Discussion
Our analyses revealed a highly dynamic social network between core units
in the C. a. ruwenzorii multi-level society. Contrary to our
first prediction, the clan tier of organization was not entirely stable
over time. Given the importance of kinship in structuring primate groups
(Silk, 2001; 2002), we predicted that this may extend to the clan tier
of organization, with core units containing related individuals
preferentially clustering (e.g., Papio hamadryas, Theropithecus
gelada , Colmenares, 2004; Loxodonta africana , Wittemyer et al.,
2005), leading to stable clans within the band. However, we observed two
major changes in our study band. First, an all-male unit formed when
seven males left the largest core unit and began to range in loose
association with the two clans. Second, one core unit moved between
clans after dispersal events involving five males. These changes show
that clans do shift in core unit composition over time, though not
frequently, and that male dispersals can cause this variation.
We found support for our overall hypothesis as both ecological and
social variables were important in determining the amount of association
among core units. Association patterns fluctuated at both the node and
network level, with the largest changes correlating to seasonal shifts
in fruit availability. As predicted, core units were more likely to
associate, and did so with a larger number of other core units, when
fruits were abundant, suggesting that food competition limits
operational group size when fruits are scarce. This increase in
association appeared to facilitate male dispersals between core units in
the band, thus creating a seasonal dispersal pattern. Our analysis of
association indices following each male dispersal event within the band
revealed that male transfers promote higher than expected dyadic
associations between interacting core units in the short-term (1-2
months post-dispersal).
Many species alter their behaviour in response to changing climatic and
resource conditions (Candolin & Wong, 2012). Our results show thatC. a. ruwenzorii is no exception. Similar to studies done on
other primates (Cercocebus torquatus, Dolado, Cooke, & Beltran,
2016; Rhinopithecus bieti, Ren, Li, Garber, & Li, 2012;Papio hamadryas , Schreier & Swedell, 2012b; Pongo
pygmaeus, Sugardjito, Te Boekhorst, & van Hooff, 1987) and
non-primates (Orcinus orca, Foster et al., 2012; Loxodonta
africana , Wittemyer et al., 2005), we found that C. a.
ruwenzorii units increase their association levels during times of peak
food availability. Food competition decreases when resources are
abundant, allowing animals to aggregate if they choose, which provides
benefits for predator avoidance (Hamilton, 1971; Sueur et al., 2011).
Species living in a multi-level society benefit from this ability to
alter overall group size in response to external pressures (Grueter &
van Schaik, 2009). For C. a. ruwenzorii , enlarged group size may
even mean an expansion of the microhabitats they are willing to take
advantage of. Adams and Teichroeb (2020) found that at Nabugabo, where
predation risk is greatest near the ground, C. a. ruwenzorii were
willing to come lower in the canopy to find food when more core units
were clustered together and predation risk was lessened. The analyses
presented here suggest that this niche expansion may occur more often in
resource rich seasons when core units are able aggregate.
Although we find correlations between seasonal fruit availability,
association patterns and male dispersal, it is important to acknowledge
that we cannot determine cause and effect between these phenomena. While
we posit that higher fruit availability leads to more clustering among
core units, which facilitates male dispersal, it is possible that males
prospect more during seasons of food abundance and that male prospecting
behaviour drives the observed changes in association patterns. Seasonal
dispersal patterns are found in many species (Likicker & Stenseth,
1992) but in most cases, this pattern emerges due to seasonal breeding
(e.g., Presbytis entellus, Borries, 2000; Suricata
suricatta , Mares et al., 2014; Chlorocebus pygerythrus, Young et
al., 2019; Rhinopithecus roxellana, Yao et al., 2011). Breeding
is not typically seasonal in black-and-white colobus monkeys (Fashing,
2011) and we do not have data showing seasonal breeding at Nabugabo.
Alternatively, it is sometimes advantageous for animals to time
dispersal to coincide with high food availability because it allows them
to compensate for increased travel, potentially in unfamiliar areas
(Pusey & Packer, 1987; Isbell & Van Vuren, 1996). This explanation is
unlikely to apply in a multi-level society like that seen in C. a.
ruwenzorii as all the core units in our band share a home range (Stead
& Teichroeb, 2019). Consequently, male dispersal between units does not
require extra travel or moving into a new, unfamiliar area. We suggest
that the best explanation for the seasonal pattern of male dispersal
that we observe in C. a. ruwenzorii is the opportunity for
prospecting provided by greater core unit clustering due to high
resource availability. The proximity of so many other core units allows
males to assess their composition (i.e., sex ratio) as well as the
competitive ability of the males there (Teichroeb et al., 2020),
potentially influencing their decision to disperse. In primates, it is
common for dispersal to occur during intergroup encounters (e.g.,Macaca mulatta , Boelkins & Wilson, 1972; Erythrocebus
patas , Rogers & Chism, 2009; Gorilla beringei , Sicotte, 1993;Rhinopithecus roxellana , Yao et al., 2011) or to groups where
prospecting has previously been directed (e.g., Colobus
vellerosus , Teichroeb, Wikberg, & Sicotte, 2011).
The persistence of high association indices post-dispersal for core
units that have males transfer between them may be a result of the
continued bonds between individuals that persist even after the
dispersal has taken place. The dispersing individual(s) likely still
have ties in their former (sometimes natal) unit, which may contain many
kin. However, over time, we see a slow decrease of association between
the units individual(s) dispersed to and from, back to the baseline
association levels that they had prior to the dispersal event. This
decrease in association may be explained by the further integration of
the dispersing individual(s) into their new unit, and/or the seasonal
decrease in fruit availability, and subsequent increase in food
competition. Future research examining how male-male genetic and social
relationships impact association patterns over short and long time
periods will provide insights into the ways that kinship structures core
unit association in tandem with ecological and social factors (e.g.,
Snyder-Mackler et al., 2014).
To conclude, our results show that in the dynamic social network of
Rwenzori Angolan colobus monkeys, core units behaviourally adapt to
changing ecological conditions by altering their association patterns.
Doing so has cascading effects on the composition of core units, and
structure of both the clan and band tiers in this multi-level society.
This type of behavioural flexibility allows animals to thrive in dynamic
environments (Candolin & Wong, 2012). Our study provides a deeper
understanding of the mechanisms underlying the formation of complex
multi-level social organizations and some insight into the intertwined
temporal effects of ecological and social variables.