Female Coordination of Group Travel in Wild Propithecus and Eulemur

Transcription

1 International Journal of Primatology, Vol. 20, No. 6, 1999 Female Coordination of Group Travel in Wild Propithecus and Eulemur Elizabeth M. Erhart 1 and Deborah J. Overdorff' 1,2 Received November 11, 1998; revised May 24, 1999; accepted July 21, 1999 Coordination of primate group movements by individual group members is generally categorized as leadership behavior, which entails several steps: deciding where to move next, initiating travel, and leading a group between food, water sources, and rest sites. Presumably, leaders are able to influence their daily foraging efficiency and nutritional intake, which could influence an individual's feeding ecology and long-term reproductive success. Within anthropoid species, females lead group movements in most female-bonded groups, while males lead groups in most nonfemale-bonded groups. Group leadership has not been described for social prosimians, which are typically not female-bonded. We describe group movements in two nonfemale-bonded, lemurid species living in southeastern Madagascar, Propithecus diadema edwardsi and Eulemur fulvus rufus. Although several social lemurids exhibit female dominance, Eulemur fulvus rufus does not, and evidence for female dominance is equivocal in Propithecus diadema edwardsi. Given the ecological stresses that females face during reproduction, we predict that females in these two species will implement alternative behavioral strategies such as group leadership in conjunction with, or in the absence of, dominance interactions to improve access to food. We found that females in both species initiated and led group movements significantly more often than males did. In groups with multiple females, one female was primarily responsible for initiating and leading group movements. We conclude that female nutritional needs may determine ranging behavior to a large extent in these prosimian species, at least during months of gestation and lactation. KEY WORDS: prosimian; leadership; group movements; female-bonded species; nonfemalebonded species; female dominance. 1 Department of Anthropology, University of Texas at Austin, Austin, Texas. 2 To whom correspondence should be addressed, at Department of Anthropology C3200, University of Texas, Austin, Texas ; overdorff@mail.utexas.edu /99/ $16.00/ Plenum Publishing Corporation

2 928 Erhart and Overdorff antroduction Most primatologists agree that the distribution of food plays an important role in determining nonhuman primate group size and social organization (Wrangham, 1980; Janson, 1988). Van Schaik (1989) demonstrated that most of the variation in female social relationships within groups could be explained by the degree of scramble or contest competition experienced by females when they feed (Sterck et al., 1997). Groups tend to be organized in an egalitarian and individualistic way under scramble conditions when food is evenly distributed so that each group member's food intake is affected equally. With little intra- and intergroup feeding competition, there is low incentive for females to establish close bonds or be philopatric: nonfemale-bonded groups (Wrangham, 1980). In contrast, despotic and nepotistic dominance hierarchies develop in groups that experience contest competition when food is clumped and easily defendable so that each group member's food intake varies. in these groups are philopatric and often establish close ties to enhance their access to resources during competitive encounters: female-bonded groups (Wrangham, 1980). In general, van Schaik's (1989) hypothesis adequately explains the variation in social organization among anthropoid primates. Most folivorous primates are nonfemale-bonded, while most frugivorous and omnivorous primates are female-bonded. However, his hypothesis does not fit the behavioral patterns observed among prosimian primates. For example, female Lemur catta have a clear, linear dominance hierarchy but seasonally incorporate leaves into the diet (Sussman, 1974; Sauther, 1994). Other species such as Eulemur fulvus are primarily frugivorous in southeastern Madagascar and have no clear linear dominance hierarchy (Pereira et al., 1990). In addition, many prosimians exhibit low rates of aggression in feeding contexts (Hemingway, 1995; Overdorff, 1998) compared to polygynous anthropoids in which male dominance over females in feeding contexts is common (Hrdy, 1981; Jolly, 1984). Male dominance has not been reported for the lemurids. Instead, female dominance, or female agonistic superiority over males, occurs in several social species (L. catta, Varecia spp., and some Eulemur spp.: van Schaik and Kappeler, 1993). However, other social lemurids do not exhibit female dominance, e.g., E. fulvus rufus (Pereira et al., 1990), and for some species there are conflicting reports, e.g., Propithecus (Richard, 1987; Wright, 1995; van Schaik and Kappeler, 1996; Hemingway, 1995). Further, the relationship between female dominance and feeding priority remains unclear, as several lemurids may have female feeding priority without female dominance, e.g., Indri, Propithecus, Phaner (van Schaik and Kappeler, 1996; Pereira et al., 1990). Female feeding priority may be the

