Since 2000, the Fundació Mona (Girona, northeast Spain; 41°54'N, 2°49'E) has devoted itself to the rescue, recovery, rehabilitation, resocialisation and sheltering of primates that have been exploited or mistreated. The FM attempts to provide these primates with better living conditions through naturalistic environments and stable social groups in order to help the animals develop behavioral patterns typical of their species, promoting their well-being and welfare.

During the daytime the chimpanzees live in a naturalistic, main outdoor enclosure comprised of a total of 5,640 m². The enclosure is divided into two areas: one is 2,420 m² and the other is 3,220 m², with a total division perimeter of 191 meters. The enclosure is surrounded by a steel fence and a 12 V electrified fence. Inside the enclosure are wooden platforms, towers and structures for climbing, resting and socializing. Two artificial water fountains provide drinking water ad libitum. The ground substrate is natural and characterized by a majority of Mediterranean and shoreline herbaceous vegetation. The vegetation is subject to seasonal changes and to the alteration of its physical environment by the chimpanzees. The following plant species are present: Verbena officinalis, Conyza sp., Plantago lanceolada, Anthemis sp., Rumex obtusifolious, Portulaca oleracea, Verbascum blattaria, Taraxacum officinale and Polycarpon tetraphylum, among others. The primates also have access to artificial termite mounds which are used for environmental enrichment. Occasionally, some native animals rest, feed or take up residence in or near the outdoor enclosure providing the chimpanzees an opportunity to hunt them. The most common species coming into contact with the naturalistic enclosure are: (1) Birds: House sparrow (Passer domesticus), Nightingale (Luscinia megarhynchos), Grey Heron (Ardea cinerea), European robin (Erithacus rubecula), Yellow-legged Gull (Larus michahellis), Black Kite (Milvus migrans), Hoopoe (Upupa epops), Common Blackbird (Turdus merula), European magpie (Pica pica), and Common Kestrel (Falco tinnunculus); (2) Mammals: European hedgehog (Erinaceus europaeus), European rabbit (Oryctolagus cuniculus), wood mouse (Apodemus sylvaticus), Brown rat (Rattus norvegicus), and European Mole (Talpa europaea); and (3) Amphibians and reptiles: grass snake (Natrix natrix), ladder snake (Rhinechis scalaris), Mediterranean tree frog (Hyla meridionalis), Perez’s frog (Rana perezi), Common toad (Bufo bufo), and the Iberian wall lizard (Podarcis hispanica); among others.

At the beginning of this study in January 2008, the sample group consisted of thirteen individuals, separated into two different groups: group A (Male group) and group B (Family group). The group of chimpanzees is formed of nine males and four females, but only 10 of them took part in the activities described here. They range from 4 to 51 years old. Between 2008 and 2011 some animals changed groups and in 2010 one of the subjects (Romie from the Family group) passed away due to natural causes. A new individual (Africa) was also introduced to group B in 2010. Table 1 shows the additional traits of these individuals: sex, age, place of birth and origin. The chimpanzees are fed juices, fresh fruit, fresh vegetables, boiled rice, nuts and other seeds four times per day and water is always available. During a routine day, animals forage and are fed food which is hidden or scattered throughout the naturalistic enclosure.

We conducted this research in accordance with all national and institutional guidelines for the care and management of primates established by FM.

Five cases of prey capture were observed between January 2007 and September 2011. The episodes of predation were observed during our ongoing Hand Laterality Research Project, which began in 2002 [23,24,25] and our on-going Rehabilitation and Resocialisation Project which began in 2006 [26,27]. All five predation episodes presented here were recorded while the researcher was present. This factor does not interfere with the results since all the chimpanzees are acclimated to the presence of the FM researchers. Three of the episodes were video and photographically recorded (episodes ID H1, H3 and H4). Two of the episodes were only photographically recorded (episodes ID H2 and H5). Data were recorded on an ab libitum basis during the predation events. Species of the animal prey, date, and time were recorded. Animals involved in the predation were also described. Regarding the H1 episode (social hunting), independent observers describe and interpret this observation the same way the original observer (MV) did. This event was videotaped, so it would be possible to test observer reliability by having three naïve observers (unaware of the original description and interpretation).

