Proximate causes of sexual size dimorphism in Colorado pikeminnow, a long‐lived cyprinid

Abstract

Evidence for sexual size dimorphism (SSD) and its possible causes were examined in the endangered Colorado pikeminnow Ptychocheilus lucius, a large, piscivorous, cyprinid endemic to the Colorado River system of North America. Individuals representing 18–24% of the upper Colorado River population were captured, measured, sexed and released in 1999 and 2000. Differing male and female total length‐(LT) frequency distributions revealed SSD with females having greater mean and maximum sizes than males. Although both sexes exhibit indeterminate post‐maturity growth, growth trajectories differed. The point of trajectory divergence was not established, but slowed male growth might coincide with the onset of maturation. Differing growth rate was the dominant proximate cause of SSD, accounting for an estimated 61% of the observed difference in mean adult LT. The degree of SSD in adults, however, was also related to two other factors. Evidence suggests males become sexually active at a smaller size and earlier age than females; a 2 year difference, suggested here, accounted for an estimated 12% of the between‐sex difference in mean adult LT. Temporal shifts in gender‐specific survival accounted for an additional 27% of the observed between‐sex difference in mean adult LT. Estimated age distributions indicated a higher number of older females than older males and more younger males than younger females in the population during the period of sampling. Dissimilarity of age distributions was an unexpected result because the male : female population sex ratio was 1 : 1 and estimates of long‐term annual survival for adult males and females were equal (88%). Future assessments of SSD in this population are apt to vary depending on the prior history of short‐term gender‐specific survival. Without recognizing SSD, non‐gender‐specific growth curves overestimate mean age of adult females and underestimate mean age of adult males of given LT. Assuming age 8 years for first reproduction in males and age 10 years for females, the adult male : female ratio was estimated as 1·1 : 1 and mean adult age, or generation time, was estimated as 16·4 years for males and 18·4 years for females.