Ecological correlates of space use patterns in wild western lowland gorillas

Abstract

The distribution of resources is a crucial determinant of animals' space use (e.g., daily travel distance, monthly home range size, and revisitation patterns). We examined how variation in ecological parameters affected variability in space use patterns of western lowland gorillas, Gorilla gorilla gorilla. They are an interesting species for investigating this topic because key components of their diet are nonfruit items (herbaceous vegetation and tree leaves) that occur at low density and are sparsely distributed, and fruits, which show high spatiotemporal variation in availability. We estimated how availability of nonfruit foods and fruit, frugivory (proportion of feeding time consuming fruit), and swamps in areas used by the gorillas influenced daily travel distance, monthly home range size, and revisit frequency to grid cells in the home range of one habituated gorilla group in Loango National Park, Gabon. Using location data from 2015 to 2018, we found that the gorillas decreased their daily travel distance as both the density of nonfruit foods and the proportion of swamps in areas used increased. Daily travel distances were shorter when both frugivory and availability of fruit were higher, yet, daily travel distances were longer when availability of fruit was low but frugivory was still high. Furthermore, monthly home range size increased as frugivory increased and monthly revisit frequencies to an area increased as fruit availability of an area increased. In conclusion, the availability of both nonfruit and fruit influenced the gorillas' space use patterns. Gorillas decreased foraging effort when food availability was high but were willing to incur increasing foraging costs to feed on fruit when availability was low. This study highlights how animals have to adjust their space use with changing resource availability and it emphasizes the value of examining multiple parameters of space use.

Daily Travel Distances of Unhabituated Grauer’s Gorillas (Gorilla beringei graueri) in a Low Elevation Forest

“…Little is known about this subspecies of gorilla, largely because decades of civil conflict have made research in the region very challenging. In contrast, an extensive body of ecological research has now accumulated on the closely related mountain gorilla (G. beringei beringei) that ranges across montane habitats (see e.g., studies of their space use [Robbins and McNeilage, 2003;Ganas and Robbins, 2005;Caillaud et al, 2014]) and we have seen a similar build-up of studies on western gorillas (G. gorilla) which are mainly found in low elevation rainforests (see e.g., recent studies on G. gorilla space use [Salmi et al, 2020;Seiler and Robbins, 2020]).…”

“…In addition, observed trends of increase in travel distances over the June-August dry season, and peaks in travel distances at the beginning of the September-December rainy season, are in line with fruit availability and consumption by gorillas in the NCA [van der Hoek et al, 2021] and with extensions of DTD during the main fruiting season by solitary male gorillas at Itebero . Similarly, studies on mountain gorillas and western lowland gorillas show that at least some family groups expand DTD in seasons with a relatively high abundance of fruits [Yamagiwa et al, 1996a;Ganas and Robbins, 2005;Masi et al, 2009;Yamagiwa et al, 2012b;Seiler and Robbins, 2020], though studies on western lowland gorillas [Seiler and Robbins, 2020] and mountain gorillas [Ganas and Robbins, 2005] show that this relationship between fruit availability and travel distances may also be influenced by other variables, such as the degree of frugivory of particular gorilla groups, the availability of non-fruit food resources, and habitat use.…”

“…By identifying these sleep, rest, and feeding locations, we can infer how these gorillas move across the landscape and derive aspects of space use such as DTD; which could then be the foundation for more advanced studies of space use by Grauer's gorillas. For example, travel distances can be indicative of food searching strategies and food resource availability, with large DTD in gorillas representative of relatively high consumption of fruit Goldsmith, 1998;Ganas and Robbins, 2005;Yamagiwa et 2012a; Seiler and Robbins, 2020] or other foods that are ephemeral and patchily available (e.g., a synthesis by Rogers et al [2004] shows that fruit, leaves, flowers, seeds, stem, bark, and pith can all be seasonal food items for western lowland gorillas [G. gorilla gorilla]). In contrast, shorter daily movements are generally linked to a more continuous and spatially homogeneous supply of food (e.g., leaves and stems of herbaceous plants for Virunga mountain gorillas [Watts, 1984]).…”

“…But we acknowledge that we have to be careful in interpreting variation in travel distances as being solely determined by food availability. For example, daily path lengths tend to increase with swamp use or availability in a given area [Doran-Sheehy et al, 2004;Seiler and Robbins, 2020], and social factors such as intergroup encounters can similarly influence the space use of gorillas [Cipolletta, 2004;Doran-Sheehy et al, 2004;Caillaud et al, 2014;Grueter et al, 2018].…”

Assessing attitudes towards gorilla conservation via employee interviews

“…This study was conducted through interviews with Ugandan employees of the Bwindi Gorilla Project ( N = 11) and the Gabonese employees of the Loango Gorilla Project ( N = 10), which are both directed by Martha Robbins (see Table 1 for details of projects). Both projects focused on studying social relationships, feeding ecology, and population dynamics of the two species of gorillas (e.g., Ganas, Robbins, Nkurunungi, Kaplin, & McNeilage, 2004; Hagemann et al, 2019; Head, Boesch, Makaga, & Robbins, 2011; Robbins et al, 2019; Robbins, Gray, Kagoda, & Robbins, 2009; Seiler, Boesch, Mundry, Stephens, & Robbins, 2017; Seiler & Robbins, 2020). The data collection protocols and training are given to staff were largely the same at the two study sites.…”