Case Teaching Notes for
“A Deadly Passion: Sexual Cannibalism in the Australian Redback Spider”

Introduction / Background

This case study is a “clicker case.” Clicker cases combine two teaching techniques, case studies and clickers, to offer an instructional strategy that allows active learning in large science classrooms. Case studies put subject matter into context, and can engage students with interesting and relevant real life examples, foster small or large group discussion, and provide genuine problems to solve. Because the case method relies on discussion, it has been primarily used in small classes, tutorials, or lab sessions. Now, however, with the addition of electronic personal response systems, or “clickers,” faculty can use case study teaching in large classrooms, fostering small group discussion in a large class and gaining immediate feedback from students.

This clicker case teaches students about the distinction between proximate and ultimate causes of behavior using the fascinating courtship and mating behavior of the Australian redback spider. The case is presented in class using a series of PowerPoint slides punctuated by questions (called “clicker questions”) that students respond to before moving on to the next slide. In this way, students work through (and understand) the distinction between proximate and ultimate causation, and make predictions for various hypotheses.

Although developed for a general biology class, this case would also be suitable for use in non-majors introductory biology or behavioral ecology courses. Prior to this case, students should be able to define behavior, distinguish between innate and learned behaviors, discuss the evidence for genetic contributions to behavior, and explain the conditions under which behavior can evolve by natural selection.

Objectives

After completing this case, students should be able to:

• Distinguish between proximate and ultimate causes of behavior.
• Explain how proximate and ultimate questions about animal behavior are linked in their evolutionary basis.
• Use natural history and experimental data to support or reject hypotheses.

Misconceptions

• Students often equate fitness with survival, and may be surprised that self-sacrifice actually increases the fitness of males.
• Students may not appreciate that a single question about behavior can have two “right” answers: one proximate, one ultimate.
• Students may not understand the difference between proximate and ultimate causes. Both proximate (how) and ultimate (why) questions are legitimate, but different, approaches to the study of behavior.
• Students may not grasp that genetic explanations for behavior are proximate, not ultimate explanations.

Classroom Management / Blocks of Analysis

This clicker case has been used in a general biology classroom of 300 students, and was preceded by a lecture introducing types of behavior and behavioral genetics. Each student was required to participate with a student response system or clicker. The case was taught in a 50-minute class period in a large, auditorium-style lecture hall. Students were asked to discuss the questions in small groups created informally within the large classroom.

Teaching the Case

Note: This presentation includes a combination of text, questions, and photos. Instructors who prefer to minimize text on slides may easily delete text and deliver this information in other formats.

Slide 1 is the title slide.

Slide 2 explains the nature of proximate questions about behavior. When we observe a certain behavior, we can ask two kinds of questions about the behavior. (1) How does an animal carry out a particular behavior? This question addresses proximate causation. (2) Why does the animal show this behavior? This question addresses ultimate causation. Proximate causation addresses how mechanisms within the animal operate to trigger a behavior and make this behavioral response possible. Proximate explanations of behavior may address genetic, developmental, physiological, or psychological bases for behavior.

Slide 3 explains the nature of ultimate questions about behavior. Ultimate causation seeks to explain why the proximate mechanisms underlying this behavior have evolved. Ultimate explanations of behavior address the ecological functions of a behavior and its evolutionary basis: the effect of the behavior on the evolutionary fitness of the animal.

Slide 4 defines evolutionary fitness. Evolutionary fitness is the key concept that underlies ultimate, or “why,” questions and explanations about animal behavior. Be sure that your students understand that fitness is relative, and depends not just on survival, but on reproductive success.

Slide 5 defines adaptive behavior as a behavior that increases relative fitness. Be sure that your students understand that this behavior must be genetically determined, at least in part.

Slide 6, the first clicker question (CQ#1) of the case, presents a natural history fact, namely, that red-crowned cranes breed in spring and early summer. Students are asked to choose a proximate explanation for this fact.

Slide 7 presents the second clicker question (CQ#2). Students are asked to choose an ultimate explanation for the same natural history fact.

