4) Social Systems - Mating Systems

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• 4) Social Systems - Mating Systems
• Mating systems have 3 components
• the number of mates an individual takes
• whether the male and female form a pair bond
• how long the pair bonding lasts

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The evolution of these 3 components ( mates,
pair bonding, length of bond) is driven
by Sexual selection and Demography Sexual
selection results from the relative investment
in reproduction Males contribute (in most
cases) only sperm Females contribute through egg
production (more costly than sperm),
gestation or incubation, and parental care of
neonates. Thus, sexual selection is normally
female choice of mate(s).
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• At one time mating systems were thought to
  represent
• cooperation between the sexes.
• More recently mating systems have been recognized
  to
• also represent conflict between the sexes.
• The conflict arises because
• males can maximize reproductive success by
• mating many times
• females maximize their reproductive success by
• producing well-adapted, healthy offspring,
• (i.e. picking the best mate, not by mating
  multiple
• times

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Lets compare the energy expenditure on mating
and parental care for (at least) monogamy and
polygamy Mating effort consists of energy spent
on courting, mating, and mate defense or
guarding Parental effort is that spent on
feeding and caring for offspring.
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There are a number of different mating
systems Mating systems in vertebrates Class
Parental Care Mating System Birds male
female monogamy Mammals female polygamy Fish
usually the male polygamy
Amphibians Lizards absent variable
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• There are different forms of polygamy
• Polygyny a male mates with more than 1 female.
• Not all males reproduce, there is important
  variance
• in male reproductive success.
• Likely to evolve when there is clumping, either
  of
• resources or females, and when there is not high
• synchrony in the receptive times of females.

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When females clump, the polygyny is
called female defense polygyny Example female
gorillas travel in groups to avoid predation
by leopards. Males defend these groups from
other males.
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When resources are clumped, the polygyny is
called resource defense polygyny Example an
African bird, the orange-rumped honeyguide
feeds on beeswax. Males defend bee colonies
from other males. Females come to the bee
colony as a food source, and copulate with
the male defending it.
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When a male defends a territory containing
enough resources, he gains in fitness by
increasing the number of mates. Polygyny arises
when a female increases her own repro- ductive
success by sharing the male with other
females due to the high quality of the territory,
rather than having a monogamous relationship in a
resource-poor area. The threshold resource level
(or territory quality level) where this
transition in optimum strategy for the
female arises is called the polygyny
threshold...
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There is one more form of polygyny -
lekking Males defend only a small area within a
communal display area. Females come to the
display area and choose among males. In each
lek there is a mating center, where
copulation occurs. Only males displaying adjacent
to the copulation center get to mate. They are
the older, stronger, more experienced males.
Young males move from the periphery of the lek
toward the center as they age.
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Fallow deer
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Lekking shows that in some territorial species,
some individuals may not breed at all. In these
cases, territorial behaviour may limit
population size. Jenkins showed this in research
on red grouse. Floater males living in poor
habitat and not holding territories did not
breed.
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Jenkins did 2 experiments to demonstrate that
territories limited population size In the
first he removed 60 males from their
territories. 55 of them were re-occupied (by
floaters) the same day. In a 2nd study area he
removed 119 males - and 111 were re-occupied
within a day. As a control, another group of 269
males were not removed. During an entire summer
only 3 males were lost (probably to predators).
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Therefore, territorial defense limits the number
of individuals that breed, and this limits the
maximum size of a population. The birds moving
into the vacant territories are floaters, not
previously able to breed. Under natural
conditions they fill in when a resident
disappears, which is rare.
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Other lekking species include bats and birds of
paradise in a bat lek there were 85 males, but
only 5 of them accounted for 79 of all
matings. In a grouse lek 2 of 8 males accounted
for 70 of all matings. Thus it really is key to
reproductive success to be a dominant male in a
lekking species.
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• What are the costs of polygyny?
• For the males
• high mating effort
• elephant seals may live 20-30 years, but they
• dominate and mate for only 2 or 3 years
• for all this effort, subordinate males may
  steal
• copulations (if you know classic blues, think
  of Howlin
• Wolfs Back Door Man and other, similar
  songs)
• For females
• high parental effort

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In species that are not polygynous, the most
common mating system is monogamy. In monogamy,
the pair bond can be short or last throughout
adult life. About 90 of birds are monogamous
(at least for the season), and less than 5 of
mammals. Monogamy occurs when resources are not
heavily clumped and/or when female synchrony in
receptivity is high. It also occurs when males
provide a substantial part of parental care. Even
in monogamy some cheating goes on. Many
broods contain offspring sired by males from
neighboring territories, so-called Extra-Pair
Copulations (EPCs).
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The next mating strategy is polyandry. In it, one
female mates with multiple males. It is rare,
since usually the female has made a great
investment, and the male little investment in
reproduction. The one situation that seems to
lead to polyandry is a female holding a large
territory with multiple nest sites. Males each
defend one nest, and raise the young.
This reverses the usual energy expenditure
pattern of the sexes. In the best studied case,
the birds clutch size is fixed (called a
determinate clutch). The only way this female can
gain fitness is by defending a large enough
territory to attract multiple mates to multiple
nests.
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• Generally, in polyandrous species the sex roles
  are
• reversed
• females are larger than males
• females defend territories
• females are brightly coloured to attract males
• males are dull coloured and do not defend
  territories

