Tuesday, April 13, 2010

Sunday, November 29, 2009

Attraction & Olfaction

The fact human females would want sex at all outside of their monthly 6-day window of receptivity is an evolutionary problem. What practical purpose does it serve? Don't females have better things to do?

Well, of course we do - but even so, those females who prescribe to a "dual sexuality" (conceptive and not conceptive) mating strategy are actually more likely to reap both the genetic and material benefits in their mate(s). There have been studies that suggest when a female is not receptive she prefers males who exhibit strong paternal and supportive attributes. These are the "nice" guys who are often pitied because they don't "get the girl." This isn't entirely true - except perhaps from a male's perspective. It really depends upon how "getting the girl" is interpreted. All but six days of the month studies have shown females to show preference towards the devout paternal male, not the raucous bad boy who is well endowed (genetically).

I suppose I should pause here to explain an important concept in evolutionary biology that's relevant to this entry. I'm inferring that he who is considered of high genetic quality is the aggressive, eyebrow-pierced, pink polo-adorned male who is incapable of articulating anything that has two or more syllables. This is true to at least one extent explained by the sexy son hypothesis. The idea is that if a female mates with a male who is considered "attractive," she will have attractive male offspring who will be perceived as having the same appeal to the next generation of women. A vibrant example of this is the male peafowl (below).

image source

One can easily see why the dual sexuality strategy would be adaptive to our female ancestors. A female prefers the male with whom she can share parental duties and, when she's ovulating, the male who can maximize her reproductive success in the long run. In human and nonhuman monogamous animals, females engage in extra-pair copulations during ovulation if the benefits outweigh the costs (ie. if the extra-pair male is much more genetically compatible and of course, if they don't get caught).

Follow your nose...
There is mounting evidence that suggests female mate choices are made through cues that reflect genetic quality ("sexy son" indicators) and material benefits ("good dad" indicators). Sexy son indicators include vocal and facial masculinity, muscles, symmetry, and body scent intensity. Good dad indicators are passiveness and feminine facial features (eg. round face and eyes).

So... what does the pill have to do with how sexy a woman perceives a man's stench?

The reason the pill has such a strong effect on female mate preference is because it keeps a high level of progestin (a substitute for progesterone) and estrogen in the female's system. Progesterone is a mellowing hormone and the sister to estrogen, the controlling, all-powerful hormone that is responsible for feelings of aggression and seduction. The pill tricks the body into thinking it is in a pregnant state, during which estrogen and progesterone are peaking. When a female is pregnant she does not continue the uterine cycle because a fertile egg is already gestating.

So what do the two key components to hormonally-based birth control pills actually do?Progestin and estrogen prevent gonatropin-releasing hormone (GnRH) from being secreted by the hypothalamus. GnRH is the signal to the pituitary gland to produce follicle-stimulating hormone (FSH) and leutinizing hormone (LH). Once GnRH is blocked, the body does not secrete FSH, which prompts the body to grow a follicle, and LH, which triggers the release of the egg. The pill essentially freezes the natural cycle so it remains at a single stage for the entire month.

The data suggests that females who are on the pill (or pregnant) do not have the same sexual palette as the non-pill user. In a study that manipulated the facial features in 20 pictures of male faces to masculinize or feminize their attributes found normally cycling women prefer less masculine faces in a long-term context compared to more masculine faces in a short-term context. Pill-users had an inverse preference and a weaker preference.

Love at first smell and contraceptives

The Major Histocompatibility Complex (MHC) is a gene family that serves as an important component to a person's immune system. It creates molecules capable of identifying pathogens, or foreign invaders, so that the soldiers of the immune system can be called upon for combat. The MHC is so essential that the olfactory cues reflecting the genetic compatibility among two individuals are thought to be based off of it alone.

Mate choice is influenced by MHC diversity through olfactory cues. MHC–peptide complexes present at the cell surface carry information about our cellular genetic make-up. When these complexes are shed from the cell surface they dissolve in our bodily fluids. This releases the peptide ligand component of the complex, which is then free to interact with other receptors (such as olfactory sensory neurons). The relevance of this has to do with the patterns of sensory neuron activation, which are unique to the structure of distinct peptide ligands. A person's distinct body odor reflects their MHC genotype, (drum roll) thus allowing us to assess the genetic compatibility of prospective mates!

This is where contraceptive pills come into play: natural selection has played a large role in making sure individuals with similar genetic material - siblings, for instance - do not breed because the resulting offspring will have weaker immune systems and be more likely subject to a whole suite of genetic faults. The cost of mating with someone who has a similar set of MHC alleles is large for the same reason. The idea is that with lots of diversity in our MHC genes, the body is ready to take on a lot of different invaders. This diversity can only be maintained by sexual outbreeding which leads to different combinations of MHC alleles representing a richer defense strategy.

Then the brilliant stinky t-shirt experiment was conceived:

In summary, (naturally cycling) women found the odors of MHC-dissimilar males more pleasant. These results yielded a negative correlation between number of shared alleles and pleasantness ratings.

