Group Living and Altruism

I. Group living only pays if the net benefit > cost

A. Costs
1. Competition for food and space (walrus)
2. Parasite and disease transmission (streblid flies, vampire bats, roosts)

B. Predation benefits
1. Predator dilution (Ridley's turtles, cicadas)
2. Predator detection - more eyes and ears (meerkats)
3. Group defense (musk ox, oryx)

C. Foraging benefits
1. Decreased vigilance time (turnstones)

2. Information center (cliff swallows, evening bats)
3. Improved prey capture success (pelicans, black-headed gulls)

4. Group hunting (wild dogs, hyaenas, lions)

II. Cooperation and altruism

A. Cooperation - both parties benefit from act
1. best explained by mutualism
2. cheating can be prevented if partners can be selective.

B. Altruism - donor pays reproductive cost while recipient benefits

1. Kin selection - Hamilton's rule rB - C > 0;

a. r = relatedness: the proportion of genes shared with another individual which come from a common ancestor; B = benefit to the recipient in terms of an increase in their lifetime reproduction, C = cost to the donor in terms of a decrease in their lifetime reproduction

b. Best example is reproductive sacrifice to help others.

i. Belding's ground squirrels give alarm calls when female kin are nearby

ii. White-fronted bee-eaters that fail nesting redirect help to clan members provides an example of how kinship is used in decision making.

c. Complete reproductive altruism

i. common among hymenoptera which are haplo-diploid. Males are haploid and females are diploid. This type of genetic system leads to relatedness asymmetry. If females mate once, then sisters share 3/4 of their genes while brothers only share 1/4 with their sisters. Mothers still share 1/2 with their daughters. Thus, sisters that help their mother rear more sisters can pass their genes on more efficiently than if they were to reproduce themselves! However, not all bees and wasps exhibit reproductive sacrifice.

ii. also occurs in termites which are diploid

iii. and naked mole rats. These subterranean mammals apparently show inbreeding, which elevates average relatedness

iv. Most recent case is snapping shrimp from sponges in Belize. Don't yet know details yet.

2. Reciprocity - you scrub my back if I scrub yours, relatedness unnecessary

a. Differs from mutualism because of delay between acts.

b. Must find way to avoid cheating.

c. Unrelated male dolphins form alliances to gain access to females

d. Female vampire bats regurgitate blood to bats that fail to feed. Related and unrelated females live together for many years, experiments in captivity show that they reciprocate feedings with unrelated bats, and that the cost of a blood meal is less than the benefit.

e. reciprocity is often modeled as the iterated prisoner's dilemma. See next week's lab.

f. common in many human societies