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I don't know what the horn is used for, if anything. Sinondendron is one of the three members of family Lucanidae, which otherwise includes Stag Beetle and Lesser Stag Beetle. Male Stag Beetles use their enlarged 'stag's-horn' mandibles to battle with other males and establish priority over females, but I don't know if Sinodendron does something similar. I've also seen it claimed that it's a character used by the female to assess the fitness of a male, i.e. the larger the horn the fitter the male, if you see what I mean! But I can't find any published evidence for that either. Sinodendron males are known to 'guard' the entrance hole to the burrow that the female makes in decaying trees, so perhaps it could have a function there too.
Looks like this is something of a mystery though, so if anyone fancies spending some hours watching one of these beetles to see what it does you could discover something new! (but you might have to wait up all night, they're active at night as well as by day).
Entomologist and biological recorder
Lat/Lng: 53.8771, -1.8734
OS grid ref: SE084423