What determines which sex is competitive and which is choosy?
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In general terms, the sexes within a species may be either competitive (i.e. compete against intrasexually for access to a resource, which is in this case, the opposite sex) or choosy. The ‘choosy’ sex therefore makes a decision about which member of the opposite sex he or she will mate with. The choice usually depends on a number of factors.
To determine which sex exhibits choice, the operational sex ratio (OSR) must be calculated. The OSR is the ratio of females to males actively looking for a mate. This differs to the standard sex ratio (males: females) as within an OSR, not all individuals may be looking for a mate. For example females actively engaging in parental care of their young are not going to be seeking a mate and therefore will not add to the OSR.
If the OSR is 2:1 (males: females) then due to the abundance of males, females will choose which males they mate with and the males will engage in intrasexual competition. If the OSR was reversed however (i.e. a 1:2 ratio of males to females) then the opposite is true, males will choose which females to mate with and the females will engage in competition between themselves.
The major contributor to changes in the OSR is time spent out of the mating pool during parental care. As parental care is often biased towards females, there will be a strong asymmetry in available mates (i.e. fewer females available to mate). This causes a male bias in the OSR leading to the production of ‘choosy’ females.
Trivers’ (1972) idea:
Where one sex invests considerably more than the other, members of the latter will compete amongst themselves to mate with members of the former.
Katydids (Bush Cricket) Example:
Female katydids show parental care in the form of egg production and investment in care of the eggs. Whereas males express parental care in a different manner – along with the transmission of their sperm, they provide a ‘protein gift’ (a sac of protein which the female can then consume during pregnancy to nourish herself).
An experiment with two conditions was devised:
- Low pollen availability – Therefore low protein availability (protein is derived from the pollen). This means parental care by the male is going to be costly; due to the lack of available protein (i.e. the male invests more into parental care). As expected, following Trivers’ idea, the males exhibit choice in the selection of their mate whilst there is competition in the females.
- High pollen availability – Therefore high protein availability, meaning that parental care by the male is fairly cheap as it is relatively easy to produce the protein gift provided as a form of parental care. As expected, the reverse is now true; females choose their mates whilst there is competition between the males.
The result of the experiment:
|Male competition (%)
|Female competition (%)
|Male Choice (%)
|Female Choice (%)
You can see that with low pollen counts there is little male competition (3.4%) which occurs in the form of calling or chirping. They also express a large amount of choice, turning away 43% of females.
As expected the opposite is true with females, high pollen means little (0%) competition and large (42.5%) choice.
Katydids in their Natural Habitat
What was tested in the above experiment also occurs in natural conditions too. During periods early in the year, pollen availability is low, therefore male parental care is expensive and competition occurs between the females – the males choose their mates. Whereas later in the year, pollen becomes more abundant and male care is relatively cheap, therefore females express choice and the males compete.
Choice & Competition in Birds
This pattern of choice and competition is also seen in birds. The sex which invests less time in parental care (and therefore the one undergoing competition) often develops brightly coloured plumages to aid in being chosen by the opposite sex.
Other factors affecting the pattern of mating competition
Although the difference between which sex invests the most time in parental care is a major factor which determines the OSR, there are also two other features which affect the ratio:
- The absolute sex ratio i.e. the number of males and females in a population
- Reproductive synchrony i.e. how close together females ovulate during the year
A good example of how the absolute sex ratio can affect the OSR is shown by the species of butterfly Acraea encedon. In this species, females provide all the parental care via the resources they put into developing the egg. Using previous knowledge we would expect the females therefore to choose their mate, however this is not the case.
This is due to a bacterial parasite which kills off only the males thus reducing their numbers, there may be as little as one male per 20 females. Females therefore have limited access to males and would rather mate with any male than be selective in which male they mate with.
Mating sites are filled with females who are more than likely to be virgins (94%). They will express interest in any passing butterfly in the hope that it is a male. They also show a very strong willingness to mate with a released male.
In a ‘Mark-Release-Recapture’ experiment performed on this species, the following was obtained:
In this experiment, a number of virgin or pre-mated females were released into a mating site. A large number of virgins were recaptured after a period of time, but only a small amount of the pre-mated individuals remained.
This showed that mated individuals were more likely to leave the mating site – this is due to the fierce competition between the females trying to mate. Once a female has mated, it is unlikely that she will mate again and thus it is more efficient for her to invest in survival and parental care.
Reproductive synchrony is how synchronised females are in respect to entering oestrus. For example, highly synchronised populations can enter oestrus within a week of each other, whereas low synchronised populations can enter oestrus all year round. This means the OSR (and thus choice and competition) can alter with reproductive synchrony.
In highly synchronised groups, where females choice would normally be expected (due to the parental care they invest), the opposite may actually be true. The large number of females requiring fertilising during a specific time period allows the males to be selective in how they mate.
In low synchronous groups, females can mate all year round, therefore they can be selective over which males they choose, this means competition occurs within the males.