This episode was recorded on 19 January 2007. It started at 17:10. Four chimpanzees took an active part in the predation: two juvenile females, one young male and one juvenile male (Figure 1). A rabbit (Oryctolagus cuniculus) came into the enclosure by chance. It was finally captured by the four chimpanzees after two attempts in which the hunters took on different roles in the action. The first attempt was made by three chimpanzees (one juvenile male and two juvenile females). The male (Bongo) pursued the prey while the females (Sara and Waty) positioned themselves for an ambush to block the rabbit’s escape route and to capture the animal (Figure 1). The second attempt was performed by all four chimpanzees (one young male, one juvenile male and two juvenile females). One of them (Juanito) performed the role of pursuer while the others (Waty, Sara and Bongo) set up an ambush in a semicircular position, blocking and capturing the prey. Once the hunting episode was finished, the other three chimpanzees joined the group: one adult female (Romie), one adult male (Toto) and one young male (Nico). All the individuals gathered around the rabbit, which was taken by its hind legs by the oldest chimpanzee (Toto) and beaten once against the ground with one fatal blow. Immediately, a sequence of exploration and brutal play behaviors started among the five juvenile and young individuals (Figure 2). The prey was not consumed, although some individuals tried to bite it.

This episode was recorded on 24 May 2008 between 15:46 and 15:59. Only one chimpanzee (Nico, male adolescent) took part in the episode. No other group individual was present during the predation episode and the prey manipulation. The capture of the prey was done opportunistically while the prey was on the ground. The prey was a young individual from the Turdus merula species. Nico hunted the prey with his hands and killed it with a blow. For five minutes Nico manipulated the prey in a playful manner. Later, using his mouth, he tore open the prey’s abdomen and ingested part of the guts, ignoring the wings, head and lower extremities (Figure 3). After the episode the prey was left in the same place.

The episode was recorded on 2 August 2008 between 17:22 and 17:38. Three individuals were involved in the hunting episode. An adult Larus michahellis specimen remained in the artificial pool of the enclosure. Charly (Group A, adult male) captured the prey with his hands. We neither recorded the killing strategy nor who was the actual killer. Immediately afterwards Marco (Alpha male) took hold of the already dead prey and carried it to one of the enclosure towers accompanied by Bongo (adolescent male) and by Charly (Figure 4(a)). Charly left the tower after one minute. Bongo and Marco remained together until the end of the episode. From 17:25 to 17:27, Marco examined the prey. During this time, Bongo simply observed without taking part. At 17:28, Bongo struck the head of the prey with his hands. At 17:29 Marco began ripping off one of the lower limbs of the prey (Figure 4(b)). At 17:32 he bit the limb of the prey to pull it apart (Figure 4(c)). Marco ended up abandoning the prey in the tower structure.

The episode was recorded on 29 October 2010 between 14:45 and 14:55. Marco (Group A, adult male) was involved in the episode but we neither detected the predation strategy, the exact moment of predation, nor exactly how he hunted. We photographically recorded the episode from 14:49 (Figure 5(a)). At that moment, the prey already appeared ripped apart with parts of its body missing, presumably ingested (Figure 5(b)). At 14:55, the prey was abandoned on the enclosure substrate.

An episode was registered on 17 September 2011 between 12:20 and 12:30. Juanito (Group B, male adolescent) was the subject most implicated in the episode but we neither detected his predation strategy, the exact moment of predation, nor exactly how he hunted. Two other individuals (Nico, male adolescent) and (Africa, female adolescent) played secondary roles in the episode. At 12:22, we started to photographically record the hunting episode. At 12:23, the prey was placed on the ground and Juanito took hold of a wooden stick to use it as a spear and began to repeatedly stab the rabbit with it (Figure 6). At 12:24 Nico rejoined Juanito. At that moment, Juanito approached the perimeter of the enclosure and threw the rabbit against the electrified fence. At 12:26 he returned transporting the prey in his mouth (Figure 7(a)). The abdomen of the rabbit was totally open with the intestines missing. One of the lower limbs was missing its muscular mass (possibly due to ingestion) and it was possible to see exposed bone. At 12:30 Juanito returned holding the prey and brought it up to his mouth, tasting it but without totally ingesting it (Figure 7(b)). Finally, the prey was discarded on the ground.