Showing posts with label sex. Show all posts
Showing posts with label sex. Show all posts
Friday, January 22, 2010
I really didn't want to get into discussing testis size.
I'm writing a paper on the choice male primates face between caring for the young they already have, and fighting other males for access to mates so they can have more young. There are, of course, other reproductive strategies they could also be investing in, like trying to impress the ladies without fighting, or impressing the ladies by being such good carers. I didn't want to get into all of these, because it just gets to big and complicated, and the relationship between the variables I have is already complicated and interested enough. But my colleague who read the paper for me thinks it is important to include something on investment in sperm competition. Sperm competition occurs when a female mates with more than one male, but only one of them will end up being the father of each baby she produces. The male with the strongest, fastest, healthiest, most numerous sperm will tend to be the father, and as such will tend to pass on his genes for big healthy sperm to the most sons. And to make lots of big healthy sperm, a male needs big testicles. Testicle size turns out to be closely correlated with how much multiple mating the females do, and in highly promiscuous species, males may dedicate a significant portion of their body mass to testes. And this is how I came to be typing search terms like "bush baby testis mass" and "monkey sperm competition" into Google Scholar. You would be amazed what Google is willing to label as a scholarly article.
Wednesday, August 19, 2009
Why don't males care?
The defining difference between males and females biologically is that males produce smaller gametes than females do. Sperm are much smaller than eggs. This difference is the starting point for most arguments describing how the sexes have evolved divergent traits. In mammal further apparently inescapable differences arise because only females harbor internal development of young, and only females lactate (while male lactation is not unknown, there are no document cases of male lactation providing meaningful nutrition to the young of a species). These three apparently fixed differences underlie most arguments describing the evolution of sexual dimorphisms found in mammals, including the tendency among mammals for females to be the sole caretakers of dependent young.
Paternal care is rarer in mammals than in most other taxa where parental care is the norm. Post-birth maternal care is found in all mammals (most fundamentally in the form of lactation), and females care alone in ~90% of mammal species. This contrasts sharply with birds, where female-only care is found in fewer than ~10% of species. Primates are unusual among mammals in that approximately 40% of genera display at least some male care, according to an older and therefore probably low estimate (Kleiman et al. 1981). Primates provide the opportunity to examine what factors lead to evolution of paternal care, even when sex specific structural factors (internal development and lactation) require maternal care.
Why do so few mammalian males engage in care? Mated individuals face the choice to continue investing in caring of current offspring (bearing fitness costs in the form of time, individual quality and mortality risk), produce new offspring with the same mate, or abandoning mate and young to seek new mating opportunities. In all mammals lack of post-zygotic and post-pregnancy investment from mothers is fatal to the offspring. Males therefore have earlier opportunities to abandon, leaving females to bear these costs for both of them, than females do. Depending upon timing and the particulars of a species' natural history, mothers may also be more likely to successfully raise the young of the abandoning male than a male could be in raising the young of his absent mate.
Males not only have greater opportunity to desert, but also greater potential payoffs. A male's reproductive success increases more rapidly with multiple matings than a female's would (although females may gain social and genetic benefits from multiple matings), and males therefore experience higher variance in reproductive success than females. This variance is often non-random, relying on traits which influence female choice or the outcome of male-male competition. These traits are necessarily expensive in order to serve as honest signals, and potentially reduce males' ability as care-givers (and longevity, reducing their reliability as care givers) as they increase their ability as competitors. Therefore males who have already mated, and therefore have the opportunity to care for their own young, are likely to also be those who could most successfully remate, and have invested heavily in the capacity to do so. A female who has mated may not be of unusually high fitness, and may not gain fitness from remating, particularly given the cost in future grandchildren associated with abandoning current dependent young. Males, lacking internal incubation, are also less certain of parentage of social young (both probabilistically and in terms of lack of individual information) than are females, further reducing the value of social offspring (measured in number of genetic grandchildren). Mated males in this standard case, have more opportunity to desert, lower risk of losing future grandchildren by deserting and higher potential for remating than do mated females. Given these conditions, it is reasonable to turn the question around, and ask not why so few mammalian males care, but why do those mammalian males who care do so?
