9.14 THIS IS HOW WE DO IT: When paternity uncertainty seems greater, is paternal care reduced?

Evolutionary theory generates numerous predictions on how organisms will make decisions about parental investment. In the previous section we saw that there should be a relationship between a male’s certainty of paternity and his investment in the offspring. When paternity certainty is low, males should benefit by reducing their parental investment and, instead, seeking additional mating opportunities.

Often, however, it can be difficult to test such predictions experimentally. The results of a manipulation can be difficult to interpret—is an observed decrease in paternal investment really a response to a perceived decrease in paternity certainty, or is it simply a consequence of the experimenter’s presence or interference?

A powerful strategy to address this challenge is to use an experimental manipulation that makes contrasting predictions in two different situations. That way, a response to the manipulation in one situation provides evidence in favor of the prediction, while a response in the second situation provides evidence for a lack of response to the manipulation. This serves as a sort of internal control in the experiment. Here’s how a researcher used this approach in two clever experiments.

The system Bluegill sunfish live in lakes and rivers in North America. Most males reach maturity at age 7 years. During the breeding season, males use their tail to create a depression in the sandy bottom and chase away almost everything that approaches this nest. Females come and lay eggs—which the male fertilizes—and leave shortly afterward. The male remains to guard the eggs and the small offspring after they hatch. Typically, he doesn’t even take a break to forage during this period of parental investment.

About 20% of the bluegill males in a population mature at age 2 years, at a much smaller size. These males, called “cuckold males,” hide near the nests of other males and attempt to sneak into the nest, fertilize the eggs, and escape without being detected by the nest “owner.”

Male bluegills are unable to distinguish between eggs they have fertilized and eggs fertilized by another male. They can, however, tell whether just-hatched offspring are their biological offspring (from a chemical cue in the offspring’s urine).

Experiment 1 The researcher randomly chose 34 nests. Around each nest he placed two glass containers, each containing two small cuckold males, and left them there for the duration of the egg-laying. The cuckold males couldn’t fertilize the eggs, but they could be seen by the male at the nest. As a control, the researcher placed two empty glass containers around each of 20 other nests.

A day after the eggs were laid, the researcher placed a glass container with a predator fish (that eats eggs and just-hatched fish) near each nest, and then evaluated parental care by measuring how vigorously the nest owner defended his eggs over the course of two 30-second periods. Parental care was measured again after the offspring hatched.

In each case, the researcher allotted a “parental care score,” reflecting the intensity of the male’s guarding of the eggs and defense of the hatched offspring.

Prediction a: The presence of the cuckold males should reduce the nest owner’s paternity certainty and therefore reduce his egg-guarding efforts.

Prediction b: After the offspring hatch, the nest owner can determine whether they are his genetic offspring, so he should not exhibit any reduction in parental care relative to males in the control group.

Results of Experiment 1 Manipulation: Cuckold males nearby, but all eggs fertilized by nest owner

Experiment 2 The researcher randomly chose 20 new nests (none from Experiment 1) and, the day after egg-laying and fertilization, he removed one-third of the eggs from each nest and replaced them with unrelated fertilized eggs from another male’s nest. Then, as in the first experiment, the researcher placed a glass container with a predator fish near the nest and evaluated parental care.

Prediction a: Prior to hatching of the eggs, the nest owner should exhibit the same egg-guarding efforts regardless of whether or not the eggs were swapped.

Prediction b: After the offspring hatch, because the nest owner can determine whether they are his genetic offspring, he should exhibit reduced parental care relative to the control males.

Results of Experiment 2 Manipulation: Eggs swapped with those fertilized by a different male.

In each of the two experiments, males decreased their parental care relative to males in the control group in response to signs that the offspring were less likely to be their own genetic offspring. The experiments provide strong evidence that genetic relatedness to offspring plays an important role in parental care by a male bluegill sunfish.