Observations
At 9:40 A.M. on September 2, 2022, we and an assistant began following NK. NK and VO were traveling as members of a relatively large party that included the alpha male of the M group, Teddy (TD). At 11:45 A.M., NK groomed VO slightly and left out of VO’s sight. At 12:20 A.M., NK slightly groomed VO and they traveled together.
At 12:24 P.M., NK and VO were resting on the ground near a tall tree of P. microcarpa near the tourist camp. At 12:37 P.M., when an adult male, Ichiro (IH) initiated a charging display (DSP) on the ground and rushed toward NK and VO, they immediately ran up to the tall tree of P. microcarpa. The assistant observed that VO fell from the tip of a tree branch, approximately 7 m above the ground. However, the moment at which VO’s crashed or landed could not be confirmed. At 12:41 P.M., we found VO lying on the ground on his back with both the eyes closed and the mouth half open, gulping and having hiccup-like convulsions every few seconds. Although the blood was noted on the orbits of the eyes, upper eyebrow ridges, cheeks, and gums, which appeared to have been drained from both nasal passages, no trauma to the head or other body parts was observed (Fig. 1). Both hands were holding the vines on the ground.
<Fig. 1>
The discovery point of VO was approximately 2 m away from the crash site in a straight line, with several fresh blood-stained fallen leaves and other debris scattered between them. When we began observing VO, an adult male, Orion (OR) stood quadrupedally approximately 3 m from VO, holding a piece of dead leaf with fresh blood on it in his right hand; IH was sitting facing VO, and another adult male, Carter (CT), was also standing quadrupedally watching VO. At 12:42 P.M., OR approached VO’s left hand, neck, and face, and sniffed and licked blood from VO’s face. Subsequently, IH and CT smelled VO’s left hand, stomach, neck, etc. CT picked up a dead leaf of Saba comorensis with blood found near the crash site, smelled it, and immediately abandoned it. OR sat in close proximity to VO, holding the blood-covered dead leaf in the right hand. At 12:44 P.M., when a part of VO’s face made slight contact with IH’s face due to convulsions, IH moved away slightly, CT and OR peered into and smelled VO’s face, and OR slightly licked the blood on it (Fig. 2). Several chimpanzees were distributed in the trees and on the ground around the 10-m perimeter of VO, but did not vocalize at all. Young males Figaro (FG) and Omaly (OL), and NK were sitting on vines and branches approximately 5 m from VO, looking down on him.
<Fig. 2>
At 12:45 P.M., pant-grunts and screams by several individuals were heard approximately 50 m to the south. CT swatted away a fly flying near VO using his right hand. VO removed his left hand from the vine. CT stood quadrupedally, looked at the VO’s face, smelled it, and licked him. VO remained on his back, raised his head slightly off the ground, and grabbed CT’s hair around the neck with his left hand.
At 12:46 P.M., CT walked away and a juvenile male Oz (OZ) approached in his place, peered into VO, sniffed him, and IH, who was sitting in close proximity to VO, reached out with his left hand and poked OZ’s head; OZ screamed and ran away. IH and OR were sitting in close proximity to VO, but they walked away one after another toward the alpha male of the M group, TD, who was approaching from 20 m south of VO. OR moved holding a bloody, dead leaf. OR and CT emitted a panted grunt against TD. VO remained lying on his back in the same place, when an adult female, Christina (XT), approached from the north, stood quadrupedally, peered into VO’s face, and sat in close proximity to him. TD and an adult male, Emory (EM), approached VO simultaneously, standing quadrupedally and peering into VO’s face and belly. TD bristled his fur. Their surroundings were quiet. OR licked VO’s face.
At 12:47 P.M., IH licked VO’s belly and face. Four adult males (TD, EM, OR, and IH) and one adult female (XT) sat close to VO. When TD peered into VO’s face, VO blinked his eyes, whimpered, showed his grimace, and reached out with his left hand to touch TD. OR and TD peered into VO’s face simultaneously and OR licked VO’s face. EM sniffed VO’s hair. An adult male, Darwin (DW), approached from the south, with bristled hairs, and chimpanzees crowded around VO. OR and EM broke off just before DW approached, and XT emitted a panted grunt and walked away.
At 12:48 P.M., DW and TD simultaneously initiated DSP. IH silently left the site. TD shook a vine and charged it bipedally. DW stood bipedally around VO and shook the vine with TD. VO sat up on the ground on his own as TD finished his display. VO walked slightly but stumbled and fell on his back. TD touched VO’s back and peered into his hips. Simultaneously, IH and OR approached and licked VO’s face. VO sat down and reached out with his left hand to touch TD’s mouth. TD bit the finger of VO’s left hand. The sitting position caused blood to flow out of both the nostrils of VO.