3 Group Movements 929 result of male deference (Hrdy, 1981), which implies that females have consistent access to food but not other resources. However, the male deference hypothesis does not seem adequate in its current form for prosimians. Jolly (1984) predicted that if male prosimians are deferential, then variance in male reproductive success should be lower, male-male competition should be reduced, and males may have an equal opportunity to mate whether they are deferential or not. Recent captive and wild studies imply that male reproductive success can be quite variable and males compete for access to reproductive females (White et al., 1996; Overdorff, 1998; Overdorff et al., 1999; Sauther, 1992, 1993). What alternative behavioral strategies could primate females implement in conjunction with or in the absence of dominance interactions to improve access to food? Sex differences in food intake occur in many anthropoid species in which females spend more time feeding, have higher feeding rates, and/or consume more food than males do (Rodman, 1977; Waser, 1977; Wright, 1984; Kinzey, 1987; Boinski, 1988). These feeding behaviors do not have to be mediated through aggressive-submissive interactions. Similarly, female coordination of group movements between food patches could result in greater control over food type and quality (Boinski, 1991). There are many anthropoid species in which females primarily coordinate group movements (Cebus capucinus: Freese and Oppenheimer, 1981; Boinski, 1993; Cercopithecus aethiops: Struhsaker, 1967; Colobus guereza: Oates, 1977; Macacafascicularis: van Noordwijk and van Schaik, 1987; Macaca mulatta: Neville, 1968; Papio cynocephalus anubis: Rowell, 1969; Saimiri oerstedi: Boinski, 1988; Mitchell et al., 1991; Theropithecus gelada: Dunbar and Dunbar, 1975), though males are often responsible for coordinating group travel in some species (Table I). The distinguishing factor between male-led groups and female-led groups is that in the former, females are not bonded and tend to migrate from their natal group, while the majority of the species in the latter are typically female-bonded and do not emigrate to new groups. Wrangham (1980) argued that when females are bonded, they can exercise more influence over group movements, implying that this ability is important in regards to resource acquisition. The majority of group-living prosimians, however, are not female-bonded (Kappeler, 1997; Table I). The purpose of our study is to describe group movements between food patches in two contrasting prosimian species in southeastern Madagascar: the large-bodied, diurnal Propithecus diadema edwardsi (4-6 kg) (Wright, 1995; Overdorff and Strait, unpublished data), and the smaller, cathemeral Eulemur fulvus rufus (2.7 kg) (Overdorff and Strait, unpublished data). Wright (1995) reported Propithecus diadema edwardsi to be female-dominant, though rates of aggression are very low (Hemingway, 1995). In contrast, Eulemur fulvus rufus is not female dominant (Pereira

4 930 Erhart and Overdorff Table I. Socioecological characteristics of primate species with female- or male-controlled group movementsa Species control Paplo cynocephalus anubisb Theropithecus geladac Macaca fascicularisd Macaca mulattae Cercopithecus aethiopsf Colobus guerezag Saimiri oerstedih Cebus capucinusi Lemur cattaj Eulemur coronatusk E. fulvus sanfordk E. f. rufusl E. rubiventerm Propithecus diadema edwardsin Males control Pan troglodyteso Gorilla gorillap Papio hamadryasq P. cynocephalus ursinusr Colobus badiuss Alouatta paliattat Bond Philopatry Neither Neither Neither Neither Neither Neither Males Males Males Neither? Males Neither F/M ratio F>M F> M F> M Equal Equal Equal Equal Equal Equal F> M F> M I/D O Diet FO O O O FO FR/I FR/I FR/FO FR FR FR FR FR/SD O FO 0 O FO FO/FR Food distribution a, Female-bonded;, nonfemale-bonded. F/M ratio, number of females (F) to males (M) in groups. I/D, (Female body mass/male body mass)100 (Smith and Jungers, 1997). Overall diet category: O, omivory; FO, folivory; FR, frugivory; SD, seed-eating; I, insectory. Food distribution:, patchy;, nonpatchy. browell (1969). cdunbar and Dunbar (1975). dvan Noordwijk and van Schaik (1987). eneville (1968). fstruhsaker (1967). goates (1977). hboinski (1988); Mitchell et al. (1991). ifreese and Oppenheimer (1981); Boinski (1993). jsussman (1974); Sauther and Sussman (1993). karbelot-tracqui (1983); Wilson et al. (1989). This study; Overdorff (1993a). moverdorff (1993a). "This study; Hemingway (1996, 1998). ogoodall (1968). "Watts (1994). qrummer (1968); Sigg and Stolba (1981). rbarton et al. (1996); Byrne et al. (1990); Henzi and Lycett (1995). swrangham (1980); Struhsaker (1980). tcrockett and Eisenberg (1987); Milton (1980).

