The impact of Masai giraffe nursery groups on the development of social associations among females and young individuals
Introduction
Mammals form groups to reduce the risk of predation, find/catch food, and protect dependent young (Clutton-Brock, 2016; Gero et al., 2013). Several types of social systems have evolved with one of the more enigmatic and complicated societies known as “fission–fusion”. Mammals living in these types of societies change the size of subgroups by means of joining and separating according to the availability and distribution of resources and social preferences (Aureli et al., 2008). The taxonomic distribution of fission–fusion social systems is diverse and includes species, such as chimpanzees, Pan troglodytes (Lehmann and Boesch, 2004), African elephants, Loxodonta africana (Archie et al., 2006), bottlenose dolphins, Tursiops spp. (Tsai and Mann, 2013), sperm whales, Physeter microcephalus (Whitehead et al., 1991), spotted hyenas, Crocuta crocuta (Holekamp et al., 1997), and giraffe, Giraffa camelopardalis (Bercovitch and Berry, 2010a, 2013a; Carter et al., 2013a, 2013b; Muller et al., 2018a; Shorrocks and Croft, 2009). The dyadic associations at the core of fission–fusion social systems are based upon multiple factors including, but not limited to, kinship, age, sex, reproductive state, body condition, the presence of dependent offspring, amount of knowledge, and behavioral state (Boogert et al., 2014; Gero et al., 2013; Godde et al., 2015; Konrad et al., 2018; Kulahci et al., 2018; McCarthy et al., 2019; Methion and Díaz López, 2020; Zeus et al., 2018).
The shifting membership of giraffe herds had long been viewed as a consequence of random associations among individuals (Foster and Dagg, 1972; Langman, 1977; Le Pendu et al., 2000), but more recent evidence reveals that giraffe are characterized by fission–fusion dynamics, whereby individuals exhibit preferences regarding herd formation that are not simply dependent upon home range overlap (Carter et al., 2013b; Deacon and Bercovitch, 2018). The strength of the dyadic associations in giraffe reflects a context-dependent situation that varies with ecological (habitat type: Muller et al., 2018a; season: Wolf et al., 2018) and reproductive (e.g., mate guarding: Brand, 2007; nursing: Malyjurkova et al., 2014; Saito and Idani, 2016) factors, as well as sex, age, and kinship (Bercovitch and Berry, 2013b; Carter et al., 2013a, 2013b), in a tandem, not independent, fashion. Giraffe herd size and structure are not only dependent upon multiple factors, but herds are both ephemeral, in that individuals rarely remain together for more than a few days (Deacon and Bercovitch, 2018; Le Pendu et al., 2000) and consistent, in that individuals do not associate randomly.
In social network analysis, demographic processes such as birth, immigration, dispersal, and death, are important factor revealing the patterns of social associations among individuals in population (Shizuka and Johnson, 2020). However, the impact of reproductive status over several months is not well examined in giraffe social networks. In mountain goats, Oreamnos americanus and female Natterer’s bat, Myotis nattereri, nursing females associate mainly with nursing females but not with non-nursing females (Godde et al., 2015; Zeus et al., 2018), and this could be explained by communal rearing of young individuals (Gero et al., 2013; Zeus et al., 2018). In giraffe social systems, crèche or nursery groups are herds composed of several female–offspring pairs (Langman, 1977; Pratt and Anderson, 1979), in which females form strong social associations with their filial offspring (Bercovitch and Berry, 2013b; Malyjurkova et al., 2014), and mothers maintain close associations over a month (Malyjurkova et al., 2014). Moreover, females in nursery groups spend more time with each other when the calves are young than when they are not lactating (Saito and Idani, 2016). Nursery group membership, unlike other types of giraffe groups (Bercovitch and Berry, 2010a; Deacon and Bercovitch, 2018; Wolf et al., 2018), is stable for up to several months (Langman, 1977).
