FIGURE 6.

Divergences in the social context of male horn dimorphism appear to drive morphological divergences in exotic populations of Onthophagus taurus. (a and b) Alternative reproductive tactics of horned (solid arrow) and hornless (dashed arrow) males. Horned males guard (a) and fight over (b) access to tunnel entrances containing breeding females. Hornless males utilize non-aggressive sneaking behaviors such as digging of interception tunnels (a, left dashed arrow), using naturally existing tunnel connections (a, right dashed arrow), or waiting as satellite males until guarding males are distracted in order to gain access to females (b, dashed arrow). Figures (a) and (b); reprinted with permission from Ref 39. Copyright 2009 Science Publishers. (c) Scaling relationship between body size (x-axis) and horn length (y-axis) in three exotic, geographically isolated populations. (d) Correlation between body size thresholds (x-axis) and densities (y-axis) in local populations across the same three exotic ranges. Theoretical models (see text) predict that body size thresholds (c) can diverge as a result of differences in local densities of competing males (d), i.e., high density populations exhibit higher size thresholds than low density populations. Data in (c) and (d); reprinted with permission from Ref 40. Copyright 2003 Oxford University Press.