Why deer emphasise their silhouettes against the snow

Gregarious cervids in extremely seasonal climates in the Northern Hemisphere tend to have summer colouration blending into the environment (e.g. https://wildadirondacks.org/images/Adirondack-Mammals-White-tailed-Deer-Odocoileus-virginianus-Cemetery-Road-Wetlands-25-June-2018-71.jpg and https://www.dreamstime.com/capreolus-roe-deers-standing-summer-meadow-sun-grass-early-dew-walking-agricultural-field-wildlife-image106400901). It might be expected that during the snowy winter they would become pale to continue to blend in during a season of particular stress and vulnerability to predation.

Instead, the winter coats of all of at least eight species of deer are partially darkened, making them more, not less, conspicuous than in summer. A ninth species, the caribou (Rangifer tarandus), shows no overall difference in pigmentation between summer and winter and pales in late winter mainly because of the fur wearing down in harsh weather.

All these species, other than the caribou, become more gregarious in winter than in summer. In the case of the white-tailed deer (Odocoileus virginianus) one of the advantages of concentrated trampling is to keep 'deeryards' free of deep snow (e.g. see https://www.maine.gov/ifw/docs/deer_yards.pdf).

It seems that the seasonal benefits of gregariousness outweigh any benefits of hiding; and the gregariousness is aided by a seasonal change in colouration towards conspicuousness. In other words, the deer become relatively showy because this attracts members of their own species, and despite this attracting predators.

I have restricted the following examination as far as possible to females, so as to avoid the complications of sexual dimorphism.

In all of these species except the wapiti (Cervus canadensis), the main tone on the torso remains at least as dark in winter as in summer. At the same time, in all species the fur actually darkens on those parts of the body most effectively contrasting with the pale backgrounds of winter. The darkened surfaces are variously the legs, neck, brisket, belly, or back, according to species. Where the ventral outline is darkened, the overall effect is the opposite of countershading.

In all of the species the ears acquire dark edges in winter (compare https://www.dreamstime.com/single-mule-deer-image-female-hay-field-autumn-image128644595 with https://www.dreamstime.com/close-up-face-mule-deer-doe-big-ears-biting-ticks-huge-group-middle-her-forehead-image205381330). The resulting increase in the conspicuousness of the ears boosts the overall conspicuousness except in the species with proportionately the smallest ears, namely the wapiti.

In Cervus canadensis, the neck remains conspicuously dark in winter. Since the torso actually pales in winter, the overall effect is of boosted dark/pale contrast on the figure as a whole, in winter compared with summer: https://www.durangoherald.com/articles/colorado-wildlife-officials-seek-solution-as-elk-herds-decline/ and https://gazette.com/news/colorado-uses-bait-to-keep-elk-mule-deer-off-highway/article_fde2eda2-6409-53f8-ac58-adc29850e75d.html.

Cervus elaphus is similar to C. canadensis but not as specialised in its colouration: https://www.newsflare.com/video/191268/animals/large-red-deer-herd-surviving-in-scottish-highlands-during-blizzard-conditions-march-2018 and https://www.istockphoto.com/photo/red-deer-herd-in-the-winter-forest-gm1091463382-292823319 and https://www.istockphoto.com/br/foto/tr%C3%AAs-magn%C3%ADficos-veado-rebanho-de-veado-f%C3%AAmea-grande-adulto-veado-vermelho-nobre-em-p%C3%A9-gm918536042-252664001 and https://www.maxpixel.net/Deer-Wildlife-Animal-Wild-Winter-Snow-Nature-5303488 and https://depositphotos.com/2846847/stock-photo-fallow-deer-in-winter-scenario.html.

Cervus nippon varies among subspecies, but is darker in winter than in summer: https://www.alamy.com/stock-photo-deer-in-the-russian-winter-forest-127735202.html and https://www.alamy.com/stock-photo-dybowskis-or-sika-deer-cervus-nippon-two-does-alert-on-snow-covered-28322608.html and https://www.agefotostock.com/age/en/details-photo/sika-deer-cervus-nippon-hinds-and-calves-standing-in-snow-knole-park-kent-england-winter/FHR-10306-00120-168 and https://www.needpix.com/photo/64965/germany-sika-deer-animals-wildlife-forest-trees-woods-winter.

