Mate Retention, Harassment, and the Evolution of Ungulate Leks
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Mate retention, harassment, and the evolution of ungulate leks T. H. Clutton-Brock Current models of lek breeding mosdy suggest tJiat males defend clustered mating territories O. F. Price because females show a preference for mating on leks. Here we argue that, in lek-breeding A. D. C MacCoU Large Animal Research ungulates, males may also gain benefits from holding clustered mating territories because Group, clusters retain does in estrus. We show that in fallow deer {Dama dama) harems are commonly Department of Zoology, disrupted by young males. Bucks that hold territories on the lek that lose dieir harems quickly University of Cambridge, regain does as other harems are disrupted, whereas bucks defending isolated, single territories Downing Street, rarely regain does the same day. The risk of harassment may also help to explain why does in Cambridge CB2 3EJ, UK estrus leave the large, unstable herds that they usually live in. Does in estrus are frequendy chased by young males when outside male mating territories. Benefits of moving to the lek (versus moving to single territories) include reduced risks of long chases. Though intrusions by young bucks are die commonest cause of does leaving male territories in our study populadon, other factors that cause does to move between neighboring harems (including disturbance by predators and persistent courtship by males) may generate benefits to males holding clustered mating territories. Once clustered mating territories have developed, the additional costs of mate choice are likely to be low, and female preferences for particular male characteristics may be likely to develop. [Behav Ecol 3:234-242 (1992)] ' I Tie different types of polygyny found tion of lek breeding is why male territories A among mammals apparendy represent should be clustered widiin one part of die forms of precopulatory or postcopulatory females' range (Balmford, 1991). A priori, one mate guarding (Clutton-Brock, 1989). Food might expect die average mating success of distribution, the risk of predation and disease, dispersed males to be higher tiian tiiat of clus- and harassment by males affect die distribu- tered ones because the latter would be likely tion of receptive females in time and space, to share mating access to a relatively small and males distribute themselves so as to max- segment of die female population. However, imize their access to mating partners (Brad- observations of leks in birds and mammals bury and Vehrencamp, 1977; Clutton-Brock, show tiiat this is not the case: large numbers 1989; Clutton-Brock and Harvey, 1978; Em- of females typically mate on leks, and females len and Oring, 1977; Rubenstein and Wrang- will move to leks from considerable distances ham, 1986). (Balmford, 1990). In lek-breeding popula- Leks, where males defend small, clustered tions, the frequency of copulations on dis- mating territories in one part of the female persed, single territories is usually low (Wiley, range, are not easily interpreted within this 1991). In our study population of fallow deer, framework (Clutton-Brock, 1989). Leks have 94% of observed matings occur on die lek, been reported in bats (Bradbury, 1977; Brad- and die most successful males may mate with bury and Vehrencamp, 1977), pinnipeds (Fay more than 50 does during a single rut (Clut- et al., 1984), and dasyurid marsupials (Lazen- ton-Brock et al., 1988). In Uganda kob (Kobus by-Cohen and Cockburn, 1988) but are com- hob thomasi), copulations on single territories Received 18 June 1991 monest among the ungulates, where they oc- are rare (Balmford, 1990; Leuthold, 1966). First revision 28 October 1991 Second revision 28 November cur in die Cervinae (Clutton-Brock et al., 1988; In several lek-breeding ungulates, diere is 1991 Schaal, 1986), the Alcelaphinae (Gosling, clear evidence diat females in or close to es- Accepted 28 November 1991 1987; Monfort-Braham, 1975), Reduncinae trus are attracted to leks once they have left 1045-2249/92/J4.00 (Balmford, 1990; Leuthold, 1966; Schuster, the large, unstable herds that tiiey usually live © 1992 International Society 1976), and, possibly, the Antiiopinae. in (Bakaford, 1990; Clutton-Brock et al., for Behavioral Ecology A central question concerning die evolu- 1988). However, a second reason males may 254 Behavioral Ecology Vol. 3 No. 3 hold clustered territories is that clusters of Table 1 territories may retain estrous females more Hourly mating rates for female fallow deer (number effectively than dispersed territories. For ex- of observed copulations divided by number of hours spent by does in each buck's territory) in relation to ample, during the 6-24 h that precede cop- total number of doe/hours spent in the harem of ulation, female fallow deer usually change ter- each back and mean harem size* ritories several times, typically moving from one territory to its closest