Species resilience in Pleistocene hominids that traveled far and ate widely: An analogy to the wolf-like canids

Journal of Human Evolution 51 (2006) 383e394

Species resilience in Pleistocene hominids that traveled farand ate widely: An analogy to the wolf-like canids

A. Clark Arcadi

Department of Anthropology, 265 McGraw Hall, Cornell University, Ithaca, New York 14853, USA

Received 18 July 2005; accepted 24 April 2006

Abstract

Morphological and genetic analyses have yet to resolve the question of whether more than one species of Homo existed contemporaneouslyin the Pleistocene. In an effort to contribute a process-related perspective to hominid phylogenetic reconstruction, this paper uses an analogy tothe northern wolf-like canids (the wolves and coyotes) to ask the question, How many Homo species should there be, given their likely behav-ioral profile(s)? In contrast to earlier comparisons to social carnivores which sought to illuminate specific aspects of hominid behavioral ecology,this paper explores behavioral constraints on the process of speciation itself. The analogy suggests that because Pleistocene Homo probablyexhibited high habitat tolerance, they would not have had the opportunity to speciate, especially in Africa. In contrast to an earlier single-specieshypothesis based on competitive exclusion between sympatric hominid species, this paper explores constraints on the process of speciation underconditions of temporary allopatry.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Hominids; Speciation; Habitat specificity; Mobility; Vicariance; Wolves

Introduction

The demonstration that two extremely divergent hominidspecies overlapped temporally in East Africa during the EarlyPleistocene laid to rest the single-species hypothesis of homi-nid evolution (Leakey and Walker, 1976). But it has proveddifficult to demonstrate unequivocally that additional clado-genic events have occurred in the genus Homo. Therefore,a fundamental question in paleoanthropologydwhether morethan one species of Homo ever existed contemporaneouslyin the Pleistocenedhas defied consensus (e.g., compare‘‘splitters’’dGroves, 1991; Schwartz and Tattersall, 1996;Klein, 1999; with ‘‘lumpers’’dHenneberg and Thackeray,1995; Wolpoff, 1999; Cunroe and Thorne, 2003).

Neither morphological nor genetic data have resolved thephylogeny of Pleistocene Homo. This is in large part becausethe taxonomic significance of morphological and genetic

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differences between closely related animals is often unclear(Tattersall, 1986, 1993; Jolly, 1993; Mayr, 2000; Ahernet al., 2005). Among extant animals, for example, ‘‘good’’ spe-cies can be nearly indistinguishable morphologically (crypticspecies), yet intraspecific variation can also be so great thatsubspecies would be mistakenly identified as species in the ab-sence of behavioral information (e.g., in baboons: Jolly, 1993).Likewise, seemingly decisive analyses of modern human (be-ginning with Cann et al., 1987; reviewed in Relethford, 2001)and fossil hominid DNA (Krings et al., 1997, 2000), inter-preted to support a recent African origin of anatomically mod-ern humans, continue to be challenged on both methodologicaland interpretive grounds (e.g., Relethford and Harpending,1994, 1995; Hawks et al., 2000; Adcock et al., 2001;Relethford, 2001; Templeton, 2002; Eswaran et al., 2005).

Information pertaining to higher-order processes and speci-ation mechanisms can be recruited to help evaluate phyloge-netic hypotheses (Foley, 1991; Turner and Wood, 1993;Conroy, 2002). For example, data relevant to the behavioralpotential for population isolation in the Pleistocene bear

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384 A.C. Arcadi / Journal of Human Evolution 51 (2006) 383e394

directly on phylogenetic hypotheses, since speciation dependson reproductive isolation. Relatively little attention has beendevoted to exploring this issue (but see Foley, 1984, 1989;Turner, 1984; Gollop and Foley, 2002; the issue has receivedmore attention for the Mio-Pliocene, e.g., Vrba, 1985, 1995;Bromage and Schrenk, 1999; Kingdon, 2003), yet a compellingphylogenetic reconstruction should address the followingquestions: What conditions could have led to sustained geo-graphic vicariance,1 and subsequent reproductive isolation,of a given hominid population? Where could this have oc-curred? What behavioral conditions would have made geneticdivergence likely, or unlikely, in an isolated population?

Comparisons to adaptively similar modern species maysuggest answers to these questions. In this paper, I use ananalogy to the wolf-like canids (Canis lupus, C. rufus, andC. latrans) to ask the question, How many Homo speciesshould there be, given their likely behavioral profile(s)? (cf.Foley, 1991). The wolf-like canids are behaviorally similarto Pleistocene hominids in three key ways: (1) they are adap-ted for endurance locomotion, (2) they have a diverse diet, and(3) they are socially flexible. These characteristics lead to highhabitat tolerance, which appears to have made the wolf-likecanids resistant to allopatric speciation. I argue that analogoushabitat tolerance would have likewise made isolation and allo-patric speciation among Pleistocene Homo unlikely, especiallyin Africa. In contrast to an earlier single-species hypothesiswhich posited competitive exclusion between sympatric hom-inid species (Wolpoff, 1971), this paper explores behavioralconstraints on the process of speciation itself under conditionsof temporary allopatry.

