The curious behaviour of the stickleback.

N. Tinbergen

Animals can be observed performing their natural behaviour, in their natural environment or in a laboratory simulation of their natural habitat. This type of research is called Ethology. It is not the concern of ethology to teach animals human directed behaviour (i.e. Tricks), usually in the laboratory. Tinbergen believed that four questions should be asked about the natural behaviour of any animal.

Tinbergen found the stickleback a convenient animal to study. It is easy to handle, relatively uncomplicated and displays many examples of fixed action patterns.

Lea (1984) described six characteristics of fixed action patterns.

Tinbergen warns against viewing animal behaviour in human terms (anthropomorphism).

The triggering mechanisms may be external 'sign stimuli', but might also require internal initiation. The internal state that initiates behaviour could be hunger or the sex drive, for example. A physiological measure of internal states can be taken. In the case of sex drive, hormone level would be a good predictor of behaviour.

As hormone levels rise, the stickleback is driven to stake out his territory, in preparation for nest building. The male stickleback, during the mating season, turns red. The red colour acts as a sign stimulus to other males, triggering aggressive behaviour. In the laboratory, this can be easily tested by presenting models of fish painted in different colours. Red males will only attack a model of a similar colour (red). The fighting behaviour is mostly display. The male stickleback will adopt a vertical position and display his spines to an opponent. He will use jerky movements, using his whole body. The male defending his territory will usually remain within its borders, attacking up to the boundary and then retreating again. If the opponent should enter the territory, then the defending fish becomes much more aggressive.

Tinbergen placed a mirror in a tank containing a red male. As would be predicted, the male attacked his reflected image, getting nowhere. Oddly, the male then would break off its attack, and would start to dig into the sand instead. This behaviour is labelled 'displacement behaviour', because it is thought the animal is displacing its energy. The fish can not effectively attack, can't escape, without losing its territory, so the pent up energy is displaced by performing the digging behaviour.

This digging behaviour is normally used to build a nest. First a hole is dug, and then this is covered in weeds. Interestingly, the hole is the sign stimulus for the weed covering behaviour. This is easily demonstrated by the experimenter filling in the hole each time it is dug. The stickleback will repeatedly dig out the hole again. Mind you, an over-zealous experimenter may fill in the hole once too often; in this case the fish will continue with the nest building process without the benefit of a hole.

The nest is nearly as long as an adult female stickleback. If an experimenter removes weed from one of the ends of the nest the male stickleback will restore the covering to its original length. Likewise, if the experimenter lengthens the nest, the male will again restore the nest to its original length.

Once, the nest is built the male needs to entice a suitable passing female back to it. Males prefer long, gravid (plump) females. This can be tested in the laboratory by presenting pairs of models to a male. Presumably, the preferred model is the one that the male spends more time with. Additionally, in the laboratory, anaesthetised females can be induced to depart with their eggs. A significant positive correlation has been found between the length of females and the number of eggs they lay. Presumably males prefer longer females, because they lay more eggs, and thereby increase the chances of more fry (baby fish) surviving. It's important to remember that males respond to sign stimuli, for example fat, overfed males, also receive the amorous attention of red males!

Females prefer males that are vigorous, and build a robust nest. Vigour is displayed by the male in the 'zig-zag dance'. The zig of the dance is an energetic thrust towards the female, whereas the zag is a slower moving away from the female. It is thought that the zig and the zag represent attraction and fear respectively. A male stickleback needs to be attracted to a suitable female, but at the same time needs to fear another fish within his territory. Laboratory studies have revealed that at the height of his mating season, the male's dance has more zig and less zag. This is characterized by energetic thrusts towards the female, and slow withdrawals. As the male hormone level declines the dance has less zig and more zag (slow movements towards the female and faster movements away).

The female, once attracted by the dance, will follow the male to the nest. At the entrance to the nest the male pokes his head into the sand (or mud). This behaviour is the sign stimulus for the female to wriggle through the nest. In the laboratory, a red model poked into the sand will cause the female to wriggle through a non-existent nest.

If the female accepts the robustness of the nest, she will lie in it, with her head and tail protruding from each end [now you see why the male was so fussy about the length of his nest]. The male now goes on to rub the tail end of the females back. After a while, the female will lay her eggs. If the female requires much stimulation, the male may break off to vent his frustration with a displacement behaviour, known as 'fanning' (described later).

Once the female has laid her eggs, she is forced out one end of the nest, as the excited male enters from the other end. The male fertilises the eggs, and then chases the female off.

Whilst the male is waiting for the eggs to hatch, he not only keeps an eye on predators, but also makes sure the water around the eggs has sufficient oxygen. He does this by 'fanning' fresher water towards the eggs, using his fins. To demonstrate that 'fanning' is not dependent upon an internal drive (i.e. hormone level), castrated males still exhibited the parental drive (i.e. fanning behaviour), even though mating behaviour was abolished.

As can be seen above, male fanning behaviour increases as the eggs mature, and ceases by the tenth day, by which time the eggs should have hatched.

When Tinbergen sucstituted the eggs for a fresh batch on the sixth day after the original batch had been layed, the fanning behaviour became less, as would be expected with a fresh batch of eggs. Interestingly, the fanning behaviour, although less than the fifth day was much higher than the first day. This demonstrates that behaviour was still affected by the first batch (ie the response to the sign stimulus of the first batch had not died out). On the ninth day the response to the first batch has disappeared, and can be seen by a drop in the fanning behaviour. The fanning behaviour will not stop completely as the stickleback is still responding to the sign stimulus of the second batch of eggs.


Return to Gary Sturt's homepage