Sexual Conflict Flashcards
Overview of sexual conflict
- Conflict can occur when whatever is the optimal outcome for eth male is different than for the female
- It can lead to each sex evolving adaptations to bias the outcome of mating towards their own interest… which in turn selects for counter-adaptations in the other sex… an arms race
Sexual conflict can occur:
- Over mating itself: a male’s reproductive success is often more limited by access to mates than by a female’s reproductive success, so for a given encounter, it will often pay a male to mate but a female to resist
- After mating, during fertilization
Water striders
- Water striders skate over the surface of streams in ponds looking for food and mates
- Males pounce on top of females to secure matings by grasping her
- Superfluous matings are costly for females - once she’s mated, lugging a male around on her back increases predation and reduces her feeding success
- Females who have already mated are selected to avoid males
- In this arms race, males evolve grasping genitalia and females evolve abdominal spines
Water striders: matching adaptations
- Across species, we see correlated evolution between male morphology to
increase grasping (elongation of grasping genitalia) and female morphology
to resist (elongation of abdominal spines) - This is an example of how adaptations in one sex are matched by counteradaptations in another sex.
- Differences in opportunities for matings and the cost of the armaments may explain why different species are at different “equilibrium levels
Sexual conflict can go the other way - males guarding paternity
- Sometimes, it pays females to gain extra-pair matings, but pays her social mate to prevent her from doing so, to guard his parternity
- In many socially monogamous birds, the social male follows the female very closely when she is fertile and chases off other males
- Females attempt to sneak off to achieve extra-pair matings
- But in some species (e.g. seabirds), the male cannot always follow the female because one individual defends the nest site while the other forages
- In this case, social males engage in frequent copulations to ‘swamp’ the sperm of rivals
Sexual conflict after mating: examples of male adaptations
- Sperm removal
- Sperm displacement
- Copulatory plugs
- Anti-aphrodisiacs
- Accessory gland proteins
Sperm removal
- In many insects, females store sperm in sacs called spermathecae
- Male damselflies and dragonflies can remove sperm deposited by rival males before inserting their own
- They physically scrape out previous sperm and replace it with their own
- Other species use inflatable penises with horn-like appendages to pack sperm of previous males into the spermatheca to give their own sperm access to fertilization ducts
Sperm displacement
Males flush out inseminations from previous males
Copulatory plugs
Males cement up the female’s genital openings after copulation (and sometimes often males)
Anti-aphrodisiacs
Males deposit scents on females after mating to discourage other males from mating
Accessory gland proteins
-Male ejaculate contains not only sperm, but a cocktail of proteins that influence behavior and physiology
-In Drosophila, for example, some AGPs:
- Incapacitate rival male sperm
- Protect a male’s own sperm from enzymatic attack in the female’s reproductive tract
- Increase female’s egg laying rate
- Decrease female’s propensity to remate
-But these are costly to females: they decrease her longevity
Sexual conflict: female adaptations
- Sperm ejection
- Sperm choice
Who wins sexual conflict?
- Theoretical models suggest that the outcome is a never-ending evolutionary chase leading to rapid evolutionary change by both parties
- For example, male accessory gland proteins in Drosophila are estimated to
evolve at twice the rate of non-reproductive tract proteins - But: can male adaptations be kept in check by evolving female counter-adaptations?
Who wins sexual conflict? Example of Drosophila
- For 47 lab generations of Drosophila, they selected the most successful males and females and bred them
- In one line, there was intensesexual selection: each vial had three males and one female
- Strong selection for males who were successful at sperm competition and females who can cope with male-male competition
- In a second line, sexual selection was eliminated altogether: one male and one female spent their whole lives together
- In the absence of competition, this male is guaranteed paternity, and selection on HIM should be for traits that maximize his partner’s lifetime reproductive success
- Monogamous line males did evolve to be less harmful to females: decreased courtship and mating rate
- Females in turn evolved to be less resistant to harmful male traits, and they had longer lives
Who wins sexual conflict? An example from waterfowl
-Female vaginal pouches morphology included a number of ‘dead-end’ pouches
- Dead end size cavities
- Likely do not function in sperm storage
- Sperm deposited in the pouches would have a longer distance to travel to fertilize an ovum and may be more easily ejected by the female
Also included clockwise spirals
- Full 360-degree twists of the vagina that can be eliminated with the elongation of the oviduct
Interestingly, male phalluses also spiral in a counter-clockwise direction:
- Suggests that the spirals function as a anatomical barriers to exclude the phallus
- Suggests coevolution in which males with longer phallus and higher forced extra-pair copulations would select for females to have more elaborate vaginas
Evidence about waterfowl
-Coevolutionary analysis: phallus length is positively correlated with the frequency of forced extra-pair matings
- Suggests the phallus has evolved to enhance a male’s ability to force intromission
-However, female reproductive traits have coevolved with male morphology
- Phallus length also correlates with the number of pouches and spirals
- This likely reduces the change of male intromission without female cooperation
