The Placenta: a Battleground in Sexual Dynamics

Sexual selection is a fun topic in biology because it explains the evolution of all kinds of non-functional, sometimes bizarre and counter-productive traits.  It is a form of natural selection in which the presence of a given trait allows individuals to out-reproduce others.  Secondary sexual traits include anything from the ornate feathers on peacocks and birds of paradise, antlers on stags, and even eye span in the stalk-eyed fly (yup, those are eyes at the end of those stalks, not antennae):

Basically, the ladies are choosy, so any male with a unique trait or the ability to grow/make one gets to mate.  In some cases when a trait gets to be too common (*cough* beards), they lose their appeal. 

I say it's the females who are choosy, but this really depends on the level of commitment given to offspring.  In species where the female cares for the offspring, she is the choosier partner and the males have ostentatious secondary sexual traits, including being larger in stature.  On the other hand, in species in which males commit more to parenting, like seahorses for example, it's the females who have to compete for the best mates and so they are the larger and showier sex. 

 

Another development of sexual selection, particularly of vivipary (live birth, as opposed to egg laying), is parent-offspring conflict.  We sometimes jokingly refer to the developing fetus as a parasite, but this is actually far truer than people think.  Parent-offspring conflict in utero is basically the conflict between mom, who wants to ensure she has enough resources to give to all offspring, and baby, who wants to hog all the resources for itself.  This has led to a dynamic in which offspring evolve tactics to drain mom's resources and mom counter-evolves to ensure she can allocate resources to all offspring (note: this is a MAJOR simplification of this process).  This is the process through which the placenta evolved.  But the evolution of the placenta creates a shift in parental investment.  Rather than putting all her resources into eggs that need to be fertilized (and are therefore very valuable), females became able to allocate resources after mating, lowering the risk of investing too much in offspring from a less-fit male.  Presumably, this meant that females could mate with multiple males of varying genetic fitness, and that the offspring from the fitter males would hog more resources than the offspring from the less-fit males.

 

A green swordtail, from
the family Poeciliidae

Biologists at the University of California, Richmond looked at this phenomenon in the live-bearing fish family Poeciliidae (which include swordtails, guppies, and mollies).  Species of this family are unique in that they are closely related, but some possess placenta while others do not (while still producing live offspring).  That means that females of some species have to contribute more to offspring pre-fertilization, with the risk that the males they choose end up being duds.  This allowed them to look at how the evolution of the placenta influences sexual dynamics. 

An example of a fish placenta. Source.  I apologize
to any squeamish readers.

By looking at 150 different species within this family of fish, the researchers found that the absence of placenta was associated with showy secondary sexual traits in males like bright coloring and larger size, while the presence of placenta reduced the number and intensity of these traits.  But on the other hand, males of placental species were smaller than females, with larger ahem appendages, facilitating sneaky copulation and circumventing female choice.  Females also mated with a larger number of males, therefore avoiding or diluting the effects of any duds. 

These results mean that parent-offspring conflict, and the evolution/counter-evolution tactics used by mother and offspring, have influenced mate choice and male sexual characteristics.  My Biology of Sex prof told us that the absence of gaudy sexual characteristics in humans was due to both parents contributing equally (arguably) to parenting.  The results from this study seem to indicate that it might be a bit more complicated than that.  Clearly, what's going on on the inside has a pretty large effect on what goes on on the outside.