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A Deadly PassionSexual Cannibalism in the Australian Redback Spider
Erin Barley and Joan SharpSimon Fraser University
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Proximate questions about behavior
• Proximate questions address the mechanisms that produce a behavior: the environmental stimuli that trigger a behavior and the genetic and physiological mechanisms that make it possible.
• For example,–How does an animal carry out a
particular behavior?
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Ultimate questions about behavior
• Ultimate questions address the evolutionary significance of a behavior: how a behavior increases the evolutionary fitness of the animal demonstrating it, helping it to survive and reproduce in its environment.
• For example,–Why does the animal show this
behavior?
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What is evolutionary fitness?
• Evolutionary fitness measures how many viable, fertile offspring an individual (or an allele) leaves in the next and subsequent generations, relative to others in the population.
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Adaptive behavior
• An adaptive behavior increases an individual’s evolutionary fitness relative to other individuals in the population.
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CQ#1: Red-crowned cranes breed in spring and early summer. Choose a proximate explanation:
A. Breeding is most likely to be successful in spring and early summer.
B. Increasing day length triggers the release of breeding hormones.
C. Ample food is available for chicks at this time.
• The Correct Answer is B• A and C are evolutionary explanations for
the behavior- they provide clues as to what allowed the birds to survive and reproduce. So they are Ultimate explanations.
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A. Breeding is most likely to be successful in spring and early summer.
B. Hormonal changes in the spring trigger breeding behaviors.
C. Breeding is triggered by the effect of increased day length on the birds’ photoreceptors.
CQ#2: Red-crowned cranes breed in spring and early summer. Choose an ultimate explanation:
• The correct answer is A.• B and C are both physiological and
therefore proximate explanations.
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Sexual cannibalism
In some species, one sex (usually the female) consumes the other during sexual reproduction.
Australian redback spiderPraying mantis Scorpion
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Sexual cannibalism in Australian redback spiders
• University of Toronto’s Maydianne Andrade has been studying Australian redback spiders for over a decade.
• Her goal: To explain sexual cannibalism in the Australian redback spider.
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Courtship in the Australian redback spider
• The male courts the much larger female for up to eight hours.
• He strums on the strands of her web as he slowly approaches her.
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Courtship in the Australian redback spider
The male has two specialized legs or palps, each of which is used to transfer sperm to the female.
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Mating of Australian redback spiders
A. The male mounts the female and inserts one of his palps into one of the female’s sperm receptacles.
B. The male moves into a “headstand.”
C. The male somersaults and dangles his abdomen in front of the female’s jaws.
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Mating of Australian redback spiders
• The female begins to feed on the male, liquefying and slurping up the contents of his abdomen.
• After the male is finished with his first palp, he disengages, performs a brief courtship, inserts his second palp, and somersaults again to dangle his abdomen in the female’s jaws.
• The female finishes eating the male.
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Link to redback somersault video (Courtesy of Andrade Lab)
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http://www.youtube.com/watch?v=F5WbtVGNpCI&feature=player_embedded#!
(You can view this video by copying the link into a new web page)
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Mating of Australian redback spiders
• The female produces an egg sac with 40-300 eggs.
• Each egg is fertilized by a separate sperm.
• If a female has mated with more than one male, a single egg sac can contain eggs fertilized by different males.
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Behavior of female redbacks
• Maydianne Andrade was interested in explaining the behavior of both females and males.
• Why do females cannibalize males during copulation?– First, we consider three hypotheses to explain
the female’s behavior.
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Behavior of female redbacks
Hypothesis 1: Mistaken Prey• Females mistake males for prey.
To test the hypothesis:• Compare female behavior when feeding
on prey to her behavior when eating her mate.
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CQ#3: If the Mistaken Prey hypothesis is correct, what would you predict?
A. Females would sometimes attack males as soon as they enter her web.
B. 65% of females would cannibalize their mates.
C. Females would only attack males when they somersault to dangle in front of her jaws.
(pause to consider answer before advancing the slide)
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CQ#3: If the Mistaken Prey hypothesis is correct, what would you predict?
A. Females would sometimes attack males as soon as they enter her web.
B. 65% of females would cannibalize their mates.
C. Females would only attack males when they somersault to dangle in front of her jaws.
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What does happen?
Prediction: • Females sometimes attack males as soon as they enter
her web.
Results:• A female only eats a male after he somersaults to dangle
in front of her jaws. In contrast, she often attacks prey as soon as they enter the web.
Conclusion:• The female does not mistake her mate for prey.
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Behavior of female redbacks
Hypothesis 2: Mate Rejection• Females eat males that are unsuitable as mates.
To test the hypothesis:• Compare the quality and mating success of
cannibalized vs. non-cannibalized males.
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A. Females would attack males only during copulation.
B. Cannibalized males would be smaller and produce fewer offspring than non-cannibalized males.
C. Cannibalized males would father twice as many offspring as non-cannibalized males.
CQ#4: If the Mate Rejection hypothesis is correct, what would you predict?
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A. Females would attack males only during copulation.
