If you were an alien in a spaceship orbiting Earth, and via a peculiarity of your alien species’ information technology, the only human document you could intercept and read was the Manual on Uniform Control Devices for Streets and Highways, 2009 Edition, and you used the page about non-vehicular warning signs to get a sense of the sorts of lifeforms that populate the planet, I think you’d get a really skewed view of things.
Even if you can comprehend the idea of a silhouette, the disparity in the way animals and humans are represented is too great, and while we, as humans, understand this implicity, the aliens may not, thinking Earth is full of complex zoological life that’s intermixed with a society of robots that use anti-gravity tech to support their perfectly spherical heads.
I guess there’s no way to stylize these animals effectively and have them retain their distinctive looks, like we have with people? Also, it kind of makes me wonder why we need to, say, differentiate between a moose and a bighorn sheep or donkey or bear on these signs; wouldn’t a generic beast icon that just means “big animals here” do the job as well? We don’t differentiate the types of people that may be crossing a street (there’s no PhDs or baking hobbyists or part-time models crossing signs, after all) so why do we do that with animals?
You’re not likely to be on a different kind of alert for a sheep than a cow, are you? I mean, maybe? Horses are certainly faster, and bears are scarier, so perhaps there are reasons? And elk sure are pointy.
Also, I bet the aliens would be really baffled by the handicapped person icon. Why does that robot have such a big ass, they’re probably wondering.