Chapter 11: Mating Behaviour
11.1 Sexual Selection Favours Characteristics That Enhance
1. Darwin proposed natural selection as a mechanism to explain the evolution of
adaptive traits in species.
2. Primary sexual characteristics: the genitalia and organs of reproduction.
1. Darwin made copious notes on the morphological differences between the
male and female genitalia.
3. Secondary sexual characteristics: morphological differences between the sexes
that are not directly involved in reproduction.
1. For example, in birds such as widowbird, only males have colourful plumage
and a very long tail feathers.
1. Darwin observed that such elaborate and exaggerated traits, in
conjunction with complex behavioural displays and vocalizations, are
often involved in conspecific interactions during the mating season.
4. Secondary sexual traits puzzled Darwin because they did not seem to fit into his
theory of natural selection.
1. Exaggerated morphological and behavioural traits should be energetically
expensive to produce and maintain, and they can make individuals more
obvious to predators and so reduce their survivorship.
5. In On the Origin of Species, Darwin first proposed that exaggerated male traits
might be advantageous for reproduction rather than for survival.
6. In The Descent of Man and Selection in Relation to Sex, Darwin in fact
hypothesized that these traits might arise from a different form of selection.
7. Sexual Selection: a form of natural selection that acts on heritable traits that
8. Mate Competition: selection in which one sex competes with other member of
the same sex for access to the other sex for reproduction.
9. While such competition is an important aspect of sexual selection, Darwin
recognized another aspect as well: females may be choosy.
1. Thus proposed that males may also compete among themselves to increase
their attractiveness to females. In this way, he envisioned that females often
play and active role in reproductive decisions in mate choice.
1. Mate choice: selection by one sex for member of the other sex for
WHY TWO SEXES?
10. One fundamental difference between males and females is the size of their
1. Anisogamy: Males tend to produce many small, motile gametes (sperm),
while females tend to produce much larger, nutrientrich, and nonmotile
2. Isogamy: in many algae, fungi, and unicellular protozoans; however, all
individuals produce similarsized gametes. Appears to be the ancestral form. 11. Geoff Parker and colleagues developed a model to answer this question. This
model is based on the following the assumptions:
1. In the ancestral marine environment, individuals in a population produce
2. Each parent has a fixed amount of energy to allocate to gamete production,
resulting in a sizenumber trade off: as the number of gametes produced
increases, their size will decrease
3. Zygote viability is related to its size. Larger zygotes have higher viability
because they contain more resources for survival.
12. Parker considered the fitness of small, large, and intermediatesized gametes.
Small gametes have a numerical advantage: they will create the most zygotes.
13. On the other hand, large gametes always produce zygotes with the highest
survival. Intermediate zygotes have neither advantage and so have the lowest
1. There is disruptive selection against intermediatesized gametes. The result is
high fitness for either “protomales”, in which produce many small gametes,
or “protofemales” which produce fewer large gametes.
BATEMAN’S HYPOTHESIS AND PARENTAL INVESTMENT
14. Bateman was one of the first researchers to examine sexual selection in males and
females using fruit flies,
15. In a series of experiments, Bateman paced equal numbers of virgin females and
males in milk bottles for three or four days and allowed them to mate.
1. Adults varied in age from one to six days old. Females can take up to four
days to become sexually mature, while males are sexually mature within 24
hours of eclosion (emergences of the adult from the pupal case).
1. Each adult was heterozygous for a different dominant mutation, and so
Bateman’s could determine the parents of threefourths of the offspring by
16. Across all experiments, 96% of the females produced progeny. In contrast only
79% of the males successfully produced progeny.
1. From these data, Bateman concluded that males had a higher variation in
reproductive success than females.
1. Bateman inferred that the intensity of sexual selection, as measured by the
variation in reproductive success, was, in general, higher on males than
2. He also inferred that this increased intensity of sexual selection was due
to malemale competition.
