The nuptial tubercles are evident only in the male during 
the spawning season (Figure 1). Spawning begins when water 
temperatures reach about 16°C, and this varies with the habitat 
and year. Males with developing tubercles can be caught in the 
Ottawa District as early as April 13th, and the tubercles are 
fully developed through late May to June and into July or even 
early August. In late July and August, males begin to lose 
their tubercles, and by early September only scars remain. The 
large tubercles are arranged in three rows across the snout, 
often seven or eight in the lower row (range 4 to 15), usually 
seven in the middle row and usually four in the upper row. In 
addition, there are up to 11 large tubercles on the chin. 
Minute tubercles are found on the top of the head and as a 
single row on the larger pectoral fin rays. 
Other features also characterize the breeding male, in 
particular: a spongy, slate-blue, wrinkled pad forming a broad 
band at the nape and tapering to the dorsal fin; a darker co¬ 
louration notably of the head and dorsal fin which may be jet 
black; light bands behind the head and below the dorsal fin with 
intervening areas dark; a thickened, blunt, first dorsal fin 
ray; and the swollen membranes of all the fins. Males are 
larger than females. In contrast, the females are quite dowdy 
and lack the tubercles, pad and darker colouration. Their vent 
region is swollen, no doubt to aid in egg deposition, and there 
is some swelling of fin membranes, particularly of the anal and 
pelvic fins which are near the vent. 
The male fathead chooses a suitable site for spawning in 
shallow water. This is usually the underside of a log, branch, 
twig or bank and rarely a lily pad. Artificial objects like 
planks will also be used. The male then chivvies a passing 
female into position below his chosen site, pushing and snapping 
at any female who shows reluctance. The male then takes up a 
position usually on the left side of the female. He then pushes 
the female in circles until he can lift her up on her side using 
his back. This action brings the female in close contact with 
the spawning site, the eggs are extruded and, being sticky, 
adhere to the underside of the site. The male fertilizes the 
eggs and drives away the female. The male selects several 
females and repeats this process with each one so a nest will 
contain several batches of eggs in various stages of develop¬ 
ment. 
The male defends the nest site using his tubercles to butt 
and drive away other fishes including members of his own spe¬ 
cies, leeches and even turtles. He turns slowly with jerky 
movements so that all possible routes egg stealers might use are 
checked out. Despite valiant attempts, eggs are lost when the 
male is energetically driving away several intruders at once and 
one is able to sneak in while he is distracted. The repeated 
spawning with several females may be necessary to replace lost 
eggs. 
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