The clear answer to this question is that these conditions, or at least the fitness inequalities they imply, are not universal. Under certain conditions, males may gain more by continuing to invest in existing offspring than by attempting to produce additional offspring. Where biparental care is necessary for production of successful young, the opportunity cost associated with abandonment and competitive risk taking increases. Under the same condition, male reproductive success is likely to increase less sharply with multiple matings, at least in cases where certainty of paternity is fairly high. If this results in a decrease in non-random variance in male reproductive success, it is likely to also decrease the potential benefit to competing for new matings, and in investing in the weaponry necessary for that competition.
This by the way, was another start to an intro that didn't quite work out. The problem isn't that the analysis is internally flawed, but that it raises issues I don't have the data to address, and doesn't really lead to the question I can answer.
Paternal care is rarer in mammals than in most other taxa where parental care is the norm. Post-birth maternal care is found in all mammals (most fundamentally in the form of lactation), and females care alone in ~90% of mammal species. This contrasts sharply with birds, where female-only care is found in fewer than ~10% of species. Primates are unusual among mammals in that approximately 40% of genera display at least some male care, according to an older and therefore probably low estimate (Kleiman et al. 1981). Primates provide the opportunity to examine what factors lead to evolution of paternal care, even when sex specific structural factors (internal development and lactation) require maternal care.
Why do so few mammalian males engage in care? Mated individuals face the choice to continue investing in caring of current offspring (bearing fitness costs in the form of time, individual quality and mortality risk), produce new offspring with the same mate, or abandoning mate and young to seek new mating opportunities. In all mammals lack of post-zygotic and post-pregnancy investment from mothers is fatal to the offspring. Males therefore have earlier opportunities to abandon, leaving females to bear these costs for both of them, than females do. Depending upon timing and the particulars of a species' natural history, mothers may also be more likely to successfully raise the young of the abandoning male than a male could be in raising the young of his absent mate.
Males not only have greater opportunity to desert, but also greater potential payoffs. A male's reproductive success increases more rapidly with multiple matings than a female's would (although females may gain social and genetic benefits from multiple matings), and males therefore experience higher variance in reproductive success than females. This variance is often non-random, relying on traits which influence female choice or the outcome of male-male competition. These traits are necessarily expensive in order to serve as honest signals, and potentially reduce males' ability as care-givers (and longevity, reducing their reliability as care givers) as they increase their ability as competitors. Therefore males who have already mated, and therefore have the opportunity to care for their own young, are likely to also be those who could most successfully remate, and have invested heavily in the capacity to do so. A female who has mated may not be of unusually high fitness, and may not gain fitness from remating, particularly given the cost in future grandchildren associated with abandoning current dependent young. Males, lacking internal incubation, are also less certain of parentage of social young (both probabilistically and in terms of lack of individual information) than are females, further reducing the value of social offspring (measured in number of genetic grandchildren). Mated males in this standard case, have more opportunity to desert, lower risk of losing future grandchildren by deserting and higher potential for remating than do mated females. Given these conditions, it is reasonable to turn the question around, and ask not why so few mammalian males care, but why do those mammalian males who care do so?
The clear answer to this question is that these conditions, or at least the fitness inequalities they imply, are not universal. Under certain conditions, males may gain more by continuing to invest in existing offspring than by attempting to produce additional offspring. Where biparental care is necessary for production of successful young, the opportunity cost associated with abandonment and competitive risk taking increases. Under the same condition, male reproductive success is likely to increase less sharply with multiple matings, at least in cases where certainty of paternity is fairly high. If this results in a decrease in non-random variance in male reproductive success, it is likely to also decrease the potential benefit to competing for new matings, and in investing in the weaponry necessary for that competition.
This by the way, was another start to an intro that didn't quite work out. The problem isn't that the analysis is internally flawed, but that it raises issues I don't have the data to address, and doesn't really lead to the question I can answer.
Friday, May 08, 2009
Adaptive male lactation?
It has long been known that male mammals, including male humans, are physiologically capable of lactating. Screw with a man's physiology by giving him the right hormones, or the wrong series of starvation and then plenty, and he may start to produce milk. No one, as far as I know, has ever suggested that this is adaptive, that this capacity exists because men gain some reproductive advantage through lactation. Rather, it is usually seen as a result of the fact that we share almost all of our genetic material with females, who do make good use of their lactational prowess. Male lactation across the mammalian world is largely thought to be a side effect of intersexual correlation, the tendency for the two sexes of a species to have similar traits.