5 Group Movements 931 et al., 1990), but aggression among them can be high seasonally (Overdorff, 1998). are not bonded in either species. Given the presumed energetic costs experienced by prosimian females when reproductive (Richard et al., 1991; Young et al., 1990; but see Kappeler, 1996; Tilden and Oftedal, 1997) and the possible ecological stress experienced during reproductive periods (Overdorff et al., 1999), coordination of group travel to food sources would have clear advantages. We therefore predict that female prosimians will be influential in initiating and coordinating group movements between food sources despite weak or absent female dominance and feeding priority and the lack of female bonds. DATA COLLECTION We sampled all occurrences of group movements, following Altmann (1974). We modified methods developed by Boinski (1991) and used them to determine if males or females coordinated group movements between food patches and rest sites. We noted the identity of the initiator the individual that made a start attempt within a food source, after pauses during travel <5 min or after a resting bout. If >50% of the group followed the initiator within 10 min, we compared the azimuth of the group trajectory in relation to the center of the remaining group to the initiator's position using a compass; this was a successful group movement. A false-start occurred when an individual attempted to initiate a group movement but <50% of the group members followed within 10 min. The individual that was at the leading edge of the group once the group began moving was the leader. At 10-min intervals, we noted the order of individuals to determine if the leader was the same individual or if there had been a change in leadership, in which case we recorded a new compass bearing. We distinguished between an initiator and leader because the individual may not necessarily be the same (Kummer, 1968; Rowell, 1969). Rowell (1969) found that when several male Papio cynocephalus anubis males left a feeding or resting spot and moved in different directions, group members did not follow until an older female set off in the direction of one of the males, at which time other females followed her and the group then moved to the next site. By Boinski's (1991) definition, the initiations made by these males would be coded as false starts, while the older females would be recorded as leaders because they successfully initiated group movements. Data presented in Table III refer to (1) successful initiations made by leaders and (2) unsuccessful initiations or false starts for adults in both study species. We analyzed group movement data via the G test with William's correction.

6 932 Erhart and Overdorff Table II. Group composition of study groups of Propithecus diadema edwardsi (PDE) and Eulemur fulvus rufus (EFR) Species PDE I PDE II ERF ainfant survived 24 hr. Adult males Adult females Juveniles Infants 1a PG 0 RESULTS Group Movements We observed a total of 146 and 192 successful group movements in groups I and II, respectively, of Propithecus diadema edwardsi and we recorded 165 successful group movements for the group of Eulemur fulvus rufus (Tables II and III). initiated and led group movements significantly more often than males did in each study group (PDE I: G = 47.73, df = 1, p <.0001; PDE II: G = , df = 1, p <.0001; EFR: G = 51.12, df = 1, p <.0001). This sex difference is significant whether females led their groups to rest sites or food patches (Figs. 1 and 2). There is no significant difference between the number of false starts and the sex of the initiator in either group of Propithecus (Table III; PDE I: G =.058, df = 1, p = n.s.; PDE II: G = 76, df = 1, p = n.s.) and there is a slight, but not significant trend for male Eulemur (Fig. 3; EFR: G = 4.06, df = 1, p = n.s.). In groups with multiple females, one was primarily responsible for initiating and leading group movements (Fig. 4). in groups I and II of Propithecus initiated about the same number of movements to resting sites and food patches (Table HI; PDE I: G = 1.59, df = 1, p = n.s.; PDE Table HI. Context of male and female successful initiations of group movements and false starts for study groups of Propithecus diadema edwardsi (PDE) and Eulemur fulvus rufus (EFR) PDE I PDE II ERF Male Female Male Female Male Female Leadership Rest Feed Travel False starts

