Eusociality

Question 1

Diversity of eusocial systems

Answer 1

Ecologically important:

- 14,000 spp of social insect (cf. 10,000 birds, 4,000 mammals)
- driver ant colony: 22 million ants
- Brazilian rainforest: 70-80% of insect, 33% animal biomass
- sophisticated communication: waggle dance
- specialisation

Question 2

Myrmica rubra life cycle

Answer 2

nuptial flight -> queen founds nest -> production of sterile female workers -> ca. 9 years later -> Produce winged females and males

Question 3

Eusociality describes social systems where there is

Answer 3

cooperative brood care, generational overlap, and, critically, sterile castes

Question 4

Eusociality has evolved independently many times in

Answer 4

the Hymenoptera (ants – 9500 spp, bees – 1000 spp. and wasps – 800 spp), and also in Isoptera (termites – 2000 spp.) and Homoptera (aphids).

Question 5

Eusocial species have a dominant ecological presence in most parts of the world

Answer 5

, and have evolved remarkable specialisations associated with their social way of life.

Question 6

Evolution of eusociality – There are two hypotheses for the evolution of eusociality from solitary ancestral forms

Answer 6

Staying at home/
Sharing a nest

Contemporary subsocial halictine bees follow the ‘stay-at-home’ model, while parasocial Polistes and stenogastrine wasps follow the ‘share-a-nest’ model.

Question 7

Staying at home: from a solitary parasitoid that guards its nest against parasites, it is proposed that young stayed at home to help their mother defend and build the protective nest.

Answer 7

As nests become more elaborate, the benefit of staying and helping raise siblings could outweigh the benefit of independent breeding. Remember that full siblings are as closely related as offspring in diploid organisms, and from the mother’s perspective, offspring are more closely related than grand-offspring.

Question 8

Sharing a nest: in many wasps, nests are founded by a group of cooperating females, very often sisters. In primitively social wasps, all females reproduce, but in others one ‘queen’ dominates reproduction, setting the scene for the evolution of workers.

Answer 8

The ecological factors leading to shared nesting are probably similar to those described above: predation/parasitism and nest-building costs. Again, relatedness will ensure that non- reproductive females would benefit from the queen’s reproduction.

Question 9

Sharing a nest: in many wasps, nests are founded by a group of cooperating females, very often sisters. In primitively social wasps, all females reproduce, but in others one ‘queen’ dominates reproduction, setting the scene for the evolution of workers

Answer 9

. The ecological factors leading to shared nesting are probably similar to those described above: predation/parasitism and nest-building costs. Again, relatedness will ensure that non- reproductive females would benefit from the queen’s reproduction.

Question 10

The Hymenoptera exhibit haplodiploidy: males develop from unfertilized eggs and are haploid; females develop from normally fertilized eggs and are diploid.

Answer 10

Males form gametes without meiosis, so all his sperm are identical, while females form gametes through meiosis, as is usual in sexually reproducing organisms. This has important consequences for relatedness among individuals in the colony.

Question 11

CHECK OUT SLIDES FOR

Answer 11

RELATEDNESS IN HAPLODIPLOID SPECIES

Question 12

In hymenoptera, workers are always female. In diploid termites they may be either sex.

Answer 12

In eusocial aphids, colony members are clonal (genetically identical). Haplodiploidy results in interesting conflicts among colony members over reproductive options.

Question 13

Conflicts over sex ratio/

Queens are equally related to sons and daughters and so should invest equally in reproductive sons and daughters (1:1 ratio of investment). Workers are more closely related to sisters (0.75) than brothers (0.25) and so prefer to invest more in sisters (3:1 ratio of investment).

Answer 13

A comparison across 21 ant species by Trivers and Hare (1976, Science 191: 249-263) suggested that workers win this conflict. But, preferred sex ratio of each party will also be influenced by:

(a) Local Mate Competition - if sons compete with each other for matings it should pay to produce fewer of them, e.g. fig wasps
(b) Queen mating frequency – if a female mates with multiple males, workers will be less closely related to each other, on average.