- Simon Fraser University
- University of Kentucky
Summary
We review parental investment decision theory and provide an experimental test of the decision rule used by male bluegill sunfish (Lepomis macrochirus) in allocating parental investment to their young. The alternative decision rules tested are: (1) invest according to brood size (number) only; (2) invest according to past investment only; (3) invest according to both brood size and past investment; and (4) invest according to neither brood size nor past investment. By manipulating brood size independently of a male's cumulative investment in the brood, and by measuring each male's defensive behavior against a model predator, we found that male bluegill invest according to both brood size and past investment. This result is consistent with recent theory that past parental investment devalues adult future reproductive value, and that animals should therefore invest according to the value of their brood relative to that of their own expected future reproduction.
Methodology
Study site. Lake Opinicon, a 900 ha mesotrophic lake in southern Ontario (Canada), holds a large native population of bluegill sunfish (Keast 1978). Our study involved a breeding colony located 1 m deep on a flat rock shelf, Big Rocky Point (Gross and Nowell 1980). The shelf is covered with fine gravel and has little vegetation. The lack of vegetation and the threat of predators excluded "cuckolders", a small-bodied alternative male phenotype (Gross 1982), from spawning at this site. Thus, all broods in the nests were fathered by the parental males. Manipulations. Approximately 60 bluegill males began to construct nests on 29 June 1984. Spawning began late the next day and continued for a few hours into the morning of July 1. After the spawning, we assigned nests randomly to one of four samples: Early (n=7), Late-1 (n= 15), Late-2 (n= 10) and Control (n = 18). The Late-2 sample, an addition to the SargentGross design, is discussed below. Each nest was marked with a small (5 cmx 7 cm) numbered tile; nests with unusually few or many eggs were not used.
For the Early sample, brood size was reduced in the afternoon of July 1 (Fig. 2). Approximately 50% of the brood (as judged by eye) was removed from each nest using a plastic scoop and SCUBA. For the Late samples, a 50% reduction was made in the morning of July 5, after all the eggs had hatched (and males had ceased fanning). To control for the disturbance of removing brood, at the time of both the first and second reductions all nests not being reduced in brood size were intruded upon with the plastic scoop. The exception is the Late-2 sample which, while otherwise treated the same as the Late-i sample, was not manipulated with the plastic scoop during the first reduction. By comparing the Late-1 and Late-2 samples, we could determine whether the act of intrusion with the plastic scoop had an effect separate from that of reducing the brood. All possible perturbations on the male nesting cycle were therefore controlled.