1. A Deadly Passion Sexual Cannibalism in the Australian Redback Spider
Erin Barley and Joan Sharp
Simon Fraser University 1

2. Proximate questions about behavior
address the mechanisms that produce a behavior
the environmental stimuli that trigger a behavior and
the genetic & physiological mechanisms that make it possible
For example,
How does an animal carry out a particular behavior?

3. Ultimate questions about behavior
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?

4. What is evolutionary fitness?
Evolutionary fitness measures how many viable, fertile offspring an individual leaves in the next and subsequent generations, relative to others in the population.

5. Adaptive behavior
An adaptive behavior increases an individual’s evolutionary fitness relative to other individuals in the population.
If you can adapt to a changing environment, you are more likely to survive.

6. CQ#1: Red-crowned cranes breed in spring and early summer
CQ#1: Red-crowned cranes breed in spring and early summer. Choose a proximate explanation:
Breeding is most likely to be successful in spring and early summer.
Increasing day length triggers the release of breeding hormones.
Ample food is available for chicks at this time.
Proximate questions address – the mechanisms – the environmental stimuli that trigger a genetic and physiological response
Best answer is B
Remember: Proximate questions address – the mechanisms –
the environmental stimuli that trigger a genetic and physiological response.
Addresses the question HOW.

7. CQ#2: Red-crowned cranes breed in spring and early summer
CQ#2: Red-crowned cranes breed in spring and early summer. Choose an ultimate explanation:
Breeding is most likely to be successful in spring and early summer.
Hormonal changes in the spring trigger breeding behaviors.
Breeding is triggered by the effect of increased day length on the birds’ photoreceptors.
Remember: Ultimate questions address – the evolutionary significance – to help it
survive and reproduce.
Addresses the question WHY.

8. Australian redback spider
Sexual cannibalism
In some species, one sex (usually the female) consumes the other during sexual reproduction.
Praying mantis
Australian redback spider
Scorpion

9. 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 Australian redback spiders.

10. 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.

11. 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.
legs or mouthparts

12. Mating of Australian redback spiders
The male mounts the female and inserts one of his palps into one of the female’s sperm receptacles.
The male moves into a “headstand.”
The male somersaults and dangles his abdomen in front of the female’s jaws.

13. 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.

14. Redback Feeding and Courtship
For other videos, you might consider trying:
• redback somersault video
• sex and the redback spider
• sexual cannibalism in the redback spider
• the Australian Redback Spider’s feeding and courtship rituals 14

15. Mating of Australian redback spiders
The female produces an egg sac with 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.

16. 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.

17. 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.

18. CQ#3: If the Mistaken Prey hypothesis is correct, what would you predict?
Females would sometimes attack males as soon as they enter her web.
65% of females would cannibalize their mates.
Females would only attack males when they somersault to dangle in front of her jaws.

19. 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.

20. 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.

21. CQ#4: If the Mate Rejection hypothesis is correct, what would you predict?
Females would attack males only during copulation.
Cannibalized males would be smaller and produce fewer offspring than non-cannibalized males.
Cannibalized males would father twice as many offspring as non-cannibalized males.

22. 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.

23. 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.

24. CQ#5: If the Hungry Lover hypothesis is correct, what would you predict?
Females would only eat males that somersault to dangle in front of their jaws.
Cannibalized males would be smaller and produce fewer offspring than non-cannibalized males.
Females would be more likely to cannibalize males if their diet has not been supplemented.

25. 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.

26. CQ#6: Choose a proximate explanation for female cannibalism:
The female eats the male because he dangles his abdomen in front of her jaws.
The female can produce a larger egg sac if she eats the male.
The female gains nutrients by eating her mate.
Remember: Proximate questions address – the mechanisms –
the environmental stimuli that trigger a genetic and physiological response.
Addresses the question HOW.

27. CQ#7: Choose an ultimate explanation for female cannabilism:
The female eats the male because he dangles his abdomen in front of her jaws.
The female gains nutrients from eating the male.
The female copulates longer while eating her mate.
Remember: Ultimate questions address – the evolutionary significance – to help it
survive and reproduce.
Addresses the question WHY.

28. 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.

29. 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.

30. CQ#8: If the Paternal Investment hypothesis is correct, what would you predict?
Consuming the male would significantly increase the number and mass of eggs in the female’s egg sac.
The male’s food value would be low because his mass would be only 1-2% of the mass of the female.
Eggs in the same egg sac could be fertilized by more than one male.

31. Behavior of male redbacks
Prediction:
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.

32. 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.

33. CQ#9: If the Nuptial Gift hypothesis is right, what would you predict?
Non-cannibalized males would copulate longer than cannibalized males.
Cannibalized males would father more offspring than non-cannibalized males.
65% of females would cannibalize their mates.

34. Behavior of male redbacks
Prediction:
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!

35. CQ#10: Choose a proximate explanation for male self-sacrifice:
The male’s somersault is triggered when he inserts a palp in the female’s sperm receptor.
The male is providing nutrients to his offspring.
A male that is cannibalized fathers twice as many offspring.
Remember: Proximate questions address – the mechanisms –
the environmental stimuli that trigger a genetic and physiological response.
Addresses the question HOW.

36. CQ#11: Choose an ultimate explanation for male self-sacrifice:
The male increases the length of copulation by sacrificing himself.
The male’s self-sacrifice is an innate, genetically programmed behavior.
The male is providing nutrients to his hungry mate.
Remember: Ultimate questions address – the evolutionary significance – to help it
survive and reproduce.
Addresses the question WHY.

37. Is self-sacrifice adaptive for the male?
Evolutionary fitness
Escape
Self-sacrifice
Benefit:
Live longer
Cost:
Lower reproductive success, with almost 0% chance of finding a new mate
Benefit:
Doubled reproductive success
Cost:
Near certain death

38. Slide Credits Slide 1, Slide 8—Center, and Slide 27
Description: Female and male Australian redback spiders.
Author: A.C. Mason
Source: Andrade Lab webpage.
Link:
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:
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:
Permissions: Used with permission of copyright holder, Jon Brierley.

39. Slide 8—Right
Description: Photograph of scorpion (Centruroides suffusus).
Author: Drini (Pedro Sánchez)
Source: Wikimedia Commons
Link:
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:
Permissions: Used with permission of copyright holder, Maydianne Andrade.
Slide 10 and Slide 36
Description: Female and male redback spiders.
Source: University of New South Wales, Faculty of Science, News.
Link:
Slide 11 and Slide 35
Description: Male redback spider’s pedipalps.
Source: Andrade Lab webpage.
Link:

40. Slide 12
Description: Drawing of male’s somersault during copulation.
Source: Forster LM 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 Published by CSIRO PUBLISHING, Collingwood Victoria, Australia.
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:
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: