Evolution III - contents

• Sexual selection vs. natural selection

• Male and female strategies

• Alternative mating tactics

Sexual dimorphism

• The large, male California sea lion, is distinctive from the surrounding, smaller females (from Kardong).

Sexual dimorphism• e.g. in peacock

•

Sexual selection

• Differential reproduction owing to variation in the ability to obtain mates (as opposed to natural selection, which is related to the differential survival of individuals in nature, depending on non-sex related traits) – see Futuyma – Evolutionary Biology

Males are often less selective than females

Australianjewel beetle

maletoad

Biologists exploiting animal desire

• sea

elephant

Anisogamy - Sperm and oocyte of hamsters (enlarged 4000 times – from

Alcock)

Differential reproductive success in males and females

• Female oocytes are a limited reproductive resource, whereas male sperm is not

• Therefore, males have the potential to sire many more offspring than females

• In humans, the male world record is held by Moulay Ismail the Bloodthirsty (Sultan of Morocco 1672-1727), who is said to have fathered 888 children

• Men produce between

100 and 300 million

sperm per day.

Female reproductive success in humans

• According to Guinness World Records 2001, the highest officially recorded number of children born to one mother is 69. This was the first wife of Feodor Vassilyev (1707-1782) of Shuya, Russia. Between 1725 and 1765, in a total of 27 confinements, she gave birth to 16 pairs of twins, seven sets of triplets, and four sets of quadruplets. 67 of them survived infancy.

Females must be more choosy because

• costs of producing female gametes are higher

• costs of pregnancy are higher (e.g. in mammals)

• females often invest more into parental care

• costs of mating with partner of poor genetic quality are higher in females

Thus, a general pattern across the animal kingdom is

• males compete for females and try to mate with as many as possible (emphasis often on quantity)

• females are choosy and try to find the highest quality partners (emphasis on quality)

How to impress females

• 1. look nice

How to impress females

• 2. bring them presents

(as e.g. in scorpion flies (right)

and Bittacus (below))

How to impress females

• 3. offer yourself as a present (e.g. preying mantids, some spiders: females cannibalise

males during or after mating –

thus detracting attention

from other possible sex

partners. also you give your

body as a resource for offspring)

How to impress females• 4. sing them songs (picture from Alcock)

Number of songs in repertoire

Days to finding partner

European warblers

If none of this works - cheat

• Satellite strategies – younger or “weaker” males often position themselves strategically near an attractive male, then sneaking up on females when the large male is un-attentive or otherwise engaged

Great Plains toads,longhorn sheep, horseshoe crabs

Alternative male mating tactics

• different males in the same species use different strategies to gain access to females

• satellite strategies are one example

Alternative male mating tactics – “rape”

• In scorpion flies, some

males will not bring

presents, but mate with

unwilling females.

Usually “losers”,

fertilisation success

typically low.

FIGURE 7.12 Barn Swallows (Kardong, after Andersson, Moller)

Females often prefer exaggerated male traits

Why do females prefer exaggerated male traits?

• “Good genes hypothesis” – extravagant traits indicate biological fitness (because less fit males would be less able to afford such traits).

• One special case of this hypotheses is that such traits indicate ability to cope with parasites.

• “Runaway selection hypotheses” – females prefer sexually attractive traits which they pass on to sons, which makes sons more fit (but only because the sons in turn will also be more attractive to females).