The Role of Risks in Mammalian Combat: Zebra and Onager Fights

ndpole (Rana

(1981,) Z. Tierpsyochl. , 56, 297 -304 @ 1981 Verlag Paul Parey, Berlin und Hamburg ISSN 0044-3573 i InterCode: ZLTIAG

: Rana pipiens

in

releasing the

:roiection to the :otaris and

i:ite

greer.r

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o1

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Conseroation dnd Research Center, National ZooLogical Park, Srnitbsonian I nstitwtion, Front RoyaL, Virginia

E-r'oming 82601, [:iison. \fliscon-

The Role of Risks in Mammalian Combat: Zebra and Onager Fights

By JoEr Bcncnn Receioed: September 6, 1980

Accepted: ApriL 13, 1981

Abstract Risks associated with the loss of balancc n,ere in-rportant factors constraining offensive tactics during the fights of two species of large mammalian herbivores, zebras (Equus burcbeLLi) and onagers (E. hemionls). Evenly n-ratched adversaries faced greater risks than disparately matched ones. Rishs were assumed often, but -r''hetl-rer or not they were capitalized on depended upon xn opponent's fighting ability. Behavior patterns such as rearing increased the probability of scorir.rg successful bites, but they aiso carricd the greatest rishs. Retaliations were more likely when such patterns were used.

Introduction Despite recent interest in animal combat by evolutionary theoreticians (Grrsr 1974; MIvNARD SMrrH and PRICE 1973; MavNARD SMrrH 1974) there are few quantitative data on the costs of behaving dangerously (i.e. escalated fighting tactics). Very little empirical evidence has been reported regarding the risks associated with various fighting tactics and the dynamics of mammalian combat. Polygynous male mammals from a number of orders generally possess secondary sexual characteristics, some of which are used in combat (ErsnNnnnc 1966; Gnrsr 1.978a; FIeRvry, KRveNecH and CrurroN-BRocK 1978; Rens 1977). For instance, males of many species of ungulates fight with horns, antlers, or rusks (Bnnnrrrr 1977; CrurroN-BRocK, ArnoN, GrnsoN and GurNNnss 1979; Reus 1975; Wlrrr:e^rx 1974). The family Equidae is a puzzling exception to this pattern because secondary sexual characteristics u.S. copyright clearance Center Code Statementi

AA44-3573I81I5604-0297.802.50/0

298

.forr Bnncrn

are not collspicuous despite intense intrasexual competition for female mates (KrrNcrr aird I(rrNcBr 1967; KrrNcrr 1977). This absence has led some scientists to beiieve that eqr-rid fights are not only harmless and non-damaging

(LEurHoro 1977), but that their mating systems approach the monogamous end of a continuum of breeding strategies (ArnxeNorn et al. 1979). . Extant equids lack conspicuous specialized weapons. Sexual dimorphism rn canines was pronounced in equids during the middle Tertiary (i.e. Miobippus acutidens; CssonNr 1918) and it also occurred during the late Tertiary (SrrNNnn 1.972). Males in some species had relatively larger canines than that observed in living equids but the canines became progressively smaller during more recent epochs (Brncrn, in prep.). Since canines are variable and relatively

small in modern equids, racrics employed during combat are of parricular iirterest because they occur with morphological structures that serve other functions as well. For instance, reeth are used not only for grooming a.d the cropping and mastication of food but also for biting during agonistic encouuters (see KrrNcnr and KriNcrr 1967,1968).

Biting is one of the more g€neralized fighting techniques available t, mammals. It is widespread in many species of carnivores, rodents, primates (Hanvav et al. 1978; BeNrs and Popu.qu 1925; I(nuur 1972; Scn,e,rrrn 1922; svrtoNs 1978;wllsoN 1975) and it occurs often in ungulates among weaponless. fema,les (Bannnrrr 1977; Wax,rwrER pers. comm.). Since biting play, a major role in fights berween male equids ir is of interest to deteimine the factors that constrain the use of biting maneuvers. In this paper I address the following quesrions: 1) what tactics lead to successful biies; 2) why aren't they used more often; and 3) how much risk is involved in using these'tactics?

A^ns11er.s to questions such as these will aid in not only understandi,g the role of risk in the evolution of equid combat, but in other mammals as *ell.

