Why Do Males Gather to Show Off in One Spot? The Females Come There

At certain sites males gather in a cluster of small adjacent territories, each defended against neighbours, and display there through a season, day after day, in the same place year after year. The territories contain nothing a female needs, no food, no nest site and no shelter, so she is not choosing a place at all. She visits, watches, mates with one male and leaves, and she receives no help from him afterwards, since he stays to keep displaying. That separation between mating and everything else is the defining feature and is what makes the behaviour so unusual.

The behaviour is scattered across unrelated groups, which suggests it evolves repeatedly:

• •Several grouse species, whose displays are the classic examples
• •Birds of paradise and manakins, with elaborate acrobatic routines
• •Ruffs, where males come in distinct permanent types
• •Hammer-headed bats and a handful of other mammals
• •Some antelope, notably the kob and the topi
• •Various insects, frogs and fish that gather and call or display

The striking result of the arrangement is how concentrated the mating is, with observation at grouse sites repeatedly finding that a small number of males, sometimes one or two, achieve the large majority of matings while most present achieve none at all. That skew is far greater than in most mating systems and raises an obvious question about why the unsuccessful males bother to attend, spending the season displaying, losing weight and exposing themselves to predators for nothing. Suggested answers include that a male can only succeed by being present and improving with age, that attending lets him learn and establish position for later seasons, and that a large gathering attracts more females so that a poor share of many is better than all of nothing.

The same patch of ground is used for decades, and in some documented cases for well over a century, which is longer than any individual lives and raises the question of how the location is transmitted. Young males appear to learn it by attending as spectators, occupying peripheral positions for several seasons before competing, so the knowledge passes between generations by attendance rather than by inheritance. That has an unfortunate consequence for conservation, since a population reduced below the point where a gathering still forms loses the tradition entirely and does not restart it even if numbers recover. Several grouse populations have been lost that way, and restoring them has required moving birds from an active site.

Since the site offers no resources, the female is assessing the male alone, and what she gains is not obvious, which has occupied researchers for decades. One explanation is that the vigour of a display is hard to fake and reveals genuine condition, since a sick or poorly fed animal cannot sustain the effort, so choosing the best performer means choosing good genes for the offspring. Another is that females copy each other, since a female is more likely to choose a male she has seen chosen, which would amplify small initial differences into the extreme skew observed and has been demonstrated experimentally. A third is that gathering simply makes comparison cheap, letting her assess many males in one visit rather than searching.

Males cluster at a traditional site holding nothing a female needs, display there through a season, and provide no help afterwards, so mating is separated from every other part of life. A handful of males take most of the matings and the majority get none. The behaviour has evolved independently in grouse, manakins, bats, antelope, frogs and insects, which suggests the conditions producing it recur.