Note, however, this test examined the preference of exclusively normally cycling women. When the same sweaty t-shirt test was performed on women who are taking contraceptives they found surprising results. These women didn't show a preference for the scent of males with dissimilar MHC genes. In fact, these women showed a higher preference for males with MHC genes similar to their own. If your body believes you to be in a pregnant state, this odor preference could be advantageous. Our primitive brains yearn to be surrounded by genetic similarity (ie. relatives) during this stressful and dangerous part of our life cycle.

Past studies suggest there are consequences to females preferring genetically similar males as long-term partners. Couples with dissimilar MHC genes are more satisfied and more likely to be faithful to their mate. Those who have similar MHC genes report less satisfaction and more wandering eyes. There are also fertility problems encountered by MHC-similar couples, who have a much higher probability of recurrent spontaneous abortions (RSA). There have also been studies that highlight the importance of odor perception as it plays a significant role in maintaining attraction within relationships.

So if a female acquires her male counterpart after beginning the pill, what happens when the woman goes off of it? Something I personally will never know. I'll take my sweaty man stench without the extra hormones, please.

Thornhill, R., Gangestad, S. W., Miller, R., Scheyd, G., McCollough, J. K., and M. Franklin. Compatibility complex genes, symmetry and body scent attractiveness in men and women. The Research & Education Division of The Fragrance Foundation.

Havlicek, J. and S. Roberts. 2009. MHC-correlated mate choice in humans: A review. Psychoneuroendocrinology 34: 497 - 512.

Sunday, August 23, 2009

The adaptive benefits of producing non-functional sperm

Scientists were scratching their heads while looking for the evolutionary explanation as to why the males of a butterfly species possessed predominantly non-fertile sperm.

What the heck is going on?

(image source)

Promiscuous mating regimes are great to research if you're interested in the coevolutionary arms race between male and female reproductive interaction. Usually it is evident that males will have evolved to manipulate female receptivity and females to resist the manipulation. An interesting example lies in the green-veined white butterfly (Pieris napi). Male butterflies transfer fertile and non-fertile sperm to their female. The non-fertile sperm requires less energy to produce and is used because females possess a sperm-storage organ that, when full, switches off the receptivity of the female. This enforces female monogamy because she cannot successfully mate with another male until she finishes the period during which she is not receptive (due to the full sperm storage organ). This period of time is called her refractory period.

However, females have also evolved to combat the standard lengthy refractory period by shortening it significantly. It is advantageous because it allows females to mate more and therefor have a higher reproductive output. Researchers found a positive genetic correlation between non-fertile sperm transfer and female refractory period. In other words, in the context of a promiscuous mating system, sexual conflict yeilds short female refractory periods and high proportions of non-fertile sperm in males. They also found that these traits were heritable and when the mating system was manipulated, the traits would also fluctuate.

This finding is consistent with previous findings related to male-female sexual conflict: selection on female reproductive traits directly affects male traits and vice versa.

Nina Wedell, Christer Wiklund, and Jonas Bergström. 2009. Coevolution of non-fertile sperm and female receptivity in a butterfly. Biology Letters.

Wednesday, July 22, 2009

Mating Systems 101

In a monogamous mating system, consistent pair bonds are formed between two individuals with sexual exclusivity. Polygamy is a general term used to describe a system in which an individual mates with 2 or more partners of the opposite sex. Two subsets exist within this definition. In a polygynous system, one male mates with two or more females. In a polyandrous system, one female mates with two or more males.

Polygynandry is a mating system defined by the sexual sharing among two or more females and two or more males. This term is often used interchangeably with promiscuity, but it differs because pair bonds form among specific individuals. In a promiscuous regime, no pair bonds are formed.

Across the board, mating systems develop to optimize the reproductive success of individuals. Monogamy will prevail when it is most fitness enhancing for those involved. For example, if females are dispersed, which is the case in some mammal and many bird species, monogamy will likely be the dominant strategy observed. If females are forced into a small area, such as female elephant seals cramped along a shoreline, males will use the opportunity to sequester females and maximize their reproductive output.

Monogamy will also prevail if the fitness of the offspring is dependent upon biparental care. In many bird species, the male and female are both fully capable of incubating the eggs and feeding hatchlings. If only one parent is involved, the probability of the chicks’ survival is compromised. Female-enforced monogamy is when a female eliminates the risk of other competing females from copulating with her chosen mate. A deep-sea angler fish male is born as a small, helpless sperm packet who lacks a digestive system and is entirely dependent upon his destine host female. He swims though the deep ocean in search of her, and has specialized mouth parts that assist in him in his permanent attachment to her.

Polygyny is by far the most commonly observed mating system in Animalia, and its presence is based on the extent to which males can monopolize females. This model applies to species in which the territory of a male contains useful resources for the female and her offspring. The environment plays a significant role in this mating system because females are easier to sequester when resources are spatially clumped. For example, female elephant seals in the Falkland Islands typically rear offspring and feed along the shoreline. Males fight one another in an effort to establish a more dominant hierarchy ranking which allows them access to the largest territory of shoreline. Once established, he is considered a harem holder, and mates with all the females who nest in his territory (a heram group). In polygynous mating systems, male reproductive success is a function of pre-copulatory strategies such as fighting in the example above. Males aim to mate with as females as possible, and females choose the male based on the quality of the territory he protects. A very small percentage of alpha males are successful harem holders among polygynous populations. The remaining male elephant seals roam the periphery of the territories, and are called satellite males.

