Ceiba pentandra Density and Basal Area
We found that Ceiba pentandra grows in our study area at a mean density of 178.4 ± 33.2 trees/km2, with a mean basal area of 3 ± 0.3 m2/ha.
Animal Species Recorded in Videos
Between October 2014 and December 2017, we recorded 8112 camera days, while monitoring Ceiba pentandra bark-peeling sites (with one camera day being 24 h of continuous monitoring by one camera). We recorded 12,234 videos in which 44 species of mammals were present, including 8 species of primates (Pan troglodytes verus, Papio anubis, Cercocebus lunulatus, Chlorocebus sabaeus, Cercopithecus lowei, Cercopithecus petaurista, Procolobus verus, Galago senegalensis); 7 species of rodents (Histrix cristata, Atherurus africanus, Thrionomys swinderianus, Cricetomys gambianus, Funisciurus pyrropus, Heliosciurus rufobrachium, Protoxerus stangeri); 12 species of bovids (Syncerus caffer, Tragelaphus euryceros, T. scriptus, Cephalophus sylvicultor, C. niger, C. dorsalis, C. rufilatus, Philantomba maxwelli, Kobus ellipsiprymnus, K. kob, Hippotragus equinus, Alcelaphus bulselaphus); 10 species of small and medium-sized carnivores (Crossarchus obscurus, Ichneumia albicauda, Atilax paludinosus, Herpestes ichneumon, Genetta genetta, G. thyerri, Mellivora capensis, Profelis aurata, Civettictis civetta, Canis adustus); 2 species of big carnivores (Panthera pardus, Crocuta crocuta), pangolins (Manis tricuspis, M. tetradactyla), ardvaarks (Orycteropus afer), hippopotamuses (Hippopotamus amphibius), and elephants (Loxodonta cyclotis); and 3 species of hogs (Hilochoerus meinertzeghani, Potamochoerus porcus, and Phacochoerus africanus).
Animal Species That Peel Ceiba pentandra Bark
After more than 3 years monitoring the trees, the only species we recorded peeling bark on Ceiba pentandra were chimpanzees and baboons. Both chimpanzees and baboons peeled long stripes of bark of Ceiba pentandra, but while the baboons consumed mainly the bark itself (N = 29 bark only consumption, 6 = cambium consumption), the chimpanzees always left the bark aside to consume the exposed cambium (N = 38). Of all the remaining species of mammals, only two more primates and two rodent species showed interest in C. pentandra bark. Guenons (Cercopithecus lowei and C. petaurista), giant pouched rats (Cricetomys gambianus), and squirrels (Funisciurus pyrropus) bit small round portions of bark (<4 cm in diameter) from the thinner parts previously peeled by chimpanzees or baboons, at stages 2 and 3 of healing.
We recorded 54 videos of baboons and 130 videos of chimpanzees peeling the Ceiba pentandra bark between August and November 2016, representing 35 events for the baboons and 38 for the chimpanzees. We recorded these videos at 14 different sites across the home-ranges of four communities of chimpanzees (Lapuente et al. 2016). Both baboons and chimpanzees peeled the bark of the same tree at only 2 of the 14 sites and at different times during the same season, with baboons always peeling the bark first and chimpanzees doing so 2 and 3 weeks later. Of the remaining 12 bark-peeling sites, 4 were used exclusively by baboons and 8 exclusively by chimpanzees. We recorded chimpanzees of all age/sex classes peeling the bark. We never recorded adult female baboons peeling bark but adolescent and juvenile females and male baboons of all ages did peel bark (Table I).
For infants, we did not consider the length of the cut because they always used the cuts previously made by the adults. We found no significant interspecific or intraspecific differences for the duration of the event (Mann–Whitney U, P > 0.05, Electronic Supplementary Material [ESM] Table SI), except for juvenile baboons, which is much shorter than for adult chimpanzee males (W = 131.5, P = 0.025) and adult chimpanzee females (W = 192.5, P = 0.031). When comparing number of cuts, we found no significant interspecific or intraspecific differences (Mann-Whitney U, P > 0.05, ESM Table SII), while for cut length, we found significant differences only when comparing adult chimpanzee males, which made significantly longer cuts than juvenile chimpanzees (W = 18, P = 0.035) and juvenile baboons (W = 51, P = 0.004) (ESM Table SIII). However, the mean duration of bark-peeling events in adult chimpanzees was twice that in adult baboons, and the mean length of the cuts was also greater in chimpanzees than in baboons (Table I).
