarters and flanks, revealing that non-lethal lion attacks occur most often from the rear. No claw marks were observed on calves (0–1 year), suggesting that calves rarely survive lion attacks. In the adult age class (>5 years), claw-mark prevalence was significantly higher among females than males. We observed substantial variation in claw-mark prevalence across study areas, indicating that lion predation risk may be heterogeneous within Serengeti. We find that claw marks are an important source of data on interactions between lions and giraffes.

Introduction

The lion Panthera leo is the most important predator of the giraffe Giraffa camelopardalis (Berry, 1973; Dagg & Foster, 1982), yet the relationship between these species has been rarely studied. Lions may be the primary cause of death for giraffe calves (Dagg & Foster, 1982), which suffer an estimated 58–73% mortality in the first of year of life (Foster & Dagg, 1972; Leuthold & Leuthold, 1978; Pellew, 1983a). Although giraffe mortality drops off substantially after 1 year of age (Pellew, 1983a), lion predation remains a significant mortality factor for subadults and even for adults (e.g. Hirst, 1969; Pienaar, 1969), which weigh 800–1200 kg (Owen-Smith, 1988) and reach heights of up to 4.5–5.5 m for females and males, respectively (Dagg & Foster, 1982; Pellew, 1983a).

Direct observations of lion attacks on giraffes are rare. In a 3-year study of Serengeti lions, Schaller (1972) observed only 10 such attacks, none of which led to a kill. Consequently, little is known about the effects of lions on giraffe mortality and behavior. What is known is largely anecdotal or inferred from short-term studies of giraffe demography or from carcass records. If the majority of attacks are occurring in conditions not conducive to direct observation, such as at night or in dense vegetation, then alternative sources of data will be required.

In this paper, we examine lion predation on giraffes by applying a novel methodology that has been used primarily in marine biology: predation marks on live animals. Underwater predation is difficult to observe directly (Bertilsson-Friedman, 2006); thus, predation marks visible on surfacing animals are an important source of data on predator–prey interactions. While predation marks cannot be used alone to estimate predation rates, they can be used to identify predatory species (e.g. Corkeron, Morris & Bryden, 1987; Cockcroft, Cliff & Ross, 1989), to elucidate attack behavior, to infer which age–sex classes of prey are better able to evade predation (e.g. Corkeron et al., 1987; Heithaus, 2001) and to examine variation in predation risk over space and time (Heithaus, 2001; Bertilsson-Friedman, 2006).

Similarly, the predation-mark method can increase the sample size of lion predation events on giraffes. Only a portion of lion attacks are fatal (Schaller, 1972; Funston, Mills & Biggs, 2001) and surviving prey may incur bite wounds or claw marks. Lion claw marks are distinctive and can be differentiated from marks inflicted by other predators (Figs 1 and 2). Claw marks are observed on giraffe carcasses (Schaller, 1972) and on live giraffes (Fig. 2). Interpretation of claw marks, however, requires caution. For example, an absence of claw marks in an age–sex class could indicate that all attacked individuals die, that no individuals are attacked or that too few individuals were sampled. Claw-mark patterns provide meaningful data when supplemented with other observations.

Figure 1.

Examples of lion Panthera leo claw marks. (a) Fresh claw marks and lacerations on the rumps of 2 live zebras Equus burchelli illustrating the parallel incisions diagnostic of lion attack. (b) Claw marks on an eland Taurotragus oryx carcass visible on the rump and flank, anterior to the anogenital region. Marks are also inflicted post-mortem during feeding. Predation marks can easily be distinguished from wounds/scars caused by disease or wire poaching snares. Photographs in (a) were provided by P. Jigsved and that in (b) was provided by D. Rosengren.

Figure 2.

Examples of lion Panthera leo claw marks observed on Serengeti giraffes Giraffa camelopardalis. Healed claw marks on a giraffe (a) hind leg, (b) rump and (c) flank–rump area. Multiple parallel marks are present in each of these cases, but in contrast to the images in Fig. 1, lion claws rarely cause severe lacerations on giraffes and healed marks can be difficult to detect. Partial tail amputation (d) was always accompanied by claw marks on the tail, hind leg and/or rump, suggesting lions, rather than spotted hyenas Crocuta crocuta, were responsible for missing tail tips. Tail amputations may affect the giraffe's ability to swat tsetse flies and other insects or to dislodge Acacia thorns from the skin.

