Chapter 14The Evolution of Social Behavior

(1st lecture)

SocietyA group of individuals of the same species that is organized in a cooperative manner, extending beyond sexual and parental care

Colonial invertebrates form aggregations of individuals

for mutual benefit

For instance, to the right is a man-of-war (phylum Coelenterata). It consists of four different types of polyps, which specialize in either

flotation, feeding, defense, or reproduction

Sociality in FishForm complex schools

Some form colonies that actively defend nest sitesSome species are territorial

A school of fish changes leadership when it changes direction

The leaders at the left (stippled) are shifted to the flank when the schoolmakes at 90 degree turn, as shown in the sequence in the center and at

the right

The colonial bluegill nests in groups

Each male defends a territory bordered by the nest sites of other males. He attempts to protect his nest against bass (blue circle), bullhead catfish (green

arrow), snails, and pumpkin-seed sunfish (red arrow)

Sociality in amphibians and reptiles

Amphibians and reptilesDevelop large communal sites for calling females

Some exhibit parental careSome species form hibernacula (garter snakes)

Some species spend considerable time and energy engaged interritorial defense

Males of a tropical frog (Dendrobates galindoi)wrestle for possession of a territory

In most cases, spacing between frogsis maintained by repetitious calling

Sociality in birds

The majority of bird species are monogamous territorial breeders

Many aggregate in feeding, migratory and roosting flocks

Well-organized breeding colonies are common

The most complex avian social systems are those involving cooperative breeding, in which non-parents share in the rearing

of young

Sociality in mammals

Majority of species are solitary, where the most complex social unit is the mother with her young

Some species are social during the mating season, when a male monopolizes several females (= polygynous mating system)

However, complex social organizations have evolved in species within virtually all mammalian orders, especially in marsupials,carnivores, ungulates and primates that live in relatively open

habitats. ==> This indicates that social behavior evolved largely as a defense against predators and protection of resources through group territory

Social behavior in the Hamadryas baboon

A large group of baboons is departing from the communal sleeping rock (leftbackground) on the way the the feeding and watering sites. The procession is

beginning to break up into the basic social units, which consist of single males andtheir harems of females and offspring. Aggressive interactions are frequent and

animated. The 2 males in the foreground threaten each other--the one on the right isusing a hostile stare, while his opponent responds with a more intense gaping display.

Sociality in insectsMost insects are not social. However, some species of insects (e.g., bees, ants, termites, and wasps) display a

remarkable type of social organization, referred to as eusociality

Eusocial insect colonies consist of:sterile workers that labor on behalf of reproductive individuals

sterile worker that cooperate in the care of youngat least two generations of individuals

Colony members use a variety of visual and chemical signals forAlarm

Simple attractionRecruitment of individuals to a new food source or nest site

GroomingGreeting rituals consisting of the exchange of oral and anal liquid food

Exchange of solid food particlesRecognition of nestmates and members of particular castes

View into a termite colony

Primaryqueen

Primarymale

Reproductive nymphs

Soldier

Secondaryqueen

Haplodiploidy and the degree of mother-offspring genetic relatedness

Haplodiploidy and the degree of sister-sister genetic relatedness

The high proportion of genetic relatedness between sisters promotes the evolution of altruism among sisters, and hence

the evolution of eusociality.

Altruism: helpful behavior that raises the recipient’s direct fitness, while lowering the donor’s direct fitness

Accordingly, altruistic females are presumed to be really helping their sisters (future queens) and only incidentally

helping their mothers

Altruism in bees

A female bee that has just stung a human. Notice that she devisceratesherself as she tries to leave her victim.

The Naked mole rat: an example of a eusocial mammal

Found in Ethiopia and KenyaIn the Family Bathyergidae (includes Guinea pigs, chinchillas, porcupines)

Colonies: Typically 75 - 80 rats, including workers, defenders, and a breeder (queen)

Queen is very aggressive (other females reproductively suppressed until queen removed)

Inbreeding high, relatedness = 0.81 (0.50 = siblings)

Control over reproductive suppression: Not pheromonal, but instead due tophysical contact (when queen removed, reproductivity released)

Genetic, ecological, and behavioral factors that appear topromote the evolution of eusociality