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234 900 mm are exceedingly rare. The largest P. lucius captured in the multiagency Upper Colorado River Fishes Recovery Implementation Program was recorded at 1240 mm TL and only 7 of the more than 5000 P. lucius captured since the 1970s have exceeded 900 mm TL (C. McAda, US Fish & Wildlife Service, personal communication). The historical record is too scant to make valid state- ments about past fish sizes other than to speculate about maximum sizes that were probably attained (e.g., Miller 1961). Two other fish species of the Colorado River, the razorback sucker, Xyrauchen texanus, and the humpback chub, Gila cypher, have been described as `bizarre' because they exhibit large nuchal processes that are located dorsally, posterior to the head. These pronounced humps look similar but are not homologous structures. The X. texanus hump is bony, formed mostly by enlarged neural and interneural bones (La Rivers 1962), while the G. cypha hump consists of a muscular mass (Miller 1946, Figure 1). The function of these nuchal humps is un- known, though Miller (1946) speculated that the G. cypha hump provides a hydrodynamic advan- tage in swift currents. Miller reasoned that water flowing over the hump would produce a down- ward force on the fish, aiding the fish in main- taining position on the bottom. Flow at the benthic boundary layer is slower and would re- quire less exertion to swim or maintain position in a river. Unfortunately, Miller (1946) was unable to observe G. cypha swimming in nature and little was known about its habitat use. La Rivers (1962) offered a similar explanation for X. texanus, hypothesizing that flowing water would push the sloping head downward while the bony keel pro- vided increased stability in fast flows, thus steadying the fish against the bottom. However, La Rivers (1962) also was unable to test his hypoth- esis. Yet, their assumptions that nuchal humps are hydrodynamically advantageous adaptations to swift currents are often cited (e.g., Cole 1994, Si- gler & Sigler 1996, Moyle & Cech 2004). Additional information about the adaptations and life histories of Colorado River fishes gathered in the last 20 years has prompted us to reevaluate the function of large nuchal humps. It is now understood that humps in both species are formed later in life at a time when adult G. cypha pre- dominantly occupy eddies and adult X. texanus C ? 6, tl Figure 1. Four endangered Colorado River fishes: (a) Xyrau- chen texanus, (b) Gila elegans, (c) Ptychocheilus Lucius, and (d) Gila cypha. Fish lengths are not to scale, however drawings were made from actual fish specimens courtesy of Jim Beard, U.S. Fish & Wildlife Service. inhabit eddies, deep runs, and slack waters (re- viewed by U.S. Fish & Wildlife Service 1990, 1998, unpublished observations). These are large, but quiet habitats where adaptations to fast flows are not required. Conversely, related native cyprinids (Gila elegans and G. robusta) and catostomids (Catostomus latipinnis and C. discobolus) that prefer fast flowing habitats do not have such large humps. Body shapes of these species suggest low pressure drag, which would be more adaptive to high mobility and fast flows (Webb 1984). Such body shapes tend to have the transitional point between laminar and turbulent flow located far back from the leading edge. In contrast, the tran- sitional point in G. cypha and X. texanus occurs on the anterior leading portion of the body, a mor- phology not conducive to reducing drag (i.e., the