Date of Award


Degree Type


Degree Name

Doctor of Philosophy


I examined the effects of offspring stage, brood size, and parental age on the patterns of brood defense in nesting male smallmouth bass (Micropterus dolomieui), a centrarchid fish with male parental care, in Lake Opeongo, Ontario. Parental defense against conspecific models of equal length ((+OR-)1 cm) was used as a measure of parental commitment to the brood. Based on Williams' principle, or the trade-off between present and future reproduction, parental males should adjust their level of brood defense to reflect the value of current offspring relative to their expected future reproduction.;Parental defense increased as a model intruder approached the nest and as offspring progressed from egg to hatched embryo stages (wrigglers), and peaked prior to 'swim-up' of fry from the nest. This pattern of brood defense over a nest cycle demonstrates the changing value of present offspring relative to the parent's expected future reproduction within a season.;In response to manipulating brood size, parental males increased their defense for augmented broods and decreased their defense for reduced broods, relative to a control. This demonstrates that parental males can adjust their level of brood defense to reflect future prospects of reproductive success.;Long-term data on population estimates and tagged nesting males in Lake Opeongo provided the necessary life history information to predict patterns of parental defense based on the reproductive value of males. Reproductive value at the age of first nesting for males in different age groups was partitioned into current reproduction (wrigglers in the nest) and to future reproduction (residual reproductive value). This index indicated that young nesting males (age 4, 5, and 6) should be more aggressive than older males (age 8+). The prediction was not supported; older males were more aggressive than young males. This discrepancy can be interpreted as either reduced reproductive effort in young males or increased effort in older males because of possibly larger resource budgets. The life history of this population suggests that size-based energetic constraints are important in determining patterns of brood defense between different age classes.



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