17. His experiment suggested that a male’s reproductive success increases more
strongly with the number of mates obtained than does female reproductive
1. Bateman concluded that reproductive success for a female is primarily limited
by egg production, because female can obtain sufficient sperm to fertilize all
her eggs from a single mate. 2. In contrast, sperm production should not limit male reproductive success.
Instead, the number of matings a male obtains primarily limits male
18. This situation sets the stage for greater variation in male reproductive success
and thus more intense sexual selection on males.
1. In sum, the evolution of malemale competition and female choice follows
from competition for access to gametes of the opposite sex.
19. Bateman’s hypothesis: female reproductive success is most strongly limited by
the number and success of eggs that she can produce, while male reproductive
success is limited by the number of mates he obtains.
1. This difference between the sexes has become a foundation of sexual selection
20. Trivers expanded on the concept by identifying all forms of parental investments
as a key difference between sexes, in addition to gamete size.
21. Parental Investment Theory: the sex that has a greater investment in offspring
production should be choosier when it comes to mates.
1. The other will then experience more intense sexual selection.
1. As a result of sexual selection, we expect that males will often exhibit
exaggerated traits used in competition for females.
2. The traits ay be morphological or involve behaviours such as vocalization
and courtship rituals.
22. Weapon: exaggerated morphological traits used in malemale competition.
23. Ornaments: exaggerated morphological traits used to attract females.
ANTLERS AS WEAPONS IN RED DEER
24. Within this population, individual males will defend and mate with several
females each year. Antlers are present only in males and are used in aggressive
contests to defend females from rival males.
25. Researchers take blood samples from calves and adults for genetic analysis to
determine individuals’ reproductive success.
1. They also collect and weigh the antlers shed by males each year to determine
their mass (size). When individuals die, post mortem analyses provide
information on body size (hind leg length).
1. These researchers determined that males with larger antlers had higher
breeding success in any given year, as well as over their lifetime.
1. In fact, male body size per se had little effect on breeding success.
26. These data indicated that antler size is significantly correlated with male
breeding success in this population: males with larger antlers sire more offspring
because they obtain more mates.
1. In red deer we see a positive correlation between weapon size and
WEAPON SIZE AND MATING SUCCESS IN DUNG BEETLES
27. In many beetles, males possess a hornlike projection that females lack. 28. Pomfret and Knell studied the role of these horns in competition for mates in male
dung beetles. Males fight with one another to defend dung piles, while females
dig subterranean tunnels to mate and raise their offspring.
1. Observed much variation in body size and horn size among males and
conducted an experiment to determine whether large horns provided an
advantage in these fights.
29. They staged fights between males in artificial nesting arenas (similar to ant
farms.) consisting two panes of glass placed 5mm apart, filled with soil and dung.
30. Male body length was measured with calipers, and horn length was measured
from photographs of the head. The winner was deemed to be the male that
successfully entered and remained in the tunnel after 24 hours, and most winners
subsequently mated with the female. The loser was excluded from the tunnel and
thus mating opportunities.
1. For small beetles, the larger male or the male with the longer horn won the
most contests, In contests between large males, large horn size alone was the
most important predictor of success. In all contests, the greater the difference
in horn length, the more likely was the winner to have the longer horn.
1. These findings suggest that the maintenance of long horns in male dung
beetles can be explained by sexual selection driven by malemale
ORNAMENTS AND MATE CHOICE IN PEAFOWL
31. Males posses extremely large tails whose feathers contain brightly coloured
ocelli, or eyespots. Because males fan out their tail when displaying to females, it
seemed likely that females mate choice involves tail morphology.
32. Beofre peahens select a peacock mate, males must compete to establish and
defend a display site. Many males aggregate on a lek.
1. Lek: a location that lacks many resources, where they display to females
using behaviours such as vocalizations and fanning of the tail. Males fight for
status and preferred positions on the lek.
33. Loyau, Jalme, and Sorci examined mating behaviour in a population of peacocks
at Parc Zoologique de Cleres in France. The researchers captured and color
banded males and females in the early spring.