Slide 8 introduces sexual cannibalism. The instructor should point out that it is challenging to determine the ultimate advantage of extreme behaviors, and none is more extreme than sexual cannibalism. A number of male arthropods, including the praying mantis, scorpion, and Australian redback spider, may be devoured by their mates before, during, or after copulation. Is sexual cannibalism adaptive for the female, perhaps as a source of nutrients? Can self-sacrifice possibly be adaptive to the male, or is he simply unable to escape predation by his hungry mate? The unusual courtship and mating behavior of male redback spiders has provided one fascinating set of answers to these questions.

Slide 9 introduces the University of Toronto’s Maydianne Andrade, who researches proximate and ultimate questions about sexual cannibalism in redback spiders. Males are cannibalized by their mates in about 65% of matings (Andrade 1996).

Slide 10 describes the tiny redback male’s courtship of his much larger mate. Australian redback spiders show extreme sexual dimorphism: the mass of an average male is only 1–2% that of a female. Males are also short-lived: 4–8 weeks vs. 2 years for females (Andrade 1998). Males stop feeding when they leave their juvenile webs to search for females. Many redback males are eaten by ants or by other spiders while searching for a female’s web; over 80% die without finding a mate. When a male successfully reaches the web of a female redback spider, she is his only reproductive opportunity. The chance that he will be successful in finding a second mate is virtually zero (Andrade 2003). Once on a female’s web, a male Australian redback spider undertakes a lengthy courtship, strumming on the strands of her web for up to eight hours as he slowly approaches her.

The large spider shown in the photo on Slide 10 is the female, identifiable by the red hourglass marking on her underside. The female typically hangs upside down in her web, making the hourglass easily visible to potential predators. Near her head is the much smaller male.

The photo on Slide 11 shows the male spider’s two palps attached to his cephalothorax (fused head and thorax). The palps are specialized for sperm transfer and are the two dark masses to the right that look a little like boxing gloves. Each palp can be used to transfer sperm and ejaculatory fluid into one of the female’s two sperm receptacles.

Slide 12 describes the mating. If courtship is successful, the male mounts the female (who is hanging upside down), and inserts a palp into one of her sperm receptacles. As he begins copulation, the male somersaults to dangle his abdomen over the female’s jaws. In most cases, the female begins to feed on the male, ingesting the contents of her mate’s abdomen throughout copulation (Andrade 1996).

Slide 13 completes the description of mating in the Australian redback spider. Male redback spiders have a specialized abdominal constriction that develops during courtship, protecting the heart and other vital organs (Andrade et al. 2005). This adaptation allows the male to survive, escape, and return for a second round of copulation. When he is ready to insert his second palp, the shrunken male dismounts and performs a second, brief courtship. As he begins his second copulation, he again somersaults and his mate finishes eating him.

Slide 14 links to Dr. Andrade’s movie of the redback male somersaulting and being eaten by the female (*.mov file format, ~1.4 MB). The video will be more effective if you download it into Windows Media Player or a comparable software platform and play it as a full-screen video. The movie is also available on Andrade’s website at http://www.utsc.utoronto.ca/~mandrade/index_files/Page332.htm. Note that there are two videos on this webpage; use the second video (black and white, about 35 seconds). (The first video is flashier, but does not show the somersault and misinterprets some of Andrade’s research.)

Video Description: The female is hanging upside down, and is oriented so that her large round abdomen is in the background and her small (hard to see) jaws are in the foreground. The much smaller male is on top of the female, and faces the same direction as the female. The male inserts a palp in the female. At about 20 seconds, the male begins to somersault toward the camera. He can be identified by the dark stripes on his pale abdomen. His abdomen is then positioned in the female’s jaws, and the female becomes quite agitated as she begins to feed on him.

