Does anyone know a reliable way to distinguish, photographically, deer and white-footed mice?

I’ve been having no luck in finding a reliable way to distinguish deer mice and white-footed mice in photographs. I’ve read about differences in feeding habits, habitat, behavior, fur colour, size – but all of these seem to have significant overlap and aren’t necessarily captured in a photograph. I don’t feel confident making an ID even when I have a mouse to identify in-hand! Does anyone have any good suggestions?

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You may find the comments on this observation useful:
https://inaturalist.ca/observations/141030992

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Where their ranges overlap, there is no way to distinguish them with accuracy from a photograph. In such cases, it’s best to ID to the genus level only.

In hand, a function of ear length, body weight, and tail length has been shown to be the most reliable method (see here; potentially not accurate in regions with different subspecies). That paper discusses some of the issues with relying on things like pelage color or hindfoot measurements, as suggested by other sources.

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Those two species are tough. In the Southwest US we have used hind foot length as a characteristic — 22 mm or greater = leucopus, less than 22 = maniculatus. Also maniculatus has a more sharply defined bicolor tail and generally a richer fur color. But that may not work elsewhere. There are some habitat differences also but that’s not highly reliable.

Incidentally our Southwest manics are not even considered that species any more, so where you are can make a difference.

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Well timed for my own obs of one or the other. (At first I thought white footed, and now I lean deer, but either way I bumped back to genus because I now know enough to know I don’t know :/)

Is tail to body length reliable at all? That seems the most promising based off what I’ve seen so far.

As a side note, the darn things are so cute, but getting way too bold at my site. The guy sat on the toe of my boot and nibbled the laces today…that’s a new one.

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I remember talking to someone once about using hair colour and contrast on various parts of the body (tail, sides, chin, I think?) but it seemed like the teeth or something were the most reliable feature.