Primate analogies and their limitations

Although debate persists concerning the nature of biologi-cal species and how to define them (Claridge et al., 1997;Wheeler and Meier, 2000; summarized in Groves, 2001),most theorists agree that some degree of allopatric isolationis essential to speciation (following Mayr, 1963; and includingvicariance, peripatric, and parapatric models: Futuyma, 2005).Sympatric or stasipatric speciation mechanisms (cf. Groves,1991) are more difficult to model and less commonly invoked(Vrba, 1995; Turner, 1999; Futuyma, 2005). Ideally, hypothe-ses about speciation should include proposals concerning pop-ulation isolation, genetic divergence, and recontact. But this isnot easily achieved for extinct populations known from smallsamples of fossils. For these ‘‘morphospecies,’’ crucial biogeo-graphic information regarding ecotones, geographic barriers,and range boundaries is difficult to obtain (Grubb, 1999),and taxonomic decisions rely primarily on quantifying mor-phological and genetic differences within and between fossil

1 Following the usage in Vrba (1995), I use the term ‘‘vicariance’’ to refer to

fragmentation in the geographic distribution of a species that, if sustained, can

result in speciation. Bromage and Schrenk (1999: glossary) credit Grubb

(1999) with a usage of vicariance, not adopted here, which appears essentially

synonymous with the term ‘‘allopatric speciation.’’

samples (e.g., Schillaci and Froehlich, 2001; Harvati et al.,2004).

Efforts to model hominid reproductive isolation have fo-cused on the use of extant primate species as analogs, withthe choice of an analog species depending on its phylogeneticand ecological profile (Jolly, 1993, 2001; Holliday, 2003).Jolly (2001) cites two criteria in support of his preferencefor baboons over apes as analogs for hominids, beyond thefact that their relative phylogenetic distance will help avoidthe error of mistaking parallelism for homology. First, he ar-gues that the best models for thinking about hominid taxon-omy will be clusters of minimally differentiated species thathave experienced recent radiations (2001: 180). Second, spe-cies that are ecological generalists are preferred (2001: 195),presumably because ecological flexibility will permit a widegeographic distribution within which allotaxa can beestablished.

But while baboons may constitute a good primate speciescluster for modeling hominid population dynamics, the ba-boon analogy nonetheless contains two important shortcom-ings. First, the preference for a species that has manifesteda recent radiation into distinct allotaxa restricts us to imagin-ing that Pleistocene Homo were distributed as allotaxa ina way similar to that now found among baboons, which exhibitrelative geographic isolation and narrow hybrid zones. How-ever, for reasons discussed below, Pleistocene Homo maynot have been divided along ecotones as baboons and manyother primate species are today. Second, although baboonsare ecologically flexible compared with many other primates,they may nonetheless be relatively specialized compared tohominids. This possibility stems from the proposition thathominids, at least by the emergence of Homo around 2 mya,were radically different ecologically from any known primate(Potts, 1994; Milton, 1999).

The unusual (for a primate) nature of hominidbehavioral ecology

At least three lines of evidence imply that hominids weredramatically different from other primates ecologically. First,at least by the Pleistocene, bipedality appears to have repre-sented an adaptation to walk and run farther and more effi-ciently than other primates can (Rodman and McHenry,1980; Haeusler and McHenry, 2004; Bramble and Lieberman,2004). Despite the long-running debate regarding the relativeefficiency of australopithecine bipedalism (Taylor andRowntree, 1973; Rodman and McHenry, 1980; Wolpoff,1983; Jungers and Stern, 1983; Lovejoy, 1988; Steudel,1995; Richmond et al., 2001), the postcranial morphology ofPleistocene Homo was certainly more like that of modern hu-mans (Brown et al., 1985), and increases in both limb lengthand body mass imply reduced costs of locomotion (energy/unit time) and transport (energy/unit distance)(Taylor et al.,1970; Steudel, 1995; Leonard and Robertson, 1997a). Im-proved locomotor efficiency, in turn, would have offerednew foraging opportunities (Carrier, 1984), and was probablyaccompanied by significant increases in range size and

385A.C. Arcadi / Journal of Human Evolution 51 (2006) 383e394

reproductive dispersal potential (Foley, 1992; McHenry, 1994;Leonard and Robertson, 1997a, 2000; Foley and Elton, 1998;Anton et al., 2002).

Second, by the emergence of Homo roughly 2 mya, hominiddietary breadth must have increased in response to the substan-tially higher energetic demands of relatively large brains(Shipman and Walker, 1989; McHenry, 1994; Aiello andWheeler, 1995; Leonard and Robertson, 1997b; Ungar et al.,2006). Higher nutritional requirements would have demandeda novel ecological solution(s) to access more easily digested,higher-calorie foods. Two possible strategies, not necessarilymutually exclusive, would have been an increase in meat con-sumption by hunting (Washburn and Lancaster, 1968; Carrier,1984; and many others) and/or scavenging (Schaller andLowther, 1969; Blumenschine, 1987; Domınguez-Rodrigo,1999), and cooking (Wrangham et al., 1999). In both cases,greater dietary breadth is implied. Increased meat consumptiondoes not eliminate plant-based nutrition, but rather would be ex-pected to supplement it, and perhaps even require the addition ofnew plant products (Cachel, 1997). In the case of cooking, whichmay date to the early Pleistocene (Wrangham et al., 1999; butsee Brace, 1997), many more plant foods would become avail-able because high heat breaks down otherwise indigestiblehard fibers and can neutralize plant toxins (Leopold and Ardrey,1972). Increased dietary breadth, in turn, would make new eco-logical communities accessible to hominids, eliminating at leastsome barriers to individual ranging and dispersal (Potts, 1994).

