The Sex Life of the Male Fathead 
Brian W. Coad 
Ichthyology Section 
National Museum of Natural Sciences 
Ottawa, Ontario K1A 0M8 
On several occasions I have been brought specimens of a 
small fish by concerned, curious and disgusted students. These 
fish, they insist, are horribly diseased and, in evidence, they 
point to the nasty bumps on their heads. Far from being sick, 
however, these fish are at the peak of their sexual prowess and 
use the bumps, known as nuptial or breeding tubercles, to drive 
away other fishes from the nest site. 
This particular fish is the Fathead Minnow ( Pimephales 
promelas), which is common and easily caught in ponds, lakes and 
muddy streams of the Ottawa District. Its reproductive behav¬ 
iour and anatomy have been well studied both for their general 
interest to biologists and because this fish has been pond- 
reared as food for cultured Smallmouth Bass in the United 
States. The sex life of male fatheads is a fascinating study in 
behaviour and structural adaptions. 
Figure 1. A male Fathead Minnow (Pimephales promelas,), 69.6 mm 
total length, from a stream draining Lac au Foin, Gatineau Park, 
Quebec, June 28, 1982 (NMC82-0335). Illustration by Susan 
Laurie-Bourque. 
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