In Table 3, significant differences are apparent in all variables between the left and right Femur, Tibia and Fibula, with the right side showing greater development in all cases. Additionally, LT exhibits a higher standard deviation compared to the other variables, followed by LBT, LE, LF, LM1, LM2 and LD, in that order (Table 3).
In Fig 3, it can be observed that LT and LBT are the variables that present the highest coefficients of variation and both are related to the tibia, given that the biceps femoris, semitendinosus, vastus lateralis, gastrocnemius and popliteus muscles have their origin or insertion in spaces determined by these variables
(Polly, 2007) and their joint functioning determines the close relationship that exists between them. On the other hand, the variables LE, LF, LM2, LM1 and LD, are associated with the femur and the fibula, presenting a certain relationship between them, based on their variations in size and function
(Ankel-Simons, 2007), since the adaptation of the axis (Fig 1) allows bending in multiple directions in order to reduce the harmful consequences of a new mechanics and this determines the locomotor behaviors of primates and the positional behaviors
(Carlson, 2005).
Likewise, the muscles that are inserted in the proximal end of the femur allow to extension of the limb by turning it around the head
(Polly, 2007), in such a way that the muscles that are related to the body of the femur and the axis, indistinctly determine the conditions of the variables. The diaphysis, for its part, functions as an engineering beam, providing limb stiffness, particularly against bending and torsional loads
(Argot, 2002), allowing the femur to resist bending loads in the caudolateral/craniomedial direction and allowing the bone to act as an energy absorber when running or jumping in a pronograde position. This makes the geometric construction of the femur ideally suited to resist high bending loads below the femoral diaphysis, particularly in the mediolateral plane
(Sargis, 2002).
The LBT and LF variables, along with their standard deviations, should be highlighted as they meet the two previous conditions between the tibia and fibula. The length of the tibia depends on the activity carried out by the muscles, with only a small percentage of this load being supported by the fibula
(Funk et al., 2004). This explains why the fibula’s length and diameter are always smaller. As the longest and widest bone in the leg region, the tibia is responsible for weight bearing in primates such as
Plecturocebus caquetensis. It is connected by two slightly mobile joints (synovial joints) at the proximal and distal ends
(Ankel-Simons, 2007). In arboreal primates where jumping is part of locomotion, the proximal end of the tibial shaft is more mediolaterally compressed
(Fleagle and Meldrum, 1988). The insertion of the quadriceps femoris muscle determines speed and agility by allowing rapid extension of the knee (
Salton and Sargis, 2009;
Ruff and Runestad, 1992;
Szalay and Sargis, 2001).
The length of the fibula affects the agility and range of movement of the lower limb required for quadrupedal arboreal locomotion
(Marchi, 2007). It is mobile due to the characteristics of the proximal and distal tibiofibular joints, which have adapted to perform rapid and sudden movements (
Barnett and Napier, 1953;
Salton and Sargis, 2009). However, the fibula’s low robustness in comparison to the tibia is because of its position and its relatively minor role in bearing mechanical loads
(Marchi, 2007).
The other variables obtained from the femur, tibia and fibula of
P.
caquetensis, with coefficients of variation less than one, do not show significant differences between them. This is because there is a total synergy between structures and joints for the transmission of force
(Marchi, 2007) associated with all these variables. For example, the variable LC reflects a mechanical adaptation to resist flexion forces during jumping
(Ford, 1990) and the variables LTM, LTMa, Lca/T, ATM and ATMa are related to the vastus lateralis and quadriceps femoris muscles, which are also linked to knee extension associated with jumping
(Salton and Sargis, 2009).
It is important to note that the differences between the right and left bone structures in all cases determine specific aspects of movement asymmetry. While there are no recognized references to this in primates, it is indicated in humans that high impacts and constant bone tension can lead to size gain
(Nikander et al., 2010). Given that
Plecturocebus caquetensis is a primate that jumps and moves on small tree branches, its posture may reflect this. However, detailed studies in its natural habitat, similar to those carried out by
Cubi and Llorente (2021) or in the past by
Colell (1992) focusing on the hind limbs, are necessary to identify the reason for this preferred posture.