The above graph shows that as reproductive synchrony increases (i.e. Smaller period of oestrus), sexual dimorphism of the males decreases.
At B, you can see reproductive synchrony is high and sexual dimorphism is low, males are therefore of similar size to the females (low sexual dimorphism). This is due to the small amount of competition which occurs between males, thus there is little/no selection for increased male size.
At A, the opposite is true. Females express choice and the males engage in competition, this means that larger males (High sexual dimorphism) are more likely to win in competition and fertilise the females, thus selection for increased male size occurs.
Predict the competing and choosy sex in the following and post your answers in the comments section below:
a) 2 spot ladybirds: Males can mate every day, females need to mate every 7 days. There is a male killing bacterium which has caused a population sex ratio of 2 females for every male.
b) Seahorses: On fertilisation, females transfer the egg to the male to care for.
c) Belastomatid bugs: Females lay eggs on the back of males. At the start of the season, all male backs are empty, however as the season progresses, only few males have spaces left.
Adaption to Intrasexual Competition for Mates
Intrasexual competition (e.g. Male vs. Male), is competition between one sex, over which the other sex has no direct control. This leads to sexual dimorphism, meaning a possible increase in body size or the addition of weapons (e.g. Antlers) and/or ornaments (e.g. brightly coloured plumages).
However, increased size has consequences; for example, in deer. Male deer put a lot of resource investment into developing in size. This leads to increased male mortality as they are not investing their resources in winter survival.
To survive the winter, large amounts of fat deposits may be needed to last in the cold, however, as males invest their resources in body size they remain vulnerable to the cold weather and experience a higher mortality rate.
Large male size in respect to females isn’t always beneficial, for example male insects are often smaller than their female counterparts. For instance, in the case of male insects mobility and signalling intensity is more important than large body size. A small male insect is much more mobile and thus able to fertilise more females.
Instead of engaging in raw competition, insect often attract females by being ‘smarter’ for example, water mite males attract females to them by ‘pretending to be food’. The males create vibrations in the water which imitates those made by their prey. The females move towards the source of the vibration, thinking that it is food, but instead they are actually being attracted to the male. The male can then mate with the female – a large body size was not required to mate with the female.
Intersexual Selection: Choice of Mates
Intersexual selection is where the rarer sex can choose their mate. These choices may be made by direct benefits:
- ‘Nuptial gift’ in some insects
- Good territory with good shade and/or resources, etc. e.g. nests made by male birds
- How well the other sex will be able to feed offspring
- A lack of parasites, also known as – ‘Contagion avoidance’
Or indirect benefits, typically genetic benefits, which result in better offspring.
Selection of Male Birds with Longer Tails
An experiment on male bird tail lengths showed that, males with longer tails on average have more nests than those with shorter tails. This suggests they are more preferred by the females, but why is this?
There are two main ideas behind by long tails may be selected for, these are:
1. Long tails are costly to produce and they may therefore indicate good, viable genes which will ensure healthy offspring. As only healthy males can produce the longest tails, they are obviously the individuals with the best genes. As tail length cannot be altered by the male (it is encoded for by his genes) it is an honest signal of genetic viability.
2. Genes for fertility of the offspring are being selected for (‘Sexy sons’ hypothesis). Continuing the example, long tailed males get more females. The gene which makes females find the long tail appealing becomes associated with the long tails. Females continually select for longer tails, the sons inherit the genes for longer tails from the father, and the son in turn is selected for by females due to his longer tail. This can lead to a great exaggeration of the desired characteristic, such that it actually begins to impede on the fitness of the male. This is known as Fishers’ Runaway process.
An experiment on daily weight gain of offspring, compared to male tail length showed that as tail length increases, the daily weight gain of the offspring also increases. This is an indicator that the male with the longer tails are passing beneficial, good genes to their offspring – hence selection for by females.
The Lek Paradox
The problem with the above ‘good genes’ idea, is that eventually continual selection for a certain characteristic will lead to a lack of choice as the characteristic reaches its upper threshold i.e. The majority of males within the population would have the upper threshold of the characteristic. Choice between these males is therefore no longer functional; this is known as the Lek paradox.
As a solution for this Hamilton and Zuk created the hypothesis that selection fluctuates with parasite resistance. As new parasites arise, the males adapt in different ways, possible resulting in new colour expressions in plumages. The females obviously want to select those resistant to parasites as they do not want to become infected themselves, they therefore select for the new plumage as opposed to the old one, meaning choice remains functional in a population.
Are All Choices Adaptive?
Is there always a reason for the female’s choice? Occasionally females can make choices on a mate not dependent on whether their offspring will be genetically superior, but instead make a choice due to sensory bias.
Sensory bias is the inadvertent preference of a mate due to certain visual, audio or olfactory stimuli. For example, if a female can hear a certain pitch better than another, and a male performs his mating call in that pitch, the female may exhibit sensory bias in making her choice towards that male. Males can pick up on these sensory biases and use them to their adva