Third, with increasingly larger brains, Pleistocene Homowere likely to have been increasingly emancipated from modalforms of social organization. That is, we should expect thatthey were even more behaviorally flexible than other primates,and therefore less likely to be constrained by ecological bar-riers, and more likely to be ecologically heterogeneous. Al-though most nonhuman primate species can be characterizedby a modal form of social organization (e.g., organized in mo-nogamous, one-male/multifemale, or multimale/multifemalegroups), it is clear that they manifest tremendous intraspecificvariability in behavior and social organization associated withecological variability (Fuentes, 1999; Arcadi, in prep). Withincreased relative brain size, Pleistocene Homo were possiblyeven more socially labile, perhaps more easily altering theirsocial behavior and organization, both within and betweengroups, to meet novel ecological demands (Foley and Lee,1989; Rodseth et al., 1991).

Taken together, these behavioral characteristics suggest thatall Pleistocene Homo had the potential to utilize a wide rangeof food resources found in varying abundance, and could travelfar to obtain them, as compared with other primates. There isevery reason to suppose, and some geological and archaeolog-ical data to support the idea, that Pleistocene Homo exhibitedhigh habitat tolerance (Kingston et al., 1994; Potts, 1994).Cultural developments such as the evolution of linguisticcapacities, increasingly sophisticated tool technologies, thecontrolled use of fire, clothing, and shelter would have contin-ued to increase habitat tolerance in Middle and Upper Paleo-lithic hominids (i.e., access to higher latitudes and altitudes:Turner, 1984).

These considerations lead us to question the applicability ofprimate models for understanding hominid population dynam-ics, because the potential for genetic differentiation in geograph-ically separate populations would have been dramaticallyreduced in hominids compared with other primates, despite theirvast geographic range. This is because dispersing populationswould have relied on behavioral flexibility, and not geneticdifferentiation, to succeed in colonizing new environments, asthe following examination of speciation theory shows.

Implications of habitat tolerance for the establishmentof allotaxa

The allopatric speciation model (see references above) stip-ulates that speciation results when gene flow between popula-tions is diminished or interrupted, and founder effect and/orselection result in the evolution of pre- and post-mating isolat-ing mechanisms.2 However, continued divergence and specia-tion are not an inevitable consequence of allopatry, andrequisite degrees of genetic and morphological divergenceare quite variable (Mayr, 1963, 2000). Thus, geographic sepa-ration is not a sufficient condition for speciation (Vrba, 1999).A substantial body of empirical evidence shows that speciescohesion can be maintained under conditions of allopatry,sometimes for very long periods of time (e.g., several millionyears in the polytypic salamanders, Ensatina eschscholtzii:Moritz et al., 1992; Wake, 1997). In some cases, allopatricpopulations can manifest minimal geographic variation inspite of dramatically different ecological circumstances (e.g.,Tanysiptera galatea: Mayr, 1992). Thus, while selection canproduce morphological differences between separated popula-tions of a magnitude sometimes observed between species,even limited gene flow can sustain polytypy without speciation(reviewed in Rice and Hostert, 1993). For speciation to occur,conditions must prevail that promote genetic divergence be-tween founder and parent populations, and this divergencemust be either directly or indirectly linked to specific materecognition systems (Paterson, 1985).

Vrba (1980: 73e80, 1995: 30e31; see also Eldredge, 1979:14) has argued that the likelihood of speciation depends on thedegree to which a species is environmentally specialized (the‘‘effect hypothesis’’). This hypothesis assumes that dispersalis an inevitable and constant feature of animal populations(see also Corbet, 1997; Grubb, 1999). Based on an analysisof species richness in terrestrial mammals, she found that eco-logically specialized taxa tend to be more speciose than envi-ronmental generalists. The theoretical explanation for this

2 It is beyond the scope of this paper to engage the debate over whether iso-

lating mechanisms are selected for directly (e.g., using the recognition con-

cept, emphasizing specific mate recognition systems: Paterson, 1985) or are

epiphenomena of divergent selection regimes (Mayr, 1963, 2000; see Rice

and Hostert, 1993, for a review of laboratory studies of speciation mechanisms

that point to the importance of diminished gene flow combined with multifar-

ious divergent selection). The important point for this discussion is that geo-

graphic separation sets the conditions for divergence, such that further

character displacement can potentially follow if recontact occurs.


finding is that narrow habitat specialization (‘‘stenotopy’’) pre-cludes easy colonization of new ecological communities, sothat peripheral population isolates will survive only rarely,and only if they differ genetically from their parent stock.By contrast, peripheral populations of species characterizedas environmental generalists (‘‘eurytopy’’), by being more be-haviorally flexible, can more readily colonize diverse habitatswithout the necessity of differing genetically from their paren-tal stock. Thus, dispersal will more often result in populationfragmentation in a stenotopic species because the failure topersist in most new ecological settings will produce gaps inthe species’ distribution. Likewise, climate-driven ecologicaldisruptions are more likely to fragment stenotopic than eury-topic populations. Thus, eurytopes are more likely to maintainpopulation continuity during range expansion, even as theycolonize new ecological communities.