Polyandry has been considered an evolutionary problem because it was thought to challenge one of the major underpinnings of sexual selection. It is the assumption that the disparity between male and female gamete investments (anisogamy) has favored the selection for diverging mating habits between males and females. Females are thought to be the “choosier” sex because they invest more energy into generating eggs, while males tend to favor quantity over quality and their fitness will not be lowered if they mate with a female of low genetic quality.

This line of thinking has gone under extreme scrutiny recently, as there has been accumulating molecular evidence revealing multiple paternity (polyandry) to be a common practice in animals. Multiple hypotheses have developed to explain this. The material benefits hypothesis predicts females who mate with more than one male are privileged to access more resources, parental care, and a limitless sperm supply. The genetic benefits hypothesis broadly predicts females who are mated with multiple males will produce offspring who are the result of superior ejaculates. If the offspring are males, they potentially have the same high quality sperm and will be able to heighten the fitness of the mother.

There are unique cases that do not fit in the categories described above. For example, we see an investment role reversal in Red Phalaropes of Alaska. Females exhibit traditionally masculine qualities. They are bigger and more colorful than males and play no role in incubating or caring for the young. The female spends about a week courting a single male and laying her eggs in his nest. Females of this species are able to generate eggs quickly, and after a week the female begins searching for another mate. Males incubate the eggs and nurture the chicks alone.

Polygynandry and promiscuity are similar yet distinctly different mating systems. Polygynandrous females regularly form pair bonds and copulate with several males at the same time as males form pair bonds and copulate with several females. The polygynandrous European badger lives in social groups with as many as five mothers and five fathers that produce multiple-paternity litters. It is thought the primary reason for this is described by the resource dispersion hypothesis: polygynandrous groups may arise because limited optimal resources may force more than one breeding pair within a territory. There is usually a dominant pair and a number of subordinates within a social group. In some species the subordinates do not mate, but in many cases subordinate females produce litters of mixed paternity.

Promiscuous species are those in which pair bonds do not form and males and females are likely to copulate with more than one individual of the opposite sex. It is a unique system because subordinate males have the opportunity to access females and male reproductive success is a function of post-copulatory strategies such as sperm competition and cryptic female choice. For example, when brown headed cowbirds live in areas with abundant resources, territorial lines are not well defined and promiscuity is often reported. The brown headed cowbird is a nest parasite that lays its eggs in other birds’ nests and provides no parental care for its young. Promiscuity may be the obvious choice given the birds’ behaviors and their lack of parental care.

Tuesday, July 14, 2009

Traumatic Insemination in aptly named 'Harpactea sadistica' spider

A violent but evolutionarily effective mating strategy has been spotted in spiders from Israel. Males of the aptly-named Harpactea sadistica species pierce the abdomen of females, fertilising their eggs directly in the ovaries. This has been described as a "traumatic insemination strategy," in which insemination wounds are created by male genitalia in areas outside the genital orifice of females. It is practiced because it gives the first male a reproductive advantage by bypassing structures in the females' genitalia.

These tactics have been observed in insects such as mites, bedbugs, and flies, but this study was the first time that it was documented in spiders. Typically, spider males deliver their genetic package via sperm that manually inserted using a pair of appendages called pedipalps.

The sperm are then held in a receptacle between the ovipore and ovary known as a spermatheca until an egg is released. However, the spermatheca is a "last in, first out" structure, so that if any further males inseminate a female, the last mate's sperm is the first in line to fertilize an egg.

Milan Rezic, an entomologist at the Crop Research Institute in Prague, has spotted a spider circumventing this problem by delivering sperm directly to the ovaries via holes that the males bore directly in the females' abdomens. The male possesses a pair of emboli, appendages modified for piercing females.

The way in which the male H. sadistica inseminates the female is choreographed and complex. The male taps the female, subdues her, and wraps himself around her to properly position the sex organs. He then alternates between the two organs, piercing and injecting the sperm on one side, then the other. The physical marks left are two neat rows of holes in her abdomen.

An analysis of the females of the species has shown that relative to other spiders, their spermathecae are atrophied, or shrunken. In an apparent case of co-evolution, they seem to be slowly shrinking into nonexistence now that their purpose is being bypassed by the males' more direct approach. This is yet another example of the co-evolutionary arm's race we see between many male and female species, in which males evolve to more efficiently inseminate the female and displace other males' seminal fluid, and females evolve to be able to control who inseminates her precious eggs.

Dr. Rezac foresees the race continuing. He suggests that a means to avoid the injury caused by the males might drive the evolution of secondary genitalia nearer to the ovaries, which have been observed in some spiders and butterflies.

Rezac, M. 2009. The spider Harpactea sadistica: co-evolution of traumatic insemination and complex female genital morphology in spiders. Proceedings of the Royal Society: Biological Sciences 276(1668):2697-701.