Mineral content as a basis for food selection by western lowland gorillas in a forest clearing

The forests in northwest Republic of Congo contain a number of herbaceous swamp clearings that provide foraging sites for lowland gorillas (G.g. gorilla). A 10-month study at the Maya Nord clearing (Parc National d'Odzala) showed that feeding activities occupied 72% of the time visiting gorillas spent on the clearing. They fed on four plant species: Enydra fluctuans (Asteraceae), Cyperus sp., Pycreus mundtii, and Rhynchospora corymbosa (Cyperaceae) among the 45 species recorded on the clearing. These clearing food species have higher mineral contents (especially Na and Ca) than the dominant Marantaceae species (Haumania liebrechtsiana) that constituted a staple food plant for gorillas in this forest. They also have higher potassium contents and contain less lignin than non-eaten clearing items/species. Finally, the most actively searched for clearing food (Enydra fluctuans) was characterized by the highest amount of Na and Ca. These results suggest that the mineral content (especially in Na, Ca, and/or K) could determine the feeding selectivity of gorillas at the clearing. They also tend to confirm that the amount of fiber plays a deterrent role in food selectivity, as has been found by many authors. The high density of gorillas in that region could result from the combination of the large areas of Marantaceae forests that provide abundant though monotonous food, and the number of clearings that provide sufficient mineral supplies. Clearings should thus be considered as key habitats for the conservation of gorillas.     

Ecological correlates of space use patterns in wild western lowland gorillas

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.     

Sex-biased dispersal in western lowland gorillas (Gorilla gorilla gorilla)

We explored two hypotheses related to potential differences between sexes in dispersal behaviour in western lowland gorillas (Gorilla gorilla gorilla). Direct observations suggest that immature females have more opportunities to move between breeding groups than immature males. The distribution of kin dyadic relationships within and between groups does not, however, support this hypothesis. At larger geographical scales, dispersal is likely to be easier for males than females because of the solitary phase most blackbacks experience before founding their own breeding group. However, previous work indicates that males settle preferentially close to male kin. By specifically tracing female and male lineages with mitochondrial and Y-chromosomal genetic markers, we found that male gorillas in the 6000 km2 area we surveyed form a single population whereas females are restricted to the individual sites we sampled and do not freely move around this area. These differences are more correctly described as differences in dispersal distances, rather than differences in dispersal rates between sexes (both sexes emigrate from their natal group in this species). Differences in resource competition and dispersal costs between female and male gorillas are compatible with the observed pattern, but more work is needed to understand if these ultimate causes are responsible for sex-biased dispersal distances in western lowland gorillas.     

Exchange of objects between humans and captive western lowland gorillas

An experiment was carried out to assess the ability of captive western lowland gorillas (Gorilla g. gorilla) to exchange objects with a human partner. Before the exchange itself, individuals had to learn to give an item to the experimenter. Four individuals out of nine performed active giving. After that familiarization with giving behaviour, two kinds of exchanges, of increasing complexity, were tested with gorillas. Simple exchange (give C to obtain D) was the first procedure and six gorillas out of nine succeeded. Double exchange (give B to receive C, then give back C to obtain D) was performed only by three individuals and triple exchange (give A to receive B, give back B to receive C, then give back C to obtain D) was performed only by two individuals. Different types of errors were scored, as well as social constraints on learning. Results showed that (1) naive gorillas are able to get a reward (D) in an experimental exchange situation; and (2) the complex rules of multiple successive exchanges were not easily understood by gorillas.     

Ant eating by eastern lowland gorillas

Evidence was obtained that Eastern lowland gorillas feed regularly on ants in the lowland forests of eastern Zaire. The six species of ants that may have been consumed by the gorillas were identified as Ponerinae, which possess a painful sting. Gorillas consumed the ants in both rainy and dry seasons, and fragments of ants were found in fecal samples from gorillas of all age-sex classes. Field signs indicated that the gorillas search intensively ants in primary and ancient secondary forests. Such searching may stimulate the gorillas to range through various types of vegetation and, thus, it may contribute to elongation of the distances that they travel daily.     

