Parental investment in threespine stickleback, Gasterosteus aculeatus

Parental investment, defined as any parental activity that1 increases the survival of offspring at a cost to the parent, is a useful concept for examining the selective bases of parental behavior. To maximize its lifetime production of surviving offspring, a parent should adjust its level of risk in a parental investment depending on the value of its future "prospects" in relation to its present young. as present young increase in value, either by number or age, a parent should expend more risk in a parental investment so long as the effectiveness of its behavior does not diminish. This will often be the case for a parent that defends a nest containing eggs. The prediction of an increase in parental risk for more eggs or older eggs has been tested using two natural populations of threespine stickleback, Gasteros teus- acule^tus L. Male sticklebacks that were guarding nests were presented with a dummy predator, the prickly sculpin Cottus asper, and their responses were measured. Those males that remained within their nest area and attacked the dummy sculpin had a larger number of eggs or older eggs than those males that deserted their nests and never attacked the dummy. In the population that is sympatric with sculpins, males that initially attacked the sculpin's head had older eggs than those which avoided the head but attacked the tail area. The level of the male's responsiveness, and associated risk, was recorded in a series of quantitative measures. The time it took a male to return to its nest, as well as the time to attack the sculpin dummy, was shorter for males with a larger number of eggs or older eggs. The number of bites at the dummy in the first minute after the initial attack increased as the egg number and egg age increased. Changes in male risk were in the predicted direction and none of the responses could be associated with any single biological or environmental factor other than the number or age of the eggs.