1. Social and Eusocial Behavior
Why do animals help others at the
potential cost of their own survival
and reproduction?
Alka Nokhwal
Ph.D., M.Sc. (Zoology)
Senior Research Fellow
NCVTC, ICAR-National Research Centre on Equines,
Hisar, Haryana

2. Social behavior consists of a set of interactions among
individuals of the same species.
Wide
range of
sociality
Some animals rarely if
ever interact with one
another, even when it
comes to issues of
parental care (Ex.
mosquitoes and polar
bears).
Highly social organisms
live together in large
groups, and often
cooperate to conduct
many tasks (Ex. packs
of wolves and schools of
fish). The most highly
social animals form
tightly knit
colonies(Ex. all ants
and termites, some
bees and wasps etc.).

3. Social Behavior is Adaptive
Many social behaviors of animals are adaptive,
meaning that being social ultimately increases an
animal’s fitness — its lifetime reproductive success.
 One example of how social behavior is adaptive is
aggregation against predators. This concept applies to
caterpillars feeding together on a leaf, a herd of
wildebeest, schools of fish, and flocks of birds.

4. Wildebeests gathered into groups are more protected from predators than any
solitary wildebeest.

5. • Living in groups
Balance of conflict
and cooperation
Costs and Benefits
• Benefits exceed
the costs and
risks of social life
Social cooperation
favored

6. Altruism
An altruistic act is one that increases the welfare of another
individual at an actual or potential cost of the individual who
performs the act.
The benefit of an altruistic behavior is ultimately measured in its
effect on an animal’s lifetime reproductive success. Evolutionary
biologists and animal behaviorists have sought to identify the
mechanisms that can explain what some have called the “problem of
altruism.” Natural selection operates against individuals who reduce
their own fitness. Altruism by definition decreases the fitness of the
individual, so how can this behavior persist?

7. Reciprocity
existence of
altruism
benefits of
altruism can
outweigh the
costs of altruism
in long term

8. • Eusociality
The evolution of social behavior at its most intimate and
complex degree is found in eusocial animals. Eusocial
species live in colonies. Only a relatively small fraction of
the animals in the colony reproduce; the non-reproductive
colony members provide resources, defense, and collective
care of the young.
 The list of known eusocial animals includes ants, termites,
some wasps, some bees, a small number of aphid and thrip
species, two species of mammal (the naked mole rat and the
Damaraland mole rat), and multiple species of reef-dwelling
shrimp.

9. Eusocial behaviour

10. Eusocial animals share the following four
characteristics:
Adults live in groups,
Cooperative care of juveniles
(individuals care for brood that is
not their own),
Reproductive division of labor (not
all individuals get to reproduce)
Overlap of generations

11. Other types of social interactions
• Social behavior between
parents and offspring (e.g.,
birds, Halictine bees
Subsociality
• Social behavior among
members of the same
generation (e.g., most bees)
Parasociality

12. Which Animals are Eusocial?

13. • Termites are thought to be highly developed social insects
that live inside the food they consume (rotting wood;
Thorne 1997). Termites are diploid insects that engage in
intricate social behaviors like nest building and territorial
defense.
• most recent eusocial organisms to be found- shrimp,
aphids, and thrips.
• The Synalpheus shrimps' eusociality has at least two distinct
origins. As internal parasites on tropical sponges, these
marine shrimp thrive in colonies of several hundred closely
related diploid individuals. Given that dispersing to found
new colonies is riskier than remaining in the natal nest, the
variable distribution of the host sponges may have aided in
the evolution of eusociality within this group.

14. • Thrips are small haplodiploid insects in the order
Thysanoptera. 300 of the approximately 5,000 species
build nests in plants called galls where they feed on the
plant tissue. Of these, six species can be categorized as
being eusocial since they have militaries with varying
morphologies that protect the galls from
kleptoparasites.
• Like thrips, social aphids live in plant galls or hollow
stems and feed on plant tissue. These tiny hemipterans
can breed parthenogenetically while having diploid life
cycles, and several species have been described with
strong soldier morphs.

15. There are at least two species of vertebrates
that could be considered eusocial, the naked
mole rat and the Damaraland mole rat.
• Both species are diploid, highly inbred and
live in harsh deserts with patchy food
resources.
• Most individuals help to raise siblings or close
relatives that are born to a single reproductive
female (the queen).