34. They measured the length of the tail and male body size by measuring their tarsus
(a bone in the lower leg that provides an index of body size) and photographed
each male’s tail when open to count the number of ocelli.
35. The researchers will then recorded malemale interaction during the breeding
season, along with the number and duration of male tail displays to females, the
number of vocalizations and the number of copulations.
36. The research team found intense competition for display sites, as only 45 of 61
males successfully defended a site. They also found significant positive
correlations between the likelihood of defending a display site and both body size
and tail length.
1. Larger males and those that had longer tails were more successful in
acquiring a site. 2. Females were very selective in their choice of mate. Only 12 mates obtained
copulations, and over onethird of all copulations were obtained by a single
1. In general, males that had high display rates and the most ocelli obtained
the most copulation, but these were not necessarily the largest males.
37. The research team concluded that both mate competition and mate choice has
played a role in the evolution of the peacock.
1. First only a subset of males successfully defended display sites, a behaviour,
that could also favour larger tails.
2. Females strongly preferred males with high display rates and tails with many
1. Together, male competition and female choice have favoured the evolution
of males with long, elaborately ornamented tails and high display rates.
THE ORIGIN OF SEXUALLY SLECTED TRAITS: THE SENSORY BIAS
HYPOTHESIS IN GUPPIES .
38. Sensory Bias Hypothesis: the hypothesis that female mating preferences are a
byproduct of preexisting bases in a female’s sensory system.
1. These biases are presumed to have evolved in a nonmating context, with
males evolving traits to match those biases.
1. One test of this hypothesis involves guppies. Females prefer to mate with
males that display the greatest amount of orange colouration, which is
derived from carotenoids obtained in the diet.
1. Carotenoids are an important nutrient in many species, and they play
a key role in immune system function.
2. Prior work revealed that both natural and sexual selection has affected
the strength of female preference and male colouration across
3. In stream populations where predators risk is high, female preference for
orange is reduced, and males display less orange body colour than do
males in other population with low predation risk.
2. Helen Rodd and colleagues noticed that guppies in the wild are attracted to
and voraciously consume orangecoloured fruits that fall into the water. These
fruits contain high levels of carotenoids, which are rare in other foods. Rodd
and colleagues tested the hypothesis that guppies have an innate preference
for orange food items because they contain carotenoids and that this pre
existing bias favours males that display orange colour on their body.
1. This hypothesis makes a basic prediction: both male and female guppies
should be attracted to orangecoloured object because their colour is
associated with the presence of carotenoids.
3. They placed small discs on a leaf in the water held in place by a small rock.
For five minutes, the researchers note all guppy approaches, pecks at the
object, and the sex of the individual.
1. To standardize environmental conditions, the researchers also conducted a
similar laboratory experiment, 4. In both experiments, male and females guppies exhibited a strong attraction
to the orange and red discs, and the strength of the preference for orange was
correlated with female mating preference for the colour orange.
5. In populations with the strongest females preference for orange males, both
sexes displayed the strongest attraction to orange discs.
1. These experiments indicate that both male and female guppies display an
innate preference for orange objects. This finding provides support for the
hypothesis that female mating preferences is linked to a preexisting
preference for orange food objects.
MALE MATE CHOICE IN PIPEFISH
39. Depending on circumstances, males can also be selective, and this choosiness has
favoured the evolution of female secondary sexual traits in some species.
40. Sexrole reversed species: a species in which females compete for males, who
invest heavily in parental care.
1. Females transfer their eggs to the males’ eggbrooding structure. The male
then fertilizes the eggs and cares for them as they developthat is, males
1. Because males invest more heavily in offspring than females as a result of
their high levels of parental care, sexual selection theory predicts that
males should be the choosy sex and that femalefemale competition for
mates be intense. We might also expect females to display secondary
41. Berglund and Rosenqvist have been studying deep snouted pipefish. Females
posses a secondary sexual ornament: a temporary striped pattern on the side of
the body that is displayed only during competition between females and when
courting males, a behaviour known as a courtship dance.
