Mating Slugs

2017 March 11

Sam, Rosie, and I were out walking in the woods on October 9, 2016, when Rosie spotted these two slugs, huddled together in a little slime-filled pit in the ground. She wanted to know what they were doing.


Well, that’s an easy one to answer. They’re mating, is what they’re doing.


The next immediate question was, which one was the male and which one was the female?


And the answer is “Yes”. Or maybe, “Both”. Those two bulging white objects waving around are slug penises, and you may note that there are two of them. Like most slugs, these are hermaphrodites, so they are both male and female, and each one is trying to inseminate the other.


The thing is, this can be sort of a duel. See, a given slug’s eggs only can get fertilized once, but it can fertilize the eggs of multiple other slugs given a chance. So, we can get into these situations where it is to a given slug’s advantage to mate multiple times as a male, but keep its own eggs from being fertilized for as many encounters as possible. Its own eggs would eventually get fertilized after it ran out of sperm and no longer had any incentive for putting things off, although it will generally get fertilized before that if it encounters a more skilled “penis fencer”. Sex amongst the hermaphrodites is very competitive.

So anyway, for a lot of animals the competitive nature of sex leads to clear-cut males and females, with the females specializing in making sure eggs hatch and get a chance to grow up, while the males specialize in seeing to it that the get fertilized. But, for whatever reason, a small (but not trivial) fraction of animals never divide up the functions, so we have things like terrestrial slugs, snails, earthworms, and a variety of other animals that stay hermaphroditic. Along with plants – a huge fraction of flowering plants are hermaphrodites. It seems to me that most of the hermaphroditic land animals are so slow-moving that they almost might as well be plants. Which suggests that the hermaphrodite lifestyle makes the most sense when you are either very slow or completely immobile, and therefore are quite likely not to encounter a lot of different mates.

Incidentally, these slugs look a bit different from slugs I’ve posted pictures of in the past. I think they are Gray Field Slugs, Deroceras reticulatum.

3 Responses
  1. Carole permalink
    March 11, 2017

    Fascinating and great documentation.

  2. March 21, 2017

    The clickbait tagline practically writes itself.

  3. Kathleen Bradley permalink
    March 23, 2017

    When I see two patrol cars pulled up together like these slugs, I always thing: copulation.

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