FINGER-sized stickleback fishes usually live in schools but during the breeding season their males form territories and become solitary and highly aggressive. Concurrently, with the change in behaviour, they also assume brilliant nuptial colours, eyes changing to shining blue, the back from a dull brownish becomes greenish and the underparts become red. If any intruder, especially male of the same species, enters his territory, the owner assumes a threatening posture.

He immediately rushes towards him with raised dorsal spines and gaping mouth. If the intruder is unmoved, then the defendant instead of biting becomes still and pointing its head down, stands vertically in the water and jerks its body in such a way as if it were about to bore its snout into the ground below. Strangely, this unusual posture is normally more than enough to drive away the intruding stickleback.

If the three-spined male stickleback is not defending territory, he builds nest by hollowing a pit out on the riverbed and pressing pieces of nest material into it, taken from sea vegetation, like strands of algae. All these are bound together with a glue-like substance secreted from a male's kidneys. Once this is done he bores a tunnel by wriggling his body through.

Sticklebacks are members of a small family of marine, brackish and freshwater fish with torpedo-shaped body. Their body is scaleless, but may possess rows of scutes (bony plates) at the side. They are known for constructing elaborate nests, like that of birds. The three-spined stickleback, having a series of separate sharp spines on the back preceding the dorsal fin, builds a nest on riverbeds whereas 10-spined species builds a domed nest up in the water-weeds.

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Once the task of nest-building is complete, the male embarks upon the job of finding and attracting a mate, for which he must overcome his innate aggression.

On the other hand, this is also the time when schools of females visit male territories, many with egg-filled bellies, to find a suitable place and a caring father in whose protection they can deposit their eggs. Since the females are drab in colour they do not elicit much aggression from males, who by now have become very colourful with a brilliant shining blue back and dark red belly.

Once the females are spotted, males start approaching them in a peculiar zig-zag dancing movement. If any one of the females is in a mood to spawn, it turns towards the male and follows him to his nest with her nose up, while others may be chased away.

After reaching the nest the male entices her to enter by thrusting his snout into the entrance. If she is not impressed she will swim away and if she is she enters the nest, meanwhile, the male prods the base of her tail, stimulating her to shed eggs. As the task of egg-laying is complete, the male chases her away and enters the nest himself, discharges his sperms over the eggs and then looks for another female.

The 10-spined stickleback also welcomes as many females as possible inside the nest, but some imposters take advantage of it. Some dull-coloured males, who look like females, also enter the nest along with them and fertilise the eggs already laid, thereby cuckolding the host.

Among the fishes only a few species look after their young, but when they do it is often the father who does the work. Perhaps, the reason is that the eggs are generally fertilised outside the female's body by males. Once it is done, females in most of the species abandon their eggs, leaving all the responsibilities to males. Fish fathers take good care, fanning water over them so that they get enough oxygen, keeping the nest clean and also fighting the predators who dare to come near the nest.

It has been observed that even the female fishes prefer those males that are already guarding eggs, because they are likely to spend more time protecting the young, and less time in courting new females.

Spines of the stickleback are a very effective weapon against its enemy. In fresh water, trouts prey upon three-spined sticklebacks, but the spines often stick into their throats, making swallowing a painful affair. Experiments have shown that if a variety of species are kept in the tank along with the sticklebacks, the trouts prefer smooth minnows in place of spiny food.

This feature was published on December 7, 2003