8 Bizarre Facts About Animal Reproduction

There’s no rule that regulates how animals obtain the necessary genetic material to reproduce—and as a result, there is a lot of weird animal sex going on out there.

1. BANANA SLUGS MATE USING PENISES ON THEIR HEADS.

Banana slugs are huge, they eat your garden, and they leave a slimy trail as they squirm across the ground. Their method of reproduction is no more pleasant than their appearance: The scientific name for one species of banana slug is Ariolimax dolichophallus, the second part of which means “long penis,” and it’s apt. The invertebrates’ penises can be 6 to 8 inches long—the entire length of the slug’s body. When it’s time to get down, the penises grow out of pores in the head. It isn’t unusual for the penis of one slug to get stuck inside the other during copulation; to solve this uncomfortable situation, the slug on the receiving end will eat the penis that’s stuck inside it.

2. MOUSE SPERM IS BIGGER THAN ELEPHANT SPERM.

Sorry, dudes: The size and shape of a male animal’s sperm is more related to the biology and mating practices of the female—which is why mouse sperm is longer than the sperm of the biggest land animal on Earth, the elephant.

Two major factors in sperm size include the literal length and size of the female reproductive tract and how many mating partners the females have. Mice produce big sperm because, in most species, females have a lot of partners. The grouping of sperm can help to increase its competitiveness in promiscuous animals. Accordingly, the deer mouse has sperm equipped with small hooks that helps them clump together in groups of up to 35 sperm to fight their way to the egg. Grouping sperm allows the cluster to swim in a straighter line to the egg; the ideal number is seven. Too many spermatozoa will eventually start working against each other. Meanwhile, female elephants have a very long reproductive tract (more on that below), so male elephants have evolved to produce more but smaller sperm to combat the risk of dilution. To follow the reproductive strategy of mice sperm, elephant sperm would need to be scaled up enormously to make a difference.

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