Slide 15 provides additional information about mating in Australian redback spiders. A single egg sac produced by a female can contain 40–300 eggs, each of which is fertilized by a different sperm. In the wild, up to six males have been observed on the web of a single female, and a female can mate with more than one male during the mating season (Andrade 1996). Because of this, a single egg sac can contain eggs fertilized by different males. It is important that students recognize that a male’s fitness depends not just on his ability to mate with a female, but on the percentage of eggs within the sac that are his. This photo shows the much larger female hanging upside down in her web. Immediately below her is the much smaller male. To the right is an egg sac containing somewhere between 40 and 300 individual eggs.

Slide 16 introduces Dr. Andrade’s research on female cannibalism in the Australian redback spider. When studying courtship behavior, it is important to consider explanations for female and male behavior separately. This is because a behavior that maximizes a female’s fitness does not necessarily maximize the fitness of her mate, and vice versa. This case examines the female’s behavior first, as it is easier to understand than the male’s behavior. Three hypotheses are presented for the female’s behavior. Note that each hypothesis has both proximate and ultimate elements. It is recommended that you not distinguish between proximate and ultimate elements as you work through the hypotheses, as students will be asked to identify proximate and ultimate causes for the female’s behavior (Slides 26 and 27) after they have considered all three hypotheses (Slides 17–25).

Slide 17 presents Hypothesis 1 (“Mistaken Prey” hypothesis) that the female redback spiders mistake males for prey. This hypothesis can be tested by comparing female behavior when feeding on prey to her behavior when eating her mate.

Slide 18 presents the third clicker question (CQ#3), which asks students which results would support this hypothesis. Note, students are not asked to identify which piece of information is factually accurate, but rather which piece of information would support the hypothesis.

Slide 19 identifies the correct prediction, and presents some relevant results. A female never eats a male until he somersaults and dangles his abdomen in her jaws. In contrast, she often attacks prey as soon as they enter the web (Andrade 1998). Students should compare the results with the (correct) prediction in order to reject or support the Mistaken Prey hypothesis. In conclusion, the results do not support the Mistaken Prey hypothesis.

Slide 20 presents Hypothesis 2 (“Mate Rejection” hypothesis) that females eat males that are unsuitable as mates. This hypothesis predicts that females would eat low-quality males that are smaller in size or mass, or less vigorous than non-cannibalized males. Also, females would eat these low-quality males prior to copulating with them. This hypothesis can be tested by comparing the quality and mating success of cannibalized and non-cannibalized males.

Slide 21 presents the fourth clicker question (CQ#4), which asks students which results would support this hypothesis.

Slide 22 presents the results. Andrade (1998) did not detect any differences in size, mass, or condition between cannibalized and non-cannibalized males. In direct contradiction to the predictions of this hypothesis, cannibalized males father, on average, twice as many offspring as non-cannibalized males (Andrade 1996). In conclusion, the results do not support the Mate Rejection hypothesis.

Slide 23 presents Hypothesis 3 (the “Hungry Lover” hypothesis) that females eat their mates because they are hungry. This hypothesis can be tested by comparing the cannibalism rates of two groups of females. One group feeds naturally, and is assumed to be food-limited. The other group has its diet supplemented with additional insect prey.

Slide 24 presents the fifth clicker question (CQ#5), asking students which results would support this hypothesis.

Slide 25 presents the results. Andrade (1998) supplemented the diet of one group of female redback spiders and allowed a second group to feed naturally. Redback spiders are known to be food-limited in the wild. When copulating males somersaulted onto the jaws of their mates, 62% of the unfed females were cannibalistic while only 29% of the fed females ate their mates. In conclusion, the results support the Hungry Lover hypothesis.

Slide 26 (CQ#6) asks students to select a proximate explanation for female cannibalism in the Australian redback spider.

Slide 27 (CQ#7) asks students to select an ultimate explanation for female cannibalism. At this point, check to be sure your students have grasped the difference between proximate and ultimate explanations. If a large number of them get these two questions wrong, spend more time on this concept.

Slide 28 introduces Dr. Andrade’s investigation of the ultimate cause of male self-sacrifice. In most species with sexual cannibalism, males behave in ways that reduce their risk of being eaten. Such males are less likely to court hungry females, and leave quickly once copulation is complete. In contrast, male redback spiders actively solicit their own consumption. Andrade’s research suggests that the male’s somersault is an adaptive behavior that increases his reproductive success (Andrade 1996).

