Bili Apes have a very flat face, a wide muzzle, and their brow-ridge runs straight across and overhangs. They seem to turn grey very early in life, but instead of turning grey-black like a gorilla, they turn grey all over. They have brow-ridges running straight across and overhanging; uniform gray fur independently of age and sex, which suggests that graying takes place early in life-opposed to all known gorilla species, where only males gray as they age (graying restricted to their backs).
Bili Ape skulls have the prominent brow ridge and may have a sagittal crest similar to that of a robust great ape, or gorilla, but other morphological measurements are more like those of chimpanzees. However, chimpanzee skulls are 190 to 210 millimeters long, but four of five Bili Ape skulls measured more than 220 millimeters, well beyond the end of the normal chimpanzee range. It should be made clear that only one of the many skulls found at Bili had a sagittal crest, thus it cannot yet be considered typical for the population.
The Bili Ape has been reported to walk upright, bipedally, at times, with the looks of a giant chimpanzee. Later observations by Hicks, however, reveal that they are knuckle-walkers like other chimpanzees that only occasionally walk bipedally. Their footprints, which range from 28 to 34 centimeters, are longer than the largest common chimp and gorilla footprints, which average 26 cm and 29 cm, respectively. Hicks' team has, in a year and a half of study, found no footprints longer than 30 cm, and most have been smaller.
Female Bili Apes, however, have genital swellings similar to other chimpanzees.
In local parlance, the great apes of the Bili Forest fall into two distinct groups. There are the 'tree beaters', who disperse high into the trees to stay safe, and who easily succumb to the poison arrows used by local hunters. Then there are the 'lion killers', who seldom climb trees, are bigger and darker, and who are unaffected by the poison arrows used by locals.
In some ways, the apes behave more like gorillas than chimpanzees. For example, they build ground nests as gorillas do, using interwoven branches and/or saplings, bent down into a central bowl. However, they frequently nest in the trees as well. Often ground nests will be found beneath or in proximity to tree nests. Their diet is also decidedly chimpanzee-like, consisting mainly of fruits (fruiting trees such as Strangler Figs are visited often).
The Bili Apes do not howl at the moon, instead they pant-hoot and tree-drum like other chimpanzees.
Behavior toward humans has baffled and intrigued scientists. There is little to no aggression, yet no fear either. "Gorilla males will always charge when they encounter a hunter, but there were no stories like that," about the Bili Apes, according to Ammann. Instead, they would come face-to-face with their human cousins, stare intently in half-recognition, then slide away quietly. Hicks' group later confirmed and somewhat expanded those observations, saying that when they encountered a large group of Bili Apes in the deep forests (far from the roads and villages), they not only approached the humans, but would actually surround them with intent curiosity. Hicks clarifies the issue as follows: the apes within 20 km or so of the roads flee humans almost without exception. The adult males show the greatest fear. Further from the roads, however, the chimpanzees become progressively 'naive'.