Behavioral Ecology Blog

More recently, researchers have started to recognize that while sperm production is cheap relative to eggs, it’s production is by no means trivial. This adds complexity to the otherwise simple ejaculatory rule. While the current paper by Leif Engqvist in a forthcoming issue of Evolution hints at this complexity, in reality I suspect that the decision is much more complicated.

ABSTRACT: Theory predicts that when sperm compete numerically, selection will favor males who vary the number of sperm they transfer with the immediate level of sperm competition. In this study, I measured male mating investment in response to both female mating status (virgin vs. mated) and the number of foreign sperm stored by females in a previous mating in the scorpionfly Panorpa cognata. Female sperm storage was manipulated by interrupting copulations at different time points. Female mating status did not significantly influence male mating investment, but resource-limited males invested strategically in relation to the amount of sperm stored by females in a previous mating. I found continuously decreasing male investment in response to increasing amounts of competing sperm. These results demonstrate an unprecedented male ability to assess the number of sperm stored by females. As a result, males are capable of an extraordinarily fine-tuned reaction to the intensity of sperm competition.

Now I’m not so interested in getting into the nuts and bolts of this paper, but basically what happens is as the amount of sperm contained in focal females spermatheca increases, ejaculatory investment decreases. This is modeled here as a linear function.

This linear model is exactly what I think is suspect… Now suppose you were are a male, and the lucky recipient of sloppy 13th’s. Now there is a heap of sperm stored in there- no way you can numerically compete right… What’s your strategy? Well you’re not gonna turn her away, but you’re not gonna invest a lot either… Now take that female that has just one mating under her belt- you have a much better chance of fertilization here- and as such you should probably invest a whole lot- especially if females are rare.

Aside from this issue of linearity- there are other issues.

1. Male condition- The authors suggest that males in poor condition (nutritionally limited in their case) should be more selective. Their diminishing returns curve is steeper than males in good condition. ON THE OTHER HAND- poor quality males are not likely to be selected by choosy females, so their 1st mating might reasonably be their last- this argument might suggest that poor quality males might be less selective and drop their whole load in the 1st available females. Opposite predictions, hmmm… I wonder….
2. Female availability- all bets are off in terms of varying the amt. of sperm per ejaculate when matings are rare. In this case, males should bet the house and go all in…
3. Social relationship/mating system- what if the female in question is your pre-existing mate. You might have more to lose, and thus invest more sperm, in this case, regardless of the amount of sperm in her spermatheca.

So anyway, I think that the recognition of “conscious” ejaculation is interesting, and a great start, but I predict that in the coming years scientists will show that the “go for the gusto” ejaculation strategy is much more like “go for the gusto whan females are rare, it’s your woman, female has few matings etc, but scale back sperm investment after that”

One might think that cognition/decision making in insects and other animals isn’t complex enough to allow for such decisions, but we’ve been surprised in the past, huh.