Earth & Conservation

Top 10 Most Virile Creatures on Earth

A survey of over 100 animals shows which ones have the most impressive goods.

| Koorosh B Photography, Flickr
| Koorosh B Photography, Flickr

Sperm and male sex organ extremes are revealed in a new study of more than 100 mammal species. The study also answers a question that has baffled scientists for years: Which is favored more during evolution, sperm size or sperm count? Quantity is key for mammals such as this roaring male lion, which can mate up to 60 times per day. The findings are published in the Proceedings of the Royal Society B.

Lead author Stefan Lüpold explained that he and colleague John Fitzpatrick of Stockholm University were "able to tease apart the relative importance of sperm size and number at different body sizes and show that in small species, both traits seem to respond to selection, because probably both of them can significantly influence competitive fertilization success."

"By contrast, in larger species, where ejaculates have a much higher risk of being diluted or sperm being lost within the much larger female reproductive tract, sperm number becomes so much more important than sperm size that the focus of selection seems to shift almost entirely toward sperm number," Lüpold, now at the University of Zurich, told Discovery News. He and Fitzpatrick conducted the research when they were both at the University of Manchester.

Read on to see which mammals bring the most impressive sexual goods to the table.

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Blue whale spouting off Moresby Island, British Columbia | John Calambokidis, Cascadia Research Collective

Blue whales have many claims to sexual fame. The scientists said these enormous marine mammals could ejaculate about 35 pints of semen in one go. Blue whale males also have the animal kingdom's largest penis, reaching 8 feet in length.

| Roderick Tuk, Flickr

The Argentine blue-bill duck has one of the largest penises relative to overall body size. The penis can extend to about the same length as the animal itself (at or over 17 inches) when fully erect.

The penis of barnacles (Balanus glandula) also merits mentioning in the "largest penis relative to body size" category, since the sexual organ can be 40 times a single barnacle's body length, according to the researchers.

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| Gilles San Martin, Wikimedia Commons

Lüpold said bushcricket males have the largest testicles relative to body size. The testicles account for 13.8 percent of the insect's overall body mass.

Among mammals, Rafinesque's big-eared bat has the largest testes relative to body size, since they account for over 8 percent of the bat's total body mass.

| John Durban, NOAA

"The North Pacific right whale has the largest testes ever recorded," Lüpold said, explaining that each weighs over 1,102 pounds.

He added, however, that "in relative terms, these testes are fairly big for a mammal, but are not exceptional in a broader context."

That's because the testicles only account for approximately 1 percent of the extremely large marine mammal's total body mass.

| CSIRO, Wikimedia Commons

Tiny fruit flies make the list because they have the longest sperm of any animal studied so far. Each individual sperm measures nearly 58 millimeters long.

Matthew Gage of the University of East Anglia, who was not involve in the study, told Discovery News that "one of the most elegant and logical explanations for why there are two sexes in most species on planet Earth comes from ‘the theory for the evolution of anisogamy.'"

He explained that this theory, developed by scientist Geoff Parker in the 1970s, holds that there are two fundamental demands acting simultaneously on the evolution of any species' gametes. There's a need for some to be "nutritive for maximizing offspring survival," Parker held, and there's also a need "to be competitive for maximizing" fertilization success, especially when there is competition from other males.

For this small fruit fly, clearly bigger sperm have proven to be more successful.

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| Derek Keats

Like whales, elephants merit multiple mentions on this list. First, both African and Asian elephants have the largest testicles among mammals in absolute terms (meaning not relative to body size). Testicles of the African elephant weigh about 10 pounds combined.

African elephants also have some of the greatest numbers of sperm per ejaculate. When mating, the male may release well over 200 billion sperm.

As for why elephants have such high sperm counts while other species have larger yet fewer numbers of sperm, the answer has to do with female size.

Gage explained, "Since elephants have bigger female reproductive tracts than mice, it therefore appears that sperm dilution in the fertilization arena is also vitally important for shaping the evolution of sperm form and function."

He added that "when dilution becomes more of a demand, males are selected to prioritize much greater investment into increasing sperm numbers to win fertilizations."

| Frank Vincentz, Wikimedia Commons

Among the largest ejaculates relative to body size belongs to the boar Sus scrofa.

Lüpold said that males of this boar "seem to produce ejaculates of over 50 billion sperm, which is well over 100 times a human ejaculate."

European hare males also have some of the greatest sperm numbers relative to their body size.

| Bayer Crop Science, U.K., Flickr

Norway rats engage in polygynandry -- a reproductive strategy that occurs when two or more males have an exclusive sexual relationship with two or more females. In short, each individual has to compete a lot on a regular basis, even at the sperm level, when mating.

To achieve fertilization success, Norway rats have very long sperm, measuring at 189 micrometers. The house mouse is also high up on such a list, with each sperm measuring about 124 micrometers.

| Wikimedia Commons

Golden hamster males have a mighty appetite for both food (as this photo shows) and sex. They make the list on two counts.

First, these hamsters, like Norway rats, have very long sperm. In this case, each sperm measures 187 micrometers.

Second, "The golden hamster had the largest testes relative to body size" in the new study, Lüpold said. The testicles account for 3 percent of the rodent's overall body mass.

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