For nonhuman primates, what exception exists regarding weight-based cage size requirements?

The correct choice highlights a specific exception related to brachiating species. Brachiation is a form of movement that involves swinging from limb to limb, which is characteristic of certain types of primates, such as gibbons and siamangs. For these species, weight-based cage size requirements take into account their unique locomotion needs.

Brachiating species require more vertical space and structural complexity in their enclosures to accommodate their natural behaviors. Standard weight-based calculations might not adequately reflect their requirements for physical exercise and mental stimulation, as their needs transcend simple horizontal space associated with body weight.

In contrast, juvenile species, old age species, and species requiring social grouping do not have specific exemptions tied directly to the physical dynamics of their locomotion patterns in the same way that brachiating species do. Each of these categories may have other considerations in their care, but the specific needs of brachiators to engage in their natural method of movement sets them apart in this context regarding cage size.