- University of Utah
Summary
Male bluegill sunfish are shown to have two alternative mating strategies: cuckoldry or parental care. Cuckolder males first mature at age 2. They follow a developmental sequence of sneaking and then mimicking female behavior to deceptively gain access to spawnings. Males who become parentals (construct nests, attract females, provide brood care) delay maturation until age 7. The parental investment of these males is parasitized by the cuckolders. This system is an example of a truly parasitically dependent mating strategy in vertebrates. A natural selection model is developed to predict the equilibrium frequencies of the two male types. A preliminary test of the model provides qualitative agreement.
Methodology
Field observations and experiments were conducted from 1976 through 1979 on a natural population of bluegill inhabiting Lake Opinicon, Ontario, Canada (14). Colonies contain 8-150 males with nests approximately rim to rim in 0.4-2.5 m of water. Data on spawning behaviors were collected by observers on towers placed near shallow colonies, and by skin and scuba diving in deeper waters. Spawning begins when a school of gravid females arrives at an established colony of males; females enter the nests singly and release eggs. At that time, males not previously associated with the nests intrude and release sperm.
We identified three male behavioral types in a random sample [focal-animal sampling] of 72 fish studied at three colonies during spawning. "Parental" males constructed and remained in nests, were highly territorial to nest intruders, and were light in body color with dark yellow-orange breasts. "Sneaker" males remained close to the substrate and showed rapid nest entry and exit. They were nonaggressive and light in body color. "Satellite" males showed slow nest entry, were sometimes aggressive to fish of equal size, and had a dark body with dark vertical bars. The coloration and movement of satellite males mimicked female behavior (16). We tested the hypothesis that these behavioral categories are mutually exclusive by examining the behavior of a random sample of 41 additional fish during their spawning. All test fish were successfully placed in a category. The labels parental, sneaker, and satellite were chosen simply because they are behaviorally descriptive. An additional randomly sampled 148 males were identified in the colonies by these behavioral criteria and then were captured, and their lengths, weights, and ages were determined. They gave three distinct age groups (Fig. 1A). Each group differed statistically with respect to age, length, and weight (pairwise two-tailed t tests; P < 0.001). Satellite males were intermediate to the small sneaker and larger parental males. There was a notable absence of 6-year-old males at the colonies.