I am therefore skeptically excited to read that males of two species of fruit bats, one in Malaysia, and one in Papua New Guinea, are said to have "well-developed lacteriforus ducts and underlying mammary tissue similar to that found in lactating females" and that milk has been "expressed" from a large number of male bats.
It is not actually known whether these males are feeding young, and if so how commonly and to what effect, but this is the closest suggestion I have yet seen of the possibility of adaptive male lactation.
I am therefore skeptically excited to read that males of two species of fruit bats, one in Malaysia, and one in Papua New Guinea, are said to have "well-developed lacteriforus ducts and underlying mammary tissue similar to that found in lactating females" and that milk has been "expressed" from a large number of male bats.
It is not actually known whether these males are feeding young, and if so how commonly and to what effect, but this is the closest suggestion I have yet seen of the possibility of adaptive male lactation.
Friday, April 10, 2009
Sex-biased longevity
Ive spent much of the last couple of weeks working on a paper on sex-biased longevity in primates.
Here is a draft of a portion of an introduction to said paper. Any similarities between this and the final published paper are purely coincidental:
The difference in longevity between the sexes of a population depends upon the selective forces each sex experiences, as well as the degree to which common genetic material limits independent demographic evolution. Sex-biased longevity has been proposed to arise from difference between the sexes in selective forces as diverse as reproductive physiology, care of offspring, parasite risk, mortality associated with reproduction, genomic stability and late-life support from kin. But measurements of sex-biased longevity have been made for relatively few species, and we have little sense of the degree to which sex-biased longevity is constrained by shared genetics or phylogenetic conservatism.
Most organisms, and likely all mammals, experience an evolved rapid increase in mortality and decrease in fertility at advanced ages, limiting longevity. The force of selection against mortality at a given age depends upon the likelihood of surviving to that age, and the mean remaining reproduction of individuals who do survive that long. Reproduction and survival are expected to drop to zero at similar ages, but reproduction can be direct (fertility) or indirect (care of offspring and kin effects).
Captive populations, the source of most demographic data on non-humans, will reflect sex-differences in evolved capacity for longevity more so than do wild populations, which tend to die younger (there are exceptions). For understanding the influence of experienced mortality patterns on evolved capacity for longevity, it is useful to make the distinction between extrinsic and intrinsic mortality. Extrinsic mortality is generally said to be that caused by the environment, while intrinsic death is driven by a failure of the organism's internal processes. In practice this distinction is difficult to make, as all environmental risk is influenced by the organism's characteristics and behaviors, and the timing and risk of intrinsic failure is inevitably influenced by an organism's history and environment. None-the less, much of our theory on the evolution of longevity is based on this intrinsic/extrinsic distinction. In general higher extrinsic mortality will lead to increased intrinsic mortality. However increased extrinsic risk at age x will influence evolved mortality at many ages, and not necessarily lead to a spike in intrinsic mortality centered at age x. This leaves overall sex-biased longevity as our best measure of sex-biased mortality when employing data from captive populations. Sex-biased longevity in captivity is highly correlated with sex-biased longevity in the wild even though the causes and timing of mortality may vary between species.
Much of the discussion of sex-biased longevity has focused on the idea that if one sex provides extended care to descendants, that sex should gain greater selective benefit from increased longevity. Allman et al. suggest that in primates, males who are the primary caregivers tend to live as long or longer than their females, while in species with little paternal care females tend to live longer. The Grandmother Hypothesis and its derivatives use indirect reproduction by post-menopausal women to explain their post-reproductive lifespan and therefore their tendency to live longer than men despite earlier decline in fertility. The Patriarch hypothesis instead argues that women's post-fertile survival is explicable based on late-life reproduction in males and the non-independence of male and female longevity.
Sex-biased longevity can also result from differences in mortality risk between the sexes. Trivers proposed that in species with frequent or intense male-male conflict, males incur significant mortality risk and therefore don't live as long as females. Alternatively, if females experience increased mortality in producing or rearing young, males should be expected to live longer. If either sex is the predominant disperser, and mortality risk during dispersal is high, this too could cause inter-sex differences in capacity for longevity. Finally, a variety of hypotheses have been put forward arguing that either the larger sex, or the smaller sex, should tend to live longer.
Using comparative primate life-history data, we examine the variability of sex-biased longevity in primates. We further examine the degree to which level of paternal care, grandmaternal care, male-male conflict, and mass dimorphism predict sex-biased longevity. Finally, we examine the patterns of correlated evolution among these variables in a phylogenetic context.
