Love is a Battlefield - and so is the Spermatheca
You heard me right - spermatheca. It even sounds kind of battlefield-y. If someone told me that the colosseum was originally called the spermatheca, I wouldn't give it a second thought. But what is it really?
The spermatheca is a part of the female insect reproductive system that can store sperm until the lucky lady insect decides she wants to fertilize her eggs. How convenient would that be?! Some bees and ants will have a few hours set aside once they reach sexual maturity where the only thing on their schedule is sex, then use all that stored sperm to produce a lifetime's worth of offspring over the span of years.
These are the dual spermathecae of Spelaeomyia mirabilis, a species of fly. Diptera (flies) can have anywhere from 0 to 4 spermatheca depending on their phylogenetic family.
While it's possible for insects not to have spermatheca, it's not common. More common is having multiple spermathecae, but we think that the default is having one. With multiple spermathecae, each organ is often different in size or shape, though we still don't know exactly why this is. Maybe to separate the Prince Charming sperm from the Prince Mediocre sperm? Sounds like too much work for me, but female insects have it down pat! Whether one or many, inside each spermatheca is a battlefield - sperm fight not only other sperm, but also their host, all in the name of reproduction. Or, in evolutionary terms, fitness.
Fitness - The ability of an individual to survive and reproduce
Sperm competition - Competition between the sperm of two or more different males to fertilize one egg
Sperm vs Sperm
We might think of sperm competition as a race to get to the egg(s), but scientists in the fields of animal behaviour and evolutionary biology know that it is bigger than just racing - since female insects store the sperm from multiple matings before deciding to fertilize, we see many other kinds of competition. Males will generally evolve traits that allow them to push any previous mate's sperm out of the spermatheca and also prevent their sperm from being pushed out by someone else.
Some insects have evolved to create larger sperm. Drosophila bifurca, for example, has huge sperm. Insanely huge. This fly's sperm can be up to 20 times the length of its body! What does this have to do with sperm competition? Easy - bigger sperm are harder to displace from the spermatheca, especially when they're up against smaller sperm. When another male's sperm comes along and says, "Beat it, buddy," this sperm just rolls its eyes. Since it is so energetically costly to make such large sperm, such insects might have a limited number to work with (or will have to spend a lot of time foraging in order to gain extra energy), but the evolution of this trait says that the pay-offs are well worth it.
This a microscopic photo of sperm leaking out of a dissected tsetse fly teste. While this fly's sperm isn't quite as incredibly large as the sperm of Drosophila bifurca, they are definitely more plentiful. Can you think of situations when having less, higher-quality sperm would be better than having many mediocre sperm?
Sperm isn't the whole ejaculate story, though: the other components of seminal fluid are also very important in sperm competition. Many male insects have an extra part to their reproductive tract - the accessory gland. Accessory glands add different chemicals to the seminal fluid that can help the sperm to combat rival sperm they encounter in the spermatheca. Some accessory glands produce chemicals that can even kill or weaken rival sperm. We don't see two sperm fighting when we look in through a microscope, but chemical warfare is just as exciting.
Penis vs Sperm
You best believe it - in some species, the penis joins the fight against the rival sperm. You might ask, "Isn't that like a single person fighting Godzilla?" My answer would be yes. In dragonflies and some other insects, the penis is designed to scoop out rival sperm or scrub the female's reproductive tract clean. By removing rival sperm, the dragonfly increases the chances of its sperm being used for fertilization of the female's eggs.
One species of dragonfly, Callosobruchus maculatus, wandered off the beaten path a bit with its penis adaptations. Instead of working as a scoop or a scrub brush, it inflates inside the female, the narrow end entering into the spermatheca and pushing any sperm already inside to the back. Since the female reproductive system operates on a "last in, first out" principle, the sperm closest to the opening of the spermatheca is most likely to fertilize the eggs.
You might not suspect it at first, but male dragonflies and damselflies often have very abrasive tactics in bed. Pun intended.
Contrary to the "last in, first out" principle, we see some weird techniques in cricket and grasshopper species. They have developed genitalia (and, ermm, techniques) that allow them to spray their ejaculate to the back of the spermatheca. This is suggested as a mechanism for "flushing out" rival sperm, but, as of right now, this flushing technique is only a proposed mechanism and further research is needed to explain exactly what kind of advantage it gives.
Sperm* vs Host
In some cases, a male's ejaculate will not only harm rival sperm, but also the female he's trying to inseminate. It's not the sperm itself that does this (which is why it's not really sperm vs host), but the chemicals released from the accessory gland we talked about earlier. These chemicals have been shown to shorten the lifespan of fruit flies and also dissuade them from re-mating. In some cases, they can speed up the fertilization and egg-laying process so that the female is more likely to fertilize her eggs with the sperm that has just entered. With all this, you'd think that insects, at least the females, would try to keep sex to a minimum, but even in species where we know ejaculate is damaging, we still see multiple matings! No, it's not just that the sex is good - this suggests that, despite everything, the pros still outweigh the cons. Right now, we think that multiple matings must convey some genetic advantage to the female's offspring.
Mid-summer, goldenrod is covered with these soldier beetles. They might be foraging, but it's almost more likely that they're getting it on like the individuals in this photo. As with many other insects, males and females both have multiple partners. Inside this female's spermatheca is a war raging on.
Allie Lavalle - Coleoptera Cantharidae - Discover Ichneumonidae
As you can see, sperm are truly ruthless. Starting to have nightmares about war-like, soul-sucking sex? That's not where it ends for insects. Thankfully, there's a lot that the females can do to sway these battles. That, however, is a topic for another day.
If you'd like to learn more about sperm competition in insects, I would strongly suggest the following paper - it has some great summaries of the relevant literature and is a fantastic kick-off point for further research. Heads-up it covers a lot of information that I didn't have space to cover in this post. If you find it too dry, don't worry - I'll make another post covering more sperm competition and sexual conflict soon!
Danielsson, I. (1998). Mechanisms of Sperm Competition in Insects. Annales Zoologici Fenniciis 35:241-257