The establishment of a 'normal' population and its behavioural maintenance in the guppy-Poecilia reticulata (Peters)

Populations of the guppy—Poecilia reticulata (Peters)—were established with a single male and female fish per compartment. In spite of excess food, regular water changes, constant temperature and continuous aeration, the populations eventually reached a stable asymptote which was proportional to the size of the compartment, particularly when the populations were visually isolated from other populations. A weight asymptote was achieved some time after the population plateau. There was a tendency for females to exceed the numbers of males at the asymptote. Fish from the asymptotes were rearranged to relatively increase or decrease the populations, on the expectation that the populations would revert to the original asymptote. This did not happen. It was hypothesized that there was a learned behavioural response which prevented reproduction in the altered populations, but not an ovarian ‘carry‐over’ for the ovaries of decreased and increased populations were distinguishable. The learning hypothesis was substantiated in fish of a single generation which were crowded, then uncrowded, and continued to show reduced courtship and increased aggressive behaviour.