Transcript Territoriality and Mating Systems

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
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).
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
Let’s 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.
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 &
Amphibians
usually the male
polygamy
Lizards
absent
variable
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.
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.
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.
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 reproductive 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”...
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.
Fallow deer
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.
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).
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.
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.
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’
Wolf’s “Back Door Man” and other, similar songs)
For females…
• high parental effort
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).
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 bird’s 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.
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
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!
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
Male behaviour (and mating system) depends on male status:
Status
bachelor
monogamous
polygynous
Territory defense
weak
stronger
very strong
Plant cover
sparse
medium
dense
Territories with dense vegetation have the most insects that
birds use to feed their young.
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 doesn’t have to share, and may get help raising the
young (Yeah, sure!)
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.
Dominant 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).
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.
How do the females choose?
Females choose after visiting the territories of several males.
Once the choice is made, it’s 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 male’s territory.
Why only three? What are the considerations that determine
the female’s choice?
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
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.
Here’s a model developed by Orians (1969) for how
a female should make the choice to maximize her fitness…
A female’s fitness (reproductive success) increases with an
increase in male territory quality.
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.
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.
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...
Now consider sexual selection…
In natural populations, as you’ve 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
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.
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
weren’t, it couldn’t survive carrying around this handicap.
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, 47:143-147.