Transcript Slide 1

HERDS
Simple aggregations...
or cooperative groups?
Basic categories of animal interactions
Grades of social structure (ground squirrels)
Female
territory
male
territory
Spermophilus lateralis
Spermophilus armatus
Spermophilus columbianus
Grades of social structure (ground squirrels)
Marmota flaviventris
Cynomys ludovicianus
Wolf (Canis lupus)
-- dominant pair, extended family
Packs of up to 40, consisting of a breeding pair (alpha
male & female) that mate for life, plus non-breeding
offspring of several generations.
Strict social hierarchy,
maintained by ritualized
display and fighting among
lower ranking members
Alpha female suppresses
reproduction in lower
ranking females
Non-breeders help rear young.
Wolf (Canis lupus)
Cooperative hunting of large game
Food sharing
Maintain large territories defended against other packs.
Pack size and social cohesion is dependent on
resource base. Non-breeders often disperse.
Lion (Panthera leo)
cooperative females, male exploiters
Prides up to 40 individuals – related adult females, dependent
young, and 1 male, or a male group (often related).
Multiple adult females reproduce.
Sexually dimorphic, males 40% bigger.
Males are dominant, but there is generally little aggression
except during pride takeovers.
Lion (Panthera leo)
Females cooperative in hunting large prey and in
rearing young (“communal” nursing).
Females stay with their natal pride, young males are
expelled near puberty. They become solitary or form
male “coalitions” (usually groups of related males).
Resident male(s) remain in
control until killed or evicted
during pride takeover by
another male or coalition.
New males kill dependent
cubs (to bring females into
estrus) and evict any older
immature males.
Spotted hyena (Crocuta crocuta)
Female social dominance
Clans of up to 100 individuals, predominantly related
females and dependent offspring. Fewer adult males.
Multiple females breed.
Some cooperative hunting, but often individual.
Lots of aggression during feeding
Communal dens,
but no communal rearing of young
Spotted hyena (Crocuta crocuta)
Females are entirely dominant over males. The youngest
lowest status female is dominant over the highest ranking
male.
The extreme aggression and large size of females has
involved hormonal “masculinization”. Including male-like
external genitalia. This causes mechanical difficulties in
reproduction (both copulation and birth)
Cubs aggressive from birth,
with competition & siblicide
involved in establishment of
social hierarchy.
Meerkat (Suricata suricata)
autonomous collective
Bands of up to 30 individuals, both sexes, no apparent
social hierarchy. Forage individually within a cohesive group.
Band defends territory against other bands.
Multiple breeding pairs. Females stay with natal group,
males tend to disperse.
Cooperative anti-predator
vigilance (sentinels).
Food sharing. Cooperative breeding
(baby sitting and nursing).
African elephant (Loxodonta)
Stable matriarchy
Herds of variable size up to 20 animals, including related
adult females and their dependent young. Led by a
“matriarch” (oldest female). Females stay with natal group,
males leave at puberty.
Males are solitary or
form separate bachelor
herds.
Promiscuous mating
Female groups very stable –
travel and forage together,
strong mutual defense.
Cooperative care of young.
Mantled howler monkey (Alouatta palliata)
Troops of variable size (up to 20). Multiple adult females form
the stable social core. Not all are close kin. 1-3 males.
Social hierarchy is linear, with males dominant, but top
ranks fluctuate.
Territorial -- troop boundaries
and spacing determined
through male vocalization
Young of both sexes disperse
from natal groups to form or
join other groups.
Mating promiscuous.
Male mating success
dependent on rank.
Males form alliances
to achieve rank and
acquire mating
opportunities.
Yellow baboon (Papio cynocephalus)
Troops of variable size (up to 100). Related females and
dependent young with multiple adult males.
Males substantially larger than females.
Females stay with natal group,
and form stable social core
with dominance hierarchy.
Female rank is inherited.
Males leave natal troop to join others.
Male hierarchy very dynamic
determined by age structure.
Low ranking “strange” males
socialize strongly with females.
Mating promiscuous,
often solicited by females who
prefer strangers.
Dominant male aggressively guards females in estrus.
Males form fighting alliances to achieve rank.
Common Chimpanzee (Pan troglodytes)
Variable group size, mulit-male multi-female group. Groups are
‘fusion-fission”. Female sexual swellings: promiscuous mating.
In bisexual groups, males form
the social core and are dominant
within a strict hierarchy.
Males remain with natal group,
females may disperse.
Male-male alliances are formed to establish rank and to
increase mating success. Alliances not always based on kinship.