Here is a draft of a portion of an introduction to said paper. Any similarities between this and the final published paper are purely coincidental:
The difference in longevity between the sexes of a population depends upon the selective forces each sex experiences, as well as the degree to which common genetic material limits independent demographic evolution. Sex-biased longevity has been proposed to arise from difference between the sexes in selective forces as diverse as reproductive physiology, care of offspring, parasite risk, mortality associated with reproduction, genomic stability and late-life support from kin. But measurements of sex-biased longevity have been made for relatively few species, and we have little sense of the degree to which sex-biased longevity is constrained by shared genetics or phylogenetic conservatism.
Most organisms, and likely all mammals, experience an evolved rapid increase in mortality and decrease in fertility at advanced ages, limiting longevity. The force of selection against mortality at a given age depends upon the likelihood of surviving to that age, and the mean remaining reproduction of individuals who do survive that long. Reproduction and survival are expected to drop to zero at similar ages, but reproduction can be direct (fertility) or indirect (care of offspring and kin effects).
Captive populations, the source of most demographic data on non-humans, will reflect sex-differences in evolved capacity for longevity more so than do wild populations, which tend to die younger (there are exceptions). For understanding the influence of experienced mortality patterns on evolved capacity for longevity, it is useful to make the distinction between extrinsic and intrinsic mortality. Extrinsic mortality is generally said to be that caused by the environment, while intrinsic death is driven by a failure of the organism's internal processes. In practice this distinction is difficult to make, as all environmental risk is influenced by the organism's characteristics and behaviors, and the timing and risk of intrinsic failure is inevitably influenced by an organism's history and environment. None-the less, much of our theory on the evolution of longevity is based on this intrinsic/extrinsic distinction. In general higher extrinsic mortality will lead to increased intrinsic mortality. However increased extrinsic risk at age x will influence evolved mortality at many ages, and not necessarily lead to a spike in intrinsic mortality centered at age x. This leaves overall sex-biased longevity as our best measure of sex-biased mortality when employing data from captive populations. Sex-biased longevity in captivity is highly correlated with sex-biased longevity in the wild even though the causes and timing of mortality may vary between species.
Much of the discussion of sex-biased longevity has focused on the idea that if one sex provides extended care to descendants, that sex should gain greater selective benefit from increased longevity. Allman et al. suggest that in primates, males who are the primary caregivers tend to live as long or longer than their females, while in species with little paternal care females tend to live longer. The Grandmother Hypothesis and its derivatives use indirect reproduction by post-menopausal women to explain their post-reproductive lifespan and therefore their tendency to live longer than men despite earlier decline in fertility. The Patriarch hypothesis instead argues that women's post-fertile survival is explicable based on late-life reproduction in males and the non-independence of male and female longevity.
Sex-biased longevity can also result from differences in mortality risk between the sexes. Trivers proposed that in species with frequent or intense male-male conflict, males incur significant mortality risk and therefore don't live as long as females. Alternatively, if females experience increased mortality in producing or rearing young, males should be expected to live longer. If either sex is the predominant disperser, and mortality risk during dispersal is high, this too could cause inter-sex differences in capacity for longevity. Finally, a variety of hypotheses have been put forward arguing that either the larger sex, or the smaller sex, should tend to live longer.
Using comparative primate life-history data, we examine the variability of sex-biased longevity in primates. We further examine the degree to which level of paternal care, grandmaternal care, male-male conflict, and mass dimorphism predict sex-biased longevity. Finally, we examine the patterns of correlated evolution among these variables in a phylogenetic context.
Monday, December 29, 2008
A conjecture on the link between babies and lack of sex
I am told by those who have children that one of the many sacrifices couples make to raise a baby is opportunity for sexual intimacy. I don't have kids, so don't know from personal experience, but it certainly makes sense. Between exhaustion, vehement interruptions and company in the house, it can be hard for a couple to find the time, privacy and energy to maintain their pre-parental levels of activity. I have had friends say that it seems very much like the baby is plotting to destroy its parents' sex lives. It has just occurred to me that in a sense, this could be very true.
