Adult male giraffes adopt two different lifestyles and mating strategies. Instead of being a reflection of age, as once thought, something more complex and intriguing is going on, revealed by the shade of their fur.

Dark-furred male giraffes roam the plains on their own or in small “bachelor herds”, while those with lighter-colored coats stay with the larger herds of females and young. Biologists assumed light coloring indicated a young giraffe not yet ready to strike out on his own.

However, Dr Madelaine Castles of the University of Queensland has found this is not always the case. Using 12 years of observations of giraffes from Etosha National Park, Namibia, she reports in Animal Behavior that “Most, but not all, male giraffes darken with age,” and “Only a small proportion of old male giraffes become very dark.” Even more surprisingly, some actually become lighter with time.

Fur color is a sign of dominance, Dr Castles told IFLScience. Male giraffes will fight over access to mates, but a light-colored male will back away from conflict with a darker one. Castles added other factors, such as height, may also influence decision-making, but color is definitely important.

“We think that darker, more dominant male giraffes use an often-successful but risky mating tactic, roaming between groups of giraffes looking for sexually receptive females,” Castles said in a statement. “In contrast, the lighter, less dominant males may be making the best of a bad situation... remaining with females in the hope of getting lucky when a dominant male is not around.”

Researchers studying the way monkeys or finches signal dominance have conducted experiments dyeing their subjects and watching how other members of the species change their behavior. The impracticality of trying something similar with the world's tallest creature, however, leaves many questions unanswered.

One of these questions is the cause of the color changes, with testosterone, diet and even heat stress possible factors. In particular, there is the question of what advantage a previously dark individual gets out of lightening its fur, thereby signaling to others they don't need to fear it. How darker males find females at suitable times is also unknown. Dr Castles noted giraffes use pheromones and ultrasound, but neither is well understood.

“We haven't been able to follow individuals,” Castles told IFLScience. Consequently the team can't be sure whether a giraffe that has lost color rejoins a herd. “It might potentially be the case,” she added. “They need to use a different tactic,” once they are no longer dark enough to fight off rivals. Staying with a herd may provide protection against predators as well as prospects for opportunistic sex.

Although parallels can be found in birds and invertebrates, Castles noted few examples have been reported of mammals using coloring to signify dominance, adding to the giraffe intrigue.