Males engage in lethal raids involving incursions into
neighboring groups.
Killer whale (Orcina orca)
Pods of variable size, consisting of one or more
subpods (female kin groups with dependent young), and
adult males. Several pods may form larger less cohesive
clans.
Both sexes stay within pods. Stable social hierarchy.
Females seek non-pod members as mates.
Males seek to mate outside their kin-group pods
Cooperative hunting, wide range of cohesive behaviors
Sperm whale (Physeter macrocephalus)
Pods of variable size, stable core of female kin plus
dependent young. Multiple females breed.
Extreme sexual dimorphism – adult males much larger
reflecting male competition for access to females.
Young males form bachelor herds. Adult males solitary,
or associate with females seasonally. Intense male
competition for mating opportunities.
Female cooperative behavior – baby sitting during adult
feeding dives, mutual defense, variety of cohesive social
behaviors, including individualized sounds for identification.
Summary of social group patterns
Matriarchal social organization
multi-generation female kin groups,
Rarity of male/female pair bonding
Social roles of males and females differ,
males are often “asocial” or even “antisocial”
Banded mongoose (Mungos mungo)
EVOLUTION OF SOCIAL SYSTEMS
What are the advantages?
Greater efficiency in resource use
Reproduction (finding mates, cooperative rearing)
Ultimately, increased fitness (genetic survival) either
Directly – through offspring and decendants
Indirectly – through direct fitness of kin
Social systems arise when:
Reproductive success is dependent on age
(young animals gain inclusive fitness by
remaining with mother to help raise younger sibs)
Resources are limited
(cooperative group foraging)
Cooperative social behavior usually involves
kin selection (“nepotism”)
societies are formed around kin groups
strongest cooperative interactions involve close kin
EUSOCIALITY
the most extreme social structure
Naked mole rats (Heterocephalus glaber)
Subterranean colonies of up to several hundred
males and females. One reproductive female (queen),
one or more reproductive males and many
non-reproductive animals of both sexes – multiple
generations of offspring.
Strict social hierarchy, distinct castes, with division
of labor. Queen suppresses reproduction in subordinates
through pheromones.
Specialized functional castes
REPRODUCTIVE
CASTES
Queen
Male consorts (1-3)
Dispersers (both sexes)
leave to establish new
colonies
NON-REPRODUCTIVE
CASTES
Soldiers
(both sexes)
Workers
(both sexes
Reproductive specialization – increased fecundity
Mole rat “queen”
(and workers)
Breeding females
Non-breeding females
and males
Basic categories of animal interactions
ALTRUISM – helping another at a cost to oneself
Altruistic behavior usually involves kin and includes
a “selfish” component through kin selection
(increasing inclusive fitness)
Helping unrelated individuals
Reciprocal Altruism – helping with the expectation of
future return. – involves “sharing”
Example – male fighting alliances
cooperation between unrelated males
to gain (and share) mating access
Bottlenosed dolphin
(Tursiops truncatus)
Example -- food sharing in common vampires
(Desmodus rotundus)
Common vampire
(Desmodus rotundus)
Small stable groups of 8-12 bats, adult female and dependent
young. Adults within a group have mixed kinship, some are
non-relatives.
Males solitary or in “bachelor groups”. Promiscuous breeding,
Dominant males defend breeding territories that include 1 or
more female groups.
Common vampire (Desmodus rotundus)
Obligate blood feeders – after a blood meal water is
excreted rapidly leaving a stomach full of concentrated
blood
Cannot survive more than 48h without food.
Feeding success is variable, largely dependent on age
and experience.
Young bats may have failure rate of 33%, so there is a
high probability of missing a meal for two consecutive
nights
BUT mortality is low, because of food sharing
Food sharing in the common vampire (Desmodus rotundus)
After returning to the roost,
females groom each other
to determine who has
fed successfully and to
solicit food sharing through
regurgitation.
Females will share food with their semi-dependent young.
(extended maternal care)
Adult females also form stable “sharing partnerships”
of two categories:
between related females (I.e., kin selection)
between unrelated adults – reciprocal altruism
Food sharing in the common vampire (Desmodus rotundus)
Food sharing in the common vampire (Desmodus rotundus)
How can reciprocal altruism between non-relatives
evolve?
The cost to the donor must be relatively small
compared to the benefit to the recipient
Reciprocity (the reversal of donor-recipient roles)
must occur over short time periods, in the same
“currency”
“Cheaters” must be identified and punished!