BIO3052 Lecture Notes - Lecture 11: Mating Plug, Traumatic Insemination, Ablation
30 May 2018
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REPRODUCTIVE BEHAVIOUR I
POST-COPULATORY SEXUAL SELECTION
• Exaggerated secondary sexual morphologies in certain male species can be detrimental as these
ae taits that ould pesual iease thei death ad is hee ot osistet ith Dais
theory
• Seual seletio is the esult i epodutie opetitio o taits that iease a idiiduals
capacity to gain a mate under competition
o More extreme males have higher mating success eg. long tails in Widow bird
o There are two types of sexual selection:
1. Intersexual selection
2. Intrasexual selection
o Is the female choice on sexual displays
o Eg. females peacocks choose males in terms
of the prettiest tail feathers
o The more extreme males (brighter/prettier)
have higher mating success
(traits are a spectrum often with many
intermediates)
o Armaments/behaviours used in male-male
competition
o All traits are in males to fight other males for
access to females (mating)
o Most likely to cause early mortality due to
physical injuries
o Eg. antlers in deers
• Why are males typically the sex doing the fighting or extravagant sexual signals? Why not
females?
o Because males invest less in parental care
o Bateas piiple: feales ae liited i gaete podutio, ales ae ot. Geeall
all females will reproduce but not all males will (gradient for males is much steeper)
hence males need to maximise fitness
o Anisogamy drives differential investment across the sexes where females have much
larger gametes than males -> are already investing much more into reproduction than
males (also have gestation and usually post-birth maternal care)
o Females are a limiting resource over which males must complete
• Coseuee of itese ale opetitio fo atigs: feales ofte ate oe tha oe
during a given reproductive cycle:
o Even females of socially monogamous species will have more than one mating
o Eg. black swans have 15% extra pair paternity
o Eg. humans ~46% in Australia – however this depends on contraception where
fertilisation is kept at a minimal in Western countries
• Implications of multiple mating by females (Polyandry):
o This puts sperm of males in direct contact and competition with each other within the
reproductive tract of the female
o Sexual selection can continue to act after mating has taken place
o Competing for fertilisation
o Achieving copulation with female is only buying a ticket raffle then another whole bout
of competition occurs post-copulation
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o Female fruit flies typically have sperm of more than one male inside them at any time
(sperm storage)
• Competition for fertilizations remains even after copulation:
Type of sexual selection
Pre-copulation :
-competition for copulation
Post-copulation :
-competition for fertilisation
Intersexual selection
Female choice
Cryptic female choice
Intrasexual selection
Male-male competition
Sperm competition
• Phenotypic variation is the fuel upon which sexual selection acts upon
o This extends to the ejaculate of males eg. in green sea urchins, there are consistent
difference among males in sperm head length
o Ejaculate is made up of many components which exhibits phenotypic variation and can
be under sexual selection
o Sperm: testes size (number of sperm), quality (size, viability, shape, motility, speed)
o Seminal fluid: volume, protein constitute (amongst the fastest evolving proteins known
ith oe s of epodutie poteis
o Muh of ales epodutive success is decided by how good his sperm are at competing
for fertilisations (post-copulatory sexual selection)
• Post-copulation sexual selection:
Cryptic female choice
o Females might discriminate against certain sperm i.e. have choice
of gametes
o Much harder to study
o Two possible ways to discriminate between male sperm:
1. Sperm dumping:
Process by ejecting/dumping sperm from males she
doest like
Eg. feral chickens are promiscuous and will mate with
alpha and subordinate males however will more likely
eject subordinates sperm
2. Biased use of sperm storage:
Eg. Field crickets can mate with up to 10 males and when 2
rival males inseminate the female, she will store more
sperm from unrelated males compared to related
Eg. Many spiders have separate copulatory openings and if
she mates twice, the sperm from the second male is stored
in the 2nd organ and female can preferentially use sperm
from either 2 organs (usually choose non-related male)
Sperm competition
o Male-male competition after copulation where sperm competition
selets fo adaptatios that iease a ales shae of pateit
o Easier to study
o Eg. male aedeagus (genitalia) in beetle species has spines which
inflicts damage inside female reproductive tract
Has different degrees of spines where greater spines have
an advantage and do better in competition
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o Relative size of testes relative to body weight is a good index of
how likely sperm competition is shaping mating
(eg. chimps have higher index because they are very promiscuous
-> a lot more competition)
o Eg. fruit flies testes are 10.6% of their body weight (also have ~6cm
long sperm)
o Why have large testes?
Larger testes = more sperm
Raffle principle: male can boost chances of paternity by
having more raffle tickets (sperm) increases chance of
success
o Eg. can experimentally evolve testes size by enforcing selection
under sperm competition (in yellow dung fly)
Experimental design: start with one population then split
into 8 sub-populations that are representative of the
original population. 4 groups are under monogamy (no
competition/selection) and 4 groups are under polyandry
(heavy sexual selection) for next 10 generations
-> example of experimental evolution (artificial selection)
Results: polyandrous groups evolved larger testes due to
strong sexual selection
o Sexual selection imposed by sperm competition selects for large
testes as it increases a ales shae i the affle he spe of
two males are competing for paternity
o Male with most sperm should gain most paternity
o Large testes is an ADAPTATION under post-copulatory sexual
selection
• Genetic monogamy is very uncommon
• Typically, this competition is more fierce for males than females
• Sexual selection favours traits that increase reproductive competitiveness
• Sexual selection can operate via male-ale opetitio Ita‐seual o feale hoie Ite‐
sexual)
• Intense reproductive competition continues beyond copulation
• Plaes seletio o the ale ejaulate: post‐opulato seual seletio
• Post‐opulato seual seletio ats o pheotpi aiatio i the ejaulate
Via two possible modes:
Cryptic female choice
Sperm competition
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Why not females: because males invest less in parental care, bate(cid:373)a(cid:374)(cid:859)s p(cid:396)i(cid:374)(cid:272)iple: fe(cid:373)ales a(cid:396)e li(cid:373)ited i(cid:374) ga(cid:373)ete p(cid:396)odu(cid:272)tio(cid:374), (cid:373)ales a(cid:396)e (cid:374)ot. Cryptic female choice: females might discriminate against certain sperm i. e. have choice of gametes, much harder to study, two possible ways to discriminate between male sperm, sperm dumping: Process by ejecting/dumping sperm from males she does(cid:374)(cid:859)t like. Eg. feral chickens are promiscuous and will mate with alpha and subordinate males however will more likely eject subordinates sperm: biased use of sperm storage: Field crickets can mate with up to 10 males and when 2 rival males inseminate the female, she will store more sperm from unrelated males compared to related. Many spiders have separate copulatory openings and if she mates twice, the sperm from the second male is stored in the 2nd organ and female can preferentially use sperm from either 2 organs (usually choose non-related male)