Methods Data ot.t combat rvere collected at the Conservation and Rescardr Cer-rter of the National Zoological Parh (Sn-rithsonian Ir.rstitution), a 3,100 ,.cre preserve in the Blue Ridge Mountains of virginia. Hcrds of 14 (5 r-r-rales, 9 fen-rales) onagcrs (Eqtus hemionus) and 10 (3 ma1es, 7 femalcs) zebras (I. burchelLi) t'ere studied in spacious semi-natural enclosures, 3O-45 acres in size. Additional date on 3 zebra fights were obtained by analyzing KrrNcrr and KrrNcrr's (1958) film on E. burcbelLi in Africa. A total of 4 zebra and 5 onager fights were recorded and ranged in duration from 1O to 65 n-rin. Each fight v.as considered an independent statistical event. Thus, 9 different data sets were represented each of which contained x and y motor actions for respective participants pcr fight.

All encounters wcre analyzed *'ith the aid of a video-tape playback system (Solry AVC-3450 and SLO-34C). Motor actior.ts vere identified for each individual during a fight and then transcribed sequentially for the duration. Thesc actior-rs were then numbered and arranged serially for ead-r individual so that the preceding, simultaneous, and subsequent behaviors q.,ere known.

Animals involved in combat were judged to be of equal or unequal stature based on body size comparisons and willingncss to interact at the beginning of a fight. In situations where both animals initially approached the other they were judged to be equally matched.

Dominant or subordinate status was assigned after the encounter. Anirnals were of unequal stature when one avoided the initial approach of the other.

1.

s

The most salierrt

:',

and "circle fighting" cludes kneeling on borl and then charging e:pl to bite its rear legs. C Posture' and standing to bite at the rear leg'. conditions, those u'ith 1972; KnuuK 1972). S txctics of onagers and ; Individuals n'ho I and who failed to re:r observed during the 9 322 were attempted r than a body's length 77 % af the time (n Successful bites .'' maneuvers were uscd. more likely to be hir:. mals off-balanced io: .-.

balanced for 1,680 : , In small primitive un:, rind muntjacs (Cerr ii.: horn: Antilocaprid.re : elaphus) individuals ic agonistic interactions 1980). It thus aDDe.i:increasingly importa:::

In

equids the r;

balance were re;rrini sumable functions a.

.:

intimidating rivlls,'., 1967;MowtLI\tAN ls-Table 1; L Pattern used to baLance

in

rivaLs

attempted biting

other patterns

'!

No prior action.

::

Zebra and Onager Combat

299

Results and Discussion

1. Salient Features of

Successftrl 'Fighters,

The most salient features of onager and zebra combat are bitirg, rearing, and "circle fighting" (KrrNcrr- and KrrNcrr 1967). This latter bef,avior iricludes kneeling on both legs_ to prorect them from bites (the 'Turtle posture,) and then charging explosively toward the posterior er-rd of a rival and tryirr[ to bite its rear legs. combatants occasio.,illy fight by biting in the .Turtll Posture' and standing occurs o,ly when one charges ot ,n o=pporent's flank to bite at the rear legs. Legs are indeed vulnerable io bites ,rrd, ,rd., natural conditions, those with wounds probably suffer greate. predaiion (ScHeurn 1972; Kxuut< 1972). si,ce there were no perceptible differences in fighting tactics of onagers and zebras, I will report on them together.

Individuals who were more effective at biting oi avoiding being bitte, and who failed to rerrear were defined as winr-reri of fights. bf 3,0"1 z r.r, observed during the 9 fights, 2,608 were acts leading to" or avoiding bites, 322 werc attempted bites, and BZ were successful bites. con-rbatanrs* closer ,!:2 !o.dy': length a.d oriented eirher facing one another or parallel spent 77 % "of the time (n : 19.13 min) with their teelh bared. successful bites were rare and they were usually achieved when various maneuvers were used. For instance, individuals ki-rocked off-balance were more likely to be bitten than were those that maintained their balance. Animals off-balanced for 68 s were bitten 21 times while animals that were balanced for 1,680 s were bitten only 66 times (p { O.OO1 ; 72:26.9; df : 1).

In small p.rimiti_ve ungulates such as iuikers (Bovidae: cep6')1oo;rs maxu.,elli) ,rnd muntjacs (cervidae: Muntia*rs ree,esi) and large bodied ,p.cies 1pro',g: horn: Antilocapridae; AntiLocapra americana nr-rd..i dee. cervidn";'-crroi, elaphus) individuals losing their balance were also at a disadvantage during ,rgonistic interactions (Bennnrrr 7g77; Kttcrtnrt 1g74; Rarrs 1.g75; surrrc 1980). rt thus appears that behaviors causirg a rival's loss of balance become increasingly important in the offensive .eperioires of combatants.