Evolutionarily, there are many ways in which the interests of the parent and the interests of the offspring are aligned. The fitness of both are improved if the baby grows, thrives and go on to produce its own offspring. They share many genes, and anything that is good for the one is at least a little bit good for the other. But some things that are good for the fitness of the parents are a net selective loss for the baby. Such as having another baby come along too soon. The parents of course are equally closely related to all their offspring, and therefore will tend to distribute care and resources between their children in a way that maximizes the number of future grandchildren. But the baby is twice as closely related to herself as she is to her full sibling; she is much better off monopolizing her parents' time and resources for longer than they might desire. The parents' fitness is maximized by having an interbirth interval just long enough to get a good return on their investment in this offspring, without unduly diminishing their opportunity to have more children in the future. The child's fitness is maximized by having the parents wait somewhat longer, until the diminishment of their future reproductive chances for each additional day waited is twice the per day increase in her own fitness gain. The technical term for this disalignment of interest, appropriately, is parent-offspring conflict.
At first glance, the advantage in the conflict over the length of the interbirth interval would seem to be distinctly on the side of the parents, rather than the sessile, pre-sentient, altricail lump of chub, digestive organs and breathing apparatus. But oh, those breathing bits can very easily be used to make sounds. Sounds that communicate desperate dire need to protect and nourish the baby. Sounds that cannot easily be ignored. What harm if one screams just a little bit louder, a little more frequently, screams and cries with slightly less provocation, and makes the parents increase their interbirth interval just a little while longer?
Before you label me a conspiracy theorist, let me be clear. I am not implying that the babies of the world are 'trying,' in any intentional way, to deprive their parents of sex. They don't have to try. It comes naturally to them.
Evolutionarily, there are many ways in which the interests of the parent and the interests of the offspring are aligned. The fitness of both are improved if the baby grows, thrives and go on to produce its own offspring. They share many genes, and anything that is good for the one is at least a little bit good for the other. But some things that are good for the fitness of the parents are a net selective loss for the baby. Such as having another baby come along too soon. The parents of course are equally closely related to all their offspring, and therefore will tend to distribute care and resources between their children in a way that maximizes the number of future grandchildren. But the baby is twice as closely related to herself as she is to her full sibling; she is much better off monopolizing her parents' time and resources for longer than they might desire. The parents' fitness is maximized by having an interbirth interval just long enough to get a good return on their investment in this offspring, without unduly diminishing their opportunity to have more children in the future. The child's fitness is maximized by having the parents wait somewhat longer, until the diminishment of their future reproductive chances for each additional day waited is twice the per day increase in her own fitness gain. The technical term for this disalignment of interest, appropriately, is parent-offspring conflict.
At first glance, the advantage in the conflict over the length of the interbirth interval would seem to be distinctly on the side of the parents, rather than the sessile, pre-sentient, altricail lump of chub, digestive organs and breathing apparatus. But oh, those breathing bits can very easily be used to make sounds. Sounds that communicate desperate dire need to protect and nourish the baby. Sounds that cannot easily be ignored. What harm if one screams just a little bit louder, a little more frequently, screams and cries with slightly less provocation, and makes the parents increase their interbirth interval just a little while longer?
Before you label me a conspiracy theorist, let me be clear. I am not implying that the babies of the world are 'trying,' in any intentional way, to deprive their parents of sex. They don't have to try. It comes naturally to them.
Saturday, November 22, 2008
Reader JTE asks:
Q:What does
two individuals with the same genotypes, except for those genes determining sex (which is some species don't exist, where sex is environmentally determined),
mean?
A: I'm glad you asked.
It means that if I had one missing or dysfunctional gene on my Y chromosome (or was XX instead of XY), I would be phenotypically female, but the rest of my genome would be the same as it is now. A great many aspect of my physical, chemical, social and mental being (my phenotype) have been altered by the effects of this one gene, which acts as a sex switch. Switch on maleness, and a whole bunch of aspects of phenotype are altered. Don't switch it on, and you get a different phenotype.
In some species, there are no X and Y chromosomes, or anything equivalent, to act as a sex switch. Instead, whether an individual develops as a male or a female is determined by the environmental conditions which prevail at a certain point in development. In alligators for example, there is no genetic determination of sex. Instead, if the temperature around the egg is above a certain temperature at a certain point in development, the alligator becomes one sex (I think male, but I don't actually remember). If it is ?colder? than that temperature, you get a female alligator. Many of the aspects of the switch are the same, only the first step of the switch is very different.
