The presence of white bellies on dark-colored mice living on dark lava flows can be attributed to a combination of camouflage and predation dynamics. Typically, the reasoning would be as follows:
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Camouflage: The white belly can help in blending with the lighter-colored surfaces that are found in the environment, such as sandy areas, rocks, or light gravel. This countershading allows the mice to remain less visible to predators from above, as their light undersides blend in with the brightness of the ground when viewed from a higher vantage point.
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Predation: Visual predators are often selective based on contrasting colors; having a lighter belly decreases the chances of detection when viewed against various backgrounds. The dark back helps the mice blend into the darker lava flow, while the white belly aids in remaining hidden against lighter substrates.
In this case, the most accurate option would be: White bellies are an important part of camouflage.