Technique Used to Peel Bark
To start the cut, both species used their canines, but while baboons bit small portions of bark from different parts of the buttress spine, chimpanzees always made a transverse first cut of several centimeters in the upper part and pulled to tear a long stripe of bark using their hands or teeth, exposing the cambium underneath (Table II). After this initial cut, chimpanzees continued pulling pieces of bark either with their hands (22 of 38 events) or their teeth (16 of 38 events). The chimpanzees used their hands to pull the bark significantly more often than baboons did (W = 462.5, P = 0.003). The chimpanzees were the only ones that used their fingernails to scrape the cambium exposed, and while there was no significant difference between the use of fingernails and the transversal teeth scraping (W = 654, P = 0.05271), they scraped significantly more longitudinally with teeth than with fingernails (W = 1265, P = 1.623e-05). Chimpanzees scraped the cambium with teeth significantly more longitudinally than transversely (W = 1427.5, P = 2.034e-10). On the other hand, baboons scraped significantly more transversely than longitudinally (W = 169, P = 3.252e-09) (ESM Table SIV, videos 1 and 2).
Once the cut was open, the baboons left the bark torn into small pieces around the buttress, while the chimpanzees left long strips rolled at the base of the buttress or lying around the tree (Figs. 4 and 5). The two species consumed the exposed cambium in different ways. While the baboons scraped exclusively with their teeth, the chimpanzees always used both their fingernails and teeth. Most of the scraping made by baboons was transverse to the spine of the buttress (93.2%, N = 689), while the chimpanzees scraped mostly longitudinally (95.1%, N = 656) and only made transverse scrapes at the bottom of the cut when it was too close to the ground to scrape longitudinally with their teeth (5%, N = 32) (Fig. 6, Table II).
The total number of scrapes per event was similar between the two species (Table II), but since chimpanzees made much longer longitudinal scrapes, the quantity of cambium obtained was probably higher.
We found stone tools beside 85 of the peeled trees. However, these tools were probably used for stone throwing, since we found stone throwing traces in most of the 85 trees associated with stones and we did not record any tool use in relation to the bark peeling at any of the sites.
In 2016, we recorded 73 events of bark-peeling by chimpanzees with our camera-traps, the first on August 5, 2016 and the last on November 26, 2016. Counting Ceiba pentandra trees that had stage 1 traces of bark-peeling along transects and reconnaissance surveys, we found the earliest signs at the end of the month of June (data from 2 years) and the latest in mid-December (data from 4 years). We found the highest number of freshly peeled trees in October and November, over three consecutive years. We never found freshly peeled trees between January and May (dry season). Thus, we found that this behavior is displayed only during the rainy season and the beginning of the dry season, peaking at the end of the rainy season, when the trees are still full of leaves, and ceasing completely when the trees start producing flowers in mid-December.
Bark Peeling Behavior in Other Chimpanzee Communities over Time
Chimpanzees of all age and sex classes peeled bark (Tables I and II). We recorded videos in four known chimpanzee communities (Odissey, Achean, Trojan, and Aeneid), with most of the individuals already identified (Lapuente et al. 2016). We also found traces of bark peeling on Ceiba pentandra trees across neighboring areas to the north and west, where at least two more communities live (communities A, K, Fig. 7). We found new bark-peeling traces in each of these home ranges in 2015, 2016, and 2017.
Tree Species Selection
From October 2014 to May 2015 we found 958 trees that had evidence of debarking by animals, of which 857 were Ceiba pentandra (89.5%). Trees of other species were debarked mainly by elephants based on tusk marks, footprints, and other traces or by unknown animal species (Table III).
Additional Uses of Ceiba pentandra by Chimpanzees in Comoé National Park
We observed Comoé chimpanzees consuming the fruits, flowers, and young leaves as well as the cambium of Ceiba pentandra. All the observations of fruit, flowers, and leaves consumption were during the period in which they do not peel bark. Bark peeling stopped in December, once Ceiba pentandra trees started flowering, but when they had not yet produced leaves and fruit. By the beginning of the bark-peeling season, the end of June, no more flowers, fruits, and few young leaves of Ceiba pentandra were available. We also found chimpanzee nests in Ceiba pentandra trees four times during the dry season.