In equids the two behaviors mosr responsible for opponents losing balance were rearing and..attempted_ biting (T;bre 1). Rearinj hud oth., p..l sumable fr.rnctions as well such as blocking bites, breakirg Lit. g.nspr, ^*,r,i intimidating rivals (see Frrsr and Mcculroucn 1976; Krrirclr nnd K.r*"r,.

1967;MonuruaN 1974).

TabLe

1:

Pattern used to cause baLance

in

Use

a

of offensive behavior patterns during equid combat

loss ot

rivaLs

tota

t

Tactics immediately preceding successfuI bites

N

rearing biting other patterns attempted

31 99 I

22.6

jmbalance

72.3

neck raised posture

5.1

direct bite+ persistent bitesr* quick bites+f c

'r No prior

tota

t

N

actions preceded the bite.

irc

t

ing

'r'r' See text.

21 6 5 1t 19 3

30.9 B.B

2.3

20.5 Z?.g

L.t

300

Jorr Brncrn

Tactics other than those causing a rival's loss of balance were also used prior to biting opponents. One strategy involved a series of successive bite attempts (Table 1). Individuals who bit most often successively won all the fights (p < 0.011' x2 - 8.67; df : 1). Another ractic used to score bites was the "rapid reversal" which entailed changing from a defensive (i.e. turning the head away) action to an offensive one (i.e. bite) before an opponent could do the same. Some individuals were successful at biting simply because they reacted more rapidly than did their rivals. Again, individuals judged to be winners were also quicker at "rapid reversals" than were losers (p ( 0.01;

:

8.00; df : 1).

Thus, there was a premium on nct only persistence but also on qgickness.

These two categories are conceptually distinct and were separated by examining the sequential use of each opponent's offensive and defensive patterns.

The difference between quickness and persistence was judged by the actions of the receiver. Quickness was determined by the frequency of the sender's bites prior to the receiver's shift from defensive to offensive patterns. In contrast, the persistence of the sender was based on the frequency of successful bites resulting from successive offensive patterns by irs opponenr. Persistence depended on how rapidly an opponent acted offensively without the use of defensive patterns. Quite obviously, speed was an important component in each category.

7'?

2. Offensive and Defensive Maneuvers Since there were winners and losers in contesrs, one could ask why an individual deemed a loser could not better its performance by improving its offensive maneuvers. The answers lie first, in an opponent's use of defensive tactics, and second, in how much escaiation and risk of r iounding may be tolerated (DervrrNs 1976; Gusr 1971, 1978 a; MevNe,no SurrH 1976; SvuoNs 1978).

Successful fighters maximized their offensive gains and minimized those of rivals (by preventing damage to themselves) through their use of defensive postures. For example, an adult male onager judged to be a winner (encounrer

#5,Table2) shifted from offensive to defensive tactics in an effective manner and successfully kid<ed his opponent in the head and chest 6 times out of 1'2 attempts with his rear legs. In contrast, his adversary attempted 13 rear leg kicks, all of which failed. The successful onager also avoided bites with the Turtle Posture. His opponent was unsuccessful at thwarting bites and was bitten twice while assuming (slowly) the Turtle Posture.

3. Risks and Limits in Combat

A limit may indeed be imposed Fights were not completely escalated were much more common, probably their chances of success and thev also

on offensive tactics by defensive ones. into biting affairs. Threats and chases because potential combatants assessed underwent risks. If injurious risks were

110t lllvo.

(Crurror

simplistic speciiic i:

(Bannrrr Svuors 1 In e.

associatec number .'

tion. Th',

rearinq ::

its

incre:

sense.

-\:

ed furti-. departirs risk bec: minimize ir.rr-olr-ed.

S:

1e-9:

roi::i

Zebra and Onager

io

used

\-e bire -on all tes \\-as

ing the ,uld do

e thev Irobe ' ^ ^1.

diness.

<amin,tterns.

Lctions

:nder's n concessfu

I

istence

Combat

301

not involved it could be predicted that animals would fight continuously (CrurroN-BRocK et aL.1979; Gersr 1978b; MevNeno Surrn 1.976). This is a simplistic prediction when applied to vertebrates or invertebrates, since intraspecific fighting may result in deadly wounds, fatal falls, and wasted energy (BennErrr 1977; Gvrsr 1967; Hverr and Se.rnoN 1978; RrrcHrnr 1978; SvnoNs 1978; WrrrrNsoN and SueNx 1976).

