Salmon speed up sperm

When animals compete for mates it’s usually the largest, strongest specimens that are successful and drive off the competition. But the underdog may have a hidden hand to play. Working on Chinook Salmon, Michael Bartlett and Patrice Rosengrave have found that less dominant males can compensate for being lower down the social pecking order by adding something to their seminal fluid that dramatically accelerates their sperm and boosts their chances of reproductive success…

Michael - We know from previous research that when males compete in order to be successful in reproduction and have offspring they are able to make these really rapid adjustments to their ejaculates in terms of the number of sperm they might produce or how fast those sperm are able to swim. What we don't know is how they're able to make these adjustments particularly over such rapid time frames so faster than they're able to make new sperm for example.

Chris - That sounds extraordinary that the animals can actually change the behaviour of their sperm in response to the social milieu. So Patrice, How did you actually pursue this because this is not, this is pretty tricky to try and get to the bottom of whats going on here I would think.

Patrice - So we wanted to do was we manipulated the social status of the fish. We caught the fish that were returning upstream to spawn. We paired up the fish of equal size and we placed them in this confined Raceway. So then we wanted to observe the behaviour of the fish so we got a GoPro camera and we were able to make behavioral observations to determine which fish was the most dominant. We then placed the dominant male with the dominant male, and then the subdominant male with the subdominant male we measured their spare motility and we counted how many sperm cells they had.

Chris - Michael, when you do this, is this reflected in the sperm that you recover from the animals? Does that change in the way you would predict based on these observations showing that the social cues are affecting the behaviour of the sperm?

Michael - Yes when a male is dominant in status and then changes to subdominant status so the risk that sperm competition is higher, we see an increase in sperm velocity in those males.

Chris - And Patrice, have you got any idea as to how they're doing this because the changes are so rapid that they're not just making new super sperm de novo, they're physically changing the behaviour of their existing sperm aren't they.

Patrice - Yeah exactly. So what we think is there's some component of the seminal fluid that is upregulating sperm function. That's what we now need to explore what is in the seminal fluid, is it a protein, is it an enzyme that interacts to manipulate sperm function.

Chris - Could you get at that Michael, by say taking sperm from a dominant male, filtering out the cells so you have just the seminal fluid and adding that to the sperm of a less dominant animal and seeing if he gets a fertility boost or vice versa, accordingly.

Michael - Yes and so that was kind of the next stage in our experiment. We took sperm cells from dominant males and subdominant males and we recombine them in the seminal fluid of males of the other status. And what we saw was that when you put sperm cells from a dominant male into the seminal fluid of a subdominant male you see an increase in sperm velocity on average and the opposite effect when you take sperm from subdominant male which generally has faster sperm, We see that you get a decrease in sperm velocity when they're incubated in the seminal fluid of a dominant male.

Chris - One would infer then Patrice that there's something which is secreted into the seminal fluid in response to the fish's perception of how threatened or how dominant it is or isn't. And that is having this enlivening effect on the sperm. Is it reflected in fertilisation success though, if you take that enlivened, accelerated sperm does it have a greater prospect of fertilising eggs.

Patrice - Yeah that's exactly right. So we found that the subdominant males that had seminal fluid that sped up sperm, we found that if we added that to the dominant male sperm it also increased their fertilisation success so yeah you're exactly correct that there was a fitness effect.

Chris - So what do you think the implications of this are?

Michael - I would say that the seminal fluid the idea that it's implicated in this process has been floated for some time but we've lacked, kind of, experimental evidence showing that that's the case. So what we've done here is been able to provide evidence that really shows that seminal fluid is involved. Now what we really need are investigations looking at seminal fluid composition and then some kind of experimental work that links the component of seminal fluid to sperm function.

Chris - Patrice is there any effect on the female as well because is it just the sperm cells that respond to this admixture of whatever goes into the seminal fluid or is there the possibility that when the sperm actually come to do the job of doing the fertilisation they are enhanced beyond just what the sperm can do.

Patrice - Right. It's a really good question. We're not quite sure yet. So the eggs are the female have this fluid also around them called ovarian fluid. We're not quite sure yet how seminal fluid might interact with that fluid around the eggs. But there is a possibility that there could be an interaction between those two fluids and that could certainly influence sperm behaviour and potentially competitiveness between the sperm of different males.