At 12:50 P.M., an adult male and the son of XT, Christmas (XM), approached and lifted VO’s chin slightly with his right hand to inspect the face and oral cavity. VO had been gulping. At 12:51 P.M., TD groomed the back of VO. AZ and NK were sitting in a tree 3 m above the ground, and OM and EM were sitting 5 m above the ground, looking down at VO and others. At 12:54 P.M., TD groomed the head of VO, and XM searched for a dead leaf with blood on it (Fig. 3). At 12:57 P.M., an estrous adult female, Effie (EF), sniffed and left VO.
<Fig. 3>
At 12:59 P.M., TD stopped grooming VO and laid down. DW and EM were seated 3 m away from VO. At 13:00, XM groomed TD. At 13:02, NK moved to the ground, approached VO, peered into VO, and explored the blood on the ground. VO was lying on his back with the mouth half open. At 13:03, NK sat 5 m away from VO. XM touched VO’s left hand. VO laid on his back with his left foot touching the forehead of XT.
At 13:07, TD, who was sitting in close proximity to VO, suddenly started to DSP silently with his fur bristling, grabbed VO’s right leg with his left hand and dragged VO slightly (Fig. 4), shook a branch of the palm tree with his right hand, and let go of VO. The surrounding individuals escaped and VO walked on his own, grimacing and wobbling for approximately 2 m. The surroundings became calm within a few seconds. VO sat down, and OR approached and licked VO’s face. At 13:10, when VO opened his mouth wide, OR placed his right index finger in VO’s mouth. VO was gagging terribly and looked sick.
<Fig. 4>
At 13:17, TD, who was lying approximately 2 m away from VO, suddenly started DSP, grabbed VO’s left arm, and dragged him for 1 m. TD sat on the ground approximately 3 m away from VO. Approximately 5 m away from VO, DW started DSP, slapping the back of NK, and NK screamed. VO approached TD, wobbled, emitted a pant-grunt, and sat in contact with TD. At 13:18, OR began grooming VO and continued until 13:37. At 13:38, VO bled and vomited blood from the nose and mouth. OR groomed VO from 13:42 to 13:47 and from 13:59 to 14:05. TD groomed VO from 13:44 to 13:49. At 14:05, as XM approached VO, TD charged on XM. VO emitted a panted grunt toward TD, who sat nearby, and approached and extended his hand to TD. TD gently bit the index finger of VO.
At 14:10, VO vomited blood out of his mouth and OR watched him. At 14:11, OR and AZ started DSP, and TD started DSP and charged toward OR and VO. TD stood bipedally and shook the bushes around VO. VO emitted a pant-grunt toward TD, but TD ran away without stopping. After DSP by the alpha male and others, many members of the party simultaneously walked off toward northeast. VO had a lot of saliva dripping from his mouth, was wobbling, walked quadrupedally, moved 5 m, and sat down. At 14:12, OR looked at the face and mouth of VO. OR sat 5 m away from VO. The voices of the group shifted away from the northeast. Subsequently, VO repeatedly approached OR on foot. At 14:43, OR, who was sitting 5 m away from VO, repeatedly shook a vine. When VO staggered closer to OR, OR groomed VO. Thereafter, OR continued to sit alone, 5–10 m away from VO, and when VO approached, OR repeatedly groomed VO for a short time.
At 15:34, OR, who was sitting 10 m away from VO, walked away toward northeast, and no other chimpanzees were observed around VO. At 16:02, 16:14, and 17:28, VO vomited his stomach contents and became limp, leaned against a tree, laid down, and occasionally gasped (Fig. 5). At 17:35, we finished observing VO. The chimpanzee party had already moved several hundred meters away and VO was left alone. At the last location, VO leaned approximately 50 m away from his crash site.
<Fig. 5>
The day after the accident, September 3, 2022, at 12:35, we began following this party. However, VO and NK were not spotted within this party. We conducted field research of the M group until September 5. On all three days, we encountered parties of approximately the same size as on the day of the accident, and although almost all adult males were found in the party, VO and NK were not spotted within the parties.
M.S. left Mahale on September 6. The local assistants did not encounter VO in the group for several weeks. On October 17, 2022 (45 days after the accident), an assistant reported that VO was found in a party with NK and that VO appeared healthy.
