• In some tropical forest ecosystems the biomass of social insects is greater than that of all other animal life combined!

• Social insects, especially ants, termites, and bees, are vital keystone species in many ecosystems.

• Some African driver ant colonies contain up to 20 million individuals!

• The genetically integral colonies of some species of ants extend for hundreds of miles!

Sociofactoids:

Insect Sociobiology

Principal Questions:

• What is sociality?

• What are its advantages compared to solitary life histories?

• How did it evolve?

What is a “social” insect?

A species that derives a fitness advantage (survival and genetic) by living in some sort of direct association with others of its kind. Especially “eusocial” insects.

2 Major Categories:

Subsocial: adult cares for brood, e.g. (mother earwig).

Eusocial: 1) cooperative brood care, 2) overlapping generations, 3) division of labor (reproductive caste, usually a“queen”), e.g. all ants.

What are the adaptive values of sociality?

• Parallel tasks (different individuals performing different functions simultaneously)

• Group response (large numbers of individuals accomplishing difficult tasks)

• Homeostasis through specialization (maintaining optimal conditions through division of labor)

Examples:

Savings in energy (especially in nest-building).

Increased security (defensive ability).

Greater food-gathering potential.

Maintenance of constant interior (nest) environment.

More efficient performance of various behaviors by the colony (cf. the individual), i.e. “multi-tasking”.

Evolution of Sociality

• Mutiply-evolved (at least 13 orders, including subsocial spp.)

Examples:

HYMENOPTERA, ants (all), many wasps & beesISOPTERA, termites (all)THYSANOPTERA, thripsHEMIPTERA (STERNORRHYNCHA, aphids)COLEOPTERA (some eusocial weevils)

• Most conspicuous & evolved: ISOPTERA & HYMENOPTERA (multiple origins)

• Important antecedents: Haplodiploidy, a genetic predisposition; central-place foraging (based from a nest); various morphological & physiological predispositions, e.g. communication pheromones, forceps-like mandibles.

Ancient eusocialityin ants.

Nothomyrmecia macrops, the most primitive living ant, from the deserts of south Australia. It lives in small colonies but is fully eusocial. Called the “dinosaur ant”, its closest relatives are known only from amber fossils.

photo: www.alexanderwild.com

Modern Concepts of Sociobiology

Haplodiploidy. A genetic system wherein one sex is diploid and one haploid. Increases sibling relatedness and the intensity of kin selection. Occurs in HYMENOPTERA and THYSANOPTERA (but not ISOPTERA!)

Altruism. Darwin struggled to explain before the age of genetics.

“Selfish Gene” concept. Genomes survive by promoting themselves, regardless of the “container”.

Inclusive Fitness. Fitness expressed through shared genes of relatives in addition to the individual of interest.

Kin Selection. Selection for genetically-based behavior pattern that lowers an individual's own reproduction but raises a relative's fitness; a genetic explanation for selfless behavior among animals.

HaplodiploidyNormal parental genetics:

Offspring related by 0.50 (1/2 each of mother & father’s genes).

Haplodiplidy:

Male haploid; one set per chromosome.

All female offspring have same paternal chromosomes, therefore related by 0.75 (“supersisters”).

• Kin Selection dictates that sisters help sisters to promote their own genes.• Haplodiploidy intensifies this effect, making sister-helping more valuable than direct reproduction.• In the extended family society, the best way to to this is to assist the mother (queen) in making more siblings (“altruism”).

Hamilton’s Rule

(W. D. Hamilton, 1962)

• A rule that determines the likelihood of “altruistic” behavior.

• Recognizes Kin Selection in the context of costs & benefits, i.e. puts sociobiology in a behavioral & ecological context.

rB - C > 0

B = benefits (in terms of fitness)C = costs (in terms of fitness)r = coefficeint of relatedness (proportion of genes shared)

Problems with the Haplodiploid Theory of Social

Evolution☞ Eusocial queens may mate with more than one male less than 0.75 relatedness, e.g. honey bees.

