Predation and the evolution of colonial nesting in bluegill sunfish (Lepomis macrochirus)

Abstract

1. Male bluegill sunfish (Lepomis macrochirus) construct nests in densely packed colonies characterized by high breeding synchrony. Females deposit eggs in the nests and males alone provide parental care for eggs and larvae. During the 7-day care period, males do not leave the nests to forage nor do they nourish the young. Parental males do actively defend their broods against predators. 2. This study investigated the effects of coloniality on predation. Bluegill brood are attacked by predators moving along the lake substrate and from the water column. Substrate-level predators include a major predator in the system, the snail Viviparous georgianus, and also bullhead, Ictalurus spp. Predation from the water column is primarily by conspecifics (94%) but also by Lepomis gibbosus and the hybrid, L. macrochirus x L. gibbosus. There is little or no predation on parental males. 3. Significant differences in brood predation are found among nests in central, peripheral colony and solitary sites. Brood loss at peripheral nests is at least three times that at central nests, and solitary nests experience greater predation than colony nests. These differences are due to effects of nest dispersion rather than to habitat characteristics. 4. Bluegill brood predation is reduced through colonial nesting as (1) peripheral nests screen central broods from snails and bullhead, (2) predators can be swamped by the high nesting density, and (3) overlapping defended zones provide cumulative defense against water-column predators and bullhead. Synchrony in breeding augments these anti-predation attributes of colonial nesting, and can also reduce conspecific predation and result in a ‘head-start’ against predators. 5. Certain costs to brood survivorship arise from colonial nesting: predation by neighboring males and ripe females, concentration of odor cues which may influence bullhead predation, and possibly fungal transmission between nests. 6. Pumpkinseed sunfish (L. gibbosus), which breed concurrently with bluegill, are relatively unsocial nesters. Pumpkinseed do not suffer the same predation pressures. As a result of morphological and behavioral adaptations for feeding, pumpkinseed are able to remove snails from their nests and probably repel bullhead attacks. As snails contribute over 50% of the estimated predation on bluegill, this difference between species is significant. 7. Brood predation is proposed as an important selective force for the evolution of colonial nesting in sunfishes. Selection should be mediated through female choice of a nest dispersion which maximizes brood survivorship. Morphological and behavioral preadaptations probably determine the type and degree of brood predation experienced by a species, and hence species-specific selection for patterns of nest dispersion.