# Mating Systems and Sexual Selection

Psychology I · Ethology · https://tryals.app/learn/psychology-i/mating-systems-and-sexual-selection

## Mating Systems and Investment

**Mating systems** are behavioral patterns shaped by resource distribution and **parental investment**: the time and energy a parent commits to rearing offspring. 

| System | Pattern | Primary Context |
| --- | --- | --- |
| **Monogamy** | 1 male, 1 female | Biparental care needed |
| **Polygyny** | 1 male, many females | Resource monopolization |
| **Polyandry** | 1 female, many males | Reversed parental roles |
| **Promiscuity** | Multiple partners | No stable pair bonds |

Parental investment theory (Trivers) predicts that the higher-investing sex is more **selective (choosy)**, while the lower-investing sex competes for access.

## Sexual Selection Mechanics

Asymmetry in parental investment drives **sexual selection**, operating via two distinct mechanisms:

- **Intra-sexual selection**: Direct competition within one sex (usually male-male) for access to mates.
- **Inter-sexual selection**: Mate choice exercised by the opposite sex (usually female choice of displays).

> **Common pitfall:** Assuming the choosy sex is always female. **Investment predicts choosiness**, not biological sex. In species like seahorses, males invest the primary parenting burden and are the choosier sex.

## Practice questions

5 of this lesson's 12 practice questions, with answers. The full set is in the app.

### 1. In seahorses, where the male carries the developing eggs and invests more per offspring, parental investment theory predicts the MALE will be the choosier sex.

**Answer:** True

**Why:** True. Parental investment, not sex, predicts choosiness - so where males invest more (seahorses, jacanas), males become the choosier sex. Assuming females are always the choosier sex is the named pitfall.

Page: https://tryals.app/practice/psychology-i/mating-systems-and-sexual-selection/in-seahorses-where-the-male-carries-the-developing-eggs-and-invests

### 2. A peacock's tail makes it slower, more visible to predators and expensive to grow. How does a trait that *lowers* survival spread?

A. Because selection counts descendants, not lifespan
B. Because sexual selection overrides natural selection entirely once it begins
C. Because the cost is negligible in practice and the appearance of expense is illusory
D. Because predators avoid brightly coloured prey, so the tail is protective after all

**Answer:** A. Because selection counts descendants, not lifespan

**Why:** Selection counts grandchildren, and a long safe life with no mates is an evolutionary dead end. The cost is doing double duty: it is the price of the ornament and the reason to believe it, since only a genuinely healthy male can carry that handicap and still be here.

Page: https://tryals.app/practice/psychology-i/mating-systems-and-sexual-selection/a-peacocks-tail-makes-it-slower-more-visible-to-predators-and

### 3. Select every statement that is TRUE about mating systems and sexual selection.

A. The male is always the competing sex, regardless of investment.
B. Elaborate ornaments evolve mainly through same-sex combat.
C. Rival stags locking antlers are settling access between themselves, with no female involved in the contest.
D. Which sex competes and which chooses can be predicted before knowing which is male.
E. Where one parent alone cannot rear the young, staying paired starts to pay for both.

**Answer:** C. Rival stags locking antlers are settling access between themselves, with no female involved in the contest.; D. Which sex competes and which chooses can be predicted before knowing which is male.; E. Where one parent alone cannot rear the young, staying paired starts to pay for both.

**Why:** Biparental monogamy, investment-predicts-choosiness, and male-male competition are all true. Ornaments evolve through inter-sexual CHOICE (not combat), and the competing sex is the lower-investing one (not always the male) - so those two are false.

Page: https://tryals.app/practice/psychology-i/mating-systems-and-sexual-selection/select-every-statement-that-is-true-about-mating-systems-and-sexual

### 4. In most species females are the choosier sex. In seahorses, where the male carries the developing brood, it is the male who is choosy and the female who competes. Why does the reversal matter more than the general rule?

A. Because it shows the driver is parental investment, not sex.
B. Because males are choosy whenever they carry the offspring.
C. Because seahorses are an exception to sexual selection.
D. Because choosiness is learned from the local mating system.

**Answer:** A. Because it shows the driver is parental investment, not sex.

**Why:** If choosiness were a fact about being female, the seahorse would be an anomaly to file away; it is the crucial test. 'Females are choosy' and 'heavy investors are choosy' predict identically everywhere except in sex-role-reversed species, which is precisely where the two come apart, and investment wins.

Page: https://tryals.app/practice/psychology-i/mating-systems-and-sexual-selection/in-most-species-females-are-the-choosier-sex-in-seahorses-where-the

### 5. Sort each behavior by the type of sexual selection it represents.

**Answer:**

- Intra-sexual selection: Two stags clash antlers for access to females, Males fight to control a breeding territory
- Inter-sexual selection: A peahen chooses among several displaying peacocks, A female evaluates male song quality before mating

**Why:** Combat among same-sex rivals (antler clashes, territory fights) is intra-sexual selection; choice exercised by the opposite sex (peahen choosing, female evaluating song) is inter-sexual selection. Competition-within versus choice-by-the-other-sex is the key distinction.

Page: https://tryals.app/practice/psychology-i/mating-systems-and-sexual-selection/sort-each-behavior-by-the-type-of-sexual-selection-it-represents