Body signals used during social play in captive immature western lowland gorillas

The play face is a well-established play signal in nonhuman primates that functions to invite play and convey a playful intent. However, recent evidence indicates that some species display repertoires of play signals that may have more specific meanings related to particular aspects of play. Furthermore, previous studies have inconsistently categorized gorilla behaviors as play signals versus actual play. Here we aim to identify behaviors displayed by two immature captive western lowland gorillas (Gorilla gorilla gorilla) at the Buffalo Zoo that meet three necessary criteria to be considered play signals. Specifically, we (1) investigate whether 21 candidate signals are significantly different from actual play behaviors, (2) and from similar signals used in non-play contexts, and (3) determine whether they predict the occurrence of social play. The results indicate that at least 18 of the 21 candidate signals have strong support for classification as play signals. These findings represent first steps in determining the function of multiple play signals in gorillas.     

Postconflict behavior in captive western lowland gorillas (Gorilla gorilla gorilla)

Postconflict (PC) behaviors, including reconciliation and consolation, have been observed in many primate and several nonprimate species. Using the PC-matched control (MC) method, PC behavior was examined in two groups (n=13) of captive western lowland gorillas, a species for which no conflict resolution data have been published. Analyses of 223 conflicts showed significantly more affiliation between former opponents after a conflict when compared to control periods, indicating reconciliation. Results also showed significantly more affiliation between the victim and a third-party after a conflict, indicating consolation. Both solicited and unsolicited consolation were observed. The majority of the affiliative interactions observed for both reconciliation and consolation were social proximity, which suggests that unlike most nonhuman primates, proximity, rather than physical contact, may be the main mechanism for resolving conflicts in western lowland gorillas. PC behavior was not uniform throughout the groups, but rather varied according to dyad type.     

Reversal of breeding season by lowland birds at higher altitudes in western Cameroon

Evidence of breeding during the dry season is given for 31 species of lowland birds in montane and semi-montane areas in western Cameroon. At least 17 of these species are shown to breed in the wet season at lower altitudes in West Africa. This reversal of breeding season may be due to the unusually heavy rainfall and high humidities which cause temperatures to decrease more rapidly with altitude than on mountains with drier climates. Low temperatures and heavy rainfall during the wet season prevent almost all montane species from breeding then, and affect similarly the lowland birds whose ranges overlap with those of montane birds. Breeding seasons of some lowland species in Cameroon have previously been considered prolonged, but separation of breeding records by altitude reveals clear seasonality.     

Maternal subgrouping in lowland gorillas

Mountain gorilla females have been reported to seek out the dominant male and to increase the time spent in proximity to him when they are caring for young infants. The study examined subgrouping phenomenon in a captive group of lowland gorilla mothers. Interaction scores failed to support the position that the male is attractive to mothers, or that they seek him out and cluster about him. The data indicate instead that the females avoid interacting with the male and rarely initiate contact with him. Subgroups and social clustering observed among the females is interpreted as preferential contact among mothers.     

Food transfers in immature wild western lowland gorillas (Gorilla gorilla gorilla)

The transfer of food items between individuals has been described in primates as serving an informative purpose in addition to supplementing the diet of immature individuals. This behaviour has yet to be described in western lowland gorillas (Gorilla gorilla gorilla), and results are presented here of observations of food transfers in immature gorillas at Mbeli Bai, Republic of Congo. The frequency of food transfers decreased with increasing immature age, while the frequency of independent feeding and processing of food increased. Transfers between mothers and infants were the most frequent, with infants attempting to take items from the mother. These attempts were not always successful and the item was relinquished on less than 50% of attempts. Mothers also took items from their offspring. The results point to the functional significance of food transfers in western lowland gorillas being informational. In a bai environment, where one species forms the majority of a visiting gorilla’s diet despite other species being available, the initiation of food transfers by immatures is proposed to serve the purpose of familiarising them with which species, and which parts of those species, may be eaten.     

