CMPS272, Winter 2007, Section 01: SexualConflict

Banana slugs are simultaneous hermaphrodites, meaning that they are both male and female at the same time. when they mate, they both insert penises at the same time (forming a yin-yang orientation) and presumably- both are donating and receiving sperm simultaneously. i.e. they are playing both mating roles when mating.

occasionally, after mating, one will chew the penis off the mating partner... when penis chewing occurs, it is often reciprocal where the partner will also chew the penis off. the penis does not grow back. this is obviously a bizarre mating behavior. my thesis examines this mating behavior as an evolved outcome due to sperm competition... a little history...

sperm competition leads to sexual conflict:

sperm competition is when the ejaculates from multiple males (easier to think male and female even if they are hermaphrodites) compete for fertilization of eggs. in many organisims, slugs included, animals have sperm storage organs and can collect sperm from multiple partners and then use that sperm later to fertilize eggs. mating and fertilization and egg laying can occur independently with respect to time. what that means is that a female can mate with multiple partners and those partners all compete via their sperm to fertilize her eggs.

sexual conflict... from the perspective of the female, sperm competition is a good thing, as she can assure fertility and increase the genetic variability of her progeny. from the perspective of each males that she mates with though- her mating with additional males is a bad thing since that reduces his probability of siring offspring. therefore there is a conflict- after mating, the female has incentive to remate, but from the perspective of her current mate, this is bad.

therefore, selection acts on males to favor males that can prevent females from remating... if he can prevent her from remating, he has higher fitness. this often leads to the evolution of unusual mating behaviors... like the penis chewing slugs.

my hypothesis is that the slugs are trying to manipulate their mating partner. say i am a slug and mate with another slug. i have given that mating partner my precious sperm and do not want them to mate again. i think, "humm... if i chew off their penis, then they won't be able to remate" (this is due to a different hypothesis about simultaneous hermaphrodites being unwilling to donate sperm to a partner that won't reciprocate: for now, accept as a given that a hermaphrodite doesn't want to mate with a slug that does not have a penis). so, me deciding to to chew their penis benefits me as i prevent them from remating and prevent sperm competition. now, as i decide to chew their penis, they have another decision to make- should they recirpocate or not? if they reciprocate, then they prevent sperm compeition in ME... so that would be good, right? they will probably ALWAYS TRY to reciprocate, but might not always be able to. for me, there is a decision to make: should i chew and get the benefit of increasing my fertilizations, BUT there is a cost of the risk of me losing my penis.

that is my first model... some factors to include... how much does me losing a penis cost me? if i can't mate again, that is bad. but what if i am an old slug that is unlikely to mate again anyway? what if i have been a whoring slug who has already mated with so many partners that it does not benefit me much to remate? etc.

i have a secondary model to develop as well.... if you have a population of penis chewing slugs, you would predict a stable frequency of slugs that have had their penises chewed off... can you have the invasion of a pure male strategy? if a mutation occured and a slug was hatched that was a male and not a hermaphrodite... would that strategy invade? the male would be able to donate sperm to the penis-less slugs, but would not be able to lay their own eggs. i have reason to believe that this MIGHT exist in nature.

at any rate- i have the verbal arguments worked out and have been thinking about this problem for several years now. i just need some people with the analytical skills to help me develop this into a full simulation. let me know if you are interested!

-- BrookeMiller? - 16 Jan 2007