Lions are a clear threat to giraffes. Observational studies indicate that giraffes alter their behavior in the presence of lions. A typical vigilance posture is shown in Supporting Information Fig. S1. When lions are nearby, giraffes avoid waterholes (Valeix et al., 2009a), where they are particularly vulnerable (Dagg & Foster, 1982; Périquet et al., 2010), and favor open grasslands over denser vegetation (Valeix et al., 2009b). Females with young calves spend a disproportionate amount of time in open habitats, sacrificing browse availability for improved ability to detect and evade predators (Young & Isbell, 1991). Calves often remain in open areas in crèche groups, while mothers travel to feed (Langman, 1977; Leuthold, 1979; Mejia, in Moss, 1982).

Carcass records from southern Africa reveal that lions kill more male than female giraffes (Hirst, 1969; Pienaar, 1969; Owen-Smith, 2008) and that predation on giraffes is highest in the mid- to late dry season (Hirst, 1969; Owen-Smith, 2008). However, it can take years to acquire a reasonably sized carcass sample and these records suffer from the underrepresentation of rapidly consumed calves (Hirst, 1969; Dagg & Foster, 1982; Owen-Smith & Mills, 2008). The mechanics and frequency of lion attacks and the circumstances in which giraffes are able to evade attacks remain unclear. Easily collected claw-mark data reduce these gaps in knowledge.

We used lion claw marks on Masai giraffes Giraffa camelopardalis tippelskirchi living in Serengeti National Park, Tanzania to elucidate lion attack behavior and predation patterns. Specifically, we studied relationships between claw marks and giraffe age, sex, herd size, height and study area, and we used supplemental carcass data from the Serengeti Lion Project to examine seasonal effects. We integrate our findings with results from prior studies on lion–giraffe interactions.

Materials and methods

Study area

Serengeti National Park forms part of the 25 000-km2 Serengeti ecosystem, a region of northern Tanzania and southern Kenya that supports large numbers of resident and migratory ungulates and, consequently, high numbers of predators, including lions (Sinclair & Norton-Griffiths, 1979). Giraffes were sampled in 3 non-neighboring areas of Serengeti during each dry season between August 2008 and November 2010 (study areas first described by Pellew, 1983a): (1) Seronera (240 km2); (2) Kirawira (210 km2); (3) Bologonja (175 km2, sampled only in 2010).

Study area comparisons focus on the well-sampled areas of Seronera and Kirawira. While giraffe density is similar in these areas (Strauss, unpubl. data), Kirawira has a lower lion density than Seronera (Packer, 1990; Mosser et al., 2009; A. Kittle, pers. comm., 2012) and high year-round densities of preferred lion prey such as wildebeest Connochaetes taurinus and topi Damaliscus lunatus. Kirawira is characterized by seasonal drainages and patches of low scrub thicket interspersed with flat open grassland, whereas Seronera is characterized by riverine areas, woodland and sloping hills. With a lower lion density, a high density of other prey and better visibility, we expected lower lion predation in Kirawira.

Data collection

Giraffes were photographed and later identified using the coat markings unique to each animal (Foster, 1966). Individual identifications, done by eye, were double-checked using Wild ID pattern-matching software for giraffes (Bolger et al., 2012). No individuals were observed in more than 1 study area during the sampling period. Most giraffes were sighted multiple times.

Using a suite of physical characteristics, including body shape, relative length of the neck and legs, ossicone (horn) characteristics and height, giraffes were categorized into 3 age classes: calf (0–1 year), subadult (1–5 years) or adult (>5 years). For a more fine-scale analysis, subadults were aged to ±1 year by comparing each individual with a sample of known-aged giraffes of the same sex. Height measurements were compared against age–height curves for Serengeti giraffes (Pellew, 1983a). We measured height with a Haglöf electronic clinometer (Haglof Company Group, Långsele, Sweden) (accuracy of ±0.1 m), calibrated by the distance from the observer to the giraffe, which, in turn, was measured with a Bushnell range finder (Bushnell Corporation, Overland Park, KS, USA) (accuracy of ±1 m). Height, from the ground to the top of the ossicones, was measured with the giraffe standing in an upright posture. Height measurements were only taken when a giraffe could be approached closely and remained still long enough for an accurate reading.

