Do monkeys check each others' blues?

A male vervet monkey may use the shade of blue on another male's scrotum as a clue to status, according to new observations of monkey encounters.

Sexually varied body colors in birds, fish, lizards, and Amphibians have inspired a lot of analysis, remarks Melissa S. Gerald of the University of California, Los Angeles. Primates also flash some outrageous colors. Yet, except for the bright colors of mandrills, they've attracted little research, she says.

Scrotal color in vervets varies considerably, from cornflower blue to a chilly white.' Gerald linked darker color to higher concentrations of a metabolic product of serotonin, a brain chemical suspected of influencing dominance.

She arranged encounters between males that had not met. In 23 pairs, males with darker scrotums usually behaved as dominant over the pale guys.

The blue cue might tell a monkey whether he has a fighting chance or is hopelessly outclassed, Gerald speculates. When she paired males with similar shades, she found them more likely to fight than males with mismatched scrotal blues.

With spray paint, Gerald upgraded some of the pale males to powder blue. The color-enhanced monkeys did not manage to achieve dominance over true blues, so Gerald speculates that the interaction involves multiple cues. Yet she did see a familiar pattern in aggression. The same-color pairs fought more readily than mismatched pairs--even though one male was just a wimp with misleading make-up.