week 3 - sexual selection

Question 1

What did charles darwin write about sexual selection?

Answer 1

depends not on a struggle for existence but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor but few or no offspring

Question 2

What is sexual selection?

Answer 2

a form of natural selection that occurs when individuals differ in their ability to compete with others for mates or to attract members of the opposite sex

Question 3

What is sexual dimorphism in animals?

Answer 3

a systematic difference in form between the individuals of different sexes of the same species

Question 4

in most examples of sexual dimorphism males are more ____ than females

Answer 4

conspicuous

Question 5

list some examples of sexual dimorphism

Answer 5

size - iguanas and humans
colouration - frogs and birds
exaggerated traits - peafowl, stalk eyed flies
calling behaviour - frogs, birds

Question 6

Example of mate choice - satin bower birds

Answer 6

A female Satin Bower Bird looks into an avenue of twigs approx. 30 cm apart built by the male which has bright shiny blue feathers. The male arranges blue objects in front of this avenue in order to attract a mate.

Question 7

Example of mate choice - peafowl

Answer 7

Peahens choose males for the quality of their trains - quantity, size, distribution of colourful eyespots. Offspring of males with more eyespots are bigger at hatching and better at surviving in the wild than offspring of birds with fewer eyespots

Question 8

What are asymmetries in sexual reproduction?

Answer 8

males tend to invest less in offspring than females
parental care
gamete size

Question 9

what is isogamy?

Answer 9

gametes of same size

Question 10

what is anisogamy?

Answer 10

some gametes (females) are large (eggs, ovules) some gametes (males) are small (sperm pollen).
Small number of large gametes and large number of small gametes

Question 11

in females the reproductive success is limited by?

Answer 11

offspring number

Question 12

in males the reproductive success is limited by?

Answer 12

number of matings

Question 13

Based on what reproductive success is limited by what 2 predictions can be made?

Answer 13

female reproductive success should level off after a certain number of mates while male reproductive success continues to increase.

Variance in reproductive success should be greater in males than females

Question 14

What are the consequences of asymmetries for males?

Answer 14

they should compete with other males over opportunites to mate - intrasexual selection

Question 15

what are the consequences of asymmetries for females?

Answer 15

they should be selective about whom they mate with because they invest more heavily in offspring - intersexual selection

Question 16

what are the 3 ways male -male compete?

Answer 16

combat
sperm competition
infanticide

Question 17

Explain about combat (3)

Answer 17

Body size is related to competitive ability
mating males larger than non mating males
some male strutures e.g. horns enhance competitive ability

Question 18

Give an example of male - male combat?

Answer 18

Male elephant seals (bulls) Mirounga angustirostris rear up and fight for access for mating females that are in oestrous. Such fighting among elephant seals can take place on the beach or in the water. They bite and tear at each other on the neck and shoulders, drawing blood and creating scars on the tough hides.

Question 19

What is sperm competition?

Answer 19

Occurs when females mate multiply, sperm from different males compete for fertilisation

Question 20

what are the 6 strategies for winning sperm competition?

Answer 20

increase ejaculate amount
increase frequency of mating
removal of sperm from previous males
sperm compete inside female
prolonged mating and guarding
copulatory plugs

Question 21

with the strategies for sperm competition you can identify two different mechanisms?

Answer 21

post copulatory

pre copulatory

Question 22

Example of an organism removing previous sperm of males

Answer 22

Insect males - dragon flies / damsel flies

Dragonflies/damselflies males have (beside the first genitalia) their secondary genitalia, which is the accessory organ on the second abdominal.

Sperm is moved from the first genitalia into the secondary genitalia just before copulation.The male bends his abdomen forward so that the first genitalia touches his secondary genitalia.

After forming a tandem and the sperm in the male secondary genitalia is ready, the male invites copulation by wing flapping and flexing the abdomen. The female responds with bending up her abdomen to the genitalia of the male and form the wheel position.

Dragonflies usually start copulation in flight. Some species, it is found that before transferring his sperm, the male will remove the existing sperm from the female genitalia which is from earlier mate, then replace it with his own.

Question 23

Evidence for prudent sperm allocation in norway rats?

SPERM COMP Theory

Answer 23

pound and gage 2004
In species where there is postcopulatory competition between males in the form of sperm competition, theory predicts that males should show prudent sperm allocation.

Specifically, males should adjust the number of sperm that they inseminate in response to variations in the risk and intensity of sperm competition.

Copulatory ejaculates were collected from 12 male rats that each mated under two experimental conditions: alone in a cage with a female and accompanied by a ‘rival’ male who observed the copulation from behind a perforated plastic screen.

As predicted by sperm competition theory, males ejaculated significantly more sperm when copulating in the presence of a rival than when copulating alone. Furthermore, the magnitudes of these ejaculate adjustments were highly predictable and were relatively consistent across individual males.