B. Cannibalized males would be smaller and produce fewer offspring than non-cannibalized males.
C. Cannibalized males would father twice as many offspring as non-cannibalized males.
CQ#4: If the Mate Rejection hypothesis is correct, what would you predict?
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What does happen?Prediction: • Cannibalized males are smaller and produce fewer
offspring than non-cannibalized males
Results:• Cannibalized males do not differ in size, mass, or
condition from non-cannibalized males. • Cannibalized males father, on average, twice as many
offspring as non-cannibalized males.
Conclusion:• Females do not cannibalize low-quality males who are
unsuitable mates.
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Behavior of female redbacks
Hypothesis 3: Hungry Lover• Females eat their mates because they are
hungry.
To test the hypothesis:• Compare cannibalism rates of two groups
of females: one group feeds naturally, while the other has its diet supplemented.
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CQ#5: If the Hungry Lover hypothesis is correct, what would you predict?
A. Females would only eat males that somersault to dangle in front of their jaws.
B. Cannibalized males would be smaller and produce fewer offspring than non-cannibalized males.
C. Females would be more likely to cannibalize males if their diet has not been supplemented.
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CQ#5: If the Hungry Lover hypothesis is correct, what would you predict?
A. Females would only eat males that somersault to dangle in front of their jaws.
B. Cannibalized males would be smaller and produce fewer offspring than non-cannibalized males.
C. Females would be more likely to cannibalize males if their diet has not been supplemented.
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What does happen?
Prediction: • Females are more likely to cannibalize males if their
diet has not been supplemented. Results:• 29% of females that were given supplemental food
were cannibalistic.• 62% of females that fed naturally were cannibalistic.
Conclusion:• Females eat their mates because they are hungry.
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CQ#6: Choose a proximate explanation for female cannibalism:
A. The female eats the male because he dangles his abdomen in front of her jaws.
B. The female can produce a larger egg sac if she eats the male.
C. The female gains nutrients by eating her mate.
Pause to consider answer before advancing slide
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CQ#6: Choose a proximate explanation for female cannibalism:
A. The female eats the male because he dangles his abdomen in front of her jaws.
B. The female can produce a larger egg sac if she eats the male.
C. The female gains nutrients by eating her mate.
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CQ#7: Choose an ultimate explanation for female cannabilism:
A. The female eats the male because he dangles his abdomen in front of her jaws.
B. The female gains nutrients from eating the male.
C. The female copulates longer while eating her mate.
Pause to consider answer before advancing slide
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CQ#7: Choose an ultimate explanation for female cannabilism:
A. The female eats the male because he dangles his abdomen in front of her jaws.
B. The female gains nutrients from eating the male.
C. The female copulates longer while eating her mate.
(While C could be an ultimate explanation, it is unlikely to offer any fitness advantage to the female)
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Behavior of male redbacks
• Female redback spiders eat their mates because they are hungry.
• But why doesn’t the male try to escape? Is self-sacrifice an adaptive behavior for the male?
• Maydianne Andrade tested two hypotheses to explain male behavior.
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Behavior of male redbacks
Hypothesis 1: Paternal Investment• Males benefit by contributing nutrients (their own
bodies!) to increase the number and size of their offspring.
To test the hypothesis:• Compare the size and mass of the egg sacs
produced by females that have eaten their mates to those of females that have not eaten their mates.
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CQ#8: If the Paternal Investment hypothesis is correct, what would you predict?
A. Consuming the male would significantly increase the number and mass of eggs in the female’s egg sac.
B. The male’s food value would be low because his mass would be only 1-2% of the mass of the female.
C. Eggs in the same egg sac could be fertilized by more than one male.
Pause to consider answer before advancing slide
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CQ#8: If the Paternal Investment hypothesis is correct, what would you predict?
A. Consuming the male would significantly increase the number and mass of eggs in the female’s egg sac.
B. The male’s food value would be low because his mass would be only 1-2% of the mass of the female.
C. Eggs in the same egg sac could be fertilized by more than one male.
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Behavior of male redbacksPrediction:
Consuming the male will significantly increase the number and mass of eggs in the female’s egg sac.
Results:• Consuming the male does not increase the number or
mass of eggs in the female’s egg sac.– This might be due to the small size of the male relative to the
female and even relative to her egg sac!
Conclusion:• The male does not benefit from contributing nutrients to
his offspring.
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Behavior of male redbacks
Hypothesis 2: Nuptial Gift• Males benefit from self-sacrifice by
increasing their fertilization success.
To test the hypothesis:• Compare copulation duration and number
of offspring fathered by cannibalized males to non-cannibalized males.
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CQ#9: If the Nuptial Gift hypothesis is right, what would you predict?
A. Non-cannibalized males would copulate longer than cannibalized males.
B. Cannibalized males would father more offspring than non-cannibalized males.
C. 65% of females would cannibalize their mates.
Pause to consider answer before advancing slide
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CQ#9: If the Nuptial Gift hypothesis is right, what would you predict?