1. Normal pipefish body coloration is quite cryptic, but the ornament makes the
female stand out and so is likely to increase predation risk making it costly.
42. Berglund and Rosenqvist tested whether this ornament affects male mate choice.
Using a simultaneous choice experimental design with aquaria divided into three
sections, they conducted an experiment that allowed a male to choose between
two females on consecutive days.
1. Females were placed in two sections that were separated by an opaque
barrier, preventing them from viewing each other. The male was in the third
section and could view both females.
1. On Day 1, the three fish were placed in the aquarium and their behaviour
2. On Day 2, the same fish were placed in the same aquarium, but now all
dividers were removed so individuals could interact. On both days fish
behaviour was filmed for ten hours.
3. On Day 1, males spent more time near the female that displayed her
ornamented more (the more ornamented female), and spent more time in courtship dancing with her. On Day 2, the male again spent more time
courtship dancing with the more ornamented female and copulated with
her more often.
1. These results showed that males prefer females that are most
ornamented. Berglund and Rosenqvist suggest that female
ornamentation reflects dominance status and overall vigour and may
make it easier for males to assess female body size.
1. Body size affects egg production, and so males should prefer
43. In all theses cases, we see that sexual selection can explain the evolution of
certain traits because they enhance reproductive success. For traits used in mate
competition, the selective advantage is easy to see: larger weapons can provide
an advantage in direct competition with rivals,
1. It is straightforward to understand how a trait favoured by one sex in their
choice of a mate will evolve in other sex. Because those that possess the most
extreme form of the trait will have high reproductive success.
11.2 Females Select Males To Obtain Direct Material Benefits
44. Parent investment theory predicts that the sex that invests the most time and
energy in offspring can benefit by being selective in its choice of mates.
45. Direct material benefits: material resources obtained by a female from mating
with a particular male.
FEMALE CHOICE AND NUPTIAL GIFTS IN FIREFLIES
46. Nuptial Gift: a physical resource such as food item that a male provides to a
female to enhance his mating success.
47. Lewis, Rooney, Cratsley studied how nuptial gifts affect female firefly preference
for males and their resulting fitness.
48. Male Photinus Ignitus fireflies produce spermatophores as a nuptial gift sperm
packaged within a protein rich structure produced by males accessory glands.
1. Once inside a female, the spermatophore disintegrates, releasing its nutrients,
which the female allocates directly to her developing oocytes.
49. Firefly courtship involves speciesspecific bioluminescent flash patterns; males
fly and flash to females sitting on nearby vegetation, and females flash in
response to attract a male to her location.
50. To investigate this question, they first examined whether there was a relationship
between male fish duration and spermatophore size. They collected 36 males and
brought them into the laboratory, where each male was weighed and video
imaged to analyze its flash pattern. Most had flash durations that ranged from 56
to 89 ms and were highly repeatable among males. 1. To quantify spermatophore mass, the researchers allowed each male to mate
and then sacrificed the female to collect and weigh the spermatophore that
2. They found a significant positive correlation between male flash duration and
spermatophore mass. The mechanism behind this correlation in unclear, but
the association does mean that females could use variation in flash duration
to choose males with the largest spermatophores.
51. Cratsley and Lewis examined the effect of flash pattern on female mating
behaviour. Because they wanted to manipulate the duration of the flashes, the
research team created simulated male flashes to which females responded by
1. To examine the effect of the duration of the male flash signal on female
behaviour, the researchers created flashes of different durations (55, 63, 7, 79,
or 87 ms) and presented them to 25 females. They found that females
responded most frequently to the longest flashes.
1. Together, these results suggest that females prefer to mate with males that
have the longest flash duration and thus likely have large spermatophores.
52. South and Lewis (2011) have recently examined this question from a comparative
perspective. Across a broad range of arthropod species, they found a positive
relationship between a spermatophore nuptial gift size and female fecundity, an
important component of fitness.