We recorded the size and composition of giraffe herds, defined as individuals feeding, socializing and/or moving together (solitary individual equals herd size of 1). Herd members could be dispersed over 1 km, but were usually within close proximity. For each giraffe, we calculated that individual's ‘mean herd size’ – a measure of social behavior. For example, if individual with identification code SF1 was observed in 5 herds of sizes 1, 5, 10, 5 and 2, SF1's mean herd size would be equal to 4.6.

Predation mark analysis

A total of 917 individual giraffes were identified during this study. Photographs of 702 giraffes (132 calves, 187 subadults and 383 adults) were inspected for predation marks. These data were used to calculate predation-mark prevalence. Individuals (n = 215) with unsatisfactory photographs were excluded. Calves were rarely excluded and males were excluded slightly more often than females because some males were seen infrequently or only at a distance.

Two classes of predation marks were recorded: unambiguous lion claw marks and ambiguous marks. We defined unambiguous claw marks as sets of parallel incisions/scars, or as long scars extending over multiple, usually adjacent, body regions. Figure 1 illustrates the appearance of lion claw marks on 2 herbivore species, zebra Equus burchelli and eland Taurotragus oryx, and Fig. 2 illustrates lion claw marks on Serengeti giraffes. Ambiguous marks were typically single, short scars usually located on only 1 body region and could not be reliably attributed to lions. Because 9.5% (n = 67) of individuals were not photographed on both sides, we applied a correction factor to take into account the probability that some individuals may have predation marks on the unphotographed side of the body. The location of each mark on the body was recorded, including body side and region (Fig. 3).

Figure 3.

A histogram of the relative frequency of claw marks observed on 8 giraffe Giraffa camelopardalis body regions defined in this study. (No marks were observed on the head.) The histogram is based on all individuals with claw marks that were photographed on both sides (n = 72). The photograph of the giraffe side profile was provided by P. Jigsved.

Additional data

To supplement predation-mark data, we examined the season and age of death of 52 giraffe carcasses presumed killed by lions between 1966 and 2011. The data are from a continuous long-term study of lions in the central woodlands and south-eastern plains of Serengeti. Means are reported as ±sd and significance was α = 0.05.

Results

Claw-mark prevalence

Results presented here focus on marks convincingly attributable to lions. An estimated 10.6% of giraffes (13.1% of giraffes >1 year old) show evidence of surviving at least 1 lion attack. The estimated prevalence of claw marks was significantly higher among adults (17.6%) than subadults (3.7%) [χ2 = 21.34, degrees of freedom (d.f.) = 1, P < 0.0001]. Of the 7 subadults observed with claw marks, 1 was a yearling, 1 was a 2-year-old and 5 were estimated to be between ages 3 and 5 years at first sighting. No claw marks were observed on calves.

We found a highly significant relationship between sex and claw-mark prevalence, with estimated prevalence higher among females (14.1%, n = 379) than males (6.5%, n = 323) (χ2 = 10.69, d.f. = 1, P = 0.001). This result is caused by sex differences among adults [adult females (22.0%) vs. adult males (12.0%), χ2 = 5.83, d.f. = 1, P = 0.016; subadult females (5.4%) vs. subadult males (2.1%), P = 0.27, 2-sided Fisher's exact test].

Predation-mark prevalence for each study area is presented in Table 1. Across age–sex classes, claw-mark prevalence was found to be lower in Kirawira than in Seronera, the 2 well-sampled areas. For giraffes of both sexes >1 year old, estimated claw-mark prevalence was 1.3% for Kirawira (n = 177) compared with 18.3% for Seronera (n = 311) (χ2 = 29.14, d.f. = 1, P < 0.0001). This result is attributable to the difference in claw-mark prevalence among adults. Figure 4 summarizes age–sex trends in claw-mark prevalence for Kirawira and Seronera.

Figure 4.

