Cooperation Under Sexual Selection: Age-Graded Changes in a Lekking Bird

Abstract

Long-tailed manakins, Chiroxiphia linearis, are birds with a lek mating system and male-male cooperation in courtship display. I studied male-male networks in a color-banded population in Monteverde, Costa Rica, from 1981 to 1987. Males displayed in scattered leks (75-300 m apart) comprising 3-15 males. Within each lek, an alpha and beta male, with strict relative dominance ranks, performed most of the courtship display. Of 50-60 active males per season, only 6-8 males were well-established alpha males in leks with consistent levels of dual-male displays (calls and dances). Demographic data suggest that males may be 8 yr of age or more before attaining beta status. Alpha tenure can last from two to at least four years. Alpha males were rarely or never seen in perch zones other than their primary perch zone (their area of dominance). Lower-ranking males maintained simultaneous affiliations with males in as many as six different zones. Each zone, therefore, acted as a hub in which males with different affiliations around the rim came into contact. Each of the six major perch zones shared at least one affiliate with each of the other zones, and roughly half the males in any particular zone were also known affiliates in one or more other zones. Marked changes occurred in male traits with increasing age and status: (1) significant declines in weight throughout the life span, without loss of dominance status; (2) a 4-yr delay in plumage maturation with distinct subdefinitive, transitional stages; (3) reduction in the number of perch zones with which older males maintained affiliations; and (4) increasing probability of copulatory success. The results are consistent with the hypothesis that subdefinitive plumages in this species serve primarily as accurate indicators of age, which in turn largely determines status, and that males queue for positions in an age-based dominance system. The results do not support the hypothesis that delayed plumage maturation involves mimicry or deceptive signaling by subdefinitive males (≤3 yr old) to high-ranking males (≥8 yr old). Of 85 males monitored from 1983 to 1986, copulations (N = 117) were distributed among 8 males. Four of these males accounted for over 90% of the copulations, with 67% accruing to one male. An index of the opportunity for sexual selection, to be used cautiously in interspecific comparisons, was estimated to range from 15.8 to 31.5 in four successive years. Direct benefits to cooperation by the beta male consisted of rare immediate copulations (N = 2) and eventual ascent to alpha status (N = 3). The high variance of mating success means few opportunities for success by younger males and should favor long-term strategies, such as cooperation and delayed plumage maturation, that enhance the prospects of future success.