Analyses of comparative species richness in a range ofmammalian taxa provide support for the effect hypothesis.The Alcelaphini (blesbuck, hartebeest, wildebeest), special-ized consumers of bulk and roughage, exhibited a radiationof 27 species over the past six million years, whereas the Ae-pycerotini (impalas), adaptable ruminants that can adjust thestructural components of their stomachs in response to diet(Hofmann, 1973: 235), evolved only 1 or 2 species duringthe same period (Vrba, 1980). Similarly, the forest-adaptedguenons (Cercopithecus spp.) evolved 18e25 species overthe last 0.5e1.0 million years (Groves, 2001; Butynski,2002), whereas the more omnivorous and habitat tolerant ba-boons (Papio) generated between 1 and 5 species during thissame period (Jolly, 1993; Groves, 2001).3 Taking high mobil-ity into consideration, stenotopic horses (e.g., Hipparion spp.:Bernor and Lipscomb, 1995; Bernor and Armour-Chelu, 1999)radiated dramatically, whereas wide-ranging and omnivorouslarge carnivores (e.g., brown bears, Ursus arctos: Nowak,1991; lions, Panthera leo: Turner, 1990) did not. Vrba(1980) proposed that, in general, we would expect carnivoresto be relatively eurytopic compared with herbivores, sincethe former tend not to be prey-species-specific (they tend tobe prey-size-specific), whereas the latter may less readily ad-just to new vegetation types.

Because Pleistocene hominids were likely to have been en-vironmental generalists that traveled far in search of diverseresources, the effect hypothesis suggests that they would nothave evolved many, if any, allotaxa across their impressivegeographic range (cf. Grubb et al., 1999).4 This conclusionis at odds with recent assessments of both Early and LatePleistocene hominid taxonomy that identify multiple contem-poraneous Homo species distributed across continuous terres-trial habitat (e.g., Schwartz and Tattersall, 1996; Wood and

3 There are insufficient fossil remains of guenons to extend the comparison

back in time (Leakey, 1988); nevertheless, the baboon fossil record suggests

that at no point during the Pleistocene did species richness in this group exceed

what we see today (Foley, 1993: Table 9.6).4 A possible exception might be island colonization, as may have occurred

with the hominids that apparently reached the Flores Islands as early as

800,000 ya and were isolated by a wide ocean barrier (Morwood et al., 2005).

Collard, 1999). However, its theoretical feasibility can be eval-uated by asking whether it is possible for a large, eurytopicmammal species to be extremely widely distributed (i.e.,more than one continent) and show statistically significantregional variation in morphology, yet resist speciation, asevinced by minimal genetic differentiation. More particularly,are there modern species that meet these criteria, while at thesame time being behaviorally and ecologically similar enoughto what we think hominids were like to support a compellinganalogy (e.g., more intuitively satisfying than one based onrodent species: Wolpoff et al., 1984: 447)?

A non-primate analogy for Pleistocenehominids: wolves and coyotes

Because of their high habitat tolerance, social members ofthe order Carnivora are good candidates for modeling behav-ioral aspects of hominid biogeography (Schaller and Lowther,1969). Of these, the northern hemisphere wolf-like canids(gray wolf, Canis lupus; red wolf, C. rufus; coyote, C. latrans)are especially intriguing because of their exceptionally widedistribution, variable morphology, and adaptive similarity toHomo. In contrast to previous carnivore and primate analogies,which have mainly focused on the behavioral ecology of car-nivory and possible interspecific interactions among Plio-Pleistocene hominids (Schaller and Lowther, 1969; Cachel,1975; Thompson, 1975; Hall, 1977, 1978; Du Brul, 1977;Stevenson, 1978; Foley, 1984, 1992; Turner, 1984; Leonardand Robertson, 1992, 1997b, 2000; Foley and Elton, 1998;Sikes, 1999; Simmons, 1999; Holliday, 2003), the goal ofthis comparison is to gain insight into the likelihood of allo-taxic evolution in later hominids.

Endurance locomotion, dietary breadth, andsocial flexibility in the wolf-like canids

Endurance locomotion. Wolves and coyotes are cursorialcarnivores that typically capture their prey after prolongedchases [compared with an ambush from concealment, as inleopards (Pantherea pardus), or a swift rush, as in cheetahs(Acinonyx jubatus)]. Wolves can sustain trotting speeds of8e9 km/hr for several hours (Mech, 1994: Table 1) and canmaintain running speeds of 55e70 km/hr for 20 minutes ormore (Nowak, 1991). Predation chases usually cover lessthan 5 km, but can be much longer: a record pursuit of20.8 km was observed by Mech and Korb (1978). Daily traveldistances range from a few to 200 km (Mech, 1970), withtypical averages of 20e30 km/d (e.g., Mech, 1970: 160;Pulliainen, 1975: 298; Ciucci et al., 1997: 811; Jedrzejewskiet al., 2001: Table 1). Dispersal distances are also variable,from a few kilometers to take over a territory adjacent to anindividual’s natal range (e.g., Mech, 1987: 65; Gese andMech, 1991: 2949), to average distances of a few hundredkm (e.g., Boyd et al., 1995: Table 1; Wabakken et al., 2001:721), to directional movements of up to record observationsof 840 km for a Rocky Mountain female (Boyd et al., 1995:139) and 886 km for a Minnesota male (Fritts, 1983: 166).