Age–sex patterns in claw-mark prevalence among giraffes Giraffa camelopardalis in the Seronera (bars) and Kirawira (markers) study areas. Sample size is presented above each data point for Seronera/Kirawira. Note that the adult age class includes all individuals between ages 5 and 25.

Table 1. Estimated predation-mark prevalence among giraffe Giraffa camelopardalis age–sex classes for 3 non-neighboring areas of Serengeti National Park
 SeroneraKirawiraBologonjaH0: S = K
nMark (%)Claw mark (%)nMark (%)Claw mark (%)nMark (%)Claw mark (%)P
  1. Predation-mark prevalence is presented 2 ways: (1) ‘Mark’ represents the percentage of individuals observed with either unambiguous lion claw marks or ambiguous marks that may have been due to lions. (2) ‘Claw mark’ represents the percentage of individuals observed with marks that could be convincingly attributed to lions. Claw-mark prevalence among Seronera and Kirawira giraffes was compared using either the chi-squared test of independence or the 2-sided Fisher's exact test. P-values are presented.
Females          
Calf340.00.0240.00.040.00.0
Subadult4511.18.9368.30.0119.19.10.125
Adult12335.826.05718.34.04553.131.70.001
Males          
Calf440.00.0210.00.050.00.0
Subadult407.55.0452.20.0100.00.00.219
Adult10335.418.43924.20.01631.30.00.002

Claw-mark characteristics and location

We observed fresh claw marks, evidenced by dried blood, on 1 subadult female. All other claw marks appeared healed or were too superficial to cause bleeding (Fig. 2). No injuries appeared severe, but subcutaneous damage could not be assessed. Giraffes with hind leg marks did not have any visible reduction in leg motion. We observed no instances of hamstringing.

Claw marks were most frequently detected on the rump, followed by the hind leg and flank (Fig. 3). Hind leg marks occurred both above and below the hock. We observed partially amputated tails on 6.8% (n = 5) of individuals with claw marks (n = 74) (Fig. 2d). Marks were rarely detected on the neck, chest or forelegs. For subadults, no marks were observed on the chest, neck, forelegs or withers. We found no significant difference in the number of claw marks observed on the left versus right sides of giraffes (χ2 = 0.43, d.f. = 1, P = 0.51). There was no significant sex difference associated with the number of claw-marked body regions (pooled for n = 2–5 body regions; χ2 = 1.40, d.f. = 1, P = 0.24); however, only females had marks on 4 or more body regions (Table 2).

Table 2. A tabulation of the number of male and female giraffes Giraffa camelopardalis with lion Panthera leo claw marks on 1 or more body regions, for individuals with claw marks that were photographed on both sides
No. of body regions with claw marksaNo. of malesNo. of females
  1. aBody regions are illustrated in Fig. 3; the maximum number of regions is 8. Multiple sets of claw marks in 1 body region count as 1 region. For example, an individual with marks on both the left and right flanks and on the tail is recorded as having marks on 2 body regions.
11119
2720
3310
401
501
6+00
Total2151

Only 2 resighted giraffes – 1 male and 1 female – appeared to have acquired claw marks during the study, both as adults. The female had marks from an earlier lion attack and thus had survived at least 2 contact attacks. This suggests that some other individuals observed with several sets of claw marks may also have survived multiple attacks.

Herd size effects

We computed mean dry-season herd size for each individual from Seronera (n = 378) and Kirawira (n = 189) that was photographed on both sides. Individuals in Kirawira were commonly observed in larger herds. In Seronera, the ‘average mean herd size’ (we calculated the mean herd size for each individual and then averaged over all individuals) was 7.99 ± 3.95 compared with 21.99 ± 9.49 for Kirawira – a highly significant difference (t = −19.45, Satterthwaite's d.f. = 221.13, P < 0.0001, independent 2-sample t-test assuming unequal variance). For Seronera, we found no difference in mean herd size between individuals with claw marks (n = 57) and those with no marks (n = 292) (t = 0.97, d.f. = 347, P = 0.33).