Slide 29 presents the first hypothesis (the “Paternal Investment” hypothesis) that males benefit by contributing nutrients to their offspring. In some other insect species, males present food gifts during copulation that result in an increase in the number and size of offspring. This hypothesis can be tested for redback spiders by comparing the size and mass of egg sacs produced by cannibalistic and non-cannibalistic females.

Slide 30 (CQ#8) asks students which results would support this hypothesis.

Slide 31 presents the results. The consumption of the male does not increase the number or mass of eggs in the female’s egg sac (Andrade 1996). The male is tiny relative to the female (1–2% of her mass) and even relative to her egg sac (2.5% of its mass). Also, several males may fertilize the eggs in a single egg sac, so one male’s sacrifice might provide nutrients to another male’s offspring. In conclusion, the results do not support the Paternal Investment hypothesis.

Slide 32 presents the second hypothesis (the “Nuptial Gift” hypothesis) that males benefit from self-sacrifice by increasing their fertilization success. This hypothesis can be tested by comparing the copulation duration and the number of offspring fathered by cannibalized and non-cannibalized males.

Slide 33 (CQ#9) asks students which results would support this hypothesis.

Slide 34 presents the results. Females copulate longer with males when they are eating them: an average of 25 minutes for a cannibalized male vs. 11 minutes for a non-cannibalized male. Cannibalized males are able to transfer twice as much sperm and thus father, on average, twice as many offspring as males that are not cannibalized (Andrade 1996). While cannibalism does not affect the number or size of eggs in a sac, it does affect the percentage of eggs fertilized by the cannibalized male. Males that are cannibalized are also more successful at plugging the sperm receptacles of their mates, preventing other males from successfully inserting competing sperm (Snow et al. 2006). These data suggest that sexual cannibalism, triggered by the male’s somersaulting behavior, is adaptive for the male even though he dies in the process. In conclusion, the data support the Nuptial Gift hypothesis.

Slide 35 (CQ#10) asks students to select a proximate explanation for male self-sacrifice.

Slide 36 (CQ#11) asks students to select an ultimate explanation for male self-sacrifice.

Slide 37 compares the costs and benefits of self-sacrifice and escape for Australian redback males. Even if a male were to survive copulation and escape, his chances of mating again are near zero as males have a short life-span, a high mortality when traveling between webs, and suffer palp damage during copulation (Andrade 1996). Thus, death during or following copulation has a negligible effect on the male’s fitness. In conclusion, self-sacrifice is adaptive for males as it increases their evolutionary fitness relative to non-cannibalized males.

Answer Key

Answers to the questions posed in the case study are provided in a separate answer key to the case. Those answers are password-protected. To access the answers for this case, go to the key. You will be prompted for a username and password. If you have not yet registered with us, you can see whether you are eligible for an account by reviewing our password policy and then apply online or write to answerkey@sciencecases.org.

References

• Andrade, M.C.B. 1996. Sexual selection for male sacrifice in the Australian redback spider. Science 271 (5245): 70–72.
• Andrade, M.C.B. 1998. Female hunger can explain variation in cannibalistic behavior despite male sacrifice in redback spiders. Behavioral Ecology 9(1): 33–42.
• Andrade, M.C.B. 2003. Risky mate search and male self-sacrifice in redback spiders. Behavioral Ecology 14(4): 531–538.
• Andrade, M.C.B., L. Gu, and J.A. Stolz. 2005. Novel male trait prolongs survival in suicidal mating. Biology Letters 1: 276–279.
• Snow, L.S.E., A. Abdel-Mesih, and M.C.B. Andrade. 2006. Broken copulatory organs are low-cost adaptations to sperm competition in redback spiders. Ethology 112: 379–389.

Slide Credits

Acknowledgements: This material is based upon work supported by the NSF under Grant No. DUE-0618570. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. We also gratefully thank Dr. Andrade for permission to use images and her research.

Date Posted: November 19, 2009.

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