Adult beetles do not grow after emergence, so a male's horn length is fixed by conditions during the larval stage. Larvae feeding well on rich decaying material pupate large; those on poor substrate pupate small, and the difference carries directly into adult weaponry.
The horn does not simply scale in proportion. Horn length increases faster than body length across the size range, an example of positive allometry, so the largest males have horns far out of proportion to those of small males. Experimental work on this species and its relatives has traced the effect to nutrition-sensitive signalling in the developing horn tissue, making the horn unusually responsive to larval condition compared with organs such as the wings or genitalia.
The fights themselves are largely non-lethal. Males use the forked horn to pry and flip rivals off a sap flow, and contests are usually settled by size assessment rather than injury, so a small male commonly retreats rather than engages.