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Sexual selection may be a major factor in the proliferation of polygamous species (Lande 1981). When males provide no resources or parental care and females have numerous males from which to choose, "extravagant" male secondary characteristics may result solely from sexual selection (Darwin 1871; Fisher 1930; Lande 1981). Dominey (1984) argued that such "runaway" sexual selection was the driving mechanism behind the explosive speciation of the polygamous cichlid fishes in the Great Lakes of Africa. However, the few studies examining cichlid mate selection in situ have concentrated primarily on species engaging in biparental care of the young (Perrone 1978; Neil 1984; McKaye 1986). Although reproductive behavior has long been hypothesized to be important in the ecology and evolution of cichlids (Lowe-McConnell 1959; Fryer and Iles 1972; McKaye 1984), factors influencing female choice and the relative reproductive success of males are poorly known for most field populations.
Several hundred cichlid species inhabit the sandy and weedy environments of Lake Malawi (McKaye 1984; McKaye and Gray 1984; McKaye, in press) and have lek-based breeding systems (Fryer and Iles 1972; McKaye 1983, 1984) with display sites that vary dramatically among species. Leks are defined as aggregates of adult males that do not contribute resources or parental care; females visit only to have their eggs fertilized (see Bradbury and Gibson 1983; Borgia 1985). During ecological studies of this sand-dwelling community, we have discovered 10 basic, sand forms built by cichlid fishes in the Cape Maclear region of Lake Malawi (McKaye 1984, in press). They range in size from giant craters 3 m in diameter (McKaye and Stauffer 1988) to small depressions in the sand and include "sand castles" (Bass 1988, see pp. 152-153 for photograph) with base diameters of more than 1 m (McKaye 1984). Because display sites vary significantly in size and shape, an experienced observer can distinguish among sibling species on the basis of the display site alone (McKaye 1984; McKaye and Stauffer, MS). Relationships among species presumed from the similarity of display sites have recently been confirmed by electrophoretic analyses of tissue samples (McKaye, J. Howard, Stauffer, R. Morgan, and F. Feresu, MS).
The term "nest" has been used to describe these display sites (Fryer and Iles 1972; Keenleyside 1979; McKaye 1983, 1984; McKaye and Stauffer 1988). However, we now prefer to use the term "bower" as found in the ornithological literature (Collias and Collias 1983; Borgia 1985; Borgia et al. 1985) to characterize these sites of courtship and spawning. They are constructed independently of the need to care for eggs and young (McKaye 1984). The female deposits eggs in the bower (nest) and then immediately picks them up in her mouth, where they are fertilized. Eggs are then brooded solely in the mouth of the female. Males engage in no parental care. The form and function of the arenas and the display sites, or bowers, are clearly analogous to bird leks (Fryer and Iles 1972; McKaye 1983, 1984). Some arenas are relatively small (30 m x 20 m); others may extend for several kilometers.
Bower size could be a critical characteristic in determining female choice and relative male success for the species Cyrtocara eucinostomus (McKaye 1983). Therefore, we examined the dynamics of female mate choice in this species, one of the zooplankton-feeding fishes, or "utaka," from Lake Malawi, which Dominey (1984, p. 236) specifically discussed in support of the sexual-selection hypothesis of speciation.
We tested two hypotheses: (1) male reproductive success improves as bower size increases; and (2) after a disturbance, density, dispersion, and sizes of bowers in this arena rapidly return to predisturbance conditions. We directly observed the behavior of C. eucinostornus males above bowers and experimentally removed bowers in order to answer five subsidiary questions. Did males above large bowers attract more females? Were larger bowers defended more aggressively, or more persistently, than smaller bowers? Were new bowers rebuilt in the same place as old bowers or were they established de novo? Were any bower characteristics species-specific? And finally, were patterns of bower initiation and construction consistent spatially within the arena? Answers to these questions would provide a basis for further examining the role sexual selection may have played in the explosive cichlid radiations common to the Great Lakes of Africa (Dominey 1984).