Animal Sperm
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Animal Sperm
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Please be respectful of copyright. Unauthorized use is prohibited.
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Guppies can store sperm for later use.
Guppies can store sperm for later use.
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The era of greyhound racing in the U.S. is coming to an end
See how people have imagined life on Mars through history
See how NASA’s new Mars rover will explore the red planet
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See how people have imagined life on Mars through history
See how NASA’s new Mars rover will explore the red planet
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Female guppies sometimes bring a little extra baggage to a relationship—in the form of a dead male’s sperm.
Female guppies ( Poecilia reticulata ) sometimes bring a little extra baggage to a relationship—in the form of a dead male’s sperm.
A new study shows that one in four wild guppies in Trinidad and Tobago are fathered by the sperm of dead males. Some of these sperm have been stored for long enough that these males could otherwise be the grandfathers of these fish -to-be.
After mating, female guppies store sperm in a specialized cavity in their ovaries, keeping it alive with small amounts of sugar until the eggs are fertilized. (Also see “Why Female Flies Eat Sperm.” )
Why store sperm? Female guppies outlive males by at least a year, so storing sperm from multiple mating partners gives the female a choice of the best genes to sire her offspring.
Since these fish fathers don’t provide any care for their offspring even when living (for that matter, mama guppies don’t tend to their offspring after birth, either), being dead doesn’t harm the fish fry after birth, according to the study, published June 5 in the Proceedings of the Royal Society B .
Yet study leader Andrés López-Sepulcre , of the École Normale Supérieure in France, and colleagues don’t know whether the guppies’ sperm quality deteriorates over time—though the stored sperm can clearly fertilize at least some eggs.
Nor do López-Sepulcre and colleagues understand why females continue to use sperm from dead males when plenty of live males are around and able to provide fresher semen. (See “Sperm Recognize ‘Brothers,’ Team Up for Speed.” )
Sperm storage is a common occurrence in the natural world. Many other animals have been found to store sperm, including the domestic chicken ( Gallus gallus domesticus ).
In 2011, researchers at North Carolina and Georgia State Universities found that the eastern diamondback rattlesnake ( Crotalus adamanteus ) stored sperm for an exceptionally long period of time.
The discovery was made when a rattlesnake captured from the wild in Florida in 2005 gave birth in 2010, despite the fact it’d been kept away from other snakes. Scientists initially thought it was an example of virgin birth—a form of asexual reproduction in which the female provides both sets of chromosomes to her offspring— until genetic analysis revealed two parents . ( Learn more about virgin birth .)
So how do the females tell the sperm that it’s time to do their job? By studying the common octopus ( Octopus vulgaris ), biologist Anna DiCosmo at Italy’s University of Napoli Federico II and colleagues tested a hypothesis that females emit chemicals called chemoattractants.
Males of various species can sense and seek out these chemicals—just as hordes of tired office workers shuffle toward the smell of the coffeepot every morning.
DiCosmo knew that when females of other species related to octopi release their eggs into the water, males are attracted by chemoattractants in the eggs. Following the chemical trail, the males swim over and release their sperm. (Also see “Small Squid Have Bigger Sperm—And Their Own Sex Position.” )
Although the common octopus fertilizes its eggs internally, DiCosmo wondered whether this species also used chemoattractants to rouse the stored sperm out of hibernation.
So the researchers gathered some mature octopus eggs and separated out proteins in the eggs. One particular protein, named Octo-SAP (octopus sperm attractant peptide), made the sperm go a-swimming. They reliably swam toward higher concentrations of Octo-SAP, a key requirement for a chemoattractant—suggesting that octopi use the chemicals to rouse sperm, according to a study published June 15 in the Journal of Experimental Biology .
DiCosmo has some hypotheses about why sperm storage evolved in the octopus. ( See video: shark vs. octopus .)
One is the mismatch between the time it takes for an egg to mature versus the rapidity with which a male can produce sperm. For instance, a female might not have a mature egg ready when she meets a male—which doesn’t appear to happen very often.
“Given that octopi are solitary animals, they do not have many chances to meet partners. For this reason, storing sperm is one of the only good strategies to optimize fitness. Immediate or external fertilization would result in no or just a few successful matings,” DiCosmo told National Geographic.