The social structure of giraffes in adulthood are well-studied and differ according to sex, with males primarily solitary and females roaming in herds (Bercovitch and Berry, 2010a; Carter et al., 2013a; Grignolio et al., 2019; Wolf et al., 2018). Males are considered to be reproductively mature when 7–8 years of age (Dagg and Foster, 1982), with younger, smaller, paler males forming bachelor herds or remaining in groups with females (Brand, 2007; Castles et al., 2019) and darker, older males becoming more solitary (Castles et al., 2019). In contrast, female giraffe produce their first calf when around 6–7 years of age in the wild (Bercovitch and Berry, 2017; Dagg and Foster, 1982; Pellew, 1983), and females are likely to be found in herds with other females (Wolf et al., 2018). The development of age differences in herd formation and dynamics of young individuals in giraffe has not been investigated, although Bercovitch and Berry (2010a) reported that male and female juveniles were first recorded in a herd without their mother at similar ages. Young male and female giraffe could be solitary during the daytime or join nursery groups in which the average number of individuals is about 7 or consists of 2–3 female–offspring pairs (Langman, 1977; Pratt and Anderson, 1979; Saito and Idani, 2016, 2018a). Giraffe initially establish herds with non-mother when 1–2 years of age (Bercovitch and Berry, 2010a), and giraffe herd size can be >20 individuals (Bercovitch and Berry, 2010a; VanderWaal et al., 2014). The home range and the pattern of habitat use are known to be slightly different among individuals; females and their offspring generally share the same home range at least during the first 6 months of age (Le Pendu and Ciofolo, 1999), and females with young calves (less than 6 months of age) select safer area (Caister et al., 2003).
In this study, we aim to reveal the impact of demographic processes, such as reproductive status and immature development on social bonds in nursery groups of Masai giraffe, G. c. tippelskirchi using data from five study periods over ten years. We hypothesized that the associations between females accompanied by their offspring (=<1.5 years old) would be stronger than those between females without offspring (hypothesis 1). In addition, we expected that juveniles would start to associate with other individuals outside of nursery groups while calves would remain closer to their mothers (hypothesis 2).
Section snippets
Study area
We conducted our study at Katavi National Park, Tanzania (6°63′–7°34′S; 30°74′– 31°84′E) which covers 4471 km2 of western Tanzania. The field study was conducted in an area measuring about 12 km2 in the vicinity of NP headquarters (HQ), located on the northern border of this national park near Sitalike Village-and that. Lions, Panthera leo, live in Katavi National Park and are considered a natural predator of giraffe (Mulder et al., 2007).
Data collection
Our data were collected during five study periods,
Results
The average herd size over the five study periods was 5.78 ± 4.32 (range, 1–23; n = 904). The mean SRI value for all study periods was 0.142 ± 0.07 (n = 2019 dyads). Over 75 % of possible dyads had direct connections but the mean SRI values were <0.2 for all study periods (Table 1). For all study periods, giraffe subjects were connected into one small network component, with all individuals observed at least five times in the study population highly interconnected either directly or indirectly.
Discussion
Giraffe herd composition within fission–fusion societies is flexible, but not random (Bercovitch and Berry, 2010a; Carter et al., 2013a, 2013b; Wolf et al., 2018), and also context-specific (Muller et al., 2018a). Demographic processes influence social association patterns among individuals in a population (Shizuka and Johnson, 2020), and reproductive status impacts female association patterns (Godde et al., 2015; Matsumoto-Oda, 1999; Zeus et al., 2018). Our study confirmed that female
CRediT authorship contribution statement
Miho Saito: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization, Funding acquisition. Fred B. Bercovitch: Conceptualization, Writing - original draft, Writing - review & editing, Visualization. Gen’ichi Idani: Resources, Funding acquisition.
Acknowledgements
The research was financially supported by Grant-in-Aid for JSPS Research Fellow, Japan (#17J09785 and #19J00510), Leading Graduate Program in Primatology and Wildlife Science, Japan (U04-JSPS), International Training Program of HOPE, Japan (Primate Research Institute of Kyoto University), and the Sasakawa Scientific Research Grant from the Japan Science Society, Japan (28-537). We thank the Tanzania Commission for Science and Technology (COSTECH), the Tanzania Wildlife Research Institute
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