Species and sex identification of western lowland gorillas (Gorilla gorilla gorilla), eastern lowland gorillas (Gorilla beringei graueri) and humans

Methods for the identification of the sex and species of individuals from samples non-invasively taken from humans and gorillas were established. Amplification of a segment of amelogenin (AMG), which is an X–Y homologous gene, using two pairs of primers from human AMG, revealed both X- and Y-specific bands. The possibility of sex identification was examined by typing the AMG gene using hair and fecal samples from captive western lowland gorillas (Gorilla gorilla gorilla) in Japan and hair samples from wild eastern lowland gorillas (Gorilla beringei graueri) in the Kahuzi-Biega National Park, Democratic Republic of Congo, which were sexed by direct observation. Species-specific bands of AMG in gorillas and humans were identified by restriction fragment length polymorphisms analysis. These tests could be used for sexing unidentified individuals of wild western and eastern lowland gorillas, and screening contamination of human DNA from non-invasively acquired samples.     

Mitochondrial DNA phylogeography of western lowland gorillas (Gorilla gorilla gorilla)

The geographical distribution of genetic variation within western lowland gorillas (Gorilla gorilla gorilla) was examined to clarify the population genetic structure and recent evolutionary history of this group. DNA was amplified from shed hair collected from sites across the range of the three traditionally recognized gorilla subspecies: western lowland (G. g. gorilla), eastern lowland (G. g. graueri) and mountain (G. g. beringei) gorillas. Nucleotide sequence variation was examined in the first hypervariable domain of the mitochondrial control region and was much higher in western lowland gorillas than in either of the other two subspecies. In addition to recapitulating the major evolutionary split between eastern and western lowland gorillas, phylogenetic analysis indicates a phylogeographical division within western lowland gorillas, one haplogroup comprising gorilla populations from eastern Nigeria through to southeast Cameroon and a second comprising all other western lowland gorillas. Within this second haplogroup, haplotypes appear to be partitioned geographically into three subgroups: (i) Equatorial Guinea, (ii) Central African Republic, and (iii) Gabon and adjacent Congo. There is also evidence of limited haplotype admixture in northeastern Gabon and southeast Cameroon. The phylogeographical patterns are broadly consistent with those predicted by current Pleistocene refuge hypotheses for the region and suggest that historical events have played an important role in shaping the population structure of this subspecies.     

Male genetic structure and paternity in western lowland gorillas (Gorilla gorilla gorilla)

The male dispersal patterns of western lowland gorillas (WLGs, Gorilla gorilla gorilla) are not well understood. To determine whether most silverbacks stay close to their relatives, we analyzed autosomal and Y‐chromosomal microsatellites (STRs) in wild WLGs at Moukalaba, Gabon. We obtained STR genotypes for 38 individuals, including eight silverbacks and 12 adult females in an approximately 40 km² area. Among them, 20 individuals were members of one identified group (Group Gentil; GG), including one silverback and six adult females. The silverback sired all 13 of the offspring in GG and no Y‐STR polymorphism within GG was found, as expected in a one‐male group structure. Over all silverbacks sampled, Y‐STR diversity was high considering the limited sampling area, and silverbacks with similar Y‐STR haplotypes were not always located in nearby areas. Although the misclassification rate of kinship estimates in this study was not negligible, there were no kin dyads among all silverbacks sampled. These results suggest that silverbacks born in the same group do not stay close to each other after maturation. The Y‐STR diversity in this study was similar to that of a previous study conducted in an area that was approximately 150 times larger than our study area. Similarity of WLG Y‐STR diversity between studies at different sampling scales suggests that male gene flow may not be geographically limited. These results suggest that WLG males normally disperse from their natal areas after maturation, at least, in Moukalaba. Am J Phys Anthropol 151:583–588, 2013. © 2013 Wiley Periodicals, Inc.     

