Red coloration and the evolution of aposematism in arboreal sciurids

An animal's coloration is associated with a variety of processes and
is therefore subjected to multiple selective pressures. Mammals,
especially, are typically inconspicuously colored, or cryptic, to avoid
detection by predators. Alternatively, an animal may use conspicuous
coloration to advertise the presence of an anti-predator defense. The
association between signal and defense is called aposematism. Conspicuous
black and white coloration has recently been associated with a range of
defenses in mammals, including body size (Howell et al. 2021), however red
coloration as a potentially aposematic signal has yet to be investigated
in mammals. Squirrels, like most mammals, are unable to perceive red for
use as a social signal. Here we use a comparative framework to test
whether redness could be a means of background matching, serve a
thermoregulatory function or be an honest warning of anti-predator
defenses across a global distribution of tree squirrels which vary in size
from 16g to 2.2kg in this study. We measured redness of the dorsum, the
venter and of red accents of study skin specimens of 57 tree squirrel
species (N=257) representing 25 genera. We then associated these
phenotypic variables with environmental variables using phylogenetic
generalized least squares regression.  We find that increasing
dorsal redness is associated with more humid environments and closed
canopies, consistent with prior work that coloration on this body region
under selection for crypsis (Sheets and Chavez 2020). However, we find
that ventral redness and maximum redness is associated with large body
sizes. Our findings suggest that crypsis and aposematism are not mutually
exclusive, and that aposematism may be more widespread in mammals than is
currently appreciated.