Group Behavior

Essential Question

How does living in a group affect an individual’s chances to survive and reproduce?

Fish swim in schools, birds fly in flocks, and wolves hunt in packs. Other animals live alone. In this lesson, you’ll look at the benefits and costs of group behavior and use data to evaluate how group living affects survival and reproduction. By the end, you should be able to use evidence to explain how group behavior can help or harm an individual.

Benefits of Group Living

• Group behavior: animals interacting and living together
• Many eyes: predators are spotted sooner
• Dilution: each individual is a smaller share of the targets
• Teamwork: hunting and defending together
• Shared warmth: huddling saves body heat

Many animals live or travel in groups, such as schools of fish, flocks of birds, herds, and packs. Group behavior can improve an individual’s chances to survive and reproduce in several ways. With more individuals watching, a predator is often spotted sooner, so each individual can spend less time scanning and more time feeding. When a predator attacks, each individual is a smaller share of the possible targets, an effect called dilution. Members of some groups hunt or defend together, which allows them to succeed at tasks that one individual could not. Some animals, such as emperor penguins, huddle to save body heat.

Costs of Group Living

• Competition for food, mates, and nest sites
• Disease and parasites can spread more easily
• Large groups can be easier for predators to see
• Costs can rise as the group gets larger

Group living also has costs. Group members use the same resources, so they compete for food, mates, and nest sites. Disease and parasites can spread more easily when individuals are close together. A large group can also be easier for a predator to notice. These costs often become greater as the group gets larger, which is one reason group size matters.

Weighing Costs and Benefits

• Benefits and costs both affect survival and reproduction
• The net effect depends on group size and conditions
• Evidence can show which effect is stronger
• Group behavior also affects a species’ chances to survive

Whether group living helps an individual depends on whether the benefits are greater than the costs under the conditions it lives in. Scientists evaluate this by comparing evidence, such as how much time individuals spend watching for predators, how many offspring survive, and how those measures change with group size. When group behavior helps individuals survive and reproduce, it can also help the species continue. Because the balance can change, the best group size can differ from one species or environment to another.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Safety in Numbers

A predator attacks a group of animals and catches one animal at random in each attack. Find the chance that a particular animal is caught in a group of 5 and in a group of 50. How many times lower is the chance in the group of 50? Name the benefit of group living that this shows.

Show the solution

1. Group of 5: 1 ÷ 5 = 0.20, or 20%
2. Group of 50: 1 ÷ 50 = 0.02, or 2%
3. Compare: 20% ÷ 2% = 10, so the chance is 10 times lower in the group of 50.
4. Benefit: dilution. Each individual is a smaller share of the possible targets.
5. Limit of the model: it assumes one animal is caught at random. Real predators do not always choose at random.
6. Answer: 20% in a group of 5; 2% in a group of 50; 10 times lower; dilution.

Example 2: Group Size and Scanning

Researchers measured the percent of time each bird spent scanning for predators in feeding flocks of different sizes (sample data). Describe the pattern. Read the value for a flock of 4. Find the percent decrease in scanning time from a single bird to a flock of 16. If a bird is not scanning, assume it is feeding. What percent of its time does a bird in a flock of 8 spend feeding?

Show the solution

1. Pattern: as flock size increases, the time each bird spends scanning decreases.
2. Flock of 4: 20% of the time.
3. Percent decrease: (40 − 8) ÷ 40 × 100 = 80%
4. Feeding at 8 birds: 100% − 12% = 88% of the time.
5. Reasoning: in a larger flock, more birds are watching, so each bird can scan less and feed more. More time feeding can improve an individual’s chances to survive and reproduce.
6. Answer: Scanning decreases as flock size increases; 20% at 4 birds; 80% decrease; 88% feeding at 8 birds.

Example 3: Is a Bigger Colony Better?

Researchers counted the fledglings (young birds that left the nest) per nest in bird colonies of different sizes (sample data). Write a claim, evidence, and reasoning statement about how colony size relates to reproductive success. Include the percent change from a colony of 10 to a colony of 50, and from 50 to 200.

Show the solution

1. 10 to 50 nests: (3.5 − 2.6) ÷ 2.6 × 100 = 35% increase
2. 50 to 200 nests: (3.5 − 2.2) ÷ 3.5 × 100 = 37% decrease
3. Claim: in these data, reproductive success is highest in a medium-sized colony, not in the largest one.
4. Evidence: fledglings per nest rose from 2.6 in a colony of 10 to 3.5 in a colony of 50, which is a 35% increase. It then fell to 2.2 in a colony of 200, which is 37% lower than the colony of 50.
5. Reasoning: group living has both benefits and costs. In smaller colonies, benefits such as predator detection may be stronger. In larger colonies, costs such as competition and disease may become stronger. These data show the net effect but do not show which cost or benefit caused it.
6. Answer: Fledglings per nest peaked at 3.5 in a colony of 50: a 35% increase from 10, then a 37% decrease at 200.

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Practice

Calculate how group size changes an individual’s risk, classify costs and benefits of group living, and use data to evaluate how group size affects survival and reproduction. Try it on your own first, then check your work with the answer key.

• Group Behavior Practice Worksheet (PDF)
• Group Behavior Practice Answer Key (PDF)

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Lesson Slides

• 2.6 Group Behavior Slides (PDF)

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Big Ideas

• Group behavior can help individuals survive and reproduce
• Benefits include many eyes, dilution, teamwork, and shared warmth
• Costs include competition and the spread of disease
• The net effect depends on group size and conditions
• Evidence, such as data on survival and offspring, is used to evaluate group behavior

Back to the essential question: living in a group can improve an individual’s chances to survive and reproduce through benefits such as detecting predators sooner and dilution, but it also has costs such as competition and disease. Which effect is stronger depends on group size and conditions, and data can show the net effect. That completes Unit 2. Use the button below to return to all Biology units.

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