Sexual Selection

Question 1

compare and contrast natural and sexual selection

Answer 1

• natural selection = traits evolve that enhance survival & reproduction
• sexual selection = costly traits evolve that enhance mating success
• **sexually selected traits can be detrimental to survival**, so they cannot be explained by natural selection

Question 2

describe 2 main pathways to sexual selection

(intrasexual competition vs. intersexual selection - mate choice)

Answer 2

• intrasexual competition = male competition

-> mechanisms: physical combat, displays of strength / dominance, or contests for territory / resources

=> evolution of traits that enhance competitive abilities

• intersexual selection = female choice

-> mechanisms: physical appearance, courtship displays, behavioral traits, or possession of resources

=> evolution of traits that are attractive to potential mates

Question 3

contrast variety of Pre-copulation & post-copulatory traits selected for male-male competition

Question 4

sexual selection

Answer 4

evolutionary process where selection acts on heritable traits that enhance reproductive success either through sex competition or mate choice

Question 5

costs of sexual selection

Answer 5

• predation
• energetically expensive
• cumbersome
• time spent displaying

Question 6

Why do males tend to be under stronger sexual selection for competition than females?

Bateman’s principle

Answer 6

• females = few, large, expensive gametes -> high parental investment
• males = many, small, cheap, gametes -> often little / no parental investment
• females are limiting factor to male production due to lower reproduction rates
• males compete, females choose to maximise their investment

Question 7

Bateman’s principle

Answer 7

the sex investing most in offspring production drives sexual selection for competitiveness in the opposite sex

Question 8

Resource holding potential (RHP)

Answer 8

• refers to individs ability to successfully acquire & defend valuable resources, eg. territories, food, or mates, against competitors
• in context of pre-copulation fighting, RHP plays crucial role in determining outcome of aggressive interactions between individuals competing for access to mates
• good RHP characteristics:
  -> large body size
  ->effective weapons
  ->aggression/effort
  -> coalitions

Question 9

RHP characteristics and examples in species

Answer 9

• successfully defending territories
  -> body size determines who holds highest traffic territories (marine iguana)
  -> aggression predicts past success holding territories (song sparrow)
• guard females/harems
  -> lifetime reproductive success & antler size in red deer
  -> oldest/heaviest group members mate with highest fecundity females in group
• defend resources females are attracted to
  -> larger males win exclusive access to shrubs where female Ligurotettix lay eggs

Question 10

some females fight!

why?

Answer 10

• often in cooperative groups
• over carers for their offspring

Question 11

pre-copulation: scramble

Answer 11

aquatic mating in Weddell seals…

• females are dispersed
• RHP has no benefit, experience in past seasons looking for females does!
• limited sexual dimorphism

if access to females is not defendable, RHP is not sexually selected

Question 12

pre-copulation: alternative, sneaky strategies

Answer 12

• in systems where access to females is defendable, low RHP males sometimes employ sneaky strategies
• eg. small sneaker males in Round headed goby
• eg. femae mimicry in cuttlefish

Question 13

Pre-copulatory sexual selection

overview

Answer 13

• RHP is important for protecting access to females if that access is defendable
• to have a chance, low RHP males may be selected to use sneaky strategies

Question 14

Why do males produce so many sperm cells?

(post-copulation)

Answer 14

intrasexual selection:
-> RHP: males compete for access to mates & those with higher RHP (eg. larger body size / ↑ physical strength), have adv in pre-copulation fighting

-> producing large no. of sperm allows males to ↑ their chances of fertilizing eggs during mating encounters, maximizing reproductive success even if they don’t win every competition for mates

in summary, production of numerous sperm cells by males is adaptive strategy shaped by intrasexual & intersexual selection pressures.

it allows males to compete more effectively for mates, maximize reproductive success in competitive mating environments, & increase likelihood of fertilizing eggs during mating encounters

Question 15

Post-copulatory interference: barriers

Answer 15

sperm plugs as insurance policy against rival copulations by…

• obstructing access to female’s reproductive tract & reducing likelihood of fertilization by rival males’ sperm
• largely absent in monogamous mating systems
• male ejaculates coagulation plug correlates with promiscuity e.g in primates

Question 16

how does male sperm plug correlate with promiscuity?

Answer 16

• in promiscuous mating systems, females mate with multiple males => intense sperm competition where sperm from diff males compete to fertilize female’s eggs
• males face challenge of outcompeting rival males’ sperm to ensure their genes are passed on to next gen…
• so males in some species ejaculate coagulation, which forms plug in female’s reproductive tract after mating
• coagulation serves as physical barrier, obstructing subsequent mating by other males and reducing chances of their sperm reaching female’s eggs

Question 17

Post-copulatory interference: flushing

Answer 17

• Grevy’s zebra have disproportionate large testes compared to body size to produce large quantities of ejaculate
• while guarding female, copulate hourly to flush previous sperm out of reproductive tract

flushing: females engage in urination / defecation immediately after mating

Question 18

Post copulatory sexual selection

overview

Answer 18

• sperm have traits selected to win race to egg
• other male traits are selected to interfere with race, & tilt outcome in their favour
• sexual selection can drive rapid evolution & diversity in traits, often even more so than natural selection