Height effects

We measured the height of 83 individual giraffes. Analysis focused on the 48 adults measured (males: n = 15; females: n = 33). The mean height of adult males was 5.08 ± 0.32 m (range: 4.40–5.55 m) and the mean height of adult females was 4.30 ± 0.20 m (range: 3.95–4.70 m). We found no difference in the height of adult giraffes with claw marks versus those with no marks (z = −1.06, n1 = 20, n2 = 28, P = 0.29, two-sided Mann–Whitney U-test). Ideal height measuring conditions were met more often with females, and only 4 males with claw marks were measured. Restricting the analysis to adult females did not affect the result (z = 0.11, n1 = 16, n2 = 17, P = 0.91, two-sided Mann–Whitney U-test).

Seasonality of predation

Long-term data on presumed lion kills from Serengeti showed a significant increase in the number of giraffes dying during the dry season (χ2 = 4.23, d.f. = 1, P = 0.04). Calves made up 14% of carcasses versus 86% for subadults/adults.

Discussion

Biases in the data

Marks meeting criteria for unambiguous claw marks could be reliably attributed to lions; however, lions probably inflicted some of the ambiguous marks and reported claw-mark prevalence is therefore conservative. Moreover, some marks were inevitably missed due to varying photographic conditions. Claw marks were hardest to detect on mature adult males, whose coat markings darken with age (Brand, 2007; Berry & Bercovitch, 2012), sometimes to an almost black shade (Dagg, 1968; Berry, 1973). The giraffe has very thick skin (Dimond & Montagna, 1976; Sathar, Ludo Badlangana & Manger, 2010), so some contact attacks may leave no evidence. Fine marks may fade with time. Marks on young animals may also heal differently than those on older individuals. However, all marks that were observed at the start of this 2-year study, including those on subadults, were still visible at the end.

Lion attack behavior

Factors affecting lion-hunting success include prey size, the number of lions participating in the hunt, time of day and the amount of cover (Schaller, 1972; Funston et al., 2001; Hopcraft, Sinclair & Packer, 2005). Although solitary lions can attack adult giraffes (Pienaar, 1969), groups of lions are more successful at bringing down large prey (Schaller, 1972). During this 2-year study, we observed few lion-hunting attempts on giraffes, and none that resulted in contact. Coupled with the small number of claw marks acquired during the study, this suggests that attacks with contact are infrequent.

We expected to find claw marks on giraffe hindquarters because lions regularly attack large prey from the rear, grasping with their forepaws (Schaller, 1972). Consistent with this, claw marks were predominantly located on giraffe rumps, hind legs and flanks, suggesting that most non-lethal attacks also occur from the rear. This finding also supports the hypothesis of Sathar et al. (2010) that thicker skin on the upper flank and rump of giraffes may protect against lion-inflicted wounds. Lions kill with a bite or hold to the nose or throat of their prey (Schaller, 1972) and are able to seize hold of the neck of a standing adult giraffe. Two adult females in our sample had claw marks on the upper neck region. A giraffe would be extremely vulnerable if brought to the ground, so these females presumably were not. Lions rarely attack their prey from the front (Schaller, 1972), consistent with our finding that few giraffes had claw marks on the chest, neck and forelegs. Giraffes defend themselves with front and rear kicks (Schaller, 1972; Dagg & Foster, 1982), capable of maiming or even killing a lion, and lions risk significant injury during attacks on giraffes.

Patterns of lion predation

The giraffe is not a preferred prey species of lions in Serengeti (Scheel & Packer, 1991), where smaller prey like zebras and wildebeest are abundant (Sinclair & Norton-Griffiths, 1979). Nevertheless, the giraffe's size means that it can provide a large quantity of meat. Schaller (1972) estimated that although lions killed few giraffes, giraffes made up 27.5–32.5% of the lion's annual diet in Serengeti in the late 1960s. Since then, wildebeest numbers in Serengeti have doubled (Mduma, Sinclair & Hilborn, 1999), while giraffe numbers have declined (Strauss, unpubl. data). Today, giraffes probably contribute substantially less to the lion's diet.

The lack of claw marks among giraffe calves suggests that calves are highly unlikely to survive attacks where contact is made. Carcasses of calves were found in proportion to their availability in the giraffe population (Pellew, 1983a; Strauss, unpubl. data). However, calves are quickly consumed so we expect lions kill more calves than observed. We found few claw marks on subadult giraffes, and younger subadults appear to be more vulnerable than older, larger subadults (Fig. 4).