☞ Many solitary haplodiploid species (e.g. most bees.)

☞ Termites not haplodiploid -- yet all highly eusocial!

Therefore, haplodiploidy, although a powerful selective force, may be equivoca; it is apparently neither sufficient nor necessary in explaining the evolution of social behavior. Other factors are also important.

(None fatal.)

ISOPTERA: Why eusocial if not haplodiploid?

• Inter-generational dependence: gut symbionts passed through mechanical transfer from one generation to the next (nutrient-poor food source).

• Adult longevity.

• Antecedents common with eusociality in HYMENOPTERA, e.g. communication pheromones, manipulative mandibles, nest & central place foraging.

• Other factors?

photos: BugGuide.net, Google

Some Possible Antecedents to Sociality in Insects

What factors predispose certain evolutionary lines toward sociality?

Condition/Biological Category

BehaviorGeneticsMorphologyPhysiologyProtected, central-place nesting XReliance on other individuals Xfine manipulation of environment (usually by mandibles) X XHighly effective defense system X XChemical communication & control (highly devleoped exocrine glands; pheromones) XGeneralist food habits (e.g. diet; all purpose mandibles) X X XHaplodiploidy X

Grades of Sociality.Social behavior may be as simple as certain insects of the same species nesting in close proximity to increase chances of mating or predator avoidance or it may be intricately complicated, with thousands of related individuals comprising a multitasking, perennial colony.

I

II

Eusocialitythe highest recongnized level of social evolution

Defining Characteristics

• Overlapping Generations

• Reproductive Division of Labor

• Cooperative Brood Care

Levels & Examples

• Primitive (Highly Eusocial I) vespoid wasps (yellowjackets & paper wasps), bumble bees: small, annual colonies; primitive communication

• Advanced (Highly Eusocial II) termites, ants, honey bees: large, perennial colonies; sophisticated communication.

Polistes sp., solitary founding queen

Bombus spp., colony development.

Primitively eusocial species.

Highly eusocial species.

termites: division of labor

honey bees: sophisticated communication

Caste and caste determination.

Special soldier caste in aeusocial aphid.

from Gullen & Cranston 2000

Complex caste system and determination in a higher termite. In many species a single individual may follow any number of pathways, including going backward!

What determines caste? Depending on species:

• Condition of colony (stress factors).

• Contact between nestmates

• Hormones, especially JH

• Pheromones, especially from reproductives

from Gullen & Cranston 2000

Honey bee morphological castes and development.

Sexual maturity (reproductive caste) is food-determined.

However, there is also extensive temporal polyethism, leading to several non-morphologically determined castes based on age-determined behavior and colony condition.

from Winston 1987

The importance of glands in ants.

Exocrine system of Iridomyrmex humilis showing 9 separate glands plus associated special structures.

Metapleural gland defensive function in Crematogaster inflata. Noxious secretions are given off if the ant is grabbed (with forceps here.)

from Holldobler & Wilson 1986

Inquilines & Social Parasites

Taking advantage of social insect colonies as a special concentrated resource.

Requires highly specialized means of overcoming colony defenses.

Most are non-lethal to the colony.

from Holdobler & Wilson

Complex behavioral/physiological ruse of an inquiline staphylinid beetle.

3 glands:adoption glanddefensive glandapeasement gland

Ant-simulating behavior(as perceived by attending ant)

Adoption strategy of a predatory inquiline caterpillar.

2 glands are used to:pacify an ant andencourage “adoption”.

Worker ant bringing home a Lycaenid caterpillar.

Once in the ant nest, the caterpillar commences to feed on the ant larvae.

from Holldobler & Wilson 1986

Myrmecophila, a genus of crickets that lives with ants

Mymecophila oregonensis, a native Pacific Northwest species that associates with Camponotus carpenter ants. A similar species, M. acervorum, is found in Europe.

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