FEMALE CHOICE AND TERRITORY QUALITY IN LIZARDS
53. Another direct benefits of mate choice by females is access to highquality
resources defended by a territorial male.
54. Calsbeek and Sinervo examined how territory quality affects female choice and
fitness in sideblotched lizards. Males defend territories where they display to
females from rock perches.
55. The quality of a territory is based on its rockiness: rocks provide perches not only
to display to females but also to spot predators.
1. Rocks also increase the range of “microclimates” (both hot and cool
locations) available for thermoregulation, an important physiological factor
for ecotherms like lizards.
56. Previous work demonstrated that offspring growth and survival are greater when
females lay eggs within male territories that have many rocks.
1. In this system, males compete for territories with abundant rocks, and females
prefer to mate with males who control the rockiest territories.
1. However, that preference makes it difficult to know whether female mate
choice is based on male quality or the quality of the his territory
57. The researchers manipulated territory quality after males had established their
territories but before female became receptive and selected mates. The first
mapped male territories and observed that large, dominant males settled on those
territories with the most rocks. Then the researchers moved 1040 rocks from the
territories of large, dominant males to the territories of small neighbouring
males. 1. The added rock piles increased territory quality for small males while
reducing territory quality for large males, because males remained on their
territory after manipulation.
1. This design allowed the researchers to successfully separate highquality
males from highquality territories.
2. At the end of the breeding season, after females had selected mates, females
were captured and brought into the laboratory to lay their eggs, which were
incubated under standard conditions. The researchers measured the egg
laying date and egg mass of each female.
1. They found that females strongly prefer improved territories, even though
small males occupied them. Of the 51 females in the population, 37 moved
on to improved plots. Females on improved territories laid eggs sooner
and produced larger egg masses, demonstrating a direct fitness benefit for
females that select highquality territories.
11.3 Females Mate Choice Can Evolve Via Indirect Benefits To
58. Indirect genetic benefits: Genetic benefits females can obtain for their offspring
by mating with males that have high genetic quality.
59. One way to assess the genetic quality is by selecting males based on their
secondary sexual traits.
1. This idea, that female choice can affect the evolution of male traits, has
stimulated much debate and research.
FISHERIAN RUNAWAY AND GOOD GENES
60. Wallace disagreed and considered only malemale competition to be a selective
61. Their argument about the importance of female choice persisted because there
was no clear mechanism to explain how mate choice could drive the evolution of
secondary sexual traits.
62. In 1930, Fisher wrote a milestone paper proposing a genetic explanation for the
evolution of exaggerated secondary sexual traits. Fisher assumed that females
selected mates based on a particular trait that varies among males. Such traits
evolve because of:
1. It’s fitness advantage, independent of female choice
2. Female preferences for it.
3. Thus, initially, the trait indicated that male quality, because males possessing
it had higher fitness and so were more attractive to females.
1. If female preference for a trait and the trait itself has a genetic basis,
female offspring will prefer the trait, and male offspring will express it.
1. The intensity of female preference for the trait will increase as long as
male offspring have a mating advantage by possessing it.
63. Runaway process: an evolutionary process in which a male trait co evolves with
a female preferences for it and becomes increasingly exaggerated
64. Lande formalized Fisher’s idea into the notion of runaway selection. He showed
mathematically how this process could occur through linkage disequilibrium when the genes for the trait and for female preference for the trait are genetically
linked, even if the trait was unrelated to male fitness.
1. Linkage disequilibrium occurs when the genotype at one locus is not
independent of the genotype at another locus.
65. Amotz Zahavi proposed an alternative hypothesis to explain how female choice
could affect the evolution of male traits. He suggested that exaggerated
secondary sexual characteristics allow the choosing sex to assess mate quality.
66. The exaggerated trait is “a test imposed on the individual” by natural selection.
Only highquality males should thus be able to display the most exaggerated form
of the trait and idea he called a “handicap”.
