Diet of Grauer’s Gorillas (Gorilla beringei graueri) in a Low-Elevation Forest

Abstract

Although the vast majority of critically endangered Grauer’s gorillas (<i>Gorilla beringei graueri</i>) inhabit low-elevation rain forests, current insights into this ape’s life history and ecology stem predominantly from 2 small populations ranging in highland habitats. Here, we provide an initial and non-exhaustive overview of food items of Grauer’s gorillas in the Nkuba Conservation Area (NCA), a lower-elevation (500–1,500 m) forest located between Kahuzi-Biega National Park and Maiko National Park in the Democratic Republic of the Congo. Community-based conservation efforts at the NCA aim to protect a population of unhabituated Grauer’s gorillas, which we have studied since 2014. Between 2014 and 2020, we simultaneously tracked 1–3 gorilla groups and recorded a total of 10,514 feeding signs on at least 100 plant species, ants, termites, and fungi. Vegetative plant parts (plant stems, leaves, pith, bark, and roots), especially of Marantaceae and Fabaceae, made up close to 90% of recorded feeding signs, with fruit accounting for most of the remainder and a small (<1%) number of feeding signs on invertebrates and fungi. We found that the most frequently recorded food items were consumed year-round, though fruit intake seems to peak in the September-December wet season, possibly reflecting patterns in fruit phenology. The diet of Grauer’s gorillas in the NCA differed from that of Grauer’s gorillas in highland habitat and instead showed similarities with Grauer’s gorillas at the lowland forest of Itebero and with western lowland gorillas (<i>G. gorilla</i>), which live under ecologically comparable conditions.

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

“…This is likely the result of higher fruit availability or higher frugivory of gorillas in low elevation forests than in highland forests; a pattern also observed among Bwindi mountain gorillas [Goldsmith, 2003;Ganas and Robbins, 2005]. 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.…”

“…This would be higher than the maximum daily paths of Grauer's gorillas in highland KBNP, estimated at ∼3.4 km [Yamagiwa et al, 2012a]. Moreover, as preliminary analyses of foraging behavior of gorillas in the NCA suggest that feeding signs on fruit increase over the June-August dry season and peak during the first months of the September-December wet season [van der Hoek et al, 2021], we predicted an increase in DTD be-tween June and October. Such a pattern of increased travel distance following peaks in fruit intake during June and October is also found at Itebero and highland KBNP [Yamagiwa and Basabose, 2006;Yamagiwa et al, 2012b].…”

“…For instance, many frugivorous species adjust their movements and diet to spatiotemporal patterns and fluctuations in fruit availability [Olupot et al, 1997;Hemingway and Bynum, 2005;Janmaat et al, 2016]. Beyond food availability, primates are also guided, restricted, and informed in movements by factors such as the spatiotemporal distribution of predators [Coleman and Hill, 2014] and conspecifics [Gibson and Koenig, 2012;Van Belle and Estrada, 2020], decisions regarding anticipated travel costs versus the nutritional value of the feeding item upon arrival [Ban et al, 2014], and the detrimental effects of parasites [Nunn and Dokey, 2006]. Knowledge of an animal's space use can thus be used to gain insights into their ecology and is correspondingly useful for conservation.…”

“…Gorillas living at lower elevations tend to be more frugivorous and have higher travel distances -this is even true across populations of the same subspecies [Ganas and Robbins, 2005] -than gorillas that range in highland or montane habitats [Yamagiwa et al, , 1996aRemis, 1997;Doran-Sheehy et al, 2004]. Data from Itebero [Yamagiwa and Basabose, 2006] and the NCA [van der Hoek et al, 2021] indicate that Grauer's gorillas in lower elevation forests feed on a broad variety of plant parts, with fruits consumed year-round. In contrast, Grauer's gorillas in highland KBNP feed relatively more on herbaceous and vegetative plant parts and may consume little fruit in some months [Yamagiwa and Basabose, 2006].…”

“…Yet, it is unlikely that these temporal patterns have the same strong effects on space use of gorillas as fruit availability [Rogers et al, 2004]. Given this consideration, the lack of detailed phenological data on plant parts of specific plant species in the NCA and the absence of evidence of clear seasonal patterns in consumption of non-fruit parts [van der Hoek et al, 2021], we omitted possible effects of seasonality in the availability of non-fruit plant parts from further analyses.…”