Recorded territory sizes range from 33 km2 (L.D. Mech and S.Tracy, unpublished data, in Mech and Boitani, 2003a: 21) to4335 km2 (Mech et al., 1998: Table 2.5); for packs followingcaribou migrations, ranges can swell to 63,058 km2 (Waltonet al., 2001: Table 1). It is also noteworthy that wolves arecompetent swimmers, with a record distance of 13 km ob-served by P.C. Paquet (pers. comm. in Mech and Boitani,2003b: xv).

Dietary breadth. Wolves and coyotes usually rely on mam-malian meat, but they are both known to fish (Gier, 1975: 250;Mech et al., 1998), to eat birds, lizards, crustaceans, and in-sects (Young and Goldman, 1944: 211; Gier, 1975: 250),and to consume some types of plant matter in large quantitieswhen available (e.g., fruits and grass: for wolves, see Petersonand Ciucci, 2003; for coyotes, see Niebauer and Rongstad,1975; McMahan, 1978; Wade, 1978; Knowlton and Stoddart,1992). Variability in prey size selection, though common incarnivores, is extreme in wolves and coyotes. In most areas,wolves concentrate on medium-to-large-sized (i.e., up to1000 kg) ungulates. Nonetheless, wolves can effectively huntand subsist on much smaller prey, including hares (Lepusamericanus), beaver (Castor spp.), and some small rodents (re-viewed in Mech and Peterson, 2003; Peterson and Ciucci,2003). Conversely, coyotes, which are roughly half the sizeof wolves, usually rely on small rodents, rabbits, and fruits,but will take prey as large as deer when these are available (re-viewed in Boer, 1992). In both species, wide dietary breadth isreflected in their generalized craniodental morphology com-pared with other carnivores (reviewed in Biknevicius andVan Valkenburgh, 1996; Peterson and Ciucci, 2003).

Flexibility in prey size selection in the wolf-like canids is theresult of two factors, one morphological and one behavioral.First, though relatively large, these predators are sufficientlysmall to subsist on smaller prey when necessary (by contrast,the extinction of dire wolves, C. dirus, may have resultedfrom their inability to rely on smaller prey after the dis-appearance of other North American megafaunadKurten andAnderson, 1980: 172; Stock and Harris, 1992: 78; Bikneviciusand Van Valkenburgh, 1996: 419). Second, wolves and coyotescan adjust group size to prey base, and hunt in packs for largerprey. Interestingly from the point of view of hominid evolution,wolves differ dramatically from similarly-sized carnivores inother families (e.g., Felidae, Ursidae) in being poorly adaptedmorphologically to capture large prey alone (Biknevicius andVan Valkenburgh, 1996). Although a lone wolf is capable oftaking the largest of its prey (e.g., Mech and Boitani, 2003a),lone hunters are sometimes killed by their quarry (Nelson andMech, 1985; Pasitschniak-Arts et al., 1988; Mech and Nelson,1990; Weaver et al., 1992).

Social flexibility. While they are not alone among the carni-vores in demonstrating complex social dynamics, wolves andcoyotes nonetheless display interesting parallels to extant pri-mates, and thus perhaps to hominid primates, in their socialflexibility. In particular, these canids exhibit a surprisinglywide range of group compositions. Mated pairs with offspringappear to be the most common type of social group, but otherarrangements are frequently observed. For example, the

following kinds of groups have now been documented amongwolves: one male/one female; one male/adult son/one female;one male/two females; all male groups; and large, fission/fusion groups containing multiple adult males and females(reviewed in Mech and Boitani, 2003a). Both sexes disperseand both sexes are capable of joining existing, unrelatedbreeding groups (Lehman et al., 1992), despite the risk oflethal aggression (Mech and Boitani, 2003a). Average packsizes are typically between 5 and 10 individuals in NorthAmerica, and maximum pack sizes of over 20 individualsare not uncommon (Fuller et al., 2003: Table 6.1). Similarly,although maximum observed group sizes are lower for coyotesthan for wolves, they too are flexible. While most coyote stud-ies have reported the prevalence of mated pairs and offspring,groups as large as 22 individuals have been observed whenlarge prey are available either for capture or as carrion(Camenzind, 1978; Brundige, 1993 and Sabean, 1993 inPatterson and Messier, 2001: 463; Patterson and Messier,2001).

Habitat tolerance, polytypy, and geneticdifferentiation in the wolf-like canids

In general, meat-eating carnivores tend to be eurytopic(Vrba, 1980), reflecting the fact that from the point of viewof digestion, animals are a more consistent and predictable nu-tritional resource than plants, which boast a myriad of physicaland chemical defenses and vary greatly in nutritional value.While prey animals from different ecological communitiesmay demand different capture strategies, they are unlikelyto demand particularly different processing techniques ordigestive requirements once acquired. Large and small ani-mals may be consumed in slightly different ways (Blumen-schine, 1987), but meat and bone themselves, unlike plantproducts, are comparatively uniform across species and envi-ronments, so that processing varies little within prey sizeclasses (Van Valkenburgh, 1989; Biknevicius and Van Valken-burgh, 1996).