Social context and behavior of chimpanzees to VO
The social context around VO after the accident was divided into four periods (Fig. 6). Period 1: from immediately after the accident to the time when the members, including the alpha male TD, arrived (12:46); Period 2: from the time when TD arrived to the time when most of the members including TD left (14:11); Period 3: from the time when TD left to the time when OR, who remained near VO, left (15:34); and Period 4: from the time when OR left and VO was left alone to the end of the observation period (17:35). The mean (±SD) number of individuals in close proximity during Periods 1, 2, 3, and 4 were 2.0±1.1, 2.5±1.3, 0.2±0.4, and 0.0±0.0, respectively. Periods 1 and 2 had more individuals in close proximity to VO than Periods 3 and 4 (Kruskal–Wallis test: χ2=242.62, df=3, p<0.001, η2=0.825). The maximum number (six animals) was reached immediately after the arrival of TD.
<Fig. 6>
Of the 26 individuals, except for VO, observed on the day of the accident, 14 were in close proximity to VO at least once between the occurrence of the accident and end of the observation period (Table 1). According to sex and age, the number of individuals that stayed in close proximity to VO and their mean (±SD) duration of proximity were 8 of 9 adult males (21.9 ± 26.5 min), 1 of 3 young males (0.33 ± 0.47 min), 2 of 3 child males (0.67 ± 0.47 min), 3 of 8 adult females (4.5 ± 10.8 min), and 0 of 1 juvenile female (except for 3 infants that were constantly with their mothers). When comparing adults alone, more males were in close proximity than females (Fisher’s exact test: p=0.0498). Significant differences in the duration of proximity were found among the sex and age categories, with adult males staying in close proximity to VO for longer periods of time than individuals in the other categories (Kruskal–Wallis test: χ2=10.337, df=4, p=0.035, η2=0.449; Table 2).
<Table 1>
Five adult males and one adult female were in close proximity to VO for more than 10 min (Table 2). The common behaviors found in these six individuals during proximity to VO were staying nearby/visitation and peeking. Sniffing, licking, and searching for objects with blood were clearly concentrated at the beginning of Periods 1 and 2, whereas grooming and staying nearby/visitation dominated the rest of the periods. Aggressive behaviors such as rough treatment and display behavior were observed only in TD and DW, whereas affiliative behaviors such as licking and biting softly were observed only in TD (Table 2).
<Table 2>
Symptoms of VO and cause of fall accident
We confirmed that VO avoided DSP displayed by IH on the ground at 12:37 P.M. by running up a tall tree of P. microcarpa, and fell shortly thereafter.
When VO occasionally opened his mouth, we observed dentition and no dental damage. In addition, although blood was on VO's face at the beginning of the observation of the accident (Fig. 1), bleeding was only observed from both nostrils. No trauma, including abrasion, was observed throughout the entire body of VO. We observed bloody vomiting from the oral cavity at 13:38 and 14:10. Subsequently, at 16:02, 16:14, and 17:28, VO vomited stomach contents that did not contain blood (Fig. 5). This suggests that the vomiting was not due to an internal stomach ulcer and that the bloody vomiting was either bleeding from the interior of the nasal cavity or the blood that had temporarily accumulated in the oral cavity.
Immediately after the accident, the spilled blood pooled near VO’s eye sockets, but he was blinking. While VO was lying on his side, both hands grasped nearby vines. When CT approached at 12:45, VO grabbed his hair near the face with left hand and pulled him. VO showed grimaces and emitted pant-grunts toward adult males. At 12:48, VO walked quadrupedally and continued to act voluntarily, albeit unsteadily and intermittently. This suggests that VO's consciousness was restored at least a few minutes after the accident, although VO’s loss of consciousness immediately after the accident can be determined only by external observation.
VO was not observed for some time after the day of the accident but continued to travel with NK after he was reconfirmed in the group, and no physical impairment was observed. This suggests that all the symptoms of VO caused by the accident were temporary.
Combining all the observational information, we inferred the following: VO accidentally fell while running up a branch of a tall tree of P. microcarpa to avoid DSP exhibited by IH. There were no shrubs, vines, or other vegetation between the branch and the ground that could soften the impact of the crash, and thus, we assume that VO landed directly on the ground. He may have landed quadrupedally, but the acceleration of his landing caused his face to strike the surface of a log, fracturing the nasal bone in the nasal cavity and simultaneously causing a strong concussion. From immediately after the accident to the end of the observation period, we observed convulsions or fidgeting every few seconds for several tens of seconds. In addition, throughout the observation period, VO walked unsteadily, and could not maintain a sitting posture on the ground easily. These symptoms are consistent with the temporary symptoms caused by a strong concussion.