Image via Wikipedia

When males are competing with each other for fertilization of a female’s eggs, which counts more: the number of sperm, or the size of each sperm? You might wonder why a guy can’t have it all but, alas, nature does make restrictions on how much energy one can devote to pumping out sperm. So which is best, tons of little sperm or relatively few giant sperm?

The answer, like so many things in biology, is: it depends. According to research headed by Simone Immler together with Tim Birkhead at the University of Sheffield and Scott Pitnick at Syracuse University, the way sperm compete within the female determines whether strength lies in size or numbers. (See paper here).

Mating birds. Image via Flickr.

The researchers compared sperm competition in two different groups of animals: passerine birds and Drosophila fruit flies. Sperm size was compared for 196 species of birds (sperm number could only be measured for 23 species). They compared sperm size in 18 fly species and sperm number in 15 of these species. What they found: higher risk of sperm competition correlated with more sperm in birds and larger sperm in flies.

The birds compete by a “fair raffle” system. Sperm is chosen at random to fertilize an egg, so if sperm from two males is mixed together in the female’s storage organ, the male with the most sperm has the best chance of winning. So, in this case, size doesn’t matter nearly as much as how many sperm the male can make.

In fruit flies, the story is different. When a female fly mates to a second male, his sperm will displace sperm from the previous male that were stored in the female. The sperm line up at the entrance of the storage organ in fruit fly females, waiting to fertilize an egg. In flies, bigger sperm have a better chance of pushing small sperm out of the way at this prime location. So, for flies, size matters.

A single Drosophila bifurca sperm!

Because the flies with the highest risk of sperm competition make extremely huge sperm, they can only make very few of them. In fact, the largest sperm of any animal (and this is not relative to body size) comes from a fruit fly called Drosophila bifurca. This fly is only a few millimeters long, but its sperm is more than 5 centimeters long! The sperm are so big, they can’t swim. Instead, they are bunched up into little balls that have to be pushed up to the storage organs by the female reproductive tract. Drosophila bifurca males can only make about 100 sperm in their entire lives–compare that to the millions of sperm that human males put into each ejaculate!

Related articles

• Love is (sometimes) a battlefield (nittygrittyscience.com)
• Largest known sperm create paradox of nature (livescience.com)
• Flies alter their ejaculate to get the best bang for the buck (science3point0.com)
• When it comes to sperm competition, size can matter–it’s the female who holds the aces (light-science.com)
• Glow-in-the-dark sperm shed light on sexual selection (nationalgeographic.com)