As for humans, we can reassure ourselves that at least we’re not hoarding sperm. Although it might be worth checking behind the ice cream carton in the freezer, just to be sure.
Copyright © 1996-2015 National Geographic Society Copyright © 2015-2022 National Geographic Partners, LLC. All rights reserved
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All sperm perform the same basic job: They fertilize egg cells. But in a new study, researchers have figured out that size matters, and it's largely the female that pushes sperm to be big or small.
Sperm cells come in a huge variety of sizes. For instance, the parasitoid wasp Cotesia congregata produces little swimmers that are less than one-thousandth of a centimeter long, while fruit flies make sperm with 2.3-inch (6 cm) tails that coil tightly to fit inside their tiny bodies.
In the new study, the researchers set out to determine how sperm size varies among species and what might be driving the differences.
"We have all these studies that show evidence of natural selection pushing sperm size in various species to be either bigger or smaller, but we wanted to take more of a zoomed-out view and look for trends across species," said lead author Ariel Kahrl, a postdoctoral researcher in evolutionary biology at Stockholm University.
Kahrl and her colleagues examined data from 3,200 species and discovered a governing principle that determines sperm size in a species: Females with small reproductive tracts drive the production of bigger sperm, and the need to spread sperm far and wide shrinks sperm across evolutionary timescales.
Here's why. For the most part, animals use two modes of sexual reproduction. One group — which includes mammals, insects and birds — are internal fertilizers that carry eggs inside their bodies. External fertilizers, by contrast, eject their eggs into the environment and hope for the best. Commonly, these species live in water, like fish and sea urchins. In both modes, tons of sperm are competing in a battle royal for the prize of fertilizing the egg, but the challenges of each mode exert incredible evolutionary pressure on sperm size.
"We found that external fertilizers tend to have really small sperm because they have to make a ton of it to reach the eggs," Kahrl said. External fertilization requires ejecting a cloud of sperm, typically into water. As the sperm spread, they become diluted, so the best strategy would be to produce as many sperm as possible to maximize the chance that at least one will reach an egg. Because an animal has a limited amount of energy to use for making sperm he can't afford to make them any bigger than they absolutely have to be.
It's a completely different situation for the internal fertilizers. "We think that for internal fertilizers, the female's reproductive tract influences the way sperm fight each other," said study co-author John Fitzpatrick, an assistant professor of biology who is also at Stockholm University. In internal fertilization, the sperm work in a tight space, so reproduction becomes less of a treasure hunt and more like a game of king of the hill. In this situation, bigger may be better for shoving other sperm out of the way, regardless of whether they came from the same father or different potential fathers.
"Some of these species make huge sperm, and if you're making enormous sperm, you don't make that many of them," Kahrl said. "These males coil up their sperm like a ball of yarn and pass it along."
In addition to internal and external fertilizers, the researchers examined a rarer third reproductive mode, called spermcasting. Spermcasting is like a combination of internal and external fertilization; for example, a river mussel might eject sperm into a stream, and that sperm would ride the currents until it is picked up by a stationary, filter-feeding female.
"With spermcasting, you have this dilution effect because the sperm are ejected into the water, but when the sperm enter the female, they evolve rapidly under the same types of pressures that we see in internal fertilizers," Fitzpatrick told Live Science. The spermcasters, though, have smaller swimmers, similar in size to the sperm of external fertilizers, likely because ejecting sperm into water incentivizes making more of them, forcing them to be small. But once those sperm are taken up by the female, the biggest sperm tend to win.
Despite being internal fertilizers, humans don't have monster sperm. Instead, human sperm measures a modest 0.002 inches (0.005 cm) long, well within the range seen in external fertilizers. That's because animals with bigger bodies have reproductive tracts that allow the sperm to spread out similarly to the way external fertilizers' sperm do.
The smaller the reproductive tract, the larger the sperm. And for a fruit fly, it's as cramped as it gets. "Fruit fly sperm is 20 times the length of the animal's body," Kahrl said.
The researchers published their findings 21 June in the journal Nature Ecology & Evolution (opens in new tab) .
Originally published on Live Science.
Cameron Duke is a contributing writer for Live Science who mainly covers life sciences. He also writes for New Scientist as well as MinuteEarth and Discovery's Curiosity Daily Podcast. He holds a master's degree in animal behavior from Western Carolina University and is an adjunct instructor at the University of Northern Colorado, teaching biology.