Claw-mark prevalence increased steeply from the subadult to the adult age class. Although size appears to be an important factor in escape probability, claw-mark acquisition also depends on other variables, as suggested by our height analysis and by the fact that subadult males reach the height of adult females at 3–4 years of age yet display a lower claw-mark prevalence. Bleich (1999) proposed inexperience as a cause of higher rates of fatal coyote Canis latrans attacks on young mountain sheep Ovis canadensis. Likewise, older and more experienced adult giraffes may be most successful at surviving lion attacks. In addition, the maximum age of giraffes is c. 25 years, so adults are exposed to attacks over a substantially longer period than subadults. In support of this, the majority of adults with claw marks (92.5%) were fully mature.

The observed sex difference in claw-mark prevalence in adults but not subadults requires explanation. Male giraffes suffer higher mortality from lion predation in southern Africa (Hirst, 1969; Pienaar, 1969; Owen-Smith, 2008), so we expected a similar pattern in Serengeti. Lower claw-mark prevalence among adult males may indicate increased male vulnerability to lethal attacks as has been observed in other ungulates, including Kongoni Alcelaphus buselaphus (Rudnai, 1974) and Thompson's gazelles Gazella thomsonii (FitzGibbon, 1990). A possible explanation for this pattern in giraffes is that adult males tend to be more solitary (Foster & Dagg, 1972; Leuthold, 1979; Pratt & Anderson, 1985; van der Jeugd & Prins, 2000; Bercovitch & Berry, 2010), and solitary ungulates have been shown to be at higher risk of predation (FitzGibbon, 1990). Also, adult males habitually spend more time than females in densely vegetated areas (Foster, 1966; Foster & Dagg, 1972; Young & Isbell, 1991; Caister, Shields & Gosser, 2003) that offer good cover for lions (Hopcraft et al., 2005).

As expected, Serengeti lions killed more giraffes in the dry season, coinciding with the decrease in preferred migratory prey. This is also a period when giraffes are nutritionally stressed (Hirst, 1969; Hall-Martin & Basson, 1975; Owen-Smith, 2008). During the Serengeti dry season, browse availability in midslope and ridgetop woodland areas declines (Pellew, 1983b) and giraffes shift habitat use to valley bottom and riverine areas (Pellew, 1984), prime ambush areas for lions (Hopcraft et al., 2005). The diet of adult male giraffes is nutritionally poorer than that of females (Pellew, 1984) and malnourished adult males may be particularly vulnerable to predation (Owen-Smith, 2008).

In contrast to adult males, adult female giraffes, especially mothers with young, are frequently observed in large herds (e.g. Foster & Dagg, 1972; van der Jeugd & Prins, 2000) and in open areas or areas with short vegetation (Foster & Dagg, 1972; Young & Isbell, 1991), a pattern consistent with our observations in Serengeti. Bercovitch & Berry (2010) suggested that in open terrain, increasing herd size does reduce predation risk for giraffes. In mountain sheep, similar behavior is observed: females and offspring occupy areas where they can detect and evade predation, while males occupy high-risk areas where they are more likely to encounter predators (Bleich, Bowyer & Wehausen, 1997). Consistent with this idea, claw marks were rarest in Kirawira, where giraffes commonly gather in large herds in open grassland areas. Although we did not find any relationship between an individual's mean herd size and claw-mark presence in Seronera, mean individual herd size may not be a useful measure if individuals are only likely to be attacked when temporarily alone.

If adult females generally behave in less risky ways, then why do they have the highest claw-mark prevalence? High claw-mark prevalence in adult females could be partially explained by marks acquired during calf defense. In a study of bottlenose dolphins Tursiops truncatus, Corkeron et al. (1987) observed fresh predation marks on a relatively high number of females with calves, and they suggested that female–calf pairs are more vulnerable to predation. Giraffe calves are an attractive target for lions. Mothers protect their calves by positioning them between their legs and by chasing or kicking at predators (Pratt & Anderson, 1979; Dagg & Foster, 1982). Lions have been observed lunging at nursing females to distract them from their calves, and this may be when they inflict superficial claw marks. In support of this hypothesis, we found a substantial jump in the prevalence of claw marks among females at age 4–5 years, coincident with the onset of first parturition (Fig. 4a). Injuries incurred during calf defense could also explain why only adult female giraffes were observed with marks on 4 or more body regions. In addition, the only observation of an individual surviving more than 1 non-lethal attack was that of an adult female. Observations of fresh claw marks on nursing females would provide additional support for this hypothesis.

