Download 1. Female choice of males that have exaggerated characters has

1. Female choice of males that have exaggerated characters has been explained by female
choice of males that have better genes. Explain how female choice of higher quality males
might lead to exaggerated male characters.
Female choice in sexually reproducing organisms is a very common phenomenon. As the
females are choosing which males to fertilize their eggs they are judging the male’s fitness and
quality. Since a female cannot ask a male how good a forager he is or how will his immune
system is functioning, she must infer that information from another source. In many cases this is
in the shape or colour of an exaggerated physical characteristic that often serves no other purpose
to the male except to advertise his quality as a potential mate. Females tend to pick males with a
feature that will stimulate them in some way. In finches, where males done have extra
ornamentation, females are attracted to males that have an artificial ornament applied by
researchers. This ornament is desirable for the female because it is indicating something about
the male. If ta male has a mutation that makes his attractive to females then he will get more
mates. If he fathers lots of offspring then his sons will inherit that trait and the daughters will
look for that trait in their male mates. Often times the cost of producing such a trait is great to
the male–it can increase the chance of predation and it diverts resources from other development.
So, in order for a male to have an exaggerated trait he must be a good quality male in order to
survive. At this point it becomes a reliable indicator to females about the health of the male and
it means that if a male has an even bigger ornament/brightly coloured ornament, he is even
healthier and his genes are passed on until you have ornaments such as tails on peacocks and
widowbirds that are very disadvantageous to the life span of the male, yet good indicators of
health to the female and passed on to the young.
This question is taken directly from the list of second test study questions (#6)–it should not be a
surprise......
The issue for this question is that it is advantageous for females to pick high quality males.
However, they pick males with outlandish, exaggerated characteristics. When should
exaggerated characters connote high genetic quality? Clearly, there must be an association
between male quality and the extent of exaggeration. There must be an association between the
signal that the male produces (the exaggerated character) and the females’ perception of quality.
If there is no such association then it will not be to the females advantage to pay attention to the
exaggerated male character. There are essentially two sorts of outcomes. The first is where
females in fact pay little attention to male signals. The second outcome is when females choose
to mate with males based on the degree to which some trait is exaggerated. This can only occur
if 1. The trait is costly. Males that invest more in the exaggerated character pay a greater fitness
penalty than those males that invest less. This is the notion of a handicap. 2. Males that have
higher intrinsic quality pay less of a cost than males that are of lower quality. The combination
of these two characteristics ensures that the degree of exaggeration of the male trait signals high
male quality. 3. Females mating choice depends on the expression of the character. This is the
characteristic that gives the males the incentive to invest in the exaggerated, but costly, trait.
2. The reproductive output of females of many species is often more limited than the
reproductive output of the males. Given that most females can obtain enough sperm to
fertilize all of their eggs from a single male, why do the females of many species seek
multiple mates?
This question was relatively straightforward. The book talks at length about this. Some people
gave essentially a list of reasons (e.g. the second example answer). In general, this is not the best
way to answer a question. In the first example answer, there is an attempt to organize these
reasons–they can produce indirect or direct fitness benefits. This sort of answer demonstrates an
understanding at a somewhat deeper level. This question is not precisely the same as one of the
second midterm questions, however, an answer to #2 covers much of the answer to this question.
Example 1.
There are many reasons why polyandry and extra pair copulations are advantageous to the
female. The advantages are both indirect (they make for better offspring genetically) and direct
(they help the female better care for her young) to the female seeking extra mates.
Indirect benefits to the female seeking other mates can include better genes for her
offspring; if her social partner is a good care giver but a lower quality male then she will produce
better quality offspring if she sneaks copulations with a bigger of flashier male outside her social
bond. Also, females have a greater fertility insurance when sneaking extra pair copulations. If
her partner is unable to fertilize all her eggs than she is ensuring by obtaining extra sperm, then
she will have the maximum number of offspring she can. Lastly, females increase the
heterozygosity of their offspring when mating with different males. She is ensuring a strong
amount of hybrid vigor in her young.
