Problem 18 - Animal Behavior: An Evolutionary Approach

Question

In species like boobies and egrets, parents' decisions about incubation and hormone allocation put their second or third offspring at great risk. If the second or third egg laid is destined to produce a chick that will die within a few days, why don't parents save the energy that goes into the superfluous egg, which would also enable parents to give more to their favored offspring? One possibility goes under the label of the insurance hypothesis: adults invest in a backup egg in order to have a replacement for a favored first-laid egg should something happen to that egg or to the nestling itself after the egg hatches. \(^{11}\) How would you test this hypothesis experimentally?

Step-by-Step Solution

Answer

Test by comparing chick survival in nests with and without backup eggs.

Step 1: Define the Hypothesis

The insurance hypothesis suggests that parents lay additional eggs to serve as backups if something happens to the first-laid egg or the nestling it produces. Our task is to test if these additional eggs indeed function as such an insurance policy.

Step 2: Design the Experiment

To test the hypothesis, design an experiment where you manipulate the presence of backup eggs. Create two groups of nests: one control group, where the regular egg-laying sequence is left untouched, and an experimental group, where the second or third eggs are removed.

Step 3: Monitor and Record Outcomes

In both groups, monitor the nests regularly. Record whether the first-laid egg survives to hatching and the survival rate of the chicks in both groups. Pay close attention to differences in survival and any cases where a backup egg (second or third egg) replaces the first.

Step 4: Analyze Results

Analyze the data to determine if the presence of backup eggs increases the chances of having at least one surviving chick in the nest. If nests with backup eggs have higher survival rates for at least one chick, this would support the insurance hypothesis.

Step 5: Draw Conclusions

Conclude whether the data supports the hypothesis. If nests with an intact set of eggs (including secondary and tertiary ones) show higher chick survival rates, it suggests that additional eggs serve as useful backups. Otherwise, the hypothesis may need reevaluation.

Key Concepts

Backup Eggs

In certain bird species, such as boobies and egrets, the concept of 'backup eggs' is intriguing. These backup eggs serve a vital role in ensuring that at least one chick survives even if the first-laid egg or its resulting chick encounters difficulties. The insurance hypothesis posits that parents lay additional eggs not merely out of abundance, but for safeguarding against potential mishaps.

This strategy is akin to a backup plan. If anything were to happen to the primary egg, the parents have another ready to take its place. It is a natural insurance policy. By having additional eggs, parents increase the likelihood of producing a surviving offspring.

To verify this, experimental approaches manipulate whether these backup eggs are allowed to stay in the nest. The aim is to determine if these supposedly extra eggs genuinely serve their backup purpose. By comparing outcomes with and without these spare eggs, researchers can better understand the role of backup eggs in these species.

Parental Investment

Parental investment refers to the resources—energy, time, and care—that parents expend on their offspring. In the context of birds like boobies and egrets, laying and nurturing additional eggs is a significant form of parental investment.

Every additional egg a bird lays means more nutrients and energy spent, and it might initially seem counterintuitive, considering the risks involved to subsequent offspring. However, this added investment can be crucial. If backup eggs increase the chances of at least one chick's survival, then the extra parental investment pays off.

The decision to lay multiple eggs, therefore, involves weighing the potential loss of resources against the probability of achieving successful offspring development. Successful nurturing of backup eggs reveals the high stakes and potentially wise allocation of parental effort, guided by the long-term survival strategies of these bird parents.

Experimental Design

To test the validity of the insurance hypothesis related to backup eggs, a well-thought-out experimental design is crucial. The experiment involves two main groups:

• The control group, where the egg-laying process remains intact, including all laid eggs—primary, secondary, and tertiary.
• The experimental group, where secondary and tertiary eggs are removed, leaving only the primary egg.

The nests in both groups are monitored to assess if removing backup eggs negatively affects chick survival rates. This involves measuring hatching success and chick survival through observations over time. By comparing data from the two groups, researchers discern the true role of backup eggs.

Such a design helps isolate whether additional eggs function as a safety net, providing crucial insights into parents' strategies for ensuring offspring survival.

Chick Survival Rates

Chick survival rates in these studies are a key metric for evaluating the effectiveness of the insurance hypothesis. By comparing survival between nests with and without backup eggs, researchers can assess the strategy's success.

A higher survival rate in nests with backup eggs would indicate that these eggs do indeed provide a significant advantage. The process involves observing which eggs produce chicks that survive through the vulnerable early stages of life.

Data collected involves noting when and if chicks replace fallen siblings from backup eggs and general survival until fledging. Seeing higher chick survival rates in intact nests suggests that multiple eggs truly serve as a biological insurance policy, validating why parents would engage in such a seemingly costly reproductive strategy.