In equids each offensive behavior pattern used during a fight had a risk associated with it. Risk in combat is defined on an individual basis as the number of times an animal places itself in direct jeopardy for possible retaliation. Thus, an individual having lost its own balance due to an offensive rearing tactic has not only experienced a risk but may now suffer because of its increased vulnerability to retaliation. This scheme also makes intuitive sense. An individual that rears up also increases its own chances of being knocked further off balance and subsequently being bitten. Similarly, an animal departing from the Turtle Posture when trying to bite a rival also incurs a risk because its legs are re-exposed and vulnerable to wounding. One way to minimize risks is by avoiding all interactions or simply by fleeing if already involved in one.

oi enr in

use

hl'

1n

rng lts ensile

4.

Assessment, Escalation, and Risk

Since many mammals are capable of assessing their chances of success n'hen fighting (CrurroN-Bnocr er. al. 1,979; CrurroN-Bnocr and ArnoN 1979; Gntsr t974), one could predict little risk will be incurred by a large bodied individual when facing a smaller opponent. In contrast, a smaller L.odied individual should experience relatively greater chances of injury when

a'r- be

fllo\5 thos.'

Ensir-e

)unte: anner

,ut o: i re::

Table

2: Risk, size

rh t[re I s-:.:

59962352 3/. 2 1 19 6 7112336205 # # #

on+.

itlse.

acts attempted imbalancing

:

of

retatiating bites

'r ..,i

,essec

II,-e:-

disparities, and the loss of balance during agonistic encounters. For eadr

of the 6 cncounters, figures indicate the sum of the frequencies of acts for both opponents.

Pushing was not evident in fights between individuals of equal st,rture. BA : attempted bite; IBM : imbalancing one's self; RET : bitc in retaliation scored by opponent; R : rearing; PU't : 'obvious' push. Identities of combatants in each encounter are as follows. #1 (1 V ..ld $, lyoldQonagers); #2(1 V oldonager (,2yold onagerQ); f,3(two6moold zebra $ $); ff4 (two adult olrager $ {; animal "a" was at least 9y o1d, "b" was 9y o1d); = 5 ("a" as in previous encounter, individual "c" was also a 9 y old d ) ; f 6 (same identities as in f 4). Left side: equal stature; right side: unequal stature

self

3

t3 2 \1 l BO

2t2

130

54

15

ZU

1

Pushing was nor an 'obviously' employed and detectable tactic when opponents were

equal stature.

'r':' Intersexual fights involving yearling males are similar to male-male fights with respect r the use of offensive behavior patterns.

302

.forr Bencrn

encounrerin€ a larger one providing rhar size is related positively to fighting ability. Such predictions were supported when unevenly matched adveisariei fought (Table 2). Larger bodied individuals experienced fewer risks while smaller bodied rivals lost their balance more ofren.

It has been sugrested that vrhen individuals are more evenly matched fights increase in intensity (Grrsr 1971, 1978 a; MaxNeno Surru 1974; CwrtoN-Bnocr et al. 1979) and it should follow that combatanrs concomirantiy assume greater risks. This prediction was supported by data for equids (Table 2). For example, when size disparities were small, fighters assumed greater risks while rearing. During encounrer #5 rearing occurred 36 times, but individuals lost their balance 2o times and they were then more susceptiSle to bites (see above). Retaliations were successful on 5 of these occasioni a1d off-balanced animals were subsequently (and immediately) bitten. pushi,g and biting arremprs were other behavior patrerns with associated risks (see Table 2). It was clear that evenly matched rivals faced greater risks than the larger of a disparately sized dyad. Evenly matched individuals were bittel

abo.-t 37 %

of

dauernd und effektiver Ang: \flie bei ,'das leicht zu \\ zwischen den i' den Klmpfer.. mehr moglich:

,

einer optimale:

flg fon:::'

opportunil- ior:support and tl--.,:

Donald Svrtor' preparcd s'hile I Smithsonian

P.":-

the time after losing their or,n balance whereas those of

disparate size suffered successful retaliations only 6.5 /c, of the time (p{0.01; t'9:8..7a; df :1).Often the smaller of the two opponents did not eiperience such risks, simply because they did not fight. Hence, it appeared that risks rvere assumed every time an animal acted offensively but-whether the risks rvere capitalized upon depended upon the fighting abilities of the opponenrs.

Arrx.rxr...

n.tN (1979) : S.r: humans.

In: t'.