16. Advantages/Disadvantages to Living
in Groups
Groups may form as
defense against
predation, forming a
"selfish herd"
(Hamilton 1971).
Advantages against
competitors, e.g., ant
Azteca trigona
(Adams 1994).
Acquiring food in
groups (e.g.,
raiding army ants;
Solé et al. 2000).
Increased
competition
Increased
transmittance of
parasites and
diseases
Easy detection of
the group by
predators and
parasites.

17. How did Eusociality Evolve?
• Natural selection's core element is in contrast with giving up
one's capacity for reproduction (to survive and reproduce).
• According to evolutionary biologists, the development of
eusociality followed a path that began with solitary
organisms learning the advantages of group behaviour and
eventually reached a "point of no return" (Wilson &
Hölldobler 2005) where some individuals lost their ability
to physically reproduce and could only benefit indirectly
from evolution.
• It's also important to keep in mind that the selecting
pressures at play at the time eusocial behaviour first
emerges may not be the same as those sustaining advanced
eusocial colonies (Hölldobler & Wilson 2009).

18. Primitive
Eusociality
Ecological
contributions
Kin selection
Delayed
benefits
Multi-level
selection
Point of
“No return”
Advanced
eusociality
It is important to note that they are not mutually exclusive - each
may play a different role in the evolution of eusociality in
different groups.

19. Kin Selection
• A gene can spread copies of itself in
subsequent generations either directly by
producing offspring or indirectly by promoting
the reproduction of close relatives.
• Inclusive fitness is the total of all reproductive
benefits, both direct and indirect. As a result, if
indirect fitness levels are higher than direct
fitness, eusociality may be chosen over solitary
behaviour.

20. • An altruistic act is one that helps the recipient at the expense of the performer.
• According to Hamilton's rule (Hamilton 1964), altruism is preferred if
r > C/B,
• where C is the cost and B is the gain to the recipient of the altruistic act in terms of
lifetime reproductive success (decrease in lifetime reproductive success). The
percentage of alleles shared by two people who are related by ancestry is known
as the coefficient of relatedness, or r, and it extends from 0 to 1. High degrees of
altruism within groups are necessary for eusociality.
• Inbreeding and haplodiploid sex determination are two well-known ways that
might increase ‘r’.
• Hamilton's rule is weighted in favour of raising sisters rather than kids in
haplodiploid organisms because the relatedness between full sibling sisters (r =
0.75) is higher than the relatedness between a mother and her offspring (r =
0.5).Inbreeding produces offspring that share a greater percentage of alleles,
raising r. This is typical of species that don't travel very far from their natal nest or
are more likely to mate with their siblings (e.g., termites and wild naked mole rats).

21. Delayed Benefits
• "Hopeful reproductives" — workers with the option to
stay and assist or go away and start their own nest —
are a possible intermediate step toward eusociality.
• The choice may be influenced by factors like territory,
food availability, environmental factors, and group
hierarchy. Until there is a chance to take over as
parents, Florida scrub jay young are known to remain at
the natal nest, raising siblings and improving their
inclusive fitness.
• Primitive eusocial wasp colonies, like Polistes, are
frequently passed over to dominant workers after a
queen dies.

22. Multi-level Selection
• Natural selection can happen on an individual, family (a
group of related people known as "kin"), or group level
(non-related individuals).
• Models of multi-level or trait-group selection can be used
to illustrate how the traits (phenotype) of the colony interact
with the environment to determine colony-level fitness in
eusocial organisms.
• It is still up for debate whether models of multi-level
selection or inclusive fitness models are the best way to
investigate the development and maintenance of eusociality,
particularly given the paucity of empirical data supporting
inclusive fitness in groups (Seeley 1997).

23. Ecological and Life History
Contributions
• Nesting behaviour has been suggested as a potential precondition
for the growth of eusociality, in part because it fosters circumstances
that encourage cooperative brood care (Anderson 1984). "Fortress
defenders" can work together to protect this vital resource in areas
where nest founding is hazardous or there are few territories or
spaces.
• Parental care can also play a significant role in a person's life story.
One route to eusociality in Hymenoptera is believed to begin with
solitary females participating in concurrent progressive
provisioning, which involves raising several larvae of various ages
at once.
• The next step in the transition to eusocial behaviour would be for the
surviving offspring and provisioning siblings, then for the offspring
to refrain from having children of their own.

24. Thank you