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Overview:
This lesson asks students to think about why hammerhead sharks have "hammers" and to read and analyze what scientists think about this question. Students will conclude by pretending to be researchers investigating this question further and describing their upcoming research.
Connections to the Curriculum:
Geography, life science
Connections to the National Geography Standards:
Standard 8: "The characteristics and spatial distribution of ecosystems on Earth's surface"
Time:
Two to three hours
Materials Required:
- Computer with Internet access
Objectives:
Students will
- discuss possible reasons why some sharks evolved to have hammer-shaped heads;
- read theories about why hammerheads have "hammers," and write summaries; and
- write one- to two-page reports describing research that they might do to learn more about why hammerheads have "hammers."
Geographic Skills:
Asking Geographic Questions
Acquiring Geographic Information
Organizing Geographic Information
Answering Geographic Questions
Analyzing Geographic Information
S u g g e s t e d P r o c e d u r e
Opening:
Have students look at this picture of hammerhead sharks. Ask them to hypothesize why these sharks have such unusually shaped heads, and discuss their ideas as a class. Ask students to explain how the processes of evolution and natural selection might have helped in the development of the hammerhead's "hammer."
[Optional: Review with the class the taxonomic groupings used in biology: kingdom, phylum, class, order, family, genus, species. This can be remembered by the mnemonic device: "King Phillip came over for ginger snaps."]
Development:
Have students go to PBS's Island of the Sharks: World of Sharks and select "Who's Who of Sharks." Ask them to scroll down and look briefly at the eight different orders of sharks. What initial differences do they notice between these shark orders?
Have students look at a list of the nine species of hammerheads that comprise the Family Sphyrnidae. Ask them to scroll up on this page to find out in which order Family Sphyrnidae belongs; they will find that they are in the Order Carcharhiniformes (ground sharks).
Ask students to return to Who's Who of Sharks and click on Order Carcharhiniformes, where they should read the introduction in the left column.
Reiterate that, as this page states, there are nearly 200 species of ground sharks (Order Carcharhiniformes). Included in this number are nine hammerhead species (Family Sphyrnidae). Scientists have long wondered why this small group of ground sharks evolved to have their unusual hammer-shaped heads. Explain that there are different theories about why this occurred.
Have students scroll down to Family Sphyrnidae and write answers to this question on their own paper: "According to this page, what might be one of the reasons for hammerheads' 'hammers'?"
Ask students to explore ReefQuest Expeditions: Topics in Shark Biology to read other ideas about the shape of the hammerhead shark's head. This page contains some advanced vocabulary words and phrasing, so provide students with this glossary and ask them to find the main ideas rather than the exact details of the text.
Glossary (terms that will help you understand the text):
lateral—related to the side
cartilaginous—containing cartilage
hydrodynamic—related to movement through water
airfoil or canard—airplane wing
negatively buoyant—not floating; sinking
autonomic control—involuntary (the shark doesn't have to think about it)
sensory—related to the senses
enhanced sensory acuity—sharpened senses
olfactory—related to the sense of smell
Ask students to write a brief summary of each of the two arguments made in this article.
Closing:
Hold a class discussion on the possible reasons why hammerheads have "hammers." Ask students to use information they have read on the Web as well as their own ideas about why evolution might have led to this interesting head structure.
Suggested Student Assessment:
Have students pretend that they are scientists who specialize in studying hammerhead sharks. Their lifetime career goal is to figure out why hammerheads have "hammers." They are planning a new round of scientific investigations into this question.
Ask students to write one- to two-page reports describing their upcoming research. Their reports should address the following points:
- A brief overview of hammerheads: their scientific classification, physical features, and habitat (students can return to "Who's Who of Sharks" for this information)
- Which hypothesis they will investigate (they should reexamine the ideas they have read about and choose a hypothesis to test)
- Why they believe this hypothesis is worth considering
- Extra credit: How they will test this hypothesis
Extending the Lesson:
- Have students write essays or create visual presentations about strange-looking animals, including hammerheads and other animals with unusual physical features. Their essays or presentations should explain the most likely reasons each animal evolved to have this feature.
- Have students go to the shark senses page. Ask them to read about sharks' six senses and hypothesize how a hammerhead's "hammer" might enable it to use its senses differently or better than other sharks.
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