As a consequence of their habitat tolerance, carnivores canoccupy vast geographic ranges, encompassing many ecologi-cal communities, while exhibiting comparatively low speciesdiversity. The canids exemplify this pattern. The family Can-idae includes 16 genera and 36 species, which are distributedthroughout four continents, excluding the dingo in Australiawhich is descended from the domestic dog. By comparison,the Cercopithecidae comprises 21 genera and roughly 125 spe-cies distributed across primarily tropical and subtropicalAfrica and Asia (Nowak, 1991; Groves, 2001). Wolves andcoyotes in particular occupy extensive geographic rangesand are capable of rapid colonization. Wolves remain secondonly to humans in extent of geographic distribution and, pre-sumably, habitat tolerance (Boitani, 2003). In the wake ofthe extirpation of the wolf from most of its North Americanrange, coyotes succeeded in colonizing most of that continentin less than 100 years. Both species can thrive in seeminglyany habitatddeserts, wetlands, northern forests and tundra,and all manner of agricultural, suburban, and urban habitats


in between (Moore and Parker, 1992; Fritts et al., 2003). Bothwolves and coyotes are largely unhindered by environmentalheterogeneity: they can subsist on a wide variety of foods,tolerate extreme temperature variation (�21 � to þ48 �C forwolves: Mech and Boitani, 2003b: xv), sustain high levels ofphysical exertion for extended periods of time, and adjustgroup size and composition to prey biomass and prey size,all of which have the effect of increasing the range of exploit-able ecological communities available to them.

As one might expect in a widely distributed species, consid-erable morphological variation exists across the range of mod-ern C. lupus.5 The biggest animals are found in the northernpopulations of North America and Eurasia (e.g., mean bodyweights for male and female C. l. occidentalis are as high as47.0 and 40.0 kg, respectively: Mech et al., 1998, n¼ 18 and15, SD not provided; for Eurasian C. l. communis, mean weightsare 44.7� 3.0 and 34.2� 4.2 kg, respectively: Wabakken et al.,2001, n¼ 15 and 7). The arctic subspecies (C. l. arctos in NorthAmerica, C. l. albus in Eurasia) are slightly smaller, with theNorth American C. l. arctos being distinguished by the rela-tively largest carnassials of any population. Medium sizedwolves are found in wide bands across the intermediate lati-tudes of North America (C. l. nubilis) and Eurasia (C. l. lupus).The smallest wolves are found in southern localities: C. l. lycaon,the smallest C. lupus subspecies in eastern North America, insoutheastern Canada; C. l. baileyi in Mexico; and C. l. pallipesin south Asia and the Arabian peninsula. Mean body weightvalues for some of these populations are less than half thoseof their northern conspecifics (e.g., 20.1 and 17.0 kg for maleand female C. l. pallipes: Mendelssohn, 1982, n¼ 7 and 6,SD not provided). Body weights of C. rufus, which were distrib-uted historically in southeastern North America, are at the lowend of the North American wolf range (e.g., 22.7� 3.9 and20.0� 2.7 kg for males and females: Riley and McBride,1975, n¼ 26 and 26). Canis latrans body weights historicallyfell below the ranges of North American wolves (e.g.,15.0� 2.0 and 13.2� 1.8 kg for males and females: Rileyand McBride, 1975, n¼ 15 and 17); C. latrans populationsthat have colonized the Northeast over the last 50 years (Hil-ton, 1978) are larger, overlapping the lower end of the bodyweight range of neighboring C. lupus (20.3 and 18.0 kg formales and females: Silver and Silver, 1969, n¼ 8 and 6, SDnot provided). Cranial dimensions can also vary greatly be-tween wolf subspecies, with the most distinctive wolf popula-tions found at the peripheries of the species range (Nowak,2003: 247).

However, although sufficient regional differences in bodysize and cranial metrics exist to designate some 10 subspeciesof modern C. lupus (Nowak, 1995, 2003), all workers agreethat clines and substantial metric overlap characterize regionalvariation in wolves (reviewed in Brewster and Fritts, 1995).Pelage color has proved too variable to be of taxonomic value,

5 A comprehensive table showing distinguishing features, cranial measure-

ments, body weights, and geographic ranges of 10 subspecies of gray wolf,

the red wolf, and coyotes, along with literature citations, is available from

the author on request.

with the singledbut not invariabledtrend being a north-southcolor cline among northern populations from lighter to darker(Gipson et al., 2002). Thus, while there is some debate aboutthe identities and boundaries of subspecies (usually in the con-text of conservation policy), all analyses point to the existenceof only one or two (i.e., if C. rufus is distinct from C. lupus)species distributed across a geographic range that spans threecontinents. Clinal variation and metric overlap reflect the exis-tence of extensive gene flow throughout the species range re-sulting from large territories and high dispersal distances(Nowak, 2003). Moreover, despite regional variation in mor-phology, general behavioral similarity prevails among allwolf populations.

The emergent profile of modern wolves is that of a widelydistributed, polytypic species for which subspecific designa-tions may be useful hypotheses for reconstructing biogeo-graphic history, but represent little or nothing aboutdifferences in behavioral ecology or specific mate recognitionsystems between populations (Mech, 1991: 315).

