The general absence of lateralized behaviour in howler monkeys might be explained by the fact that the ecological constraints to which they are subject to (e.g. arboreal life, type of diet) may actually be most influential in shaping their side preferences.

• Mechanical constraints of the arboreal life: 

Howler monkeys are a quadrupedal arboreal species. The arboreal substrate is discontinuous (frequent gaps between trees; i.e. risk of falling to the ground), tree-dimensional (branches can be oriented horizontally, vertically and obliquely) and heterogeneous (trunks and branches with different dimensions), which has an effect on individuals' balance. Because of that, individuals often need to adopt precarious body postures (e.g. stand on hind legs, hang, etc) that require the use of one or more extremities for additional support. Therefore, individuals may require both sides of the body equally able to face unpredictable spatial challenges.

• Type of diet does not require high cognitive or manipulative abilities:

The mechanical and cognitive complexity of a behaviour has been reported to influence the strength of lateralization. It has been proposed that motor patterns that require a high cognitive and/or manipulative processing, such is the case of novel tasks and tasks that imply complex motor coordination (i.e. “high-level” tasks), elicit stronger lateralization in individuals and populations than those that are highly practised, routinised and/or low-demanding at the cognitive and/or motor levels (i.e. “low-level tasks”), such is the case of simple reaching and pulling. This suggests that the complexity of manipulation of the foods comprising a species' diet might also have an influence on its manual preferences. Howler monkeys mainly eat leaves (56.7%) and fruits (31.4%) that practically need no manipulation prior to consumption; i.e. their diet does not require high cognitive or manipulative abilities, which might party explain the lack of side preferences with food-related motor patterns.

Possible causal agents of the few significant individual preferences in some motor patterns reported in the present study may be the presence of internal and/or external handicaps/predispositions (e.g. variation due to maternal effects, congenital diseases, injuries, etc.) and/or learning by means of experience and imitation, among others.