IRA FLATOW, host:
This is TALK OF THE NATION: SCIENCE FRIDAY. I'm Ira Flatow.
A few years back, zookeepers at Omaha's Henry Doorly Zoo discovered that one of their hammerhead sharks had given birth. Now, that's not unusual - except her tank had three females and no males. In fact, the female shark had not been exposed to males for at least three years. Now, hammerhead sharks can store sperm, and initially, researchers thought that one of the females had done just that - stored sperm inside her body and used it later to fertilize an egg.
But a new report out this week tells a much different story. The baby shark was genetically identical to the mother. There was no male genetic contribution. It's the first such time that a birth - a parthenogenesis - has been documented in a hammerhead shark. Joining me now to talk more about it is my guest George Burgess, director of the Florida Program for Shark Research at Florida Museum of Natural History in Gainesville. Welcome back to SCIENCE FRIDAY.
Mr. GEORGE BURGESS (Director, Florida Program for Shark Research, Florida Museum of Natural History, University of Florida): It's good to be with you.
FLATOW: How shocked were you or anybody else about it?
Mr. BURGESS: Well, actually I'd known a little bit about this story in advance because the office reported on this in scientific meetings several years ago. So we were anticipating the publication of it.
FLATOW: So this is a big story for shark researchers, then?
Mr. BURGESS: It's a real interest in the sense that among the vertebrate groups, parthenogenesis or asexual reproduction has been documented in all but sharks and mammals. And so it wasn't wholly unexpected in sharks, being one of the more basal groups, but of course the implications are similar to - they were in other groups as well.
FLATOW: Tell us about the ability, the talent of a shark to store sperm for so long.
Mr. BURGESS: Well, storing sperm is something we've known about for some time in sharks, and many different groups of sharks are able to do this. Obviously, the advantage of that, from an evolutionary standpoint, is to take full advantage of your opportunity to reproduce whenever it comes to you. And of course, as a female, then she - in a way sort of has the choice for fertilization in - by storing sperm when the opportunity arises.
We've known for many years that in aquaria - occasional females will give birth in aquaria where there are no males. And in many of these cases, sperm storage has either been found as the answer or hypothesized as the answer.
FLATOW: So, do you hypothesize that this parthenogenesis, not needing the male, goes on regularly in the wild, not just once in the zoo?
Mr. BURGESS: No, actually quite the opposite. At this point scientifically, all we have been able to document as a scientific community is parthenogenesis in the artificial situations of an aquarium. It has not been documented in the wild, and of course it begs the question, then, is there something about isolating these organisms in an artificial environment of an aquarium, thus contributing to the process?
FLATOW: It would be much tougher to tell in the wild, would it not?
Mr. BURGESS: Yes, it would. Of course, the way this was determined and ascertained is through genetic studies of the offspring to determine who the parents were. And in this case, the determination was only the mother. There have been genetic studies done and are continuing to be done on wild-caught(ph) animals, and a lot of groups are working on the genetics of individual broods, for instance. We know that in many - if not most - species, where a female gives birth to her pups, as they're called, it's quite common and pretty much the norm now that those are fathered by a series of fathers rather than a single parent.
Mr. BURGESS: So the question is, are we seeing this parthenogenesis in the aquarium simply because we're able to look at it more carefully, or is it simply a situation where the artificial isolation of females in aquaria is contributing to the situation?
FLATOW: Do other - are other fish able to do this? Do other fish do this?
Mr. BURGESS: Other fishes do.
Mr. BURGESS: And in fact, so do amphibians, reptiles, and birds, among the other vertebrate groups.
FLATOW: We've even heard reports, I'm sure there's, you know - you can fill me in more on this - of fishes changing sex if they have to.
Mr. BURGESS: Yes. In fact, that is the norm for certain species. For instance, groupers and other sea basses routinely change sex, some of the rasses(ph) and so on. But again, this is routine within those groups.
FLATOW: Could this be unique to sharks, or could it have anything to do with how old they are in the history of evolution, how far back they go?
Mr. BURGESS: I guess my thoughts on the matter are these - I think that it's very possible that this is a mechanism, an evolutionary mechanism, that's in place that is induced during situations when a female is basically shut out from contact with other males. In other words, under extreme situations where the female is not having an opportunity to mate, evolutionarily, it is given the shot, the opportunity, then, to parthenogenetically reproduce. Of course, there's a risk and a real cost in doing so, and that cost is reduction of genetic diversity. You're only going to get one parent's worth of genes and genetic material. And so this, of course, is what leads to inbreeding, and from an evolutionary standpoint, it's not a positive step because there's less viability of the offspring.
FLATOW: Now, I imagine that your aquarium or any aquarium can't really be that unique, you know, in having the females to with each other. Might you not expect to hear from other aquaria…
Mr. BURGESS: Yes.
FLATOW: …to report on this?
Mr. BURGESS: There have been other instances of, if you will, a virgin birth being reported in aquaria involving at least two other species. It really comes down to situations where aquaria are not including members of the opposite sex in their tanks together. And - but more importantly is, is this a strategy that is routinely employed in the real world or not?
Mr. BURGESS: And that's - the compelling question is, what was seen in this aquarium - is it occurring in the natural environment? Now we know that the species involved in this case, the bonnethead shark, the small hammerhead shark, is reproducing very nicely by traditional means of males and females throughout its range. And that species is in not particular danger from a conservation status in terms of being over-fished or killed out.
If it was, however, and there were less opportunities of males and females to get together, then parthenogenesis might be a strategy that would then be invoked in the wild.
FLATOW: Dr. Burgess, I want thank you for taking time to be with us.
Mr. BURGESS: Glad to be with you.
FLATOW: Good luck to you, have a happy holiday weekend.
Mr. BURGESS: You do too.
FLATOW: George Burgess is director of the Florida Program for Shark Research at the Florida Museum of Natural History.
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