1. How did eusociality originate?
5/13/08

2. 6/18/08: Social behavior II: Eusociality
Lecture objectives:
Be able to state the characteristics that define eusocial species
Understand how high relatedness might have promoted the evolution of eusociality
Understand how ecological factors might have promoted the evolution of eusociality R
B/C

3. Eusocial species are characterized by 3 traits
1. Cooperative care of young
2. Overlapping generations
3. Reproductive division of labor (“castes”):
Fire ants

4. Eusocial species: those in order Hymenoptera
Ants (all)
Wasps (some)
Bees (some)

5. Eusocial species: other
Naked Mole Rat
Damaraland Mole Rat
Termites (all)
Aphids (some)
Thrips (some)
Snapping shrimp (some)

6. Sterile individuals display extreme altruism
Camponotus saundersi soldiers

7. Why might sterile individuals display such altruism?
R=relatedness (between donor and recipient)
C=cost to donor B=benefit to receiver
Hamilton’s Rule: R x B > C
How did eusociality originate? Two hypotheses:
Genetic hypothesis (haplodiploid hypothesis): promoted the evolution of eusociality
Ecological hypothesis: promoted the evolution of eusociality

8. High relatedness promoted the evolution of eusociality
What causes high relatedness between individuals? 1.
Some thrips 2.
Some aphids 3.
Fertilize egg?
Bees, ants, wasps, thrips
yes no
Female
Male

9. High relatedness promoted the evolution of eusociality
How might haplodiploidy promote helping behavior?
R (female – daughter) =
A female could pass along more of her genes
R (female – sister) =

10. High relatedness promoted the evolution of eusociality
Supporting evidence: 1.

11. High relatedness promoted the evolution of eusociality
Supporting evidence: Haplodiploid workers should be more likely to favor sisters when their queen is
Workers are all full sisters (R = 0.75)
Workers have full sisters (R = 0.75) and half sisters (R = 0.25) (overall, R < 0.75)
Test this in a species where a queen may be monogamous OR polyandrous

12. High relatedness promoted the evolution of eusociality
Supporting evidence: Haplodiploid workers should be more likely to favor sisters when their queen is monogamous versus polyandrous
Prediction: Formica ant workers with a ____________ queen will bias their rearing toward sisters compared to workers with a _________ queen

13. High relatedness promoted the evolution of eusociality
Supporting evidence: 3.
Eusociality independently arose at least 12 times in the Hymenoptera (ants, bees, wasps),
but only 1-2 times in Isoptera (termites)

14. High relatedness promoted the evolution of eusociality
Supporting evidence: Monogamy should be ________ in eusocial Hymenoptera
Monogamy was the ancestral state for 8 of the independent origins of eusociality

15. High relatedness promoted the evolution of eusociality
Evidence that there might be other factors involved: High relatedness is ______________ for eusociality to exist (high relatedness may be a ____________________)
Termites are diploid
Mole rats are not typically inbred

16. High relatedness promoted the evolution of eusociality
Evidence that there might be other factors involved: High relatedness is ___________ for evolution of eusociality: There are non-social haplodiploid species
Haplodiploid but not eusocial: Solitary bees

17. Why might sterile individuals display such altruism?
R=relatedness (between donor and recipient)
C=cost to donor B=benefit to receiver
Hamilton’s Rule: R x B > C
How did eusociality originate? Two hypotheses:
Genetic hypothesis (haplodiploid hypothesis): high relatedness (high R) promoted the evolution of eusociality
Ecological hypothesis: high benefit-to-cost ratio (high B/C) promoted the evolution of eusociality

18. Ecological factors promoted the evolution of eusociality
What causes a high benefit-to-cost ratio for helping behavior?

19. Ecological factors promoted the evolution of eusociality
Supporting evidence: 1.
Example: Drywood termites:
flexible caste policy; retain ability to develop into reproductives
Prediction: Drywood termites should be more likely to disperse when costs to staying at home are high

20. Ecological factors promoted the evolution of eusociality
Supporting evidence: 2. Eusocial species tend to have a

21. Ecological factors promoted the evolution of eusociality
Supporting evidence: 2. Eusocial species tend to have a “fortress” that requires defense
Example: Gall-forming aphids have soldiers that help defend gall
Prediction: Galls with soldiers will be more likely to be attacked by insect predators than those without soldiers

22. Ecological factors promoted the evolution of eusociality
Evidence that there might be other factors involved:
7 species of African mole rats build communal tunnels; Many species of rodents build complex burrows
Wild norway rats