This document discusses research on sexual cannibalism in female Australian redback spiders conducted by Maydianne Andrade. Andrade tested several hypotheses to explain why females cannibalize males during mating and why males allow it. Her results showed that females eat males because they are hungry, as supplemented females were less likely to cannibalize. Males benefit from self-sacrifice because it doubles their fertilization success by lengthening copulation time. Andrade's work provides proximate and ultimate explanations for the sexually cannibalistic behaviors of both male and female redback spiders.

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

2. 2
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?

3. 3
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?

4. 4
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.

5. 5
Adaptive behavior
• An adaptive behavior increases an
individual’s evolutionary fitness relative to
other individuals in the population.

6. 6
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.

7. • 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.
7

8. 8
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:

9. • The correct answer is A.
• B and C are both physiological and
therefore proximate explanations.
9

10. 10
Sexual cannibalism
In some species, one sex (usually the
female) consumes the other during
sexual reproduction.
Australian redback spiderPraying mantis Scorpion

11. 11
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.

12. 12
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.

13. 13
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.

14. 14
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.

15. 15
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.

16. 16
Link to redback somersault video
(Courtesy of Andrade Lab)
16
http://www.youtube.com/watch?v=F5WbtV
GNpCI&feature=player_embedded#!
(You can view this video by copying the link into a new web page)

17. 17
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.

18. 18
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.

19. 19
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.

20. 20
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)

21. 21
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.

22. 22
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.

23. 23
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.

24. 24
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?

25. 25
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?

26. 26
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.

27. 27
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.

28. 28
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.

29. 29
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.

30. 30
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.

31. 31
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

32. 32
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.

33. 33
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

34. 34
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)

35. 35
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.

36. 36
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.

37. 37
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

38. 38
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.

39. 39
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.

40. 40
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.

41. 41
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

42. 42
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.

43. 43
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!

44. 44
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

45. 45
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.

46. 46
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

47. 47
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

48. 48
Is self-sacrifice adaptive for the male?
Benefit:
• Doubled
reproductive
success
Cost:
• Near certain
death
Benefit:
• Live longer
Cost:
• Lower reproductive
success, with almost
0% chance of finding a
new mate
Self-sacrifice
Escape

49. 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: 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.

50. 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.

51. 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.