Lecture 20: Social Systems

What factors influence a species' social system?

• Typical group size: how many individuals generally occur together

• Social organization: what is the nature of social interactions and who interacts

• Philopatry (staying in natal group): remaining in a natal group or in a natal territory

• Dispersal patterns: movement to a new group or territory to reproduce

What is the difference between honey bee and fly maggot social organization?

Honey bees are highly organized, have different job and a hierarchy

Fly maggots have no organization

Note: this indicates that a lot of members does not mean same social systems

What are some axes of social structure variation?

1. Presence/absence of in-group v. out-group behavior

2. Nature of interactions (cooperative, competitive, etc)

3. Types of individuals in the group (males, females, young)

4. Dominance hierarchy within the group

5. Reproductive division of labor and/or task specialization

What does philopatry influence?

Whether or not you will be around relatives. Family groups depend on philopatry

How do ocean species demonstrate dispersal?

• Many species in the ocean have long distance dispersal of larvae driven by currents, so individuals are not near relatives

  • Ex: After hatching, clownfish larvae swim into the water column and are dispersed by currents

How do small species demonstrate dispersal?

• On land, many small animals disperse via the wind.

  • Ex: ballooning in spiders. No direct dispersal

Does one sex disperse further than the other?

• Yes, this reduces inbreeding

  • In birds, females tend to disperse

  • In mammals, males tend to disperse

• Note: there are plenty of exceptions in both birds and mammals to the general pattern

What are two consequences of staying at home in social groups?

• Formation of family groups and cooperation.

• Risk of inbreeding

What is an example of a family where non-breeding individuals help raise the young?

• Meerkats have adult helpers that:

  • Babysit the pups

  • Feed the pups

  • Stand guard while group forages

Does evolution favor individuals that have behaviors that promote their own success?

Yes

What are meerkat adult helpers an example of?

Altruism: when an individual’s behavior provides a benefit to another individual at a cost to it self. Costs can be both negative consequences of a behavior or the missed opportunities caused by helping.

What is hamilton’s rule?

A way to explain why animals sometimes help others, even if it costs them something

What is Hamilton’s rule equation?

In Hamilton’s rule equation, what does the r, b and c refer to?

• Relatedness: The proportion of ancestry shared between two individuals through common descent

  • For sexually reproducing species, this means that each pedigree link needed to connect individuals reduces relatedness by 50%

• Benefits: The additional offspring that the recipient will have as a result of the altruist’s behavior

• Costs: The loss in offspring that an altruist will have a result of the helping behavior

What are some key things to understanding relatedness?

1. Each parent passes on 50% of its DNA to offspring

2. All offspring get 50% from each parent, which is a random assortment of the DNA that came from each grandparent

3. Each sibling gets a different subset of the parent’s DNA

4. Which sections are passed on in each egg/sperm is random

5. To calculate relatedness we multiply within a path of relatedness. The we need to sum different paths of relatedness

For meerkat adult helpers, what is the pay off?

1. They (typically) raise their full siblings

2. Pup production is easiest in large groups

3. Leaving to start your own group means you will be in a small group and you will produce few pups

What does mating with close relatives increase?

The chances that a negative genetic trait for which you are a carrier will be expressed

Are many negative mutations present in populations recessive?

Yes, meaning they have little or no impact on the phenotype when there is only one copy

What gets reduced when one sex regularly leaves social groups?

Inbreeding gets reduced

How does lion social structure actually work?

• Males: compete for access to mates

  • Solitary males or coalitions compete for access to prides of females

  • When males are deposed, the new males will kill any nursing cubs present in the pride

• Females: compete for resources

  • The core unit of the pride is group of related females. They cooperate to defend territory and hunt. They will also provide care for others’ cubs.

How can shared DNA similarity be measured?

In many ways (in this classic work using DNA ‘fingerprints). The important point is to assess similarity of variable DNA elements among individuals

What does DNA show about lions?

• For lions, DNA shows that females within a pride are relatives

• Male partners tend to be relatives

• Breeding males and females are not related to each other.

Do lion coalitions vary in size?

Yes

What is the difference between large and small lion coalitions

• Large coalitions a made of close relatives that dispersed single pride

• Small coalitions may have unrelated males.

Is there a single king in lions?

Multiple males sire cubs when there are coalitions. There is no single ‘king’ of the pride that gets all the matings.

What can larger coalitions control?

Prides for longer (avoid infanticide). They can hold larger prides and they sire more young