67. Handicap principle: welldeveloped secondary sexual characteristics are costly
to survival but reliable signals of fitness.
1. Females should prefer such males because they must have excellent genotypes
to overcome the handicap.
68. Good genes: the alleles of a high quality individual.
1. These genes may be associated with an enhanced immune system, greater
fighting ability, or increased vigour and viability.
69. The cost of the trait need not be a handicap to survival; however, such traits
simply need to be costly to produce or maintain. If the traits were not costly, then
all males, regardless of their quality, would be able to express the trait, and it
would no longer indicate quality.
1. Thus, a long, physically demanding vocal or behavioural mating display, or
physiologically costly morphologies and chemical pheromones can be reliable
indicators of male genetic quality if only the healthiest and most vigorous
males (i.e. those with the best genes) can produce them.
70. At first researchers debated the merits of Fisher’s runaway and Zahavi’s handicap
principle hypotheses by focusing on their different assumptions.
1. More recent work has shown that these models share important similarities.
In both models, the male trait initially preferred by females indicates a high
degree of male vigour and survivorship.
71. In good genes models, a genetic correlation is assumed between the female
preference for the male trait and the male “good genes” that provide enhanced
physical vigour and are indicated by the trait.
1. In Fisherian runaway selection, there is a presumed genetic correlation
between the female preferences for the male trait and the male trait itself.
2. Today, much research on sexual selection focuses on understanding
1. The traits used by females in mate choice
2. How sexually selected traits indicate male genetic quality
3. The fitness benefits of mate choice
MATE CHOICE FOR GOOD GENES IN TREE FROGS
72. Sexual section theory predicts that females should base their mating decisions n
the presence of costly male traits to obtain indirect genetic benefits. Many species
produce vocalization to attract mates.
1. For example, among frogs that aggregate in ponds, many males call to
attract females in chorus. 73. Jaquiery and colleagues tested the hypothesis that female European tree frogs
select males for genetic benefits based on their vocalizations.
1. Females initiate all matings, and males do no interrupt a pair that is mating,
so malemale competition is assumed to be weak.
2. The species is thus ideal for examining the genetic benefits of female choice.
Furthermore, the species is threatened in parts of its range, so understanding
its mating behaviour may help with conservation.
74. Each night of the 22night breeding season, the research team visited four
neighbouring ponds in western Switzerland to determine which males were
calling in the chorus. A total of 15 calling males were captured and identified
from a photographic data bank (individuals’ black lateral line has unique colour
patterns), and a buccal (or cheek) sample was taken for genetic analysis.
75. To determine male mating success and the fitness of their offspring, the team
visited the ponds every four days in daylight to collect eggs.
1. Ten offspring from each clutch were reared with unlimited food, and each
tadpole was weighed twice to record its growth rate.
2. Tadpole survival was determined at 28 days, and all individuals were then
1. The resulting genetic analysis allowed determination of the mating
success of adults
2. From these data, the researchers could determine the attractiveness of
each male, defined as the number of females with whom he mated divided
by the number of nights he called during the breeding season.
3. Only 10 of the 15 males sired any clutches, indicating that males varied in
their attractiveness to females and that females were selective.
4. Offspring growth rate was positively correlated with the father’s
attractiveness; males that sired more offspring produced tadpoles with higher
76. In amphibians, offspring growth rate is positively related to fitness, and so the
research team concluded that females do indeed select males for genetic benefits.
1. Additional work is required to determine the features of male vocalization that
attract females. The researchers speculate that these features will be accurate
indicators of male quality, because calling is so costly.
1. This example illustrates how female mate choice for a sexually selected,
costly trait can enhance her fitness. Next, we consider a different way that
females can benefit by selecting males that have the greatest expression of
a costly secondary sexual trait.
GOOD GENES AND IMMUNE SYSTEM FUNCTION IN BIRDS
77. Hamilton and Zuk proposed that parasite and pathogens play an important role