InoNs, eds.) D:r : Ba.xxs. E. l. lcmmit.tgs. le ": :,

n:::: Clctru-r-: evolution oi:'.-r Aruos, R. \!. ( fighting in rei ,:.

I-t.qu xrrr. i behavior oi

Summary

Extant equids no longer possess specialized fighting structures and their current methods of combat ii-rcorporate teeth as primary weapons, although legs are used for kicking. Several tactics were used to score successful bites.

These included unbalancing rivals and biting persistently a.nd rapidly. \Winners were not based solely on offensive actions but also on the effectiveness of both offensive and defer-rsive behaviors. In equids, as with many mammalian species, aggressive patterns of behavior carried certain riski. Those behavior patrerns with greater probabilities of r.ounding carried greater risks. As disparities in combatant abilities decreased, the risks of fighting increased and were greatest duri,g escalated encounters. At some poi"t, excessive risks outweighed their potential advantages and an individualls use of dangerous behavior patterns no longer represented an 'optimal strategy' partrcularly since the risk of injury also increaseci.

i c.

Frrsr. .I. Il

Ersr:..n;

ieral horses. Z. Grtsr. \-

I

\'. (1967) : O:r :: t92-191 . c... i1974): On i:-:-'

\\'eapons. c!)1ll:,1:

rrrn. L., P. Pr-:.

Life Straregrc..: .-f Health. Sp;::.

H,lnr-r: . ? il primate t.'::.:.

:.

Combat in t'h.

68. 1S2-211.

Zusammenfassung

Klrcurr-.

-

1-96 . K:.1 ::unicate. iS:... :{.. end U. K::: :.vdrol. 21. ---

-i8.

Die heutigen Pferdeartigen haben keine spezialisierten Kampf-Waffen; im Kampf als wichtigste \flaffen die zahte, dazu die Beine zum schlagen. verschiedene Taktiken werden eingesetzt, um erfolgreiche Bisse zu plaziere,. Dazu geh6rt, den Rivalen aus dem Gieichgewicht zu bringen sowie sie benutzten

'..:halten ,F:.::

i:..:t.'d Hr-e::.

,

Zebra and Onager

Combat

303

dauernd und schnell zuzubei{Jen. Der Sieg beruht auf einer Kombination effektiver Angriffs- und Verteidig'.rngsaktionen. \flie bei anderen Slugern ist Kampfverhalten risikobeladen. Verhalten, das leicht zu Vunden fiihrt, ist auch riskanter. Bei geringen Verschiedenheiten zwisdren den Klmpfern nimmt das Kampfrisiko zu, besonders in eskalierenden Klmpfen. Ab einer bestimmten Grenze iiberwiegen die Risiken, nicht mehr mcigliche Vorteile; das Verha.lten des Individuums verld8t den Bereich einer optimalen Taktik.

Acknowledgments The Conscrvation and Research Centcr of tl-rc Smithsoniar.r Institution creatcd a unique opportuniy for this research. I offcr special gratitude to Christer.r M. Wrnmen for his untiring support and thank Larry CorrrNs, John EIsrNrrnc, James Munr.tucH, Ingrid PonroN, and Donald SvuoNs for their conrnents on :r prior copy. The final draft of this paper was prepared vrhile I visited the Chicago Zoological Societl'. This rcsearch r-as supported by a Sn.rithsor-rian

Postdoctorai Irellowship.

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Demarkati

ltiZZy:

Surrrr, .1.

U. e.-(lszi,6.d., p.rirro,lactyla. BulI A,r. Mus. Nat. Hist. 148, 1.1.7-13a. in farmed red behaviour fighting and dominance on removal antler of /.t9so\: The effect play Study

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A'rim. irnni^.

Territories, North*est Isla.,d, Ba,rk, Cambridge' Press' Unir" Harvard tisri), s".i"tiology, the New Synthesis'

ffi'l;.',j. ili: ;;";;;

Received: Dece-:

Accepted: J,rn,o..'

National Zool0gical Author,s address: Joel Brncnn, conservation and Rcsearch center, U'S'A' 22630' Virginia Royal, Front Institution, Park, Smithsonian

Patternr .-: investigated

in:,

first year and:: conspicuous t.-i.

frequentll'eater-. years. The spat::

significantll no:.. arms. The re.u-tr

advantage to th: of secretion. lin-ri:,

Gerenuk

of the tribe { Arid Zone (I

1978 a). The st occuPy exclu>i

and their ofis

territorial r often out ( ritories for se. probabli'bec; tree and shrui L',r.

.1re

ritorial - i.

males

Coptrighr C..:-