7 Group Movements 933 Fig. 1. Male and female leadership of groups to rest sites in Propithecus diadema edwardsi and Eulemur fulvus rufus. ***p < II: G = 1.23, df = 1, p = n.s.), while female Eulemur initiated more movements to food patches (G = 10.91, df = 1, p <.001; Table III). In all cases, once a female initiated a group movement, she remained the leader until the group stopped to feed or rest in both groups of Propithecus. The only time adult male initiated a group movement in group I, he led them to a sleeping site at the end of the day. In group II, one adult male initiated 16 of the 19 observed bouts for males; however, group II's female had assumed the leadership position by the next 10-min time interval. In the group of Eulemur the initiator remained the leader in each group movement. Male Eulemur initiated and led their group significantly more often than male Propithecus did (Table III; G = 32.53, df = 2, p <.0001). DISCUSSION Based on the results of this study, female Propithecus diadema edwardsi and Eulemur fulvus rufus reliably coordinate group travel. This result is in

8 934 Erhart and Overdorff Fig. 2. Male and female leadership of groups to food patches in Propithecus diadema edwardsi and Eulemur fulvus rufus. ***p < agreement with reports of female leadership in other prosimians such as Eulemur coronatus, E. fulvus sanfordi (Arbelot-Tracqui, 1983; Wilson et al., 1989; but see Freed, 1996), and Lemur catta (Sauther and Sussman, 1993). Among anthropoids, female leadership can be explained by femalebondedness as the degree of sexual dimorphism, diet, and distribution of food varies (Table I; exception Salmiri oerstedi: Boinski, 1988; Mitchell et al., 1991). This explanation, however, is not helpful to understand female leadership patterns among prosimians since the majority of prosimians are not female-bonded (Kappeler, 1997). Why do female prosimians lead groups? Our results do not indicate that this bias can be attributed to female dominance or feeding priority as currently defined. We saw no aggression in feeding contexts for either group of Propithecus (Erhart and Overdorff, 1998; Erhart and Overdorff, unpublished data), thus we are unable to confirm the existence of female dominance or feeding priority in this species. As in other studies of Eulemur fulvus rufus (Pereira et al., 1990), we documented aggression during feeding, but males and females did not differ in the amount of aggression directed to other group members (Erhart and Overdorff, unpublished data). One

9 Group Movements 935 Fig. 3. Male and female false starts in Propithecus diadema edwardsi and Eulemur fulvus rufus. nonaggressive way in which females may be able to influence their daily foraging efficiency and nutritional intake, and possibly impact their longterm reproductive success, is by initiating and coordinating group movements to food patches (Boinski, 1991). Through these activities, females may influence several aspects of their diet: (1) which food or plant species are used, (2) which individual food sources are used, (3) how far the group travels to reach food sources, and (4) how long the group spends in any one food source. Each of these behaviors could allow females to increase their daily nutritional intake when reproductively stressed and to minimize energy costs. This appears to be the case based on the fact that the female responsible for the majority of group movements in groups with multiple females was of reproductive age, produced an infant, and also had the highest cumulative reproductive success of any female in the social group (Overdorff et al., 1999; Erhart and Overdorff, unpublished data; Overdorff and Strait, unpublished data). Why do some males in some species lead group movements? This pattern may be related to the fact that while females increase their reproductive success through food acquisition, males increase their reproductive success through mate acquisition (Wrangham, 1980). Male-led species share

10 936 Erhart and Overdorff Fig. 4. Female leadership of groups with multiple females in both species. several important characteristics: (1) there are more females than males in the group, (2) males are philopatric and thus much more familiar with their group's range than immigrant females are, and (3) food is either nonpatchy and abundant or patchy but not easily defensible (Table I). Males may also be devoting time to vigilance for predators or nonresident males. Although female Eulemur led groups more often than males, male Eulemur led groups more often than male Propithecus did. This difference may be related to the degree of territoriality and frequency of intergroup encounters. Eulemur fulvus rufus have large overlapping home ranges and encounter other groups frequently (Overdorff, 1993b), while Propithecus diadema edwardsi have smaller ranges and encounter other groups rarely (Hemingway, 1995; Overdorff and Erhart, n.d.). In future studies we plan to quantify differences between females in different reproductive stages and nonreproductive individuals regarding access to food, types of food eaten and the particular parts of plants consumed, and the nutritional components of selected plants. Another question that needs further clarification is whether by leading groups leaders actually acquire more and better foods than other group members. An understand-

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