Friday, December 05, 2008

Pregnant Males? Seahorses are Nature's Mr. Mom

          That's right guys. Male Pregnancy. It seems that seahorses have brought fathering to a whole new level. Generally in nature we may think of it as normal or usual for a female to produce eggs, give birth, and spend most of their time caring for their offspring. The male, on the other hand, makes his one contribution and can then spend the rest of his time on the couch watching football. However, over time it seems to have been more beneficial for seahorses to reverse this role, and with it the inversion of "Darwin's other big idea" of sexual selection. 

          So how does this work, you ask? Well, females haven't begun to produce sperm, but rather deposit their eggs. When seahorses mate, the female inserts her ovipositor into the male's brood pouch (which grows on the outside of the male's body). The female then deposits her unfertilized eggs into the pouch, upon which the male releases sperm for fertilization. The brood pouch is highly vascularized and specialized  for maintaining and caring for the developing embryos. For instance, the male controls osmoregulation of the salt concentrations, as well as providing oxygen and nutrients through a placenta-like structure until he gives birth. 
You can this this really well here: Seahorse Birth Video

          It seemed fit to talk about sexual selection since we just recently covered it in class. Recall, that because of the differences in reproductive strategies, that females generally invest more time and energy into reproduction then males to. Females use energy to form eggs and take time to care for their young, where as males spend little energy producing sperm and then spend little or no time caring for their young. As a result of this, the males have a higher reproductive rate and are therefore more "promiscuous". The opposite is true for the females. The final point, and Darwin's main idea, is that from this the females will enable to choose who their mates are because of the high availability, and males will have to compete for mate because low availability. Eventually, the males will develop physical secondary sexual traits to aid in being chosen by females. This usually is in the form of coloring, a behavior, or an exaggerated part of the body (e.g antlers).  

          So what does male pregnancy (role reversal) do to Darwin's sexual selection theory? Well, it would make sense for everything to be reversed, that being the males become choosy and for the females to develop secondary traits like bright colors. Although this does occur in other species with male pregnancy (such as the polygamous pipefish) it doesn't occur in seahorses. Because seahorses form monogamous pairs during breeding, an equal male to female sex ratio is created. Since there are enough mates for everyone, there is low competition. Therefore there is no need for the evolution of a secondary trait. What does agree with the role reversal, is the competitive behaviors exhibited by females during mating and courting, a trait that is normally attributed to males. In addition to this, the males then become choosy. 

          So guys, if you plan on having kids one day, be thankful you're not a seahorse. For the ladies, if evolution takes a step in this direction, perhaps we'll be the ones on the couch. If seahorses can do it, why can't we? 

Posted by: Amanda Sceusa (Week 11)

Many of the questions I received in the comments, were asking why this reproductive strategy (male pregnancy) is beneficial and how has it evolved. After doing some internet research I discovered that there is really no answer yet. I stumbled upon many proposals for this research topic, like this one HERE, which is attempting to trace the evolution genetically to try to find some answers. This research is scheduled to end in January 2009, so keep your eyes out for updates about these questions in the Spring semester. Even though there are no concrete answers yet, there are still many hypotheses about why male pregnancy actually occurs in nature. An article to follow up on some of the clues to the genetic dichotomy can be found HERE One hypothesis is over the course of time, a gene that would normally help liver and kidney function, was altered to also help osmoregulate the brood pouch. "Genetic moonlighting" is what they called it, or basically instead of solely aiding liver and kidney function, it also helped the brood pouch. Other hypotheses can be made just by thinking about the costs and benefits of this systems. There must be at least SOME benefit to this strategy if seahorses are still alive, no? We will have to wait and see...
As far as other species that exhibit male pregnancy: the Pipefish, which happens to be in the same family as seahorses; Syngnathidae which includes over 200 species. 


At 4:10 PM, Anonymous Allison Cornell said...

While I have heard of male seahorses giving birth, I have yet to watch it until that video. It looked almost as if the male was blowing bubbles out his belly! It's interesting how everything is reversed, from who incubates the eggs to competitiveness. If most everything is opposite, I'm still not sure I understand what caused this change. What is the advantage? Are females more useful for other things than males during the time the eggs are developing?

~Allison Cornell (11)

At 9:32 PM, Anonymous Anonymous said...

I've heard about this before but the seahorse birth video was still just awesome! So the seahorse has undergone a role reversal, but why? What caused the females to be more competitive? Is it more benificial for the male to carry the offspring?

Hanbing Guo

At 12:25 PM, Blogger PWH said...

Seahorses are so amazing. I did not know that seahorse males can give their offspring’s birth. I also saw the video which was very interesting. However, I am not sure that why female and male role reverse occurred at the first place.

So Jin Lee

At 12:44 PM, Blogger PWH said...

Wow that is so cool!!! Other than holding the eggs till they hatch do the male also exhibit maternal behavior as well? I truly wonder why females deposit their eggs in the male pouch and how it evolved over time?

posted by Joanne Philippeaux

At 2:35 PM, Anonymous Anonymous said...

Great post, I've heard of male seahorses giving birth, but didn't know the specifics or seen it on video. You mentioned that the males have evolved into the choosier sex. Since there is low competition for mates and low dimorphism, what are the males selecting for?

-Jane de Verges

At 4:52 PM, Anonymous Anonymous said...

Why would the fact that males are carrying the developing young make them become more choosy rather than the female? I was under the impression that the reason for this choosiness was because eggs are costly to produce and sperm was not. If the females are still producing the eggs why are they not still the choosy ones? How does this work with the fish you mentioned where the males carry the young?

-Alex Jackson

At 5:03 PM, Anonymous Anonymous said...

Awesome article. Do we know of any other male species that can give birth to offsprings?

- David Huynh

At 6:55 PM, Anonymous Anonymous said...

That's pretty cool. How are the sexual organs differentiated or are they similar?

-Duy Nguyen

At 8:19 PM, Blogger PWH said...

This article caused me to wonder why it might be that the males carry the eggs? I think that maybe it helps the females to be able to have a faster turnover rate and be able to more quickly produce her next clutch. Producing eggs is a very energy demanding process and to have to carry them around and care for them as they are developing only adds to it. So maybe this allows for the female to look towards preparing to reproduce again more quickly. What do you think?
Posted by: Lindsay Goodyear

At 8:27 PM, Anonymous Anonymous said...

The article is very interesting, I've actually never heard of any male species giving birth before. I can totally correlate it to our class lectures and see how this makes the females more competitive and the males more choosy.

- Tazneena Ishaque

At 7:19 PM, Anonymous Anonymous said...

We can theorize what steps were needed leading to this behavior. What happens after this, what can we theorize this behavior will be a pre-adaptation for? Will males grow larger pouches to carry more young? Will eggs get larger and therefor less offspring per pouch but each young may be more precocial?

Allan Eldridge


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