Bio Sisterhood: quick revision, 12 bites

Sisterhood Is a Biological Strategy

Sisters: more than family, an evolutionary edge.

**Sisterhood** in biology refers to the cooperative bonds between female kin, especially sisters. These bonds are not just social—they are **evolutionary adaptations** that increase survival and reproductive success. By helping relatives, females can pass on shared genes indirectly, a concept called **kin selection**.

Takeaway. Sisterhood is nature's way of ensuring shared genes survive.

Kin Selection Explained

Why help family? It's in the genes.

**Kin selection** is an evolutionary strategy where individuals help relatives survive and reproduce, indirectly passing on their own genes. This explains altruism among sisters: by helping a sister raise offspring, a female ensures shared genetic material continues.

Takeaway. Hamilton's rule: help when the genetic payoff outweighs the cost.

Sisters Share 50% of Genes

Half your genes, all your loyalty.

Full sisters share about **50%** of their genes, the same as parents and offspring. This high relatedness makes cooperation evolutionarily advantageous. In many species, sisters form **coalitions** to defend resources, raise young, or compete for dominance.

Takeaway. Genetic overlap fuels sisterly solidarity.

Lionesses: Sisters in the Pride

Lion prides run on sister power.

In African lions, females are the core of the pride. **Sisters often hunt together**, defend territories, and synchronize reproduction. They even nurse each other's cubs. This **cooperative breeding** increases cub survival and strengthens the pride's competitive edge.

Takeaway. Lionesses show how sisterhood builds a stronger pride.

Elephant Matriarchs and Kin

Grandmothers, mothers, daughters: a united front.

Elephant herds are led by the oldest female, often a **matriarch**. Related females—sisters, aunts, daughters—stay together for life. They share knowledge of water sources, help raise calves, and protect each other from predators. This **matrilineal society** is a prime example of sisterhood in action.

Takeaway. Elephant sisterhood preserves wisdom across generations.

Sisterhood vs. Solitary Living

Together or alone? The trade-offs.

Takeaway. Sisterhood trades individual competition for collective security.

How Kin Recognition Works

How do animals know who's family?

Takeaway. Kin recognition prevents helping strangers and reinforces sisterly bonds.

Sisterhood in Human Evolution

Grandmothers and the rise of cooperation.

In humans, **cooperative breeding** and **matrilocal** societies may have shaped our species. The **grandmother hypothesis** suggests that post-reproductive women helped daughters raise children, boosting survival. Sisterly bonds also provided childcare, food sharing, and social support.

Takeaway. Human sisterhood may have been key to our evolutionary success.

Sisterhood Isn't Always Harmony

Even sisters compete.

A common misconception is that sisterhood means zero conflict. In reality, **competition** for resources, mates, or status can occur even among close kin. However, the **benefits of cooperation** often outweigh the costs, so sisterly bonds persist despite occasional squabbles.

Takeaway. Sisterhood balances cooperation with competition.

Exam Tip: Link Kin Selection to Sisterhood

Connect genes to behavior.

When asked about sisterhood, always link it to **kin selection** and **Hamilton's rule**. Explain how shared genes make altruism adaptive. Use examples like lionesses or elephants to show real-world evidence.

Takeaway. Always tie sisterhood back to genetic benefits.

Remember: 'Sisters Share, So They Care'

A rhyme to recall kin selection.

Use the phrase **'Sisters Share, So They Care'** to remember that genetic sharing (50%) drives caring behaviors. This helps recall why cooperation evolves among female kin.

Takeaway. Shared genes → shared care.

Bonobo Sisters: Peace Through Bonds

Female bonds keep the peace.

In bonobos, **female coalitions**—often between sisters and mothers—prevent male aggression and maintain social harmony. Unlike chimpanzees, bonobo females dominate, and sisterhood is central to their peaceful society.

Takeaway. Bonobo sisterhood shows how female bonds can shape entire societies.