Effect of Age and Experience On the Filial Behaviour of Tilapia Mossambica Fry (Pisces : Cichlidae)

Abstract

AbstractThis study investigated the nature of social bond formation in fry of the maternal moutbrooding cichlid fish, 1'. mossambica. Attention was especially devoted to possible behavioural predispositions of the fry in addition to the effects of early social experience. Three different maternal models were used in the study: I) a black pit model, which previous research had indicated might be highly attractive to the fry; 2) a red rectangle, which previous research had indicated might be an inadequate maternal model; and 3) a preserved female T. niossambica. All fry were removed from their mother's mouth as eggs and hatched under artificial conditions so that they were never exposed to normal maternal stimuli. Separate groups of naive fry were tested for their responsiveness to all three models on days 10, 12, 16, and 22 post-hatching; under normal conditions fry are released from their mother's mouth for the first time about day 10 and leave their mother's presence about day 22 post-hatching. Other groups of fry were exposed to one of the three maternal models for varying lengths of time on day I, I through 6, 8, 10, or 12 and then tested for their responsiveness to all three models on day 10, 12, 16, or 22. It was found that all groups of naive fry of a given age exhibited the same level of responsiveness to all three maternal models. There was a peak of positive responsiveness on day 12 and a decrease by day 16; this pattern is similar to the pattern of responsiveness exhibited by maternally reared fry toward their real mother. Previous exposure to any of the models failed to prevent the decline in positive responsiveness observed in naive fry on day 16. Previous exposure to models also failed to have any effect, at any age, on the response of fry to the black pit model: fry which had been exposed to models responded at the same level as naive fry of the same age. On the other hand, previous exposure with any model often resulted in a significant decrease in positive responses to the red rectangle while such experience had an intermediate effect on later responsiveness to the preserved female T. mossambica. It was concluded that T. mossambica fry hatch with an initial perceptual schema that predisposes the fry to react to certain characteristics of the broody mother. However, when the fry are naive they initially respond to almost any object, regardless of whether or not the object fits their schema. If the object to which they initially respond does not fit their schema (i.e., the red rectangle), experience with the inadequate object will cause them to fail to respond to the inadequate object in the future; previous experience with objects that better fit their schema will also result in a similar lack of positive responsiveness. On the other hand, the fry will respond positively to a model that closely matches their schema (i.e., the black pit model) regardless of their previous experience. Finally, the fry's initial perceptual schema undergoes an irreversible developmental deterioration. This deterioration explains why the fry in the present study exhibited a significant decrease in positive responsiveness by day 16 regardless of their previous experience; it also explains why normally reared fry always leave their real mother by approximately day 22. It would be highly adaptive for the fry to ignore or avoid maternal stimuli after day 22 since an attraction to mouth-size holes could be fatal once they have left their mother.