Download Sociobiology

Insect Sociobiology
Sociofactoids:
• 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!
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?
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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 & bees
ISOPTERA, termites (all)
THYSANOPTERA, thrips
HEMIPTERA (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 eusociality
in 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
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.
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!)
Haplodiploidy
Normal 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
rB - C > 0
(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.
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
(None fatal.)
☞
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.
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
Protected, central-place nesting
Reliance on other individuals
fine manipulation of environment (usually by
mandibles)
Highly effective defense system
Chemical communication & control (highly
devleoped exocrine glands; pheromones)
Generalist food habits (e.g. diet; all purpose
mandibles)
Haplodiploidy
Physiology
Morphology
Genetics
Condition/Biological Category
Behavior
What factors predispose certain evolutionary lines toward sociality?
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X
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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
Eusociality
the highest recongnized level of social evolution
Defining Characteristics
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•
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Overlapping Generations
Reproductive Division of Labor
Cooperative Brood Care
Levels & Examples
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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.
Primitively eusocial species.
Polistes sp., solitary founding queen
Bombus spp., colony development.
Highly eusocial species.
termites: division of labor
honey bees: sophisticated
communication
Caste and caste determination.
Special soldier caste in a
eusocial 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 agedetermined 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.
from Holldobler & Wilson 1986
Metapleural gland defensive function in
Crematogaster inflata. Noxious secretions are
given off if the ant is grabbed (with forceps here.)
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.
Ant-simulating behavior
(as perceived by attending ant)
3 glands:
adoption gland
defensive gland
apeasement gland
Adoption strategy of a predatory inquiline caterpillar.
Worker ant bringing home a Lycaenid caterpillar.
2 glands are used to:
pacify an ant and
encourage “adoption”.
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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