Joanne Godwin asks if having vast numbers of tiny sperm is always the best strategy for males?
Check out the full paper here.
How to win at sperm competition
When a female mates with more than one male, the sperm of these rivals will be in competition for fertilisation of the ova – sperm competition1, and the female may be able to exert choice over which sperm fertilise her ova – cryptic female choice2.
Theory predicts that the male with the highest number of sperm will win sperm competition over rivals, the ‘raffle principle’3. However, resources for reproduction are limited, so there is expected to be a trade-off between number and size, therefore, sperm competition is theorised as the driving force behind the evolution of numerous but tiny sperm in males3.
But what if there are other ways of winning sperm competition? What if sperm competition is about more than just numbers?4 What other aspects of sperm form and function might give an advantage in sperm competition?
Imagine a rugby game between England and Scotland. Each team starts with 15 players
but at half time one of the English players is sent off. Who do you predict will win the match? Probably Scotland because they have the advantage of more players. However, what if the English players run faster and score more tries? Or, what if the England team are bigger and stronger and win the ball in the scrum? England could win despite having fewer players!
Despite the theory that males maximise number of sperm at the expense of size, there are notable exceptions to the rule, like the 6cm long sperm of Drosophila bifurca5, a fruit fly that is only a few mm in length. The image on the right shows one of these giant sperm cells wound into a bundle. How has this evolved? What is the benefit of increased sperm size? Is there a cost for males? Is sperm competition success determined by more than just numbers?
Can sperm competition drive the evolution of longer sperm?
How did we test this?
Our lab group uses the red flour beetle, Tribolium castaneum, to ask questions about evolution. Both males and females are highly promiscuous, and like many other insects, females store sperm, so they are a good species for investigating sperm competition.
For this question, we made use of populations in which we manipulate the sex ratio every generation (over 77 generations for this study), to increase or decrease the intensity of sperm competition. In Female-biased populations (10 males and 90 females) sperm competition is low/relaxed because males can mate with multiple females and there is little risk that she will mate with another rival male. In contrast, in Male-biased populations (90 males and 10 females) the number of females is severely limited so females will often mate with multiple males and sperm competition will be intense.
What did we find?
First, we wanted to provide evidence of the contrasting levels of sperm competition in Male-biased and Female-biased populations. We tested sperm competitive ability of males from both sperm competition backgrounds against a rival with distinctive antennae known as ‘Reindeer’ which allowed us to assign offspring paternity to either male. Males from Male-biased populations gained 65% share of paternity in competition with a Reindeer rival, compared to 45% for males from Female-biased populations (Figure A).
Do populations with contrasting intensities of sperm competition show differences in sperm length? Yes!
Figure B shows that males from Male-biased populations (red – intense sperm competition) have significantly longer sperm than males from Female-biased populations (blue – weak sperm competition). In addition, when we compared these results with the length of sperm in males from the ancestral population (dashed line) it suggests that not only has sperm length increased under intense sperm competition in Male-biased populations, but also decreased under low competition in Female-biased populations.
Sperm production is often considered to be energetically ‘cheap’ compared to eggs. However, the decrease in sperm length in males from Female-biased populations points towards a cost involved in producing longer sperm as it suggests that if males don’t need to produce long sperm, they don’t. We backed this up by showing that where males experience restricted protein in their diet sperm are significantly shorter in length compared to males on a control diet (Figure C).
These results are exciting because they suggest sperm competition is more than just a numbers game, but also shapes qualitative aspects of sperm form and function!
So, what is the advantage of increased sperm length?
Longer sperm might i) swim faster and win the race for fertilisation, or ii) block the female reproductive tract and prevent rival sperm displacing them, or iii) indicate that a male is high quality if sperm length depends on resources/condition. For now, the mechanism remains unknown, and is likely to differ across taxa depending on whether fertilisation is internal or external, and the extent of sperm storage by females, which will both alter the intensity of sperm competition.
Joanne Godwin is an Evolutionary biologist interested in understanding how competition between rivals for access to a mate, and choice of mate, (sexual selection) causes evolution in individuals and populations. For more information about this work Joanne can be contacted at firstname.lastname@example.org
1) Parker, G.A. (1970) Sperm competition and its evolutionary consequences in the insects. Biological Reviews. 45 (4), pp. 525–567.
2) Eberhard, W.G. (1996) Female Control: Sexual Selection by Cryptic Female Choice. Princeton University Press.
3) Parker, G.A. (1982). Why are there so many tiny sperm? Sperm competition and the maintenance of two sexes. J. Theor. Biol. 96, 281–294.
4) Snook, R.R. (2005). Sperm in competition: not playing by the numbers. Trends Ecol. Evol. 20, 46–53.
5) Pitnick, S., Spicer, G.S. & Markow, T.A. (1995). How long is a giant sperm? Nature 375, 109–109.