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Polyandry occurs, for example, in arctic spotted
sandpipers. With the limited growing season (and
thus food availability), females maximize egg
production by tricking males into incubating
eggs. The female abandons the male and the nest,
moves on, and ends up mating with several males.
In one month she produces five clutches of eggs
(20 total eggs). The total egg biomass is 4-5x
her body weight!
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• The last possibility is a variable mating system
  within a
• single species.
• Bachelor, monogamous, and polygynous males are
  found
• together in the same population.
• In some migratory bird species
• In the spring
• males set up territories
• males escort females around the territory
• females choose the territory in which to nest

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Male behaviour (and mating system) depends on
male status Status Territory defense Plant
cover bachelor weak sparse monogamous
stronger medium polygynous very
strong dense Territories with dense vegetation
have the most insects that birds use to feed
their young.
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Now think about female choice Is she better off
choosing a territory with an unmated male or one
in which the male already has one or more
females? Already mated males have good,
productive territories, but she has to share it
with other females. Unmated males have lower
quality territories. However, she doesnt have to
share, and may get help raising the young (Yeah,
sure!)
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Verner Willson (1966) studied marsh wrens in
western North America. This species migrates
there from more tropical areas each spring.
Males arrive first and set up territories. Domina
nt males defend the best territories, and
typically attract 2 or 3 females. Some males are
able to defend territories of only intermediate
quality. They are monogamous. Some males on low
quality territories are unmated (remain
bachelors).
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What determines territory quality? Answer the
abundance of insects that are the marsh wrens
food. The abundance of insects is
determined by the abundance of plants along the
edge of the marsh. The abundance of plants
determines the abundance of insects that feed on
them.
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How do the females choose? Females choose after
visiting the territories of several males. Once
the choice is made, its final. She stays within
the chosen territory. Males will accept any
females that choose to settle within their
territory. Usually, no more than three females
will settle within a single males
territory. Why only three? What are the
considerations that determine the females
choice?
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• A female could choose to mate with an unmated,
  bachelor
• male. The result would be a monogamous pair.
• Or
• She can choose to mate with a male who already
  has one or
• more females living within the territory.
• If she mates with the bachelor -
• Benefits
• she has the territory to herself with the male
• the male will help raise the young, but
• Cost
• the territory defended by the male is of lower
  quality

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• If she mates with a male who has one or more
  mates
• Benefit
• the territory defended by this male is of higher
  quality.
• There is more food in the territory for her
  babies.
• Cost
• She shares the territory with other females
• She gets no help in parental care from the male
• So, her decision is based on the difference in
  the quality of
• the territories among which she can choose.
• If territories are of essentially equal quality,
  she should
• always choose to mate with a bachelor. There is
  no advantage
• to sharing the male.

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Heres a model developed by Orians (1969) for
how a female should make the choice to maximize
her fitness A females fitness (reproductive
success) increases with an increase in male
territory quality.
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Given 2 territories of equal quality, a
monogamous female will have higher fitness than
a polygamous female. If a female has to share a
territory, her fitness will be lower than if she
occupied the territory alone.
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But clearly there can be a balance A polygamous
female on a very high quality territory
could have a higher fitness than a monogamous
female on a poorer one.
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What we should find in nature is that males with
better territories should mate with more females
and have more offspring. And Polygamous
females should have equal or greater fitness than
monogamous females. Here are plots of relevant
data...
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Now consider sexual selection In natural
populations, as youve seen, individuals of
one sex often compete for mates. Typically it is
males competing for females. Males that win in
this competition have a larger number of mating
opportunities, and are favored in natural
selection. Darwin called the process sexual
selection. The process results in territorial
defense elaborate form, colour, and
behaviour structural adaptations like antlers,
bright feathers, mating dances
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In the widowbird, females select males with
longer tails. This happens in birds of paradise,
as well. The result is incredibly elongated
and elaborated tails. This is called runaway
sexual selection.
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Scientists finding runaway sexual selection have
developed the handicap principle. The male that
can thrive while carrying one of these tails or
another structure resulting from runaway sexual
selection must be superior physiologically and
genetically. If it werent, it couldnt survive
carrying around this handicap.
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References Paton, D.C. and Ford, H.A. 1983. The
influence of plant characteristics and
honeyeater size on levels of pollination in
Australian plants. In Handbook of
Experimental Pollination Biology. C.E. Jones and
R.J. Little eds. Scientific and Academic
Editions, N.Y. N.Y. Verner, J. and Willson, M.F.
1966. The influence of habitats on mating
systems of North American passerine birds.
Ecology, 47143-147.