Fingerprint ridges allow primates to regulate grip

Fingerprints are unique to primates and koalas but what advantages
do these features of our hands and feet provide us compared
with the smooth pads of carnivorans, e.g., feline or ursine
species? It has been argued that the epidermal ridges on finger
pads decrease friction when in contact with smooth surfaces,
promote interlocking with rough surfaces, channel excess water,
prevent blistering, and enhance tactile sensitivity. Here, we found
that they were at the origin of a moisture-regulating mechanism,
which ensures an optimal hydration of the keratin layer of the
skin for maximizing the friction and reducing the probability of
catastrophic slip due to the hydrodynamic formation of a fluid
layer. When in contact with impermeable surfaces, the occlusion
of the sweat from the pores in the ridges promotes plasticization
of the skin, dramatically increasing friction. Occlusion and
external moisture could cause an excess of water that would
defeat the natural hydration balance. However, we have demonstrated
using femtosecond laser-based polarization-tunable terahertz
wave spectroscopic imaging and infrared optical coherence
tomography that the moisture regulation may be explained by
a combination of a microfluidic capillary evaporation mechanism
and a sweat pore blocking mechanism. This results in maintaining
an optimal amount of moisture in the furrows that maximizes
the friction irrespective of whether a finger pad is initially wet
or dry. Thus, abundant low-flow sweat glands and epidermal furrows
have provided primates with the evolutionary advantage in
dry and wet conditions of manipulative and locomotive abilities
not available to other animals.