• The means with which majority of the genetic material is passed on to the next
generations is through sexual reproduction
o Process in which two animals of different sexes (male and female) each contribute
to the formation of a gamete (single cell)
These fuse to form a zygote and provided the right conditions develops to
form a new and unique genetic individual
• Has genes common with its parents
• Gametes produced my male and female animals, sperm and eggs
are different and these differences have an impact in behaviour of
Males and Females Are Different
• Statistics ; single ejaculation contains approx. 350m sperm
o Highlights an important observation – at a single reproductive event a male
produces a very large number of gametes than a female (who produces few)
• Anisogamy – male animals produce sperm that are small, cheap, and mobile to produce.
When they are reproductively active they have a large number of sperm ready for use at
any time. This reserve can usually be very quickly replenished. The gametes of females
are large, immobile, expensive and finite (short) supply.
o Consequence ofAnisogamy – reproductive event may be more important to a
female animal than to a male.
Example: in season a lioness may mate with one of the males in her pride
every 15 minutes. In total she may mate 3000 times before becoming
pregnant. During that time she has contributed only a single egg but
billions of sperms have contributed.
• Important consequence – for social behaviour of lions. The small
value to the coupling event means that in prides with few males,
the males don’t compete to monopolize females that are in season.
Therefore, group cohesion is seen which allows males to devote
time and energy to defence of their pride. • Once fertilisation takes place another difference can be seen between males and females.
o Assuming that the cub survives till independence the lioness will not be ready to
conceive for more than 2 years.
Actual and operational sex ratios differ
• Male lion will be ready to mate with another female in the pride almost immediately
• Females who are out of the reproductive loop when they’re providing parental care
• Sex role reversal can take place, example: female spotted spiders (acititis macularia) lay
four eggs and then abandon their eggs to the care of male partner. The male partner
incubates the eggs, and doesn’t mate for that period of time with another female.
o However the female will start to mate soon and start another brood of eggs
• Ratio of imbalance in the ratio of males to females in a population even when there is an
actual 1:1 sex ratio
How do males and females maximize their output?
• Sexual selection (Charles Darwin) – the evolution of competitive advantage in members
of the same sex when they compete for the attention of the other; and the advantage in
discriminatory ability exhibited by members of the choosier sex. (Or where some
individuals gained an advantage over others of their kind in relation to reproduction)
• Males and females can maximize their outputs in different ways
• Males – can produce a huge cheap supply of gametes and father as many young as
possible; i.e. mating with many females as possible
• Females – cannot adopt the same strategy as males.
• Females – cannot increase the number of offspring’s beyond a certain limit due to the size
of her gametes
• Females – most effective way is by maximizing the quality of her offspring
o Quality of contribution that the male mate provides
His genetic contribution
Resources he can provide
Choosing a Mate • Terns are socially monogamous (pair of birds raise a brood of chicks together)
o In many bird species, when male is away foraging, provides opportunity to
engage in ECPs (extra-pair copulations)
• Mate choice related to resource provision
o No clear correlation between number of food gifts that male brings to his mate
and the number of times they copulate
o Birds copulate far more times than necessary for egg fertilization
o Terns – male benefits from frequent copulation (in lions – lioness benefits due to
low fertility rate enabled her to reduce copulations to each of her partners)
o Males of other species provide nuptial gifts as incentive to females before or
Black-tipped hangingfly – in pressure to provide right gift
• Providing an inedible ladybird will be rejected
• Females of this species decided when to copulate and for how long
o Only mate with those males who provide them with edible
gift and will remain with him as long as it takes to eat the
Spiders (male of many species) provide themselves as gifts
• Nibalism is risky.
• Might seem maladaptive at first
• But in species like the red-backed spider males are unlikely to
survive the search for a second mate and females that have recently
fed are less likely to encounter a second potential mate than hungry
females; hence self-sacrifice ensures the male spider of the
continuation of his line
• Traits that may communicate a material benefit
o A complex song can be used by passerine birds o FemaleAmerican goldfinch will choose MaleAmerican goldflinches that are
strikingly coloured (vivid yellow birds with striking black cap and an orange
The orange and yellow is a result of the deposition of carotenoid pigments
in the feathers and beak as they grow, and so by choosing brighter male
with high carotenoid levels
Why is it a good thing?
o Vertebrates can’t synthesize carotenoid so they obtain them
from their food.
• Egyptian vulture – only bird species that engage in coprophagy
(eat feces of other animals, in this case of Ungulates) poor source
of food BUT have high levels of carotenoid lutien.
o Have yellow colored face – result of lutien deposits in skin
(could be due to mate choice)
So females goldfinch may recognize the brighter males as better forages as
carotenoid is dietary in origin
• Females can minimize risks that they will contract a disease or
• Two healthy parents can better provide for offspring
Link between carotenoid and health
• Role in functioning of the immune system
• Neutralization of harmful free radicals that can damage the DNA
• Carotenoid stimulate production of T and B lymphocytes (needed
for the body’s antipathogen strategy)
• Play part in production of cytokines and interleukins 9which in
turn play part in body’s injury and inflammation)
• Choosing traits for their own attractiveness
o Male trait such as tail or horn length serves as mean by which females might
discriminate between males and identify a preferred type o Longer tail benefit – balance or agility in assistance in foraging or in predator
Could tap the sensory bias of the females and make male easier to find or
notice in crowd
o Genes will be passed onto next generation
o Sexy sons hypothesis – female chooses a long tailed male so a females son will
inherit their fathers tail length and should themselves be more attractive to
o The daughters will inherit their mothers preference and will thus benefit
o Genetic lineage is assured
o Runaway selection – ongoing selective process could result in the evolution of
increasingly elaborate traits
o Limit can be reached as trait can become detrimental
• Being choosy may not always pay off
o Older mates will not spend as much time selecting a mate as younger will (such as
o Delay of selecting can reduce reproductive output
o Females allowed to mate early are discriminating about who they mated with
o Females who delayed mating showed no discrimination and mated very quickly
with first male encountered
o Falling fertility levels in the aging female result in increased motivation to breed
• Involves the number of mates that an individual has during a mating season
• Monogamy – one male and female animal are involved in the production of offspring.