Molecular genetic analyses of C. lupus interpopulation var-iability (reviewed in Wayne and Vila, 2003) are consistentwith this interpretation. Studies employing allozyme electro-phoresis, mitochondrial (mt) DNA restriction site analysis,and genetic fingerprinting all point to low genetic differentia-tion between and within wolf populations (Brewster and Fritts,1995). For example, in an analysis of 310 C. lupus individualsfrom 16 North American localities, Lehman et al. (1991; seealso Wayne et al., 1992, 1995) found five C. lupus mtDNA ge-notypes widely distributed among populations, with only twopopulations (from Mexico and Manitoba) possessing oneuniquely. Sequence divergence among genotypes was ex-tremely low (<0.5%), with the most differentiation presentin Mexican, Alaskan, and southeastern Canadian wolves (theperipheries of the North American range) (Wayne et al.,1995: 403). A similar pattern has been found for Old WorldC. lupus populations (Lehman et al., 1991; Wayne et al.,1992). Thus, consistent with the patterns of clinal variationand statistical overlap in morphological features discussedabove, genetic analyses suggest ‘‘a large, panmictic (randomlyinterbreeding) population resulting from extensive movementsof individuals and packs.’’ (Brewster and Fritts, 1995: 369),despite significant population differences in gross morphologydetectable through multivariate analyses.

Although increasing geographic distribution is associatedwith greater species diversity in many genera (Foley, 1991),this is not the case among the wolf-like canids. Instead, acrosstheir extensive geographic range, these animals demonstrateminimal genetic differentiation, substantial clinal variation inbody size and proportions, and affinities between widelyseparate populationsdall against a backdrop of behavioralsimilarity and mutual mate recognition. Resistance to discon-tinuous regional differentiation appears to be a consequence oflarge home ranges, long dispersal distances, and a history ofrepeated waves of colonization across and between continents(Nowak, 2003). Recent molecular genetic analyses have beeninterpreted to suggest that gene flow has stifled genetic differ-entiation among C. lupus (Wayne et al., 1995; Wayne and Vila,


2003), as it has among C. latrans (Lehman and Wayne, 1991).In the words of Wayne and Vila (2003: 236), ‘‘In this sense,typological species concepts may be inappropriate becausegeographic variation in the wolf is distributed along a contin-uum, rather than being partitioned into discrete geographicareas delineated by fixed boundaries.’’

Could the same have been true of Pleistocene Homo?

Species resilience in Canis and Homo

Molecular clock estimates place the divergence of wolves,coyotes, and jackals at 2e3 mya (Wayne et al., 1995). Up tonine possible species of wolves have been identified in the fos-sil record since that branching point (Nowak, 2003: 240). Mostof these represent chronospecies since the last common ances-tor, with C. etruscus and then C. mosbachensis representingthe best described earlier Pleistocene forms (Agustı and An-ton, 2002). The relatively brief appearance of C. dirus (thedire wolf, about 30,000e10,000 ya) marks the only fairly clearperiod during which two contemporaneous wolf species werepresent in the fossil record, and some workers neverthelessconsider C. dirus a large regional variant of C. lupus (reviewedin Nowak, 1979, 2003: 240). Fossils identified as modern C.lupus date to the late Middle Pleistocene in North America(Kurten and Anderson, 1980: 171) and Eurasia (Nowak,1992; Agustı and Anton, 2002). Hence, the previous emphasison interspecific fertility in modern canids to illuminate possi-ble hominid species relationships (e.g., Holliday, 2003) hasneglected the equally interesting possibility that wolves area model of resilience to allopatric speciation in a large terres-trial mammal.

Later hominids share with wolves the suite of behavioralcharacteristics that makes species resilience in the latter possi-ble. The behavioral parallels between wolves and hominidsthus represent strong theoretical support for the hypothesisthat Pleistocene Homo evolved multiregionally as a single,widely dispersed, polytypic species (Wolpoff et al., 1984;Thorne and Wolpoff, 1992; Hawks et al., 2000; Cunroe andThorne, 2003). The central proposition of this argument isthat a large bodied, nutritionally demanding, adaptively flexi-ble, eurytopic species would not have had the opportunity tospeciate. This is because genetic differentiation would nothave been necessary to colonize the new ecological spacesopen to expanding Homo populations, and sustained vicari-ance would have been unlikely even if temporary periods ofclimate-driven isolation occurred during transcontinentalcolonization (Hewitt, 1999; Dennell, 2003). Other theoreticalapproaches have produced results consistent with this chrono-species interpretation (Foley, 1991: 4216; Grubb, 1999: 163;Conroy, 2002).

Although it is clear that Pleistocene Homo comprised manydistinctive morphotypes, both over time and contemporaneously,

6 Foley (1991) computed high estimates of hominid species richness based

on comparative primate data, but low estimates based on comparative carni-

vore data.

examination of the wolf-like canids suggests that cautionis warranted in drawing species-level distinctions based onmorphological variation. Likewise, the canid analogy suggeststhat caution is warranted in attributing the relatively low ge-netic variation documented among modern humans (Vigilantet al., 1991: Table 2; Goldberg and Ruvolo, 1997; Gagneuxet al., 1999; Krings et al., 2000; Kaessmann et al., 2001; re-views in Relethford, 2001; Pearson, 2004) to recent commonancestry (Cann et al., 1987; Vigilant et al., 1991). In bothwolves worldwide and coyotes in North America (Lehmanet al., 1991: Table 4; Wayne et al., 1992), low genetic var-iation has been linked to persistently high levels of geneflow (Wayne et al., 1995). In view of the analogously eury-topic and mobile nature of Pleistocene Homo, the wolf-likecanids provide a compelling demonstration of how gene flowcould have stifled hominid, and eventually modern human,genetic diversity (Wolpoff et al., 1984; Relethford andHarpending, 1995; Hawks et al., 2000; Relethford, 2001;Templeton, 2002).

