The researchers chose to examine the sperm of crickets, because, as with humans, you can get samples of it without having the male come into contact with a female first.
What’s the News: You might already know that sperm, which can survive for only a few hours when exposed to the outside world, can live for several days in women after ejaculation. But did you know that an ant queen can fertilize her eggs with sperm she’s stored for up to 30 years? And that organisms as diverse as birds, reptiles, and insects can hang onto sperm and keep it fresh for days, weeks, or months?
Scientists studying this ability have been trying to figure out how females do it, and in a recent paper, researchers put forth evidence showing that the ladies may be arresting the aging process, by slowing down sperms’ metabolism.
How the Heck:
- The researchers, who hail from the University of Tuebingen in Germany and University of Sheffield in the UK, decided to test one of the current models of cellular aging with sperm. This model proposes that the reason cells age is that as they go about their daily business of metabolism, they produce oxygen free radicals—oxygen ions that will react with whatever molecules are nearby and can destroy DNA, eventually causing the cell to self-destruct. The team wanted to see if sperm inside females (in this experiment, female crickets) had different rates of metabolism and free radical production than sperm that had just been ejaculated by the male.
Free radical production was lower in sperm within females
(white diamonds) than in freshly ejaculated sperm (black diamonds)
or in lab-stored sperm (grey diamonds).
- Using fluorescent markers often used in cancer studies, they measured both rates and found that sperm stored inside females for up to 26 days had lower production of free radicals and a slower metabolism than sperm outside. Production of free radicals was on average 42% lower and metabolic rate 37% lower than in freshly ejaculated sperm. They also measured the rates in sperm that they stored in the lab, from the same males, and found that their rates were the same as those of freshly ejaculated sperm, indicating that being inside the female is indeed what’s causing the effect.
- They also found that sperm’s metabolic rate while it was outside the female didn’t predict the metabolic rate it would have inside the female. That suggests that tests of sperm’s vitality might not reflect how they perform in real life.
What’s the Context:
- Whether this metabolic change is responsible for females’ ability to keep sperm around for so long isn’t actually shown by this study; establishing that being in a female changes this possible marker of cellular aging is just a first step in figuring out whether this is the mechanism. But it’s a plausible connection.
- Female crickets can keep sperm fresh for several weeks, previous observations have shown, which is about the length of time researchers left the sperm in females in this study. It would be interesting to confirm directly that eggs can still be fertilized by sperm that has experienced this drop in metabolism and free radical production.
The Future Holds:
- This is one of the first times researchers have gotten metabolism and free radical production rates from cells using this particular method, so they’d like to try similar tests with other species’ sperm, to establish what’s normal and how much variation occurs naturally.
- In future studies, they’ll work to get to the bottom of whether the metabolism drop is causing the free radical drop, and whether either is the cause of females’ sperm storage superpowers.
Reference: Anne-Cécile Ribou and Klaus Reinhardt, Reduced metabolic rate and oxygen radicals production in stored insect sperm, Proceedings of the Royal Society B, published 25 January 2012, doi:10.1098/rspb.2011.2422
Images courtesy of Eran Finkle / flickr and Proceedings of the Royal Society B