What’s in the hump?

Authors: Ward, David Lance*, Ward, Mike, Morton-Starner, Rylan

Year: 2015 (xlvii)

Abstract

The function of the nauchal 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 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. Although the reason for the evolution of the hump in humpback chub may never be resolved, the additional body depth created by the hump at adult sizes may help to limit predation. Analysis of anatomical gape and body depth relationships indicate juvenile humpback chub are not likely to be vulnerable to predation by adult chub once they reach 100 mm total length. Gape and body depth relationships also indicate vulnerability to introduced piscivores like rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta or smallmouth bass Micropterus dolomieu are likely to continue until humpback chub reach more than 300 mm total length, even with a developed hump.