Resilience to speciation versus competitive exclusion

The use of a canid analogy to support a single species in-terpretation of Pleistocene Homo evolution differs fundamen-tally from the theoretical argument for Lower Pleistocenesingle species evolution proposed by Wolpoff (1971). Theanalogy to the wolf-like canids focuses on the conditionsthat favor, or do not favor, genetic divergence between parentand daughter populations. By contrast, citing the competitiveexclusion principle, Wolpoff (1971) argued that there wouldnot have been ecological space for two Lower Pleistocenehominid species to coexist because of the ecological overlapinferred from their reliance on tool use for hunting and de-fense. Both models envision broad habitat tolerance for hom-inids. But the key difference between the two approachesis that Wolpoff (1971) attempted to show why two culture-bearing hominid species could not have existed sympatrically,whereas the canid analogy asks whether under conditions ofallopatry, which are assumed to have occurred periodicallythroughout human evolution as a consequence of normal andclimate-driven dispersal and population movements (cf. Grubb,1999), hominids would have speciated to begin with.

Significantly, persuasive theoretical objections to Wolpoff’s(1971) single-species hypothesis do not apply to the species-resilience hypothesis described here. Rather, they tend tosupport it. In his critique of Wolpoff’s (1971) argument,Winterhalder’s (1980, 1981) key insight was that hominid cul-tural abilities would have increased, not decreased, the poten-tial for sympatry between competing hominid species, becausethe ability to make and use tools would have dramatically in-creased the potential of hominid populations to partition realecological niches and, consequently, to coexist. By the sametoken, Winterhalder’s (1980, 1981) analysis of culture andniche partitioning supports the species-resilience interpretationbased on the canid analogy, because if culture increased thepotential for niche partitioning, it would by definition haveincreased hominid eurytopy. Following this reasoning, behavioral,


not genetic, variation would have made colonization of newenvironments possible, further reducing the likelihood of vi-cariance and speciation.

Problems and future directions

The canid analogy presented here offers an ecological per-spective on the question of species resilience in hominid evo-lution. Like all analogies, it is imperfect.

First, the utility of the analogy depends on the degree towhich Pleistocene Homo were in fact mobile and habitat toler-ant. For example, despite their adaptation for endurance loco-motion, these hominids may not have habitually dispersedlong distances as do the wolf-like canids. Resolving this issuewill be essential for evaluating whether a Homo populationcould have become isolated in southern Africa (cf. Klein,1999) or in Europe (Howell, 1952; Hublin, 1998) for the0.5e1.0 million years necessary to speciate (cf. Avise et al.,1998), since dispersal corridors were present even during gla-cial maxima (Howell, 1952: Fig. 1; Turner, 1984: 195; Hewitt,1999: 90; see also Simmons, 1999; Dennell, 2003: 427). Sim-ilarly, despite the increasing importance of meat to the homi-nid diet during the Pleistocene (Milton, 1999; Richards et al.,2000), the extent of dietary breadth among hominids remainsunclear (but see Ungar et al., 2006).

A second challenge to the analogy is the degree of species re-silience the wolf-like canids actually manifest. If C. rufus (orsome other peripheral population) proves to be a good biologicalspecies, despite high levels of hybridization driven by extremedisparities in population size with neighboring wild C. lupuspopulations, then the canid analogy would support a multi-line-age scenario of Pleistocene Homo evolution (see: Mech, 1970;Lehman et al., 1991; and Wayne and Jenks, 1991; in supportof C. rufus as a C. latrans/C. lupus hybrid; see Nowak, 1979,1995, 2003 for C. rufus as a separate species from C. lupusand C. latrans; and see Wilson et al., 2000 for C. rufus referredto C. lycaon as descendants of an early colonizing wolf popula-tion separate from C. lupus). A similar conclusion might be war-ranted if Lycaon and Cuon spp. are referred to Canis (Werdelinand Lewis, 2005), or if jackal species are shown unequivocallyto have split recently from the wolf-like canid clade (cf. Zrzavyand Ricankova, 2004). Finally, debate persists regarding theidentification of fossil canids (Nowak, 2003), as it does for fossilhominids, and the current picture of Canis chronospecies couldyet be reorganized into one that emphasizes a series of contem-poraneous fossil lineages.

Finally, a ‘‘species resilience’’ perspective on PleistoceneHomo evolution offers the potential for new comparative anal-yses of modern species to explore constraints on the speciationprocess. It has long been recognized that eurytopic mammalstend to be more widely distributed and less speciose thanstenotopic ones. However, detailed quantitative analyses ofmobility, dietary breadth, and genetic variation within and be-tween eurytopic species, in conjunction with fossil evidence,are needed to resolve more precisely the determinants of rela-tive species resilience. Especially interesting would be com-parative analyses of intercontinentally distributed carnivores

such as ursids, felids, and canids, as well as of widely distrib-uted primates. Coyne and Orr (2004: 425) note that relativelylittle research has been devoted to factors that prevent specia-tion. Comparative analyses of species resilience in Homo andother eurytopes, therefore, offer the possibility of contributingboth to debates about hominid taxonomy and to our under-standing of mammalian speciation in general (cf. Vrba, 1992).

Acknowledgements

This paper benefited enormously from input by Susan An-ton, Alan Turner, Lars Werdelin, and three anonymousreviewers.

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