Benefits to the female are also directly present by have many males. She may incur
benefits in the form of many nuptial gifts or spermatophores (in many insect s like butterflies)
with many partners. Also, she may get access to better territories if seeking many males. The
female may also be able to enlist the help of more males to feed heer young if she has more than
one mate. If she can confuse the paternity of her offspring enough then she is more likely to get
extra paternal care, as in Dunnocks. Additionally, if females can confuse paternity it may cut
down on potential infanticide compared to mating with only one male. Ther are many benefits to
female promiscuity.
Example 2.
The reproductive output of females of many species is often limited because it is very
costly to make an egg/ovum as compared the males that can typically fertilize theoretically all the
females in one single ejaculate. The reproductive output of females are also very limited because
most of the time the greatest responsibility of parental care i.e. lactation and providing food for
the offspring fall on her. Females are therefore prove to be polyandrous for many reasons:
1. Females that are polyandrous e.g. dunnocks can mate with 2 males so that both male partners
can help with parental care. She manipulates th circumstance so that both males believe that they
have a considerable number of offspring in her clutch of eggs and therefore have an obligation to
offer parental care. This is very advantageous to the females because it ensures the reproductive
success of her offspring. Both males provide food for the hatchlings.
2. Females also become polyandrous if they perceive that another male has good genes as
compared to their monogamous mate. The good gene hypothesis states that the female will seek
out an attractive male so that the same genes can be transferred to the next generation.
3. Females can also be polyandrous to ensure genetic variation in her offspring and to increase
reproductive success. In an attempt to reduce inbreeding, females can travel to mate with other
males, specially if a good quality is perceived in that other male.
4. Females can by polyandrous to reduce male infanticide. The more males the females mate
with, the less likely it will be for an alpha male to kill off the young if he knows one of the
offspring might be his own.
5. Females can also seek multiple males to ensure complete fertilization of her eggs. Most
monogamous males have smaller testes and therefore produce less sperm and may not be able to
complete fertilization of her eggs and increase reproductive output.
6. Females may also seek multiple mates if the significant male pair is diseased and might infect
the nest. In order to prevent sexually transmitted diseases or lice/tick infestation of the nest,
females may choose to seek out other mates to ensure a healthy nest of offspring.
3. Describe the causes and consequences of competition among males after mating has
occurred. Consider the causes and consequences from the point of view of both males and
females.
Not precisely the same as #10 from the second midterm questions, but developing a complete
answer to that question surely would have helped. This question, worth 10 points, did not
demand an answer in as great depth as other questions. I liked this answer because there was not
just a list of ways that sperm competition could occur, but an attempt to organize the
information–3 ways of competing - removal, more sperm, mate guarding. This is not an
exhaustive list and it wouldn’t include things like producing fast sperm. However, there is an
attempt to organize information–males may try to negate previous sperm, try to beat current
sperm and try to prevent future sperm, and I like that attempt at finding broader patterns.
Sperm competition occurs after mating, almost exclusively in polygamous populations –
monogamous species have no cause for such competition. Males are intent on passing on their
genes and can try to effect this is 3 ways: they can try to develop genital mechanisms (such as the
damselflies brush) to remove the previous males’ sperm, they can produce more sperm (larger
testes), or they can practice mate-guarding, as in dogs. None of these are sure ways of assuring
the males’ sperm wins, but they are effective. As a result, genitalia evolve more rapidly, males
with larger testes are favored and physiological mechanisms evolve to allow mate guarding.
From the point of view of females it is their mating with multiple males that makes such
competition necessary (virgin females are prized b/c they have been untouched by other males).
The consequence for them is that they are able to receive more parental care b/c more than one
male may believe he is the father.
4. Occasionally, one sex adopts more than one discrete type of mating strategy. Discuss the
requirements for the evolution of these alternative mating strategies.
Again a direct copy from list of second midterm questions (#8). The success of this answer is
again because they found the essential way to organize results–it depends on whether the
alternative strategy is genetically ‘determined’ or environmentally induced.
The general requirement for alternative strategies depends on if it is genetic or
environmentally induced. If it’s genetic then the fit of all strategies should be statistically the
same. If one strategy was completely advantageous, then that would end up being the only
strategy occurring. The alternate strategies also should tend to go towards an equilibrium. If one
strategy increases too much, its fitness will be reduced such that the frequency will decrease.