A. Non-cannibalized males would copulate longer than cannibalized males.
B. Cannibalized males would father more offspring than non-cannibalized males.
C. 65% of females would cannibalize their mates.
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Behavior of male redbacksPrediction:
Cannibalized males father more offspring than non-cannibalized males.
Results:• Cannibalized males copulate for an average of 25
minutes, while non-cannibalized males copulate for an average of 11 minutes.
• Cannibalized males father twice as many offspring, on average, as non-cannibalized males.
Conclusion:• Males double their fertilization success by sacrificing
themselves to their mates!
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CQ#10: Choose a proximate explanation for male self-sacrifice:
A. The male’s somersault is triggered when he inserts a palp in the female’s sperm receptor.
B. The male is providing nutrients to his offspring.
C. A male that is cannibalized fathers twice as many offspring.
Pause to consider answer before advancing slide
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CQ#10: Choose a proximate explanation for male self-sacrifice:
A. The male’s somersault is triggered when he inserts a palp in the female’s sperm receptor.
B. The male is providing nutrients to his offspring.
C. A male that is cannibalized fathers twice as many offspring.
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CQ#11: Choose an ultimate explanation for male self-sacrifice:
A. The male increases the length of copulation by sacrificing himself.
B. The male’s self-sacrifice is an innate, genetically programmed behavior.
C. The male is providing nutrients to his hungry mate.
Pause to consider answer before advancing slide
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CQ#11: Choose an ultimate explanation for male self-sacrifice:
A. The male increases the length of copulation by sacrificing himself.
B. The male’s self-sacrifice is an innate, genetically programmed behavior.
C. The male is providing nutrients to his hungry mate. This may benefit the female, but no clear
benefits to the male
Proximate
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Is self-sacrifice adaptive for the male?
Benefit:• Doubled reproductive
successCost:
• Near certain death
Benefit:• Live longer
Cost:• Lower reproductive
success, with almost 0% chance of finding a
new mate
Self-sacrifice
Escape
Evolutionary fitness
Slide CreditsSlide 1, Slide 8—Center, and Slide 27 Description: Female and male Australian redback spiders. Author: A.C. Mason Source: Andrade Lab webpage. Link: http://www.utsc.utoronto.ca/~mandrade/ Permissions: Used with permission of copyright holder, Maydianne Andrade.
Slide 6 and Slide 7 Description: Red-crowned cranes. Author: Frank J. Gualtieri Jr. Source: Wikimedia Commons. Link: http://commons.wikimedia.org/wiki/File:Tsuru1260.jpg Permissions: Released by author to the public domain.
Slide 8—Left Description: Photo of paying mantis. Author: Jon Brierley Source: University of Guelph, Arboretum Link: http://www.uoguelph.ca/arboretum/PhotoGall/PhotoRecent6.htm Permissions: Used with permission of copyright holder, Jon Brierley.
Slide 8—Right Description: Photograph of scorpion (Centruroides suffusus). Author: Drini (Pedro Sánchez) Source: Wikimedia Commons Link: http://commons.wikimedia.org/wiki/File:Centruroides_vittatus.jpg Permissions: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.
Slide 9—Right Description: Maydianne Andrade and redback spider web. Author: Ken Jones Source: Ontario Innovation Trust Link: http://www.oit.on.ca/Pages/SStories21-40/StoryUTorontoAndrade.html Permissions: Used with permission of copyright holder, Maydianne Andrade.
Slide 10 and Slide 36 Description: Female and male redback spiders. Author: Ken Jones Source: University of New South Wales, Faculty of Science, News. Link: http://www.science.unsw.edu.au/news/spider-love-little-guys-get-lots-more/ Permissions: Used with permission of copyright holder, Maydianne Andrade.
Slide 11 and Slide 35 Description: Male redback spider’s pedipalps. Source: Andrade Lab webpage. Link: http://www.utsc.utoronto.ca/~mandrade/index_files/Reproductive_biology.htm Permissions: Used with permission of copyright holder, Maydianne Andrade.
Slide 12 Description: Drawing of male’s somersault during copulation. Source: Forster LM. 1992. The Stereotyped Behavior of Sexual Cannibalism in Latrodectus
hasselti Thorell (Araneae, Theridiidae), the Australian Redback Spider. Australian Journal of Zoology. 40(1): 1–11. Copyright CSIRO 1992. Published by CSIRO PUBLISHING, Collingwood Victoria, Australia. http://www.publish.csiro.au/nid/90/paper/ZO9920001.htm
Permissions: Used with permission of CSIRO Publishing.
Slide 14 Description: Video of male’s somersault during copulation. Author: Ken Jones Source: Andrade Lab webpage. Link: http://www.utsc.utoronto.ca/~mandrade/index_files/Page332.htm Permissions: Used with permission of copyright holder, Maydianne Andrade.
Slide 15 and Slide 26 Description: Female redback spider with egg sac. Source: Pulse of the Planet Link: http://pulseplanet.com/monthlyfeature/featurestories.php?id=20 Permissions: Used with permission of copyright holder, Maydianne Andrade.
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