Horned reptiles may prefer to lie in wait

This exercise is a part of Educator Guide: Analyze a Clinical Trial and Horned Reptiles / View Guide
Saharan horned viper
Lizard and snake horns — like those of the Saharan horned viper (Cerastes cerastes) — may be less likely to evolve in more active lizard and snake species because they may make the reptiles easier to spot by both predators and prey. Paul Starosta/Getty Images

Directions for teachers:

To engage students before reading the article, have students answer the “Before Reading” questions as a warmup in class or for homework. Then ask students to read the online Science News article “Why do some lizards and snakes have horns?” Afterward, have them answer the “During Reading” questions. For deeper analysis on the evolution of physical traits and the evolutionary tree, have students discuss the “After Reading” questions. This article also appears in the January 27 issue of Science News. Science News Explores offers its own version of the same article written at a middle-school reading level. Post this set of questions without answers for your students using this link.

Directions for students:

Answer the “Before Reading” questions. Then read the online Science News article “Why do some lizards and snakes have horns?” and answer the remaining questions as directed by your teacher.

Before Reading
1. List three animals with horns of some type. What benefit might these horns provide to each animal? Write one potential benefit for each animal.

Answers will vary. Examples may include antelope, deer, certain lizards, and more. Potential benefits could include protection/defense, territorial disputes, courtship and mating rituals.

2. Evolution is the process by which one species develops and diversifies over generations into a new species. What types of changes might drive evolution? Come up with three examples — perhaps including the environment or climate — that might drive evolution.

Answers will vary, but some students may describe changes in predation, the environment (such as fractured habitat), or climate (such as increased precipitation or higher temperatures) that lead to different, more beneficial characteristics.

During Reading

1. Describe how most species of horned lizards and snakes catch their prey.

Most species of these lizards and snakes lie in wait to eventually ambush their prey.

2. How might horns on a lizard be a disadvantage when hunting?

The horns may be visible to potential prey, alerting it to the lizard’s presence.

3. What question did Frederico Banfi and his team hope to answer?

Frederico Banfi and his team wanted to learn whether a horn’s camouflage benefits differ between reptiles with different hunting styles.

4. Into what two categories did Banfi and his team group various species of reptiles in their study?

Banfi and his team group the reptiles into “sit-and-wait predators” and “active pursuers.”   

5. How many species of reptiles were included in this study? How many of those had horns on the snout, eyebrows or head?

Researchers included 1,939 species of reptiles in this study; 175 of those had horns on the snout, eyebrows, or head.

6. Researchers mapped their data onto a previously published lizard-snake evolutionary tree. About how many times did horns evolve independently? In which of the two categories of reptiles you listed in question 4 (in the During Reading section) did this trait evolve most frequently?

Horns evolved about 69 times, most commonly among the “sit-and-wait” predators.

7. According to Banfi, how might the presence of horns present reptiles a disadvantage? Under what circumstances are reptiles more likely to have horns?

Banfi explains that horns may help some species but hinder others. If the reptiles are more active, horns may hinder their movement. But if the reptiles are “sit-and-wait” predators, Banfi suggests the horns may not be such a disadvantage and may help the organism via camouflage, defense, or mate selection.

8. What are some possible next steps for scientists studying horned reptiles?

Scientist Theo Busschau points to the need for scientists to continue testing their hypotheses about the advantages and disadvantages of horns in reptiles to increase understanding about why they evolved.

After Reading

1. An evolutionary tree (also called a phylogenic map) is a kind of diagram that shows the evolutionary changes and development of different species over time. A series of organisms may exist for many generations then split, creating new species that now form new tree-like branches on a map. Check out the following example. The image or use of the term tree serves as an analogy. An analogy is a comparison between two things — usually one thing that is familiar and one unfamiliar. The analogy helps explain the unfamiliar concept by relating it to the familiar one. Do you think a tree is a good analogy to show changes in species over time? Why or why not? 

Relating a species’ evolution to a tree is a good analogy because the branching of the tree’s limbs is similar to how species evolve and diversify into new species over time.

2. How was an evolutionary tree helpful to scientists for this study? Explain.

Scientists used a lizard-and-snake evolutionary tree to note hunting style and the presence or absence of horns for different lineages of these reptiles. It helped them determine whether horned projections evolved independently and whether they were more prevalent for sit-and-wait or active predators. 

3. Convergent evolution is when two unrelated species independently develop similar strategies or traits. Give one example of convergent evolution from this article. What contributed to the convergent evolution you’ve described?

An example of convergent evolution from this article is when the different species of reptile evolve similar horns. Hunting strategy — such as a more active vs. ambush hunting techniques — contributed to this convergent evolution.