If the strategy is environmentally induced, then at some point in the males life, the
alternate strategy would by more beneficial than the standard strategy in terms of reproductive
success. Such as the case with horseshoe crabs and their satellite males being older males who
have degraded in quality and thus have a greater chance to reproduce if they use a satellite
strategy.
5. Discuss the relationship of parental care and mating system.
#3 from the list of second midterm questions. The second section of the course was about mating
and parental care. One of the practice questions invites you to think about their relation. The
presence of this question on the exam should come as no surprise.
Mating systems generally depend on the females and how they use space. Once a mating
system has been ‘selected’ it has a great influence on how/if males and females care for their
young. [Actually, I would have put parental care as one of the factors that strongly influences the
mating system.]
In a monogamous system, which occurs when females are too spread out in a stationary
territory for a male to guard or inseminate more than one, males of provide a great deal of
paternal care. If is advantageous for the male to help and ensure more of his offspring reach
adulthood (especially considering he may not find another receptive females). An extension of
this is seen in Dunnocks (and certain species of hamsters) where, if a female can confuse
paternity enough, the female may be able to enlist the help of more than one male.
In polygynous systems where a single male will guard many females, there is no parental
help offered to females as there are so many females to one male, as in elephant seals and red
deer. The females may get the indirect benefit of high quality young or direct benefits of a better
territory (red-winged black birds), but she gets no male help. [I really like the last sentence. It
show the tradeoffs–the benefits on one hand and the costs on the other.]
In a polyandrous mating system, again as in dunnocks, females may be able to enlist more
male help with increasing uncertainty of paternity. However, in many species there is no male
help at all. Sometimes direct benefits (like spermatophores) may help the female but there is no
parental investment on the part of the male. The opposite may also be true. In some polyandrous
(and polygynous) mating systems (blue gills and large water bugs) the males may take care of the
offspring entirely on his. In general, the mating system will greatly influence the type of parental
care given to young.
Actually this last sentence makes it clear why this answer does not receive full credit. The
direction of the causal arrow is drawn from mating system to parental care. In fact mating system
does influence parental care. However, the question asks you to discuss the relationship of the
two and often the direction of the causal arrow can go from parental care towards the mating
system. In fact, the two are intertwined as I described above.
I think that a profitable way to start might have been something like: Natural selection maximizes
fitness–the reproductive success of males and females. Fundamentally selection is operating
jointly on the outcome of mating and parental care. Those traits that maximize the reproductive
of males or females may sometimes be those that increase the number of mates and other times
may increase the amount of care given to offspring. When those traits conflict then you are likely
to see a tradeoff between increased mating and increased parental care. When those traits are
positively correlated, then you will see no conflict. –This would make it possible to discuss those
situations where increased mating success depends on evidence of parental care (bluegills-giant
water bugs) and those cases in which increased mating success conflicts with parental care
(elephant seals).
6. Parents frequently invest differently in their offspring–some receive more parental care
than others. What circumstances may tend to favor this and what are some of the
consequences?
This question has parts of #4, #5, and #11. The answer below is not bad. It would have been
even better had it clarified the issues of conflict of interest between parents and offspring or
among the offspring...
It is very common for some species to selectively favor some offspring over others.
There are a number of costs and benefits to the practice as well as adaptive reason why and when
it occurs.
When an organism bears young it is making a huge investment in those offspring. They
want as many offspring as possible to reach the end of parental investment period. [This is the
best sentence in the answer.] Often more young than the parent(s) can care for are ‘made.’ Often
the parents must make a choice to decrease the overall number of offspring in a particular litter or
have the fitness of all offspring reduced. As in coots, the parents will favour the continued
investment in healthy, strong young (the ones with bright red feathers) and kill/ignore the ones
less likely to make it to adulthood. While it may decrease the overall grand offspring they could
have under ideal circumstance the reality is that by brood culling they are able to ensure the
remaining ones have an excellent chance of reproducing.
Often, instead of outright killing, parent will lay eggs asynchronously. This means there
will always be an offspring smaller than the rest. Often this chick dies. It acts like an insurance
policy for the parents and if not needed it will be killed/ignored to ensure the other bigger
offspring get more resources. Especially during time of poor resources this selective strategy is
effective at maximizing parental fitness (decreasing their costs to feed so many chicks) and the
fitness of the remaining chicks.