the matriarch. African forest elephant groups are less cohesive than African bush elephant groups, probably because of the lack of predators. When separate family units bond, they form kinship or bond groups. After puberty, male elephants tend to form close alliances with other males. While females are the most active members of African elephant groups, both male and female elephants are capable of distinguishing between hundreds of different low frequency infrasonic calls to communicate with and identify each other. Elephants use some vocalisations that are beyond the hearing range of humans, to communicate across large distances. Elephant mating rituals include the gentle entwining of trunks. The bulls were believed to be solitary animals, becoming independent once reaching maturity. New research suggests that bulls maintain ecological knowledge for the herd, facilitating survival when searching for food and water, which also benefits the young bulls who associate with them. Bulls only return to the herd to breed or to socialize; they do not provide prenatal care to their offspring, but rather play a fatherly role to younger bulls to show dominance. Feeding While feeding, the African elephant uses its trunk to pluck leaves and its tusk to tear at branches, which can cause enormous damage to foliage. Fermentation of the food takes place in the hindgut, thus enabling large food intakes. The large size and hindgut of the African elephant also allows for digestion of various plant parts, including fibrous stems, bark and roots. Intelligence African elephants are highly intelligent. They have a very large and highly convoluted neocortex, a trait they share with humans, apes and some dolphin species. They are amongst the world's most intelligent species. With a mass of just over , the elephant brain is larger than that of any other terrestrial animal. The elephant's brain is similar to a human brain in terms of structure and complexity; the elephant's cortex has as many neurons as that of a human brain, suggesting convergent evolution. Elephants exhibit a wide variety of behaviours, including those associated with grief, learning, mimicry, art, play, a sense of humor, altruism, use of tools, compassion, cooperation, self-awareness, memory and possibly language. All of these behaviors point to a highly intelligent species that is thought to be equal with cetaceans and primates. Reproduction African elephants are at their most fertile between the ages of 25 and 45. Calves are born after a gestation period of