Selection of Ceiba pentandra Trees for Bark Peeling
We checked 1834 Ceiba pentandra trees in our 900 km2 study area during the study (October 2014–December 2017). Nine hundred and eight of these had traces of bark peeling and 926 had no signs of peeling.
Trees with recent peeling traces (stages 1 and 2) had significantly smaller diameters above the buttresses than those with only older traces (stages 3 and 4) (W = 35,197, P < 0.001) and untouched trees (W = 114660, P < 0.001) (Table IV).
Effect of Bark Thickness on the Selection of Ceiba pentandra Trees to Peel
We found a significant and strong positive correlation between diameter above the buttresses and bark thickness in 31 Ceiba pentandra trees (r = 0.742, df = 29, P < 0.001) (Fig. 8).
We found that 87% (N = 45) of the previously bark-peeled trees that were revisited by chimpanzees had regrown bark in stage 3 of recovery. We measured 10 samples of this green thinner bark from stage 3 traces and we found that it was a mean of 4.5 ± SD 0.97 mm thick.
Is Bark Peeling of Ceiba pentandra: Sustainable or Damaging?
Of 908 trees with bark-peeling traces over 3 years, only 2 fell down during strong windstorms, while 6 of 926 unpeeled Ceiba pentandra trees fell after storms.
Of the 53 Ceiba pentandra trees with stage 1 traces in 2014 and 2015, 4 had stage 3 traces (completely sealed the wounds with bark) 1 year later and 41 had reached stage 3 traces 2 years later. Twelve trees took 3 years to become stage 3, due to very large wounds, >20 cm wide. Only 3 of these 53 trees were revisited and peeled again by chimpanzees in the next year while 14 were revisited and peeled again 2 years after the initial peeling, once the bark had regrown and covered most of the wound. In all cases of bark-peeled trees, only the spines of the buttresses were peeled, which represented <5% of the surface of bark at that level, leaving the rest of the bark around the tree intact.
When we counted the growth rings in the section we cut from a repeatedly peeled buttress, with completely sealed wounds (Fig. 9), we found that the buttress had been peeled three times in the past 16 years and that ≥8 years had passed since the last peeling. We conducted experimental cuts in four buttresses of four different trees that we checked after 3 years, finding that a visible growth ring forms every dry season on the buttresses of Ceiba pentandra in the conditions at Comoé National Park, Ivory Coast.
We found no significant differences in growth in the 20 peeled and 20 unpeeled trees we monitored between October 2015 and October 2017 (W = 202.5, P = 0.955) (Table V).
Sustainable Exploitation of Ceiba pentandra Trees by Chimpanzees
We found 45 Ceiba pentandra trees that were peeled repeatedly during the study period, 40 of which were peeled twice and 5 were peeled three times, once per year. Of the 40 trees peeled twice, 11 (28%) were peeled 1 year after the first peeling, 19 (48%) after 2 years and 10 (25%) after 3 years. In 39 of the 45 revisits (87%) the chimpanzees peeled over previously peeled stage 3 traces, provoking the progressive deformation of the buttress’ spine, with several parallel protruding deformations of the spine (Fig. 3). Moreover, of the 908 Ceiba pentandra trees we monitored, 712 (78.4 %) had traces of at least two different recovery stages, indicating that they had been peeled twice. Of these, 485 (53.4%) had traces of at least three different stages (Fig. 10).
We recorded three videos of three different adult females and one adult male inspecting the buttresses of four different Ceiba pentandra trees and then looking up toward the canopy and leaving the spot without peeling. We also recorded four adult females and one male in four different videos checking the buttress and the canopy and then starting to peel bark. Moreover, we recorded two sequences of videos of the same group of chimpanzees checking the buttresses of a specific Ceiba pentandra tree and leaving afterwards, but the same group returned exactly 1 year later to the same tree and peeled the bark (Fig. 11, ESM video 3). In all cases, the only part of the tree peeled was the spine of the buttress, which represented on average <5% of the total surface of the bark at that level of the tree such that most of the sap would continue to flow without interruption.