Question 24

Evidence of increasing sperm investment? meadow voles

SPERM COMP THEORY

Answer 24

del barco-trillo and ferkin 2004
male meadow voles respond to a risk of sperm competition conveyed by odours of conspecific males

Microtus pennsylvanicus increase sperm investment when they mate in the presence of
another male’s odours.

Question 25

What is the sperm competition theory?

Answer 25

Predicts that males should strategically allocate their sperm reserves according to the level of sperm competition

Question 26

what is infanticide?

Answer 26

males that take over a coalition kill previous males offspring

Question 27

what is an example of infanticide?

Answer 27

lions

males form coalitions to take over a pride.
offspring of previous coalition killed by new males
male infanticide accounts for 10% lion mortality
females abort foetuses when new males take over
come back into sexual receptivity shortly afterwards

Question 28

What is another example for infanticide?

Answer 28

Langurs
They live in groups of 11-64 in tropical, dry thorn thickets to pine and subalpine forests, usually one male:multi-female. Range over 200 ha to 1200 ha. They spend up to 80 % of their time on the ground. They are diurnal and move using ‘all fours’. When a new male takes over a troop, he systematically kills all the offspring sired by the previous alpha male.

Question 29

Provide evidence for infanticide in langurs?

Answer 29

Agoramoorthy and Mohnot 1988

10 cases of infant killings and 2 cases of juvenile killings were observed in two troops of Hanuman langurs, (Presbytis entellus) around Jodhpur, India.
Age of the victims ranged from 0.2 to 48 months. Interbirth interval among females whose infants were killed is significantly shorter compared to females whose infants survived.
In this study, 7 cases support the reproductive advantage hypothesis, that infanticide is an adaptive behaviour to increase male reproductive success. The remaining 5 cases do not fit into the reproductive advantage hypothesis. In these cases, victims are over 8 months old, and as such their deaths could not shorten the interbirth interval.
It appears that by killing older infants and juveniles the males obtain an advantage in resource competition for their offspring. An alternative is that new males chase or marginalise the older infants and juveniles, which leads to 97% predominant uni-male troop structure in Jodhpur.

Question 30

Provide evidence for infanticide in bank voles

Answer 30

Ylonen and Horne 2002
If infanticide is costly to mother, natural selection should favour counter-strategies by females to protect their pups. Here the frequency of infanticide by male and female bank voles (Clethrionomys glareolus) was studied and the effectiveness of dams in deterring infanticide.
In trials in which unprotected pups were exposed to voles of both sexes, one third of male and female intruders killed pups. When mothers were present at the nest site, not a single female and only 2 of 25 males were able to commit infanticide.
Females acted aggressively towards all intruders and hence did not discern between infanticidal and non-infanticidal males and females.
Aggression of mothers against any intruder indicates that all strange individuals near the nest site of a territorial species form an equal threat to pups. However, the presence of the aggressive mother is sufficient to deter most intruders from harming offspring by keeping them at a distance from the nest site.

Question 31

Provide evidence for infanticide in zebras

Answer 31

Pluhacek and Bartos 2000

Hypothesis - Introducing a new zebra male into a herd of breeding females should increase foal mortality in comparison with herds in which the sire of the foals is still present. The younger the foal, the more likely infanticide should be. Data collected from five herds in two zoos in Czech Republic.
Nine records of infanticide and three cases of abortions that were probably induced by forced copulation. Abortions were three times more likely in herds with introduced males than with only fathers present.
The probability of foal death was greatest when the new male joined the herd just after conception and decreased with increasing time between conception and date of the new male introduction (the chance of a foal surviving was less than 5% just after conception and more than 50% at the time of delivery).
Results suggest that captive plains zebra show the highest occurrence of infanticide reported among ungulates.

Question 32

Apart from sperm competition what are 2 alternative male mating strategies?

Answer 32

developmental changes

stable polymorphisms

Question 33

what are developmental changes?

Answer 33

young males of small size may attempt to mate quickly or ‘sneakily’ with females, and change strategy later in life

Question 34

what are stable polymorphisms?

Answer 34

multiple forms (fighters vs sneakers) present in population and success is frequency dependent

Question 35

Darwin believed that females choose among males of the same species. Experimental results support Darwin’s view. What are the 3 hypotheses that can be used to interpret various examples of female choice?

Answer 35

Runaway selection models
Handicaps
Direct gain

Question 36

What is the runaway selection model?

Answer 36

genetic co-evolution of trait and choice genes – female programmed to choose

Question 37

what is a handicap model?

Answer 37

indicators of disease resistance - and Good Genes models: male behaviours and traits reveal high quality genes for offspring, disease resistance

Question 38

what is the direct gain hypothesis?

Answer 38

choice not of males but of safe mating places within leks or bowerbird bowers