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Sharks, rays, and skates—all fish in the subclass Elasmobranchii— are a beautifully diverse collection of animals. One big way they differ is in how they reproduce. They lay eggs, like traditional fish, and let them mature in a select corner of the ocean. Or, they might let the eggs hatch inside their bodies. But they can also give live birth to pups gestated like mammals: with an umbilical cord and a placenta in a uterus.
These fish, like many other members of the animal kingdom, have two uteruses.
Females are capable of reproducing asexually, without help from a male. As genetic sequencing has advanced, researchers have been finding another curious pattern: Many litters of pups will have more than one father, a phenomenon known as multiple paternity. The researchers also write that a male-dominated field may be more likely to miss a female-driven reproductive strategy, and push for more study of female reproductive biology. John Dankosky talks to the lead author on the research, Georgia Aquarium shark biologist Kady Lyons, about the vast wonderland of reproductive strategies in this fish subclass—and what a history of male-centered research may have missed.
But we are going to Woman looking sex Lyons Georgia you about some things you might not know about baby sharks and their relatives, baby rays. Pretty cute, right? And it often takes a long time to grow them. Most female sharks are pregnant for at least a year, though sometimes as long as three years. Kady Lyons. Welcome, Kady, to Science Friday. Thanks so much for being here. So I am going to bias this conversation a little bit towards the gestating animals. So moms are pregnant, just like you would see somebody on the street.
So we have a range of these reproductive characteristics. I mean, that sounds both incredibly frightening but also probably efficient, right? And so if she has less opportunities to do that, that could have some important consequences. There can be up to pups for a mature whale shark, but you can have a single pup, like in a cownose ray. They only have one pup per litter. They can have pups in both. There are some species that have a completely vestigial uterus. So they have two, but they only really use one. So they have all kinds of cool things going on. Females tend to be a little more chill and not as frisky as the males.
You talked about the males being frisky. From what we understand, the mating process for sharks and rays is pretty aggressive. Maybe we can just talk about that piece of it first. But they do have teeth. And those teeth can be very sharp. So you have males that will grasp onto females. So a lot of times when people are observing mating behavior, or not observing it directly, but you can look at those scars on females as a cue of, oh, maybe this is around when mating season is, because we see these open wounds.
But in addition, males are known for being fairly aggressive, and to gang up on females in groups. So that can be a little overwhelming, as you might imagine, for a female shark or ray. We kept finding that multiple paternity or having multiple fathers in a single litter is actually more of the norm than it is the exception. Some explanations for that, though, tend to be very male-biased. And that gets back to some of that mating behavior that we talked about, where you have very frisky males that might gang up on females. But for our paper, we wanted to flip the other side of that coin because there is a whole suite of physiological mechanisms that females are going through to actually bring that egg all the way up to a completely developed embryo that is fully functional and ready to go.
And so we were saying, hold on, she is investing a ton of energy into producing these babies.
Why would she not have a stake in that? We set up a few hypotheses, both from the male and female perspective. And essentially, what we demonstrate is that there is an equally plausible explanation that these observances of multiple paternity or having those multiple fathers in a single litter can be by female drivers. And then we go on to talk about all the cool ways that females might potentially be able to do that based on other literature, all the way from fruit flies up to complex mammals like us.
So we hypothesize that this might be happening, and very much advocate for a whole other field of research to really look into this to demonstrate that those are happening. But some of those ways can be through sperm storage. But that all occurs in the female body.
And she can actually manipulate those fluids to change the way that that sperm can move. Some other things that we hypothesize is that she might be able to regulate when she ovulates. So if she can ovulate over a long period of time, maybe if she mates with another male down the line, then she can replace that other sperm from the first male with sperm from the second male. So again, these are all hypotheses that we propose that people should look into. But they could be plausible explanations. Queen bees will mate, a lot of times, very early on, and store sperm for sometimes a very long period of time.
Is it similar? Do we know how long female sharks store sperm after mating? So definitely a huge area of research that would be great. And a lot of that has been hypothesized in species that are more oceanic. So they might not encounter mates very often. Yeah, so a lot of research about that has been done in mammals. So that intrauterine competition between mom and pup I think is going to be fantastic to really dig into further. Why is this so interesting to you? But I think all researchers, whether they know about it or not, are going to have some bias that they come with.
We have essentially one way that we do it. And sharks and rays have evolved numerous different ways on providing the supplemental nutrition. And so my interest is, well, how, and who does it benefit, and how are they able to do this?
Studying these mating systems is important because males and females have different prerogatives of what is going to increase their fitness. And so for males, it behooves them to mate with as many females as possible. Females, on the other hand, are limited because they have a certain amount Woman looking sex Lyons Georgia space and a certain amount of resources they can put into the production of young.
And so for them, that cost then becomes mating with a low-quality male or a concept called genomic incompatibility. And that basically is where not every sperm is going to be able to fertilize every egg. We talked about asexual reproduction before. Would there be reasons, you think, for a female shark to find it advantageous to essentially clone herself, even if she has access to sperm from a male?
So they artificially inseminated bamboo sharks. And they found that, even when females had access Woman looking sex Lyons Georgia sperm, some of them still chose— sure, at a lower rate, but asexual reproduction still happened. But we really just need to start using sharks and rays as model species. I think that there is going to be a lot of interplay between physiology and ecology. And that would be another exciting avenue to pursue this research, to say, well, when is it advantageous to do this type of reproduction in this particular environment or in this social context?
And part of that has to do with things that get ingrained. So Darwin is the father of evolutionary biology and a lot of modern science. Thanks so much to my guest, Dr. Kady Lyons, research scientist at the Georgia Aquarium in Atlanta. Thanks so much for ing me today, Kady. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Christie Taylor is a producer for Science Friday.
Her day involves diligent research, too many phone calls for an introvert, and asking scientists if they have any audio of that narwhal heartbeat. John Dankosky works with the radio team to create our weekly show, and is helping to build our State of Science Reporting Network.
He and his wife have four cats, thousands of bees, and a yoga studio in the sleepy Northwest hills of Connecticut. Science Friday. Latest Episode. Credit: Georgia Aquarium Sharks, rays, and skates—all fish in the subclass Elasmobranchii— are a beautifully diverse collection of animals. Further Reading about reexamining gender biases in mating research in sharks and rays. Segment Guests Kady Lyons. More From Guest. Segment Transcript. Tell us more about that. This has been a delight. Meet the Producers and Host Christie Taylor.
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