1. Introduction
Various mammalian species, including dolphins, elephants, giraffes, hyenas, and some primate species, exhibit fission–fusion dynamics (FFD), which alters the size of subgroups by splitting and merging [
1,
2,
3,
4,
5,
6]. Fission–fusion dynamics allow the adaptive adjustment of subgroup size to reduce feeding competition and improve foraging efficiency [
7,
8,
9,
10]. Among primates, chimpanzees (
Pan troglodytes) have been particularly studied as a species that exhibits a high degree of FFD, wherein members of the same group form temporary subgroups (parties) that vary in both size and composition [
11,
12].
Research on the FFD of chimpanzees has revealed several factors that drive individuals to gather into relatively large parties, such as the risk of predation, proximity of neighboring groups, and presence of receptive females [
13,
14,
15,
16,
17]. In contrast, research on factors that constrain maximum party sizes has mainly focused on ecological factors, such as fruit abundance and distribution [
13,
18,
19,
20]. However, the effects of these factors on FFD vary among populations. Chimpanzees have also shown flexibility in party size at study sites where fruit abundance varies relatively little and does not seem to constrain maximum party size [
17,
20,
21].
In addition to the factors that form FFD, potential options in party participation based on the costs and benefits that the FFD might offer to the individuals should be investigated to improve the understanding of their social structure. Chimpanzees are also known to exhibit intense intragroup aggression [
4,
22,
23], and social tensions might prevent individuals from gathering. In fact, several studies have suggested that female chimpanzees with young infants face potential costs from male infanticide and that this influences their decision about participating in parties [
24,
25,
26]. Similarly, especially for low-ranking males, leaving a party during increased social tension among party members could be an adaptive strategy, as chimpanzees display linear dominance hierarchies, and low-ranking males are often subjected to aggression [
23]. However, the effect of male aggression on the party attendance of low-ranking chimpanzees has received limited research attention. Investigating the potential role of the FFD is essential for understanding the mechanisms shaping their social systems. In this study, we hypothesized that the FFD may offer alternative tactics for low-ranking males to mitigate costs caused by disadvantages when competing for survival and reproduction against other adult males. We made the following three predictions from this hypothesis: (1) low-ranking males range alone or in small parties more frequently than higher-ranking males, (2) when there are no receptive females in the group, low-ranking males spend more time ranging alone than when there are receptive females in the group, and (3) low-ranking males attract more aggression when attending larger parties. To examine these predictions, we evaluated the effect of male dominance rank, party size, and the presence of receptive females on the party attendance of males and the frequency of received aggression within the parties. We also investigated the effect of fruit abundance to ensure that we accurately assessed the effect of social factors on the party attendance of chimpanzees.
4. Discussion
In this study, we investigated the influences of the dominance rank of each focal male, the presence of females with MS, and FAI on the size of parties that the focal males attended and the frequency of aggression that they received. In order to examine the three predictions we made from the hypothesis, we observed 10 adult male chimpanzees of the M group in Kalinzu using the focal animal sampling method.
We found that the dominance rank of males affected male tendencies of attendance at parties, with low-ranking males ranging alone and in parties with a less number of males more frequently than males of higher-rank classes. In the presence of females with MS, males of all ranks decreased their frequency of spending time alone and increased attendance at larger parties. In contrast, low-ranking males increased their frequency of ranging alone when females with MS were absent. These results suggest that low-ranking males tend to avoid attendance at parties, especially larger ones, unless they need to attend for access to receptive females.
The food abundance, represented by the FAI in this study, has been considered one of the main factors influencing the FFD of chimpanzee grouping [
13,
18,
19,
40]. However, other studies have shown that party size is not affected by the food abundance but by other factors such as the presence of receptive females [
17,
20,
21,
41]. Actually, in this study, the FAI, which represented the fruit food abundance, did not exert a significant influence on the tendency of males to range alone. Furthermore, males tended to attend smaller, rather than larger, parties when the FAI was higher. One explanation for this contradiction might be that the male chimpanzees of the M group tended to forage separately to avoid social tension when food was abundant in the forest and many food patches were available. In fact, in March 2018, when the FAI was the highest of all observation periods, chimpanzees often fed on the fruit of
Musanga leo-errerae. As
Musanga trees tended to be distributed sparsely and each tree could host one or a few individuals, we might have underestimated the number of individuals in the party. In contrast, in March 2019, when the FAI was the lowest, chimpanzees frequently fed on figs of
Ficus natalensis. In Kalinzu,
F. natalensis usually has a large canopy of 10–20 m diameter, and therefore, many individuals foraged on the same tree while forming a big party (Shibata, unpublished data). Similar results were reported in a previous study on the relationships among fruit abundance, fig abundance, and orangutan party size [
42]. Thus, to investigate the relationship between food availability and party size of chimpanzees more precisely, we need to analyze the effect of the distribution of food patches as well as the FAI.