So my colleague was pondering the fact that two individuals with similar, or even identical, genotypes can have importantly different phenotypes, based on the action of this switch. This means that whether this switch is on or off can greatly affect the actions of other genes, and therefore the effects those other genes have on the survival and reproductive success of the organism.
two individuals with the same genotypes, except for those genes determining sex (which is some species don't exist, where sex is environmentally determined),
mean?
A: I'm glad you asked.
It means that if I had one missing or dysfunctional gene on my Y chromosome (or was XX instead of XY), I would be phenotypically female, but the rest of my genome would be the same as it is now. A great many aspect of my physical, chemical, social and mental being (my phenotype) have been altered by the effects of this one gene, which acts as a sex switch. Switch on maleness, and a whole bunch of aspects of phenotype are altered. Don't switch it on, and you get a different phenotype.
In some species, there are no X and Y chromosomes, or anything equivalent, to act as a sex switch. Instead, whether an individual develops as a male or a female is determined by the environmental conditions which prevail at a certain point in development. In alligators for example, there is no genetic determination of sex. Instead, if the temperature around the egg is above a certain temperature at a certain point in development, the alligator becomes one sex (I think male, but I don't actually remember). If it is ?colder? than that temperature, you get a female alligator. Many of the aspects of the switch are the same, only the first step of the switch is very different.
So my colleague was pondering the fact that two individuals with similar, or even identical, genotypes can have importantly different phenotypes, based on the action of this switch. This means that whether this switch is on or off can greatly affect the actions of other genes, and therefore the effects those other genes have on the survival and reproductive success of the organism.
Thursday, November 20, 2008
Intersexual Correlation
A colleague wrote to ask me what I thought about an idea he'd had. He was thinking about the fact that one could have two individuals with the same genotypes, except for those genes determining sex (which is some species don't exist, where sex is environmentally determined), and end up with significantly different phenotypes. In some traits (e.g. Hair color) these two individuals would be expected to have very similar traits, in others (e.g. genital morphology) they would be expected to be very different, and perhaps in some cases uncorrelated or negatively correlated. He wondered if this might affect the ability of individuals to choose mates who would produce highly successful offspring. For instance, a female sizing up a male would have a better sense of what that male's sons would look like than what his daughters would look like. A big very masculine male might tend to have oversized and somewhat unattractive daughters. My colleage wondered if this might confuse things enough to slow down the action of sexual selection, and allow a greater genetic diversity to remain in the population than would otherwise be the case. I found htis a very interesting question, and wrote the following reply:
There is a body of literature on the degree to which natural selection on the traits of one sex will affect the traits of the other sex. People often use the term "correlated evolution" to describe this sort of thing. When there is a correlation (positive or negative) in a trait between the female expressed genotype and the male expressed genotype, I've seen the phrase "intersexual correlation." I am not terribly familiar with this literature, I'm afraid.
This recent paper is the closest thing I know of to what you are talking about.
Whether any of this would lead to a greater genetic diversity in the population, I am not sure. The effects of natural selection may be somewhat weaker, as traits that are expressed in one sex but not the other are less often expressed, and therefore less often subject to selection (an epistatic interaction in effect). In the case of sexual selection, my guess would be that as long as degrees of intersexual correlation in particular traits evolve more slowly than do what cues individuals use to choose mates, choosers should evolve to focus on characteristics that are good indicators of fitness in both male and female offspring. I think this will generally be the case, as there is clearly very strong selection against those who use misleading cues in mate choice. I am not aware of any reason to think there would be rapid change in the degree of intersexual correlation in a wide range of traits all at once. As long as there is any consistently reliable signal available, the family lines that use it should tend to do better than the population average.
It raises an interesting set of questions, I am not sure how many of them there is any literature on.
Does this answer your question? If you want more expert answers we could ask Monty Slatkin, who I am sure has thought about this in some detail at some point.
There is a body of literature on the degree to which natural selection on the traits of one sex will affect the traits of the other sex. People often use the term "correlated evolution" to describe this sort of thing. When there is a correlation (positive or negative) in a trait between the female expressed genotype and the male expressed genotype, I've seen the phrase "intersexual correlation." I am not terribly familiar with this literature, I'm afraid.
This recent paper is the closest thing I know of to what you are talking about.
