Does the North American porcupine have eye-mask colouration?

In this Post, I define 'eye-mask' (https://en.wikipedia.org/wiki/Disruptive_eye_mask) as a dark feature of animal colouration that disguises the eyes, on a figure and head that are not dark overall.

This topic has recently started to appear in the scientific literature. See:

I note that mammals possessing eye-masks tend to fall into two categories, viz.

It makes sense that the eyes are particularly inconspicuous in animals that have overall colouration designed for hiding from prey, or from larger-bodied predators. However, any functional relationship between eye-masks and warning colouration remains to be explained.

As can be seen from the examples above, eye-masks in mammals occur mainly in predatory species. They are rare in plant-eating species.

However, a possible example is Erethizon dorsatum.

Is this plant-eater one of the few rodents to possess an eye-mask? And, if so, is this related to warning colouration in this 'porcupine'?

As far as I know, these possibilities have not previously been mooted in the literature.

Warning colouration in E. dorsatum is weaker and less consistent (see https://www.inaturalist.org/journal/milewski/65187-similarities-differences-between-the-porcupines-of-different-hemispheres-erethizon-vs-hystrix-part-2#) than those in either African-Eurasian 'porcupines' (Hystrix) or the various genera of skunks (https://en.wikipedia.org/wiki/Skunk). The subtle dark/pale contrasts depend on posture and muscular movements of the skin in E. dorsatum. They are absent in some individuals, partly owing to seasonal and regional variation.

EVIDENCE OF EYE-MASK IN ERETHIZON DORSATUM

The following individuals of E. dorsatum seem to possess an eye-mask:

https://www.inaturalist.org/observations/8175871
https://www.inaturalist.org/observations/11274273
https://www.inaturalist.org/observations/10788818
https://www.inaturalist.org/observations/28411661
https://www.inaturalist.org/observations/37898442
https://www.inaturalist.org/observations/44542308
https://www.inaturalist.org/observations/33855151
https://www.inaturalist.org/observations/45805001
https://www.inaturalist.org/observations/80415227
https://www.inaturalist.org/observations/79231635
https://www.inaturalist.org/observations/78933797
https://www.inaturalist.org/observations/78111758
https://www.inaturalist.org/observations/77306649
https://www.inaturalist.org/observations/76953405
https://www.inaturalist.org/observations/69636688
https://www.inaturalist.org/observations/64210546
https://www.inaturalist.org/observations/27843255
https://www.inaturalist.org/observations/49303934
https://www.inaturalist.org/observations/47206486
https://www.agefotostock.com/age/en/details-photo/adult-male-north-american-porcupine-erethizon-dorsatum-minnesota-usa/YY4-1640900

However, the pattern is not clear-cut because, in some individuals in some seasons/regions,

Juveniles tend to be dark overall, lacking mask-colouration (https://www.istockphoto.com/photo/porcupines-gm180723145-24209220 and https://www.istockphoto.com/photo/baby-porcupine-gm157719814-22029118 and https://www.istockphoto.com/photo/baby-porcupine-gm504918032-83421755 and https://www.masterfile.com/image/en/841-03506138/a-captive-baby-porcupine-erethizon-dorsatum-animals-of and https://www.istockphoto.com/photo/baby-porcupine-gm174802022-22029114).

Some individuals are pale overall, for unknown reasons, e.g. https://www.researchgate.net/figure/An-isabelline-colored-North-American-Porcupine-Erethizon-dorsatum-from-Yukon-Canada_fig1_270904496 and https://www.inaturalist.org/observations/49305543 and https://www.istockphoto.com/photo/the-north-american-porcupine-also-known-as-the-canadian-porcupine-or-common-gm1272502547-374755167.

DISCUSSION

The anti-predator defences of Erethizon dorsatum are centred on its hindquarters, leaving the face relatively unprotected.

The carnivore most adept at killing E. dorsatum is Pekania pennanti (https://www.adfg.alaska.gov/index.cfm?adfg=fisher.printerfriendly#:~:text=Hunting%20Method,to%20descend%20trees%20head%20first). The relatively small body and elongated, weasel-like shape of this mustelid allow it to bypass the posterior spines and reach the front, whether the rodent is on the ground or climbing a tree. The carnivore thus manages to bite the face repeatedly until E. dorsatum is incapacitated.

Given that the face is, as it were, the Achilles' heel of E. dorsatum, and that most attacks occur by night, it may make sense that the rodent would benefit from its eyes being hidden. Furthermore, the inconsistency of the pattern may help to deny would-be predators - other than M. pennanti - a clear search-image in the first place.

A result of this obfuscation is that E. dorsatum is peculiarly non-photogenic even by day.

As any scrolling of the thousands of observations in iNaturalist soon shows (https://www.inaturalist.org/observations?taxon_id=44026), the photographed figure often looks almost faceless (e.g. https://www.alamy.com/close-up-portrait-of-a-porcupine-image439051825.html) unless the illumination is particularly clear.

Posted by milewski milewski, May 10, 2022 15:34

Comments

I like this theory. The porcupine is also known to be prey for the puma, which also attack the nose. I see a benefit to the porcupine in having its eyes obscured by fur coloration in order to protect that most vulnerable part of its face. It makes sense that a mask would obscure the eyes, as in raccoons and other species. It is more subtle in porcupines, but I can see exactly what you mean.

Posted by beartracker 3 months ago (Flag)

@beartracker Many thanks for this helpful comment.

Posted by milewski 3 months ago (Flag)

Add a Comment

Sign In or Sign Up to add comments