Thursday, April 12, 2012

Evolution of Sex(ual organs) Pt. 2

(Don't worry- no graphic pictures included)

In my previous post, we looked at two postulates for the earliest penis fossil records (up to 100~400 mya). Although the idea of males having sexual organs that stick out seems like an evolutionarily plausible and reasonable thing, for some animals, this seems quite extravagant and unnecessary. A specific example of this is birds. Only 3% of the bird species have penises. Knowing that having penis is the minority, the right question we can ask is: why do some birds have penises in the first place, and how are they different?

An article from Nature demystifies this phenomenon through observing ostrich penis.

Most penises become erect by blood vascular system, where the arteries become dilated that draws blood towards the penis. However, ostriches and other ratites, or flightless birds such as kiwis, utilize lymphatic system to achieve this erection. They have paralymphatic bodies below the muscle that cause erections. The origin of lymphatic erection is yet to be determined; however, they are hypothesized to be a result of divergent evolution.

Penises are one of the primary sexual traits, yet birds prove this theory wrong.

Anyone interested in reading the full article or watching an erect ostrich penis, go to the link below:


  1. I read another article about this phenomenon ( and one of the authors of the study mentioned that since the lymphatic system has low pressure, an erection cannot be maintained for an extended period of time. Interestingly, some birds that use the lymph-based mechanism have "explosive" erections, in which the lymph fluid is propelled through the penis to increase pressure briefly.

    I don't know anything about avian copulation, but I wonder if this method of erection is somehow less energetically expensive than utilizing the vascular system.

  2. I think rather than energy expense, this article is trying to show that there must be some phylogenetic connection between reptiles and birds, since reptiles are cold-blooded and birds don't use blood to get an erection. The article states "[this] could have important implications for the understanding of the shared, and divergent, evolutionary heritage of birds and reptiles."

  3. Are then penises, especially ones that use the vascular system, only prevalent among mammals? Also, how much does climate have to do with it? I was reading an article that said that in colder climates, many animals have retractable penises (i.e. the Artic hare) and others don't at all, because it become a freezing liability.

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