Whether any of this would lead to a greater genetic diversity in the population, I am not sure. The effects of natural selection may be somewhat weaker, as traits that are expressed in one sex but not the other are less often expressed, and therefore less often subject to selection (an epistatic interaction in effect). In the case of sexual selection, my guess would be that as long as degrees of intersexual correlation in particular traits evolve more slowly than do what cues individuals use to choose mates, choosers should evolve to focus on characteristics that are good indicators of fitness in both male and female offspring. I think this will generally be the case, as there is clearly very strong selection against those who use misleading cues in mate choice. I am not aware of any reason to think there would be rapid change in the degree of intersexual correlation in a wide range of traits all at once. As long as there is any consistently reliable signal available, the family lines that use it should tend to do better than the population average.
It raises an interesting set of questions, I am not sure how many of them there is any literature on.
Does this answer your question? If you want more expert answers we could ask Monty Slatkin, who I am sure has thought about this in some detail at some point.
Wednesday, June 04, 2008
Of listing and sex
Among many bird-watchers, there is an obsession with listing. Species one has seen anywhere, species one has seen in Ontario, species one has seen in Ontario in October, species one has seen through a particular window while sitting on the toilet and so on. I once had a colleague point his binoculars up at the sky for a few seconds, and say, "Awesome, I've never seen a Red-Throated Loon on the southward migration over Lake Eire before. Another tic for my list."
I keep a few place specific lists, "birds I've seen in the El Cerrito Hillside Natural Area," or, "birds of Mahopac, NY," and used to keep a life list (e.g. all the wild birds I've ever seen anywhere) but I've never really gotten into listing for listing's sake. Hard-core listers (commonly referred to as twitchers for their inability to hold still if someone suggests a bird they don't yet have on a particular list is nearby) are often far more interested in adding birds to lists than in actually looking at the birds, and will often travel long distances to find a bird, look at it for just long enough to confidently add it to their list, then begin the long journey home. Those of us who are not twitchers, but know people who are, enjoy the game of making up new types of lists and mentioning them to twitchers so that they will feel compelled to rush out and start building a bigger, longer more impressive list with more rare species. So, in that spirit, I have started to compile a copulation list, of all the bird species I have ever observed copulating. This list is necessarily incomplete, as I am only including those species which I know a particular time and place when I witnessed a copulation in the wild, but excluding all the other species that I forget where or when. This list was inspired by the activity of two house-sparrows this fine June morning.
In no particular order:
House Sparrow
California Condor
Turkey Vulture
Fairy Bluebird
Florida Scrub-Jay
Western Scrub-Jay
Steller's Jay
Common Raven
American Crow
Bald Eagle
House Finch
European Starling
Canada Goose
Mallard
Muskovi Duck
Great Egret
American Robin
Dark-Eyed Junco
California Towhee
Red-Tailed Hawk
Red-Shouldered Hawk
Willie Wagtail
I keep a few place specific lists, "birds I've seen in the El Cerrito Hillside Natural Area," or, "birds of Mahopac, NY," and used to keep a life list (e.g. all the wild birds I've ever seen anywhere) but I've never really gotten into listing for listing's sake. Hard-core listers (commonly referred to as twitchers for their inability to hold still if someone suggests a bird they don't yet have on a particular list is nearby) are often far more interested in adding birds to lists than in actually looking at the birds, and will often travel long distances to find a bird, look at it for just long enough to confidently add it to their list, then begin the long journey home. Those of us who are not twitchers, but know people who are, enjoy the game of making up new types of lists and mentioning them to twitchers so that they will feel compelled to rush out and start building a bigger, longer more impressive list with more rare species. So, in that spirit, I have started to compile a copulation list, of all the bird species I have ever observed copulating. This list is necessarily incomplete, as I am only including those species which I know a particular time and place when I witnessed a copulation in the wild, but excluding all the other species that I forget where or when. This list was inspired by the activity of two house-sparrows this fine June morning.
In no particular order:
House Sparrow
California Condor
Turkey Vulture
Fairy Bluebird
Florida Scrub-Jay
Western Scrub-Jay
Steller's Jay
Common Raven
American Crow
Bald Eagle
House Finch
European Starling
Canada Goose
Mallard
Muskovi Duck
Great Egret
American Robin
Dark-Eyed Junco
California Towhee
Red-Tailed Hawk
Red-Shouldered Hawk
Willie Wagtail
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