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Fluorescence is one way to defend against the harmful bond-breaking abilities of UV light.

I suspect in many cases it is simply the biologically cheapest 'sunblock'.

Most mammals have no concern for UV damage, as they're covered in fur.

When young, the fur often hasn't grown yet.

Also, the fur itself may degrade in UV, making it weaker, fading, and losing waterproofing and other properties the animal relies on.

And finally, the cost of fluorescence may be very small - meaning that it may have had some benefit at some point in the past, but there now no selective pressure to get rid of it.

Why is it on the belly then and not on the back? Countershading?

Many non mammals have vision that includes UV (birds, reptiles, insects ... ) if any of these hunt the mammal, fluorescence may be a way of not reflecting light the predator can see. Fluorescence must emit a lower frequency and intensity of light than could have been reflected

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