What's in the Hump?

The function of the nuchal hump on adult humpback chub Gila cypha has been the subject of many fireside conversations.  Hypotheses about the purpose of the hump range from it being a feature that confers hydrodynamic advantages in swift water to speculation about how the hump may have reduced predation vulnerability to toothless Colorado pikeminnow Ptychocheilus lucius.  We compared the histology of the head region of captive reared specimens of humpback chub, bonytail chub Gila elegans and roundtail chub Gila robusta to evaluate if histological examination could give insight into the function of the hump.  Tissues were sectioned, stained and photographed by a pathologist at the Associated Regional and University Pathologists laboratories in Salt Lake City, UT.  The forehead section in all three species of chub is composed almost entirely of skeletal muscle.  Muscle from the hump appears slightly atrophic, less well organized overall, and less vascular than muscle from the dorsal and caudal musculature.  Although the reason for the evolution of the hump in humpback chub may never be resolved, additional information about the hump may help to limit hypotheses.