Adult females may be most susceptible to lethal lion attacks in the last weeks of pregnancy and just after parturition, when females behave more like mature males: pregnant females spend more time browsing in dense vegetation to meet nutritional needs (Young & Isbell, 1991). Females also become solitary shortly before giving birth (Foster & Dagg, 1972; Strauss, pers. obs.) and keep their neonates relatively isolated from other giraffes for up to 3 weeks post-partum (Langman, 1977; Pratt & Anderson, 1979; Mejia, in Moss, 1982), thereby forgoing the vigilance benefits of additional herd members.

Index of predation risk

Studies of marine mammals suggest that predation-mark prevalence can be a useful index of predation risk (Heithaus, 2001; Bertilsson-Friedman, 2006). For example, in bottlenose dolphins Tursiops spp., predation-mark prevalence varies widely among populations, from 0 to >70% (Corkeron et al., 1987; Cockcroft et al., 1989; Bearzi, Notarbartolo-di-Sciara & Politi, 1997; Heithaus, 2001).

Likewise, lion claw-mark prevalence is 1 way to assess spatial and temporal patterns in predation risk for giraffes. We speculate that the low claw-mark prevalence observed in Kirawira is an indication of low lion-predation risk. With high densities of preferred prey available, lions probably target Kirawira giraffes infrequently. In addition, Kirawira giraffes benefit from high visibility due to low vegetation. Kirawira giraffes also aggregate in large herds, which reduces each individual's risk of predation due to increased likelihood of predator detection and a dilution effect (Hamilton, 1971; Pulliam, 1973; Bercovitch & Berry, 2010). Giraffe recumbency during the daytime was observed frequently in Kirawira but rarely in Seronera and further supports our hypothesis of low lion-predation risk in Kirawira. Further research is needed to explain large herd sizes typical of Kirawira.

The giraffe is an important food source for lions in some regions, including Kruger National Park, South Africa (Pienaar, 1969; Owen-Smith & Mills, 2008) and Hwange National Park, Zimbabwe (Loveridge et al., 2006). Where giraffes are a large component of the lion's diet, we might expect even higher claw-mark prevalence than observed in Serengeti. Alternatively, claw-mark prevalence could be lower if lions in these areas are more successful giraffe hunters or if giraffes are less adept at surviving attacks.

In summary, predation marks demonstrate that nature is indeed ‘red in tooth and claw’, even for the largest prey. Our results support prior published data on giraffe predation, suggesting that young giraffes are most vulnerable to predation and that lethal attacks increase in the dry season. We find evidence to suggest that while adult males are more vulnerable to lethal attacks, females are also likely to incur non-lethal attacks during calf defense. Our results also suggest that there is significant spatial variation in predation risk within Serengeti.

Overall, we find that in the absence of direct observation, claw marks provide an important source of data on lion predation attempts on giraffes. Unlike carcass data, claw-mark data can be collected on a large sample of individuals over a relatively short amount of time, with prompt analysis aided by continuing advances in digital camera technology and pattern-matching software. Thus, we recommend the use of claw marks to increase the sample size of lion predation attempts on giraffes. Claw-mark studies may also prove useful for other lion prey species.

Acknowledgments

We thank the Tanzania Commission for Science and Technology, Tanzania National Parks and the Tanzania Wildlife Research Institute for permission to conduct research in Serengeti National Park. M.S. was supported by an NSF Graduate Research Fellowship. Funding for fieldwork was provided by grants to M.S. from the American Society of Mammalogists, Chester Zoo, Columbus Zoo, the Explorer's Club, Minnesota Zoo, Riverbanks Zoo and Garden, and by the University of Minnesota's Graduate School, GPS Alliance and Bell Museum. We are grateful to Eric Thrane, Sara Cairns and an anonymous reviewer for insightful comments on earlier drafts of the paper.

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