Reproductive skew in the polygynandrous acorn woodpecker

Abstract

Reproductive skew models, which focus on the degree to which reproduction is shared equally (low skew) or monopolized by a single individual (high skew) within groups, have been heralded as providing a general unifying framework for understanding the factors determining social evolution. Here, we test the ability of optimal skew, or "transactional," models, which predict the level of skew necessary to promote stable associations of dominants and subordinates, rather than independent breeding, to predict reproductive partitioning in the acorn woodpecker (Melanerpes formicivorus). This species provides a key test case because only a few vertebrates exhibit polygynandry (multiple breeders of both sexes within a group). Contrary to the predictions of the models, joint-nesting females share reproduction more equitably than expected, apparently because egg destruction and the inability of females to defend their eggs from cobreeders eliminate any possibility for one female to control reproduction. For males, however, reproductive skew is high, with the most successful male siring over three times as many young as the next most successful male. Although this result is consistent with optimal skew models, other aspects of male behavior are not; in particular, the reproductively most successful male frequently switches between nests produced by the same set of cobreeders, and we were unable to detect any phenotypic correlate of success. These results are consistent with an alternative null model in which cobreeder males have equal chance of paternity, but paternity of offspring within broods is nonindependent as a consequence of female, rather than male, control.

Figure 1

The expected subordinate's share of reproduction in relation to ecological constraints (measured as the probability of successful dispersal by a cobreeder) for females (a) and males (b). The two curved lines in each panel represent the expected subordinate's share of reproduction according to transactional models based on concessions (–4, 8) (p_con) and restraints (7, 8, 14) (p_res). p_con assumes dominants are in complete control and offer the minimal necessary incentive needed to induce subordinate to remain in the group, whereas p_res assumes that subordinates obtain the maximum amount of reproduction they can without being evicted by the dominant. Thus, the white triangular areas bounded by these lines represent the potential range of values for p_sub, the subordinate's share of reproduction, consistent with transactional models. Values for p_sub within this area (but not on the lines themselves) are those predicted by mixed or “compromise” skew models (8). Values above and below this area are not predicted by either concessions or restraints models. Cobreeding/joint nesting is not predicted to occur at all in the area to the right of the dotted vertical line, defined by the probability of successful dispersal where p_con and p_sub cross. In this area, where cobreeding is predicted to be unstable, constraints are so low (i.e., the probability of successful dispersal is high) that subordinates will always do better by dispersing and breeding on their own. The horizontal broken lines determine p*_f and p*_m, the maximum level of subordinate reproduction possible for females and males, respectively, based on concessions models and, alternatively, the minimum level of subordinate reproduction possible based on restraints models. The observed level of subordinate reproduction (where subordinates are defined as the individual achieving the lower proportion of parentage) are indicated the “observed p_f ” and “observed p_m ” lines. Lines dropping vertically to the x axis (marked by the arrows) from where these values meet the p_res (females) or p_con (males) curves indicate the maximum probability of successful dispersal (d) consistent with cobreeding given our demographic and parentage estimates. Finally, the dotted lines indicate the level of subordinate reproduction implied by the basic (for females) and alternative (for males) simulation models (“inferred p_f ” and “inferred p_m ” = 0.5). These lines are also accompanied by a vertical dotted line dropping to the x axis indicating the maximum probability of successful dispersal consistent with this level of subordinate reproduction.