Monday, September 28, 2009

Male Sexual Choice; an Uncommon Occurrence

During last Tuesday’s brief discussion on the sexual selection dynamic that exists between male and female barking frogs (Hyla gratiosa), I mentioned that, as a general rule, the female gender of most species are the “deciders” when it comes to choosing reproductive partners. What was meant by this was that females are more-often-than-not the limiting sex in a population and are thus behaviorally better positioned to choose between competing males during the process of sexual selection. Today, I thought that it may be a good idea to give an example of a species that exhibits a mating system typified by reversed sex-roles; one in which males , as opposed to females, are the limiting reproductive resource.

One of the many unique things about Syngnathidae, the biological Family to which pipefish, sea dragons and seahorses belong, is that its female members lay their eggs directly on the trunk or tail of the male gender. Following the ovipositioning that occurs during copulation, the eggs remain bodily attached to the male where they are subsequently fertilized by his sperm. The male then carries the eggs as they develop, providing protection and in some instances even direct nutritional support via circulatory connections to placenta-like tissues contained within his brood pouch (note: not all Syngnathidae males have true brood pouches, in some species the eggs are attached externally to the male’s body in “sockets” that lack circulatory tie).

Because males take on the responsibility of carrying and protecting the clutch, they’re ultimately accountable for the success of the offspring. Fitness measures, such as the number of viable offspring produced from a mating session, are in part determined by the resources allotted by the male to the embryonic development of the piggy-backing eggs. Of course, another significant factor in the fitness potential of offspring is the health and condition of the female whom is contributing the eggs. It is the female’s health and current condition that the male assesses during the initial stages of mate selection.

Although both mating behavior and organismal reproductive physiology differ greatly between individual species of the Subfamilies Hippocampinae (seahorses) and Syngnathinae (pipefish and sea dragons), the process of precopulatory sexual selection is characterized by a courtship dance in which the female entices the male (sometimes for a period of days) through performing elaborate maneuvers, turns, circles and tail holding. If she appears healthy and passes muster – if she’s got the right moves – the precopulatory dance may be followed by mating as mentioned above; the female deposits eggs into the brooding pouch (or, in some species places the eggs on the male’s body).

Incidentally, if you haven’t witnessed the courtship dance of seahorses or pipefish, you are truly missing out on an extraordinary display!!! Check-out this video from the Monterey Bay Aquarium:

In addition to the fitness potential read from the females’ sophisticated chorography, the Syngnathidae males also inspect the female for other visual cues that may provide indication to as to her health, condition and genetic make up.

To better understand the male’s assessment method, Adam G. Jones, Assistant Professor at Texas A & M, examined the preference of male members of the species Syngnathus typhle (a pipefish) for females displaying indicators of high parasite load. As most folks could imagine, large quantities of parasites can affect the overall health of an animal; these freeloaders tap into their host and drink with delight the vital resources acquired through the labors of their target. Even for the pipefish, large quantities of unsightly parasites observable on the exterior of a potential mate are pointers of poor current health. The parasites are literally unsightly; unsightly because that is precisely how the male Syngnathus typhle weighs the female’s appearance – by sight. Should the male perceive large or numerous dark spots on the female, he is far less likely to choose her as a reproductive partner. This is because the dark spots, the male pipefish assumes, are parasites and may adversely affect the condition of any eggs the female may be carrying; therefore black spotted females represent a risky reproductive investment for the male. However, there is one detail that the pipefish in this experiment weren’t privy to, that is that the dark spots in Jones’s study weren’t parasites at all – they were harmless tattoos.

During a recent visit to Tallahassee, Dr. Adam G. Jones explained that the tattooed pipefish experiment was originally intended as an inquiry into the post-copulatory behaviors of Syngnathus typhle. Knowing in advance that males preferentially mated with females showing low parasite loads, Jones had suspected that males of the species may engage in a cryptic choice behavior in order to reduce impregnation by parasite ridden females. Although this experiment ultimately demonstrated no support for the parasite-to-cryptic choice hypothesis, it did exhibit the possibility for cryptic choice along lines other that parasite load, AND the experiment also provided evidence to two key phenomena. Firstly, the experiment revealed that eggs from larger females had a greater statistical tendency to become viable offspring than those received from smaller females. And secondly, the study showed that the first female to mate with the male deposited a larger quantity of eggs than did those females ovipositing afterwards.

So... The natural history of the Family Syngnathidae demonstrates that male choice does exist in the natural world. Further, Jones’s work has shown that male choice extends beyond the precopulatory selection of female mates and that the behavioral ecology of the pipefish even includes post-copulatory mechanisms of sexual selection.

Partridge, C., Ahnesjö, I., Kvarnemo, C., Mobley, K., Berglund, A., & Jones, A. (2008). The effect of perceived female parasite load on post-copulatory male choice in a sex-role-reversed pipefish Behavioral Ecology and Sociobiology, 63 (3), 345-354 DOI: 10.1007/s00265-008-0668-3

No comments:

Post a Comment