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241 were merely interested in drag and lift components of fish body shapes with large nuchal humps. We believe that the casts (rigid-body analogy) we used were adequate for determining the role that pro- truding rigid structures (nuchal humps) have on drag and lift generated by fish that would be attempting to hold their position in flowing water. Thrust values were not sought after in this study; rather, we focused on dead-drag. The use of dead- drag is justified because we evaluated rigid struc- tures and were not concerned with the action of the fleshy body or turbulence caused by fins and other structures (e.g., see Webb 1975). Our use of rigid body casts instead of fresh specimens may be criticized. However, the main objective of this study was to evaluate whether a hydrodynamic advantage might be induced solely by large nuchal humps. Nuchal humps of G. cypha and X. texanus are rigid body structures of bone and hard muscle that have been thought to aid these fishes hydrodynamically while attempting to hold their position in a stream (Miller 1946, La Rivers 1962). Thus, we used rigid casts to evaluate the effect of humps on drag and lift in order to avoid turbulence and higher drag coefficients that would result as an artifact of body fluttering or due to fins that may cause spurious readings in fresh specimens (Webb 1975). Also, we did not address shape-specific effects of humped fishes of different sizes. No doubt, an evaluation of the allometric relationships between nuchal hump size and the drag and lift produced would provide useful information about the functional consequences of developing enlarged nuchal humps. However, determining these relationships was beyond the scope and budget of our study. This study is pri- marily concerned with determining whether large nuchal humps provide a hydrodynamic advantage (i.e., negative lift with marginal drag), and offering an alternate hypothesis of their usefulness. The relatively high metabolic cost of swimming is indisputable and a direct reflection of the drag that must be overcome. For mobile organisms such as stream fishes, the cost of excessive drag is extreme, depending on habitats utilized. Fishes living in swift water must expend energy at a rate that is at least that of the product of drag and velocity (Vogel 1994). Our analysis of drag on the body casts of different fishes we studied produced results that are in agreement with habitat prefer- ences obtained by empirical studies: G. cypha and X. texanus prefer slower habitats such as eddies, backwaters and slackwaters (Tyus 1987, Karp & Tyus 1990, Sigler & Sigler 1996). However, closely related forms utilize high velocity habitats. In the case of G. cypha, a congener, G. robusta, is more streamlined, has a much less pronounced nuchal hump, and does occur in swift currents. With re- spect to X. texanus, its close relative, C. latipinnis, also is more streamlined, lives in riffles and has no hump. For the hydrodynamic advantage theory to be correct, all other main channel fish species should have a large hump to benefit swimming performance in swift currents. Our findings show that Colorado River fishes with the most stream- lined bodies are fishes that historically inhabited fast-flowing main river channels, whereas fishes with large humps are known to occupy eddies and slackwaters. In the case of X. texanus, a fish that has been occasionally observed feeding in shallow runs (Tyus 1987), enlarged pectoral fins may be an adaptation to compensate for lift created by the hump when the fish ventures into faster flows. An apparently high energetic cost would be associated with hump growth and locomotion with the presence of a hump due to the drag that is produced (Pettersson & Bronmark 1999). Al- though it might be argued that humps are relict structures that once had some purpose that has been lost, logic dictates that some benefit must accrue. One possible explanation, in view of our findings that the humps do not aid swimming or produce `negative' lift, might be sexual dimor- phism as an aid in sexual selection. However, males and females of both species produce humps; thus, we discount sexual selection as the primary factor in hump formation. Humps of males do appear slightly larger, but we attribute such dimorphism to building on an already existing structure rather than developing the hump for this purpose de novo. This is supported by a regression of nuchal hump size vs. fish total lengths for fishes that we measured at Dexter National Fish Hatchery. In this case, no sexual dimorphism was evident, and a smooth transition occurred as hump size increased with fish growth (rz = 0.984 for X. texanus; Figure 5 and r2 = 0.986 for G. cypha; Figure 6). An additional possibility is that en- larged nuchal humps are produced in these two species in response to predation.