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All sperm perform the same basic job: They fertilize egg cells. But in a new study, researchers have figured out that size matters, and it's largely the female that pushes sperm to be big or small.
Sperm cells come in a huge variety of sizes. For instance, the parasitoid wasp Cotesia congregata produces little swimmers that are less than one-thousandth of a centimeter long, while fruit flies make sperm with 2.3-inch (6 cm) tails that coil tightly to fit inside their tiny bodies.
In the new study, the researchers set out to determine how sperm size varies among species and what might be driving the differences.
"We have all these studies that show evidence of natural selection pushing sperm size in various species to be either bigger or smaller, but we wanted to take more of a zoomed-out view and look for trends across species," said lead author Ariel Kahrl, a postdoctoral researcher in evolutionary biology at Stockholm University.
Kahrl and her colleagues examined data from 3,200 species and discovered a governing principle that determines sperm size in a species: Females with small reproductive tracts drive the production of bigger sperm, and the need to spread sperm far and wide shrinks sperm across evolutionary timescales.
Here's why. For the most part, animals use two modes of sexual reproduction. One group — which includes mammals, insects and birds — are internal fertilizers that carry eggs inside their bodies. External fertilizers, by contrast, eject their eggs into the environment and hope for the best. Commonly, these species live in water, like fish and sea urchins. In both modes, tons of sperm are competing in a battle royal for the prize of fertilizing the egg, but the challenges of each mode exert incredible evolutionary pressure on sperm size.
"We found that external fertilizers tend to have really small sperm because they have to make a ton of it to reach the eggs," Kahrl said. External fertilization requires ejecting a cloud of sperm, typically into water. As the sperm spread, they become diluted, so the best strategy would be to produce as many sperm as possible to maximize the chance that at least one will reach an egg. Because an animal has a limited amount of energy to use for making sperm he can't afford to make them any bigger than they absolutely have to be.
It's a completely different situation for the internal fertilizers. "We think that for internal fertilizers, the female's reproductive tract influences the way sperm fight each other," said study co-author John Fitzpatrick, an assistant professor of biology who is also at Stockholm University. In internal fertilization, the sperm work in a tight space, so reproduction becomes less of a treasure hunt and more like a game of king of the hill. In this situation, bigger may be better for shoving other sperm out of the way, regardless of whether they came from the same father or different potential fathers.
"Some of these species make huge sperm, and if you're making enormous sperm, you don't make that many of them," Kahrl said. "These males coil up their sperm like a ball of yarn and pass it along."
In addition to internal and external fertilizers, the researchers examined a rarer third reproductive mode, called spermcasting. Spermcasting is like a combination of internal and external fertilization; for example, a river mussel might eject sperm into a stream, and that sperm would ride the currents until it is picked up by a stationary, filter-feeding female.
"With spermcasting, you have this dilution effect because the sperm are ejected into the water, but when the sperm enter the female, they evolve rapidly under the same types of pressures that we see in internal fertilizers," Fitzpatrick told Live Science. The spermcasters, though, have smaller swimmers, similar in size to the sperm of external fertilizers, likely because ejecting sperm into water incentivizes making more of them, forcing them to be small. But once those sperm are taken up by the female, the biggest sperm tend to win.
Despite being internal fertilizers, humans don't have monster sperm. Instead, human sperm measures a modest 0.002 inches (0.005 cm) long, well within the range seen in external fertilizers. That's because animals with bigger bodies have reproductive tracts that allow the sperm to spread out similarly to the way external fertilizers' sperm do.
The smaller the reproductive tract, the larger the sperm. And for a fruit fly, it's as cramped as it gets. "Fruit fly sperm is 20 times the length of the animal's body," Kahrl said.
The researchers published their findings 21 June in the journal Nature Ecology & Evolution (opens in new tab) .
Originally published on Live Science.
Cameron Duke is a contributing writer for Live Science who mainly covers life sciences. He also writes for New Scientist as well as MinuteEarth and Discovery's Curiosity Daily Podcast. He holds a master's degree in animal behavior from Western Carolina University and is an adjunct instructor at the University of Northern Colorado, teaching biology.
Stay up to date on the latest science news by signing up for our Essentials ne
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