Abstract
How much a parent invests in its offspring has been a focus in many studies assessing parental care dynamics across taxa. An animal with offspring must make investment decisions and behave in a way that will maximize its lifetime reproductive success. Biparental species face a unique conflict in terms of parental investment: the parent must take into consideration how much it invests itself, and also how much its partner invests. A lack of care by both parents may lead to brood loss, but parental care is energetically costly to each parent, resulting in cooperation and conflict. Previous studies have examined investment allocation by a single parent or how parents in a biparental system allocate their investment against a single attacker. However, very few studies have examined investment allocation decisions when a biparental pair is threatened with multiple predators that also differ in size - a realistic threat in the wild. The convict cichlid (Amatitlania nigrofasciata), is a biparental species that provides prolonged parental care, and is therefore an ideal species for examining this question. Parental defense is critical for offspring survival, and therefore, involves appropriate decision making by the breeding pair. That is, each member of the guarding pair does not simply strike at the closest attacker or attack at random, but instead assesses the threat level and responds with a consistent behavior. The objective of this research is to explore this behavioral response by a pair of parents that differed in size (i.e., a large male and small female). To investigate this research question, a constructed predatory-pair model (two model fish, differing in size) was presented to a pair of convict cichlids that were actively guarding fry, simulating a predator attack. For each experimental trial the model was presented to the pair and moved in a figure-8 pattern towards the pair for 30 seconds, pulled away for 30 seconds, and presented for a final 30 seconds. During each model presentation, the number of bites that each fish took at each of the predators was recorded, producing four scores. The four scores from the first presentation were added to the four scores from the second presentation, to produce four scores for the trial (i.e., male versus large, male versus small, female versus large, and female versus small). This procedure was replicated for 5 consecutive days. The four scores from each day were summed to produce four scores for the pair for the experiment (i.e., male versus large, male versus small, female versus large, and female versus small). The hypotheses for this experiment were as follows: (1) the larger parental fish (the male) in the convict cichlid pair will bite the large model attacker fish more than he will bite the small model attacker fish, and (2) the smaller parental fish (the female) will bite the small model attacker fish more than she will bite the large model attacker fish. A total of twelve convict cichlid pairs were used in the behavioral experiment, which was performed over a nine month period (May 2019 – January 2020). A paired t-test was used to examine the impact of predator size (small vs. large) on parental investment decisions within each sex, measured by the number of bites each parent took at each model fish during the duration of the experiment. Results revealed a consistent pattern within each sex, with significant differences in the number of bites at the model fish predators by both the male (p-value = 0.0001) and by the female (p-value = 0.0003) of the twelve pairs examined. Specifically, the large fish (the male) within a pair bit the large model attacker significantly more than the small model attacker and the small fish (the female) bit the small model attacker more than the large model attacker, supporting the hypotheses. Results from this research suggest that the size of a predator, when multiple are present, influences the decision-making of a biparental pair. This research builds off of a solid foundation of theory and experimentation and expands our understanding of investment allocation decisions in biparental fishes.