Vertical clinging and leaping

The variations in vertical leaping can be classified into three types based on differences in mid-flight posture:[1]

There is an array of clinging postures that primates use while foraging and resting in trees. These postures include sitting, bipedal standing, squatting, forelimb suspension, forelimb-hindlimb suspension and more. Stabilization is key to how a primate positions themselves while clinging. The placement of the torso depends on the weight distribution between the hands, feet, and tail.[1] Vertical clinging and leaping primates may also use their tails to grip adjacent branches in order to stabilize their positional modes.

Launch angles vary depending on height and range needs

Before leaping, primates must fundamentally consider the distance they are traveling and the stability in their launch point and landing point. A large branch is preferable to a small branch as a launch point, as the wider diameter provides a sturdier base for the jump and also reduces the energy expended during takeoff. In fact, tarsiers have been observed climb to lower heights in order to leap from and land on larger diameter branches. However, this also limits the height of the leap, as the primate must be able to land on a branch that is large enough to stabilize the animal and also minimize the energy necessary for maintaining balance during landing.[11]

Launch angle does not appear to be species-specific, and the range of angles varies between 30 and 70 degrees. The optimum ballistic trajectory angle would be 45 degrees,[11] but often the animal will need to cover more horizontal than vertical distance, or vice versa. In these cases, a trajectory other than 45 degrees would be more beneficial to give more height or more range (see photo), so vertical clinging and leaping animals adjust their launch angles accordingly to compensate for this.[12]

Ida, an early Eocene primate fossil who provided the first evidence of vertical clinging and leaping

There is evidence of vertical clinging and leaping adaptations in the postcranial skeleton of early Eocene fossils of prosimians. Lacking any common quadrupedal features, the postcranial bones and fossils of the Eocene prosimians most closely resemble the specializations of vertical clingers and leapers. Common morphological elements identified in these early fossils include a high phalangeal index, low intermembral index, femurs with cylindrical head and high, narrow patellar grooves, and fused fibula and tibia.[4] The high phalangeal index is the elongation of the phalangeal finger bones, which helps vertical clingers who require increased prehensile power in their hands to support their large bulk. The low intermembral index is the total length of the humerus and radius in percentage of total length of the femur and tibia. The features of the femur is part of an evolution of elongated femurs that helps the primates' extreme range of flexion and extension, which allows them to perform their leaps. Thus, VCL seems to be the only known locomotor adaptation in the Eocene primates, meaning it is one of, if not the earliest, locomotor adaptations.[5][2]

The earliest evidence of VCL can be found in an early Eocene adolescent primate, named Ida. She exhibited legs that were longer than her arms, suggesting that leaping was a key aspect of her locomotion. Her death is hypothesized to be linked to her vicinity to the Messel Pit, near Frankfurt, Germany. The hot magma from the earth mixing with the underground water tables led to explosions that released toxic gasses. Ida was found with a broken wrist, and it is believed that due to this, she could not leap or cling to the higher tree branches, and therefore had to stay lower to the ground where she came into contact with the toxic gasses and died.[6]