 This has lead the drills to forage for an increasingly long period and they now forage for a longer time than any other primate on the island.The biomes on Bioko Island are typically categorised into three subtypes; tropical rainforests in the lower regions, montane forest, which is typified by higher humidity, a lower temperature, and is dominated by tree ferns, and mossy forests at highest altitudes.While the Bioko drill will preferentially inhabit lowland forests, tending to larger groups in these areas, populations have been found inhabiting areas up to 2,000m above sea level in grasslands and areas with dense undergrowth.