Overall, our results regarding the tendency of party attendance matched the predictions: (1) low-ranking males range alone or in small parties more frequently than higher-ranking males and (2) when there are no receptive females in the group, low-ranking males spend more time ranging alone than when there are receptive females in the group. The current results also matched prediction (3): low-ranking males receive aggression more frequently when attending large parties. The frequency of aggression increased with the number of males in the party and low-ranking males were attacked more frequently than higher-ranking males. However, the presence or absence of females with MS, which has been considered to affect the frequency of aggression among males [
43,
44], did not have a significant effect on the overall frequency of aggression toward focal males. This might be because the characteristic of male aggression changed in the absence of females with MS and the presence of those females. When the number of males was less than seven in the presence of females with MS, low-ranking males did not receive aggression and higher-ranking males received more aggression. This was most likely because the female with MS was monopolized by high-ranking males, and the low-ranking males did not attempt to approach her even when he was in the same party. In fact, low-ranking males stayed most of the time at the periphery of the party, away from the females with MS (Shibata, unpublished data). The FAI did not have any significant influence on the frequency of aggression in the parties.
The current study supports our hypothesis that the FFD may offer alternative tactics for low-ranking males to mitigate costs caused by disadvantages when competing for survival and reproduction against other adult males. The attendance of low-ranking males at parties seemed to be affected by the aggression they received during social gatherings, even in the absence of receptive females.
Male chimpanzees benefit from parties through opportunities for social and sexual interactions with other individuals. Affiliative interactions with other males during social gatherings are necessary for forging and maintaining social bonds, which are important for achieving a higher dominance status and for intergroup conflicts [
45,
46]. Furthermore, it is necessary for low-ranking males to attend large parties to seek mating opportunities with receptive females, who tend to be found in large parties as many males gather around them. However, low-ranking males receive limited benefits and are more likely to attract aggression from other males as they compete for food or receptive females or because higher-ranking males often show dominance displays toward low-ranking males in large parties. As observed in this study, the tendency of low-ranking males to receive aggression did not decrease even when there were no receptive females in the group. Therefore, it seemed to be more beneficial for low-ranking males to range alone, especially during the absence of receptive females.
A previous study on spider monkeys (
Ateles spp.), which exhibit male philopatric social structures and strong fission–fusion tendencies similar to those of chimpanzees [
47,
48], suggested that the FFD of spider monkeys function as a form of conflict avoidance by mitigating the effect of food competition, both scramble and contest, and maintaining a low rate of aggression among adult females throughout the year [
49]. Another study on spider monkeys reported that a young male individual was frequently observed alone after receiving aggression for several weeks before being killed [
50]. Our findings suggest that male aggression of chimpanzees affects their dispersion tendencies, which are similar to those observed in spider monkeys.
However, in the present study, we did not investigate whether aggressive interactions trigger the leaves of chimpanzees, although aggressive interactions were most frequent toward low-ranking males. To understand the extent to which male aggression triggers dispersion, further studies are needed to assess aggressive interactions within parties and whether dispersion occurs after such interactions. In addition, we did not assess the possible effect of the existence of bonding partners or maternal brothers on the party attendance of individuals. These social factors are also likely to strongly affect individuals’ decisions on party attendance. Testing from a physiological perspective is also necessary to understand whether ranging alone is an adaptive choice for low-ranking males in terms of their stress levels. Hormonal analyses of individuals’ urinal/fecal cortisol levels would allow the estimation of stress levels in males when they range alone or in large/small parties. Research on the behavioral ecology of low-ranking chimpanzees, focusing on social and physiological aspects, is expected to further our understanding of the flexibility and complexity of the social structure of primates and other animals living in the FFD.