Why should I care?

In our previous discussion, we ended by wondering if the female brown trout was fated to suffer egg cannibalism by her potential sexual partners.

Salmonids are usually known for their lack of paternal care, and several explanations are usually coined to back up this idea:

1. First, sexual selection is expected to be strong, and some think that a good male is thereof better inspired to go and seek a new female rather than to care for the eggs.
2. Second, we saw that multiple mating and thus paternity uncertainty happens: it could be costly to provide care for other people's eggs.
3. Third: survival can sometimes be very high up to emergence, and it would not be so beneficial for the male to try to improve it any more.

In fine, here is what looks like an average father in salmonids :

Well, argument 2 certainly holds in this specific case, argument 1 is possibly polemic, and argument 3 is definitely wrong given the impact of cannibalism on redds.

Assume competition is relatively strong, but hey, you are here to win, assume you spent a lot of time guarding the female during the redd preparation, assume you managed to fertilize her. These assumptions represent a good 75% of the situations observed. And then, suddenly, your competitors steal your progeny, and munch your fitness in less than two minutes. And this damned female you selected, for whom you chased other males to guard her, does nearly nothing to prevent them to do so. But how could she, when they are so big and numerous compared to her, and when she already has to cover the redd with gravel?

Of course, as explained by Hanna Koko, this is not what you invested that counts, but what is your horizon, your future opportunities: the balance between costs and benefits. But here, if a male managed to protect his progeny for a few minutes, he could greatly improve his fitness.

So we went back to our observations, back in the river. We counted the number of chases after fertilization from the fertilizing male on other males. We evaluated if this potential effort to protect eggs was dependent on the level of competition around. 

The above graph summarizes hours and days of patient fish watching in the rivers of the Basque country. It relates the number of post-fertilization chases to the level of competition (operational sex ratio). We can observe that the number of chases increases initially, then decreases when the competition is too intense. Could it be an evidence of a cost/benefit balance?

Well, let's look at the benefits side. Through a statistical model, we could predict the probability to undergo egg cannibalism as a function of the number of chases. The solid line is for classical situation, where the dominant male fertilized the eggs. The dashed line shows a situation where a take-over in the dominance happened (and therefore paternity is not ensured). As we can see, in the classical situation, not only the cannibalism is lower, but it also decreases with the number of chases made by the male.

 

It does not seem very costly: it all happens in a few minutes, and it does not require a large number of chases. And it can make a huge difference between a zero reproductive success, and a happy and large family.

 

 Well, OK, these are salmon, but you know what I mean!

References

Tentelier C, Larrieu M, Aymes JC, Labonne J. 2011. Male antagonistic behaviour after spawning suggests paternal care in brown trout, Salmo trutta. Ecology of Freshwater Fish 20:580-587.