Three-spined sticklebacks (Gasterosteus aculeatus) mate in early spring in shallow fresh water, but make ideal experimental subjects because they are just as happy in a laboratory tank. There are four distinct phases to their reproductive behaviour, as follows:

• Male leaves school of fish and stakes out territory. During this period its normal colour changes slightly from an unexciting silver-grey, and it gets a pink blush on its chin and a greenish gloss on its back and around its eyes. It will also defend its territory from other males (as well as females).
• The real curiosity is that the fighting stage seems to be compulsory. Male sticklebacks defend their territory for a while even if the necessary nesting materials (see next) are freely available, and it is territorial defence rather than inter-male aggression, because if they are taken from their territory at this juncture and put in with another male they will immediately retreat.

• The male now builds his nest by digging a shallow pit in the sand, carrying the sand away mouthful by mouthful, until the depression is judged big enough (5cm across). If the experimenter fills the pit in at this juncture, it will be duly re-excavated. However, if the experimenter keeps filling in the hole, the stickleback will eventually proceed to the next stage without a sand pit.
• The next stage is for the male to collect a heap of weeds, and shapes them into a mound with its snout. When this has been done, he wriggles through the mound once, forming a tunnel.
• Transitional Behaviour: When the nest is complete, the male further changes colour. The pink blush on its chin now becomes bright red, and the back turns blue-white. And again, the the nesting stage seems to be compulsory - until the nest is complete to his satisfaction, male sticklebacks will studiously ignore the advances of even the most amorous females.

• The male now only defends its territory against other males. It patrols its borders and will vigorously attack any red object coming close (also, but to a lesser extent, silver or green). In these ritualised combats, it is usually the stickleback furthest from its own territory who loses, even if up against a much smaller opponent: 'at the nest itself he is a raging fury' (Lorenz, 1952:47).
• Female sticklebacks, meanwhile, have grown shiny and swollen with eggs as they come into season. For their part, they are attracted to red coloured objects, and will follow a red-coloured model. When a female passes the patrolling male's territory, she is therefore automatically attracted in.
• When the female enters the territory the male approaches her in a series of zigzags, repeatedly pausing and showing her his flank. The swollen belly seems to be the effective stimulus at this stage, because males with bellies swollen with food are also courted. The female then swims towards the male in a head-up posture, and the male then retreats towards the nest, beckoning, the female to follow.
• At the nest, the male makes a series of rapid thrusts with his snout into the tunnel, turning on his side as he does so and raising his dorsal spines towards the female. This prompts the female to enter the nest tunnel and she comes to a halt with her head sticking out of one end and her tail out of the other.
• The male now rhythmically prods the base of her tail with his snout. This prodding is the effective stimulus at this stage, because if the experimenter does the prodding with a glass rod the same effect is produced - even if the female has just seen the male being removed.
• The female now deposits her eggs in the sand beneath the nest, and slips out of the nest. Her belly is now no longer swollen. The male then enters the nest and fertilises the clutch of eggs (not shown).
• The male now chases the first female away. The effective stimulus here is the smell of the eggs rather than the act of recent ejaculation. He then returns to the edge of his territory in search of another. Usually three to five different females can pass through the tunnel in succession.
• Transitional Behaviour: The mating urge now subsides, the male's colour reverts to silver-grey, and it also reverts to defending its territory from both other males and females.

• The male now guards the nest from predators, and fans the eggs with his pectoral fins. This ventilating gets more intense each day until the eggs hatch.
• The male now herds the hatchlings together into a group, gently retrieving any strays in his mouth.
• Eventually, however, the brood get strong enough to go off on their own, to repeat the cycle themselves in due course, the hazards of the river permitting.