Question 19

cost of competition

Answer 19

• pre-copulation: injuries / death / energy
  -> eg. Fidler crabs can lose claws
• post-copulatory: germline DNA damage

Question 20

sexually selected traits can be extremely costly,

BUT as long as fitness benefits outweigh the costs…

they will be selected

Question 21

Explain the selection pressures that drive male sexual signalling

Question 22

Describe main benefits of female choosiness and how they can identify informative signals of male quality

Question 23

Explain how trait variation is maintained despite “Lek paradox”

Question 24

female cryptic choice

Answer 24

represents female-driven mechanisms at / after mating that bias sperm use & impact male paternity share

• selective ejection
• spermicide
• selective transport/storage

eg. Red jungle fowl cloacal contractions eject sperm of less desirable partners

eg. separate storage receptacles in flower beetle

Question 25

who (females) stores sperm longest? snake bat shark chicken

Answer 25

snake = 7 years bat = 6 months shark = 2 months chicken = 2 weeks

Question 26

If females have so much control, how do males prove themselves?

Answer 26

SIGNALLING

Question 27

what is signalling?

Answer 27

- signals are **traits** that have evolved to modify another’s behaviour in way that **benefits signaller** - for a signalling system to be evolutionarily stable, on av. both **signaller & receiver should benefit**

Question 28

costs of signalling

Answer 28

- **conspicuous** to predators - **time** spent signalling rather than feeding - growth of structure - **inhibits locomotion**

Question 29

sexy sons hypothesis

Answer 29

- females choose males with **heritable attractive traits** => **maximising their fitness** through reproductive success of their ‘sexy sons’ - attractive traits can be arbitrary features that are **not necessarily signals of quality** - **runaway selection**: a trait in 1 sex becomes genetically linked to preference for that trait in other sex => **coevolutionary feedback loop** between trait & trait preference

Question 30

deceptive signalling

Answer 30

- most of time signals are honest % benefit both signaller and receiver - however, signallers may benefit by deceiving receivers - as result of behaviour of signaller, receiver registers a certain situation that is not in reality occurring... - as result of the interaction, **signaller benefits** (while **receiver pays a cost**)

Question 31

when can deceit be stable? (in deceptive signalling)

Answer 31

- when **cost** of **ignoring an honest signal** is high - when **cost** of being deceived is low - BUT most signals are actually honest -> lies are expensive when they are found out!

Question 32

Deceitful sexual signalling is hugely costly for females...

Answer 32

1. **low quality males** produce dishonest signal of quality 2. females **deceived** in mate choice 3. females **invest in low quality offspring**

Question 33

good genes hypothesis

Answer 33

- extravagant ornaments/displays are not arbitrary traits - females choose males based on **extravagant/costly traits** because they are **honest signals of genetic quality/viability**

Question 34

what makes a signal of male quality **informative** for females?

Answer 34

**indices (can’t be faked)** - dishonesty not possible - eg. male bird with bright plumage: index of genetic quality as brightness of feathers **reflects ability to acquire sufficient nutrients**, correlates with overall genetic fitness **handicaps** - costly for low quality individs to fake - either through production of traits / social costs - costly/conspicuous/cumbersome ornaments or weapons - dishonesty possible but unprofitable - eg. peacock tail is handicap as energetically costly to grow & maintain. Male with large, colorful tail shows he has **sufficient resources & genetic fitness to overcome this cost**, making him an attractive mate

Question 35

another index eg.

Answer 35

- **red deer stags** display fighting ability to opponents by roaring - to roar well, stag must be in good condition - **body size** is causally related to resonant frequency of roars - **signals of fighting ability are indices: they can’t be faked** - stags that roar more = more success in fighting - fighting is **costly**: roaring signall benefits signaller & receiver

Question 36

carrying on with Red deer stags (LRF)

Answer 36

- larger stags produce **lower resonant frequencies (LRF)** - females use LRF as **signal of body size** - females prefer **larger** males - stags with LRF have **greater reproductive success**

Question 37

the handicap principle

Answer 37

signals show strong condition dependence: - high quality individuals can afford to produce costly signals - low quality individuals could produce signal but **cost would outweigh the benefit**

Question 38

Female choice and the **Lek Paradox**

Answer 38

- **fixation of male traits** is fundamental problem - female preference for traits = **reduced genetic variation** - without trait variation: **no longer benefit to females being choosy** - ornaments/displays & mate choice are **costly** - if **cost of mate choice outweighs benefit**; female choosiness will **not be maintained**

Question 39

How is **genetic trait variation** & **female choosiness** maintained?

Answer 39

many aspects of trait variation: - **sneaky/satellite mating** - **parasites/disease** -> parasites attracted to Hawaiian field crickets “singing” -> silent “satellite” males take advantage - **environment** - **Red Queen hypothesis**

Question 40

what would **prevent** the Lek paradox?

Answer 40

**sources of variation in male signalling** eg. changing predation pressure

Question 41

changing **predation** pressure eg.

Answer 41

- more dangerous predators of guppies **downstream** - male sexual signalling is **less conspicuous downstream**