neighbor. Where Mating rate per doe/h in relation to females change territories several times be- Total doe Mean harem fore mating and usually move to the nearest hours/buck size of bucks territory, this process can generate advantag- Year r. N r, N es to males that hold clustered territories, aris- ing from the fact that females that have en- 1986 -.324 (ns) 22 -.253 (ns) 22 tered a cluster of territories seldom leave it, 1987 -.235 (ns) 19 -.18 (ns) 19 subsequently moving between territories with- 1990 -.018 (ns) 18 -.155 (ns) 18 in the cluster. As a result, males holding ter- " Both calculated for all bucks mating on the lek at Petworth ritories in clusters have an improved chance during the rut (18-30 October). In 1986 and 1987, about of receiving females that move from other 200 copulations were observed on the lek per year, but males within the cluster (Stillman et al., in in 1990 the sample of observed copulations fell to 90 as press). In contrast, males holding isolated sin- a result of reduced time spent in observation. gle territories suffer the disadvantage that, once estrous females leave their territory, they are unlikely to return. This process will op- ments and compare the frequency of harass- erate so long as females leaving territories ment at times when females in or close to commonly move to the nearest territory, and estrus are on the lek, in single territories, and it does not depend on any preference in fe- in feeding herds. To overcome the problem males for remaining on leks, though female that estrous does are rarely seen outside mat- preferences for doing so will reinforce the ing territories, we waited until does on the lek advantages to males holding territories there. began to allow bucks to mount them (con- In the first part of this paper, we investigate firming that they were in full estrus) and then the benefits that males holding territories on gently herded them off the lek and monitored the lek may gain from the lek's capacity to their behavior. retain does in estrus. This approach to the evolution of lek breed- ing in fallow deer raises two immediate ques- METHODS tions. Why do estrous does move between ter- ritories? And why do they leave the herds where Study area and population they usually live in the first place? In both Fallow deer were present in Britain in the cases, females might move in order to choose Pleistocene, but the present subspecies is mating partners (see Balmford, 1991; Brad- thought to have been introduced from the bury, 1981; Gibson et al., 1990; Queller, Mediterranean area before or shortly after the 1988). However, though female ungulates Norman conquest. The species is widely kept avoid mating with immature males, there is in deer parks and has established itself in many little unequivocal evidence of female prefer- wooded parts of the country. Coat color is ences for particular mating partners in un- variable, ranging from white through spotted gulates and, in fallow deer, females will mate to black. Mature bucks, which develop large, as readily with unsuccessful adult males as with palmated antlers, usually weigh between 50 successful ones (Table 1). In this situation, it and 70 kg; mature does weigh from 30 to 55 is sensible to avoid the common assumption kg (Chapman and Chapman, 1975; Pember- that females move between male territories to ton and Dansie, 1983). Social behavior also assess mating partners and, instead, investi- varies. Where density is high, does and youn- gate the reasons for female movements. We ger bucks form large, mixed-sex herds num- have already suggested that harassment by bering 50 or more but of unstable member- young males is often responsible for move- ship while, in areas of lower density, does are ments of females between territories (Clutton- usually seen in small parties of 2-10. Mature Brock et al., 1989) and that does in estrus may bucks are segregated from does throughout leave the large, unstable herds that they live much of the year, also forming unstable groups in for much of the year because individual of up to 50 or more animals (Alvarez et al., bucks cannot provide adequate protection 1975; Chapman and Chapman, 1975; Schaal, from dangerous harassment in unstable herds 1986). Breeding is strongly seasonal (Arm- (Clutton-Brock et al., 1988). In the second strong et al., 1969): In late September, buck half of this paper, we investigate the relation- groups fragment and bucks move to areas of ship between harassment and female move- high doe density where they defend discon- Clutton-Brock et al. • Mammalian leks 235 tinuous mating territories or stands, often on ritories lose their does, they often abandon the edge of clearings in the forest (Chapman the territory temporarily, either returning to and Chapman, 1975). Does will only accept mixed-sex herds or, in some cases, defending mounting for a short period, and in our study resource territories under oak trees (see be- area more than 90% only mate once (Asher, low).