A training program for noninvasive semen collection in captive western lowland gorillas (Gorilla gorilla gorilla)

Staff at Omaha's Henry Doorly Zoo devised a training program for noninvasive semen collection from three male gorillas. Trained behaviors, shaped using food rewards and verbal praise, enabled the trainer to perform a cursory physical examination and collect >270 semen samples during the study period. Ejaculate volume, sperm motility, and progressive forward motion were recorded. Two of the gorillas regularly produced semen samples that were cryopreserved. Frozen semen from one gorilla was successfully used in an assisted reproductive procedure. The training program may be used as a guideline for other institutions to create similar programs. Zoo Biol 17:143–151, 1998. © 1998 Wiley-Liss, Inc.     

Use of buckets as tools by Western lowland gorillas

While all great apes have been documented to use tools, gorillas are arguably the least proficient tool users. In 2009, a Western lowland gorilla (Gorilla gorilla gorilla) at the Buffalo Zoo was observed using a bucket, which had been provided as part of normal enrichment, as a tool to collect water. We conducted a brief, ad libitum investigation to confirm the validity of the initial observation. We then carried out a systematic investigation of the behavior in 2010. We collected 72 hr of videotaped data and tested the null hypothesis that the gorillas did not differ in their prevalence of engaging in bucket-use behaviors. We documented that all four adult gorillas in the group used buckets as drinking tools; however, there was significant individual variation in frequency and type of use of buckets. Four of the eight behaviors showed significant variation among individuals. The silverback male and the youngest adult female contacted and held the bucket significantly more than the remaining two adult females. The young female carried and drank from the bucket significantly more than any other individual. Furthermore, she was observed to fill the bucket with water four of the six times during which this behavior was observed. These data provide evidence of the ability of gorillas to utilize tools, given the appropriate environmental conditions. We continue to explore the abilities of gorillas to recognize the functionality of buckets as tools.     

Paternity determination in captive lowland gorillas and orangutans and wild mountain gorillas by microsatellite analysis

Paternity exclusion studies provide useful information for testing certain theories of behavioral ecology and for the management and conservation of both wild and captive populations of endangered species. This study used eight human nuclear microsatellite loci, in the absence of species-specific PCR primers, to genetically identify the sires of 12 captive lowland gorillas (Gorilla gorilla gorilla) and 2 captive orangutans (Pongo pygmaeus pygmaeus andPongo p. abelii). Parentage assignments were confirmed by excluding all except a single potential sire for each offspring with the least two loci. Sire-offspring relationships were verified in 12 of the 14 cases, and reassigned in the case of two gorilla offspring. The orangutan paternity typing was supplemented by DNA fingerprinting. Additionally, five of the eight microsatellite loci, in conjunction with behavioral data, were used for a non-exhaustive set of paternity exclusions for five wild mountain gorillas (Gorilla g. beringei). The eight loci described in this study should be useful additions to the tools available for the study of genetics in the great apes.     

Sexual behavior in female western lowland gorillas (Gorilla gorilla gorilla): evidence for sexual competition

Previous research in gorillas suggests that females engage in post‐conception mating as a form of sexual competition designed to improve their own reproductive success. This study focused on sexual behaviors in a newly formed group of western lowland gorillas (Gorilla gorilla gorilla) housed at Zoo Atlanta. All females engaged in mating outside their conceptive periods, although there was individual variation in the frequency of the behavior. An analysis of the presence/absence of sexual behavior found females, regardless of reproductive condition, were more likely to engage in sexual behavior on days when other females were sexually active. On these “co‐occurrence” days, females were significantly more likely to solicit the silverback, but copulations did not differ from expectation. The results find further evidence for sexual competition among female gorillas and suggest that this may occur throughout their reproductive cycle rather than only during pregnancy. Am. J. Primatol. 71:587–593, 2009. © 2009 Wiley‐Liss, Inc.