Odocoileus virginianus: https://www.oao-7.top/ProductDetail.aspx?iid=59640286&pr=32.99 https://www.weareiowa.com/article/life/animals/iowa-dnr-winter-weather-impact-on-deer-population/524-e705e24d-0018-4750-bb76-87d9de2a721c and https://3.bp.blogspot.com/-aZ-oyR5qMOI/TXpoR7JjSdI/AAAAAAAAFqc/-i0OYSH1pv8/s1600/winter%2Bwhitetail%2Bdeer%2B%2528courtesy%2BSUNY%2BESF%2529.jpg and https://wpcdn.us-midwest-1.vip.tn-cloud.net/www.outdoornews.com/content/uploads/2020/02/Two-does-in-snow-8973-ps-c-NALE.jpg.

Odocoileus hemionus hemionus: https://www.123rf.com/photo_12141681_mule-deer-herd-in-deep-snow-cordillera-colorado.html and https://wildlife.org/jwm-deer-study-calls-into-question-juniper-removal-projects/ and https://www.shutterstock.com/nb/image-photo/mule-deer-herd-winter-1886553019 and https://www.istockphoto.com/photo/herd-of-mule-deer-in-snow-gm1090539484-292549638 and https://animalia.bio/mule-deer.

Capreolus capreolus and Capreolus pygargus: https://www.pond5.com/stock-footage/item/46733434-deer-roe-foraging-food-snowy-woods-4k-real-time-ultra-hd and https://www.shutterstock.com/nb/image-photo/roe-deer-capreolus-forest-bieszczady-mountains-1257330046 and https://elements.envato.com/wild-roe-deer-herd-in-a-snowfall-PVSV4H5 and https://www.photocase.com/photos/1543785-roe-deer-herd-on-snow-beautiful-life-hunting-winter-snow-photocase-stock-photo and https://www.photocase.com/photos/1511371-roe-deer-herd-over-white-snow-beautiful-hunting-winter-photocase-stock-photo and https://www.wallpaperflare.com/roe-deer-wild-animal-damm-wild-animal-world-winter-snow-wallpaper-zrmcu and https://www.dreamstime.com/roe-deer-snow-winter-whid-walking-forest-sweden-image134672284

Dama dama: https://www.123rf.com/photo_20940100_herd-of-female-fallow-deer-in-white-winter-snow.html and https://www.dreamstime.com/stock-photo-female-fallow-deer-winter-forest-image29582770 and https://www.bigstockphoto.com/image-280207912/stock-photo-fallow-deer-herd-snow-forest-landscape-%28dama-dama%29 and https://www.canstockphoto.com/fallow-deer-female-snow-winter-dama-73046022.html and https://elements.envato.com/fallow-deer-doe-on-snow-in-winter-with-space-for-c-GP7FHUC and https://depositphotos.com/126741458/stock-photo-fallow-deer-in-a-winter.html

In the pronghorn (Antilocapra americana) - which belongs to a different family from deer - the fur becomes long and changes texture in winter. This species also resembles the deer in being more gregarious in winter than in summer (see https://academic.oup.com/jmammal/article/93/4/1129/960158). However, the colouration remains the same in the snowy as in the warm season. Why does the pronghorn not conform to the trend shown by sympatric and ecologically similar deer?

פורסם על-ידי milewski milewski, אוגוסט 27, 2021 12:33 לפנה"צ

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פורסם על-ידי milewski לפני 3 חודשים (סמן)

Thanks @milewski for linking to this amazing set of information on Deer. There is a great amount of info here and I will take pleasure in reading it.

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@ram_k I am glad you find this sort of discussion interesting and I look forward to your comments.

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