Long-Term Study at Berenty, Madagascar
7.4 Population and Group Changes in the Study Population: Between-Group Competition
7.4.1 Increases in Population Density and Immigration/
Emigration
When we began our observations in September 1989, three groups of ringtailed lemurs consisting of 63 individuals existed within our study area of 14.2 ha (4.4 lemurs/ha) (Koyama et al. 2002 ). Six groups comprising 82 lemurs (5.8 lemurs/ha) were present by September 1999. The mean annual rate of population increase was 2.7 %, mainly the result of the high fecundity of adult females. During the 10-year time period, 204 infants were born, 125 lemurs died, 58 lemurs immigrated into the study population, and 118 emigrated, bringing a net of 19 new lemurs into the study population.
Immigration and emigration play a large role in the study population size, and these movements fall into several categories (Ichino and Koyama 2006 ), as follows.
(1) The most usual case is “male transfer.” Male lemurs change groups after a tenure that varies from 1–7 years (Koyama et al. 2002 ). The mean length of stay in a group is 3.1 years. Thus, males disperse from natal sites, although it is unknown how far.
From 1989 to 1999, 90 males left the study population and 53 males immigrated into it. Thus, the high population in the study area may have biased the entire popu-lation structure of the Malaza area as a “male source.” (2) “Female transfer” infre-quently occurs. Two unidentifi ed adult females immigrated into Troop C2A in 1998 (KN-group in Fig. 7.2 ), possibly because they had been evicted from other groups beyond the study area. (3) “Group invasion” or “group dismissal/eviction” to and from the study area occasionally occurs. One group invaded the study area (Troop U2 in Fig. 7.2 ), and three groups left the study area (Troops B and U2, and HSK- group in Fig. 7.2 ), from 1989 to 1999.
The adult sex ratio (adult females vs. adult males) during the birth season fl uctu-ated from 1:0.615 to 1:1.22 between 1989 and 1999 but without consistent correla-tion with time or populacorrela-tion density (Koyama et al. 2002 ). Pooled data show that the number of adult males per adult females is 0.968, suggesting that every adult male principally belongs to one of the social groups during the birth season. Meanwhile, the mean group size decreased from 21.0 in 1989 to 13.7 in 1999.
7.4.2 Female Eviction and Group Fission
“Female eviction” is a common social phenomenon in ringtailed lemurs (Vick and Pereira 1989 ; Ichino and Koyama 2006 ). It is defi ned as follows. One or several females become the target of persistent aggression by other females (targeting aggression, defi ned by Vick and Pereira 1989 ) and are eventually evicted from their group. Female eviction primarily occurs around birth seasons and in large-sized
groups comprising 16 or more lemurs with 7 or more adult females (Ichino and Koyama 2006 ).
Figure 7.2 shows the troop histories of the study population from September 1989 to January 2002. Six cases of female eviction were directly observed during the 12.5 years. In one case, the evicted females were able to rejoin the original groups (KN-G in 1998; Fig. 7.2 ). In another case, the evicted females established a new home range, and mature males joined them, thereby forming a new social group, that is, “group fi ssion” (Troop T1B in 2000). In three cases, the evicted females could not establish a stable range within the study area and eventually disappeared from the study area, as “nomadic groups” without defi nite home ranges (HSK-G in 1997, MK92-G in 2000, and KN-G in 2001). Evicted females would be expected to encounter numerous diffi culties establishing new ranges and securing mating partners because of high group density. In particular, the number of adult males per adult female tends to be low in these newly formed groups, which suggests that males hesitated to join small groups. In the remaining case, the evicted females sporadically fought with females of another group and dominated them, eventually forming one group. Such a case can be termed “group fusion”
(SH-G in 1995). If evicted females join other groups without aggressive fi ghting, it is called “female transfer”.
Female eviction usually occurs among matrilineal kin groups (Ichino and Koyama 2006 ), and group males rarely play a dominant role, indicating that female
Fig. 7.2 Female eviction and group fi ssion events that occurred from September 1989 to January 2002. Each square indicates groups living within the study area. The dotted squares indicate nomadic groups. Im , shifting of a group to the study area; Em , shifting of a group beyond the study area. Circled numbers indicate cases of female eviction/group fi ssion
eviction is the result of female within-group competition through kin selection;
these evictions may function to decrease the intensity of within-group competition by reducing the number of group members.
In addition to these cases of female eviction, four group fi ssion events occurred in the study population between 1989 and 2002 (Troops C1 and C2 in 1989, Troops C1 and CX in 1993, Troops T1 and T2 in 1993, Troops C2A and C2B in 1997;
Fig. 7.2 ). As the processes leading to these specifi c group fi ssion events were not directly observed, it was uncertain whether the females were evicted by other females or if they voluntarily left their groups. As a result of these female evictions or group fi ssions, the number of groups in the study area increased from three in 1989 to seven in 2002 (Fig. 7.2 ).
7.4.3 Between-Group Competition
High group density (42.2 groups/km 2 in 1999) should have intensifi ed “scramble competition” over food resources in the study population. In such territorial defenses, adult females, not males, play the most active role in intergroup confrontations, regardless of dominance rank (Nakamichi and Koyama 1997 ). Groups occasionally take over the secure ranges of other groups, and such cases are termed “range take-over” (Ichino and Koyama 2006 ). During our observations, several groups lost their ranges and were thought to have become “nomadic groups” (e.g., Troop B in Fig. 7.2 ).
Fig. 7.3 Regression between the number of adult females and birthrate. Birthrate generated an inverted U-shaped curve, which approximated a second-degree curve ( y = 7.42 + 26.06 x – 2.21 x 2 ) ( r 2 = 0.774, P < 0.03). The group with three adult females had a lower birthrate than those with four, six, and seven adult females (χ 2 = 5.92, df = 1, P < 0.02; χ 2 = 5.09, df = 1, P < 0.03; and χ 2 = 6.8, df = 1, P < 0.01, respectively)
Intergroup relationships may infl uence female fecundity (Takahata et al. 2006 ).
Based on the 60 group-years of data recorded from 1989 to 2001 in the study popu-lation, the birthrate follows an inverted U-shaped curve against the number of adult females (Fig. 7.3 ). These data agree with the assumption of the intergroup feeding competition (IGFC) hypothesis proposed by Wrangham ( 1980 ), and not with that of the predation-feeding competition (PFC) hypothesis put forth by van Schaik ( 1983 ).
In contrast, infant mortality rate is not consistently correlated with group size/num-ber of adult females (see fi gs. 1b and 2b in Takahata et al. 2006 ), as Jolly et al.
( 2002 ) pointed out. Figure 7.3 suggests that females of small-sized groups suffer from the stress of between-group competition. When we captured lemurs and mea-sured their body mass in 1999, the members of a small group (Troop CX) exhibited the smallest body mass values and the heaviest infection by ticks ( Haemaphysalis (Rhipistoma) lemuris ), which may have been related to their environmental and social conditions, as Troop CX inhabits the most humid area of the gallery forest and is subordinate to neighboring groups.