Why the Smallest Animals Have the Longest Sperm

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February 04, 2016 • 11,512 views

Story at-a-glance

  • As animals get bigger, sperm have a higher likelihood of being diluted or lost before reaching an egg, worsening the chances of successful reproduction
  • Nature compensated by increasing the numbers of sperm in larger animals, while increasing the size of the sperm (but not the quantity) among smaller animals
  • The fruit fly has the longest sperm of any animal — 2.3 inches, which is 20 times longer than human sperm

By Dr. Becker

Have you ever wondered about the characteristics of sperm in wild animals like lions, mice and even fruit flies? Well, maybe you haven’t, but a new study published in Proceedings of the Royal Society B uncovered some intriguing information, nonetheless.1

Take, for example, the fruit fly. It has the longest sperm of any animal (2.3 inches — 20 times longer than human sperm!).2 It turns out that the smaller the animal, the longer their sperm tends to be and vice versa (the larger the animal, the shorter the sperm).

Researchers from the University of Zurich tested two hypotheses for why this might be and revealed what’s known as the “sperm dilution hypothesis” appears to be correct.

Quantity Trumps Size as Animals Get Bigger

As animals get bigger, so too does the size of the female reproductive tract. This means sperm have a higher likelihood of being diluted or lost before reaching an egg, worsening the chances of successful reproduction.

As a result, nature compensated by increasing the numbers of sperm in larger animals, while this appears to be less important in smaller species. Among small animals, sperm size appears to be far more advantageous than sperm quantity.

Take, for instance, the sperm of mice and elephants. A mouse sperm is about 124 micrometers long compared to 56 micrometers for an elephant sperm (which is roughly the same size as human sperm).

Quantity wise, a mouse ejaculation contains about 9.5 million sperm to an elephant’s 200 billion.3 Study co-author Stefan Luepold of the University of Zurich told AFP:4

“Since elephants are bigger than mice, it seems that their sperm have a higher risk of being diluted or lost in the bigger female reproductive tract. In other words, sperm number becomes far more important than sperm size …

By contrast, sperm dilution is less of an issue in small species, which creates conditions where the benefit of longer’  — and faster-swimming — sperm may come into play.”

There are other trade-offs as well. As UPI noted:5

“More promiscuous females drive males to invest more in sperm production. Such species tend to produce more sperm and bigger sperm than species with more monogamous arrangements.

Another revelation: the manliest males tend to have lower quality sperm. When males invest in the flashy accessories used to win females — big horns, bright colors, deep voices — they pay for the upgrade with a drop in sperm count and size.”

10 Most Virile Animals

In their study on sperm, the researchers evaluated more than 100 mammal species. Here are some of the more, shall I say, impressive results, as reported by Discovery News.6

  1. Blue Whales
  2. Blue whales can ejaculate 35 pints of semen at a time, from a penis that’s eight feet in length (the largest among animals).

  3. Argentine Blue-Bill Duck
  4. This duck has an exceptionally long penis in relation to its body size; it may reach over 17 inches, which is about the length of the entire duck. Barnacles also deserve mention here; their penis may be 40 times their body length (yes, barnacles have penises!).

  5. Bush cricket
  6. Male bush crickets have the largest testicles relative to body size, as they make up nearly 14 percent of the insect’s body mass. Rafinesque’s big-eared bat earns this honor among mammals; their testicles make up 8 percent of their body mass.

  7. Right Whale
  8. The largest testes ever recorded (more than 1,100 pounds each) belonged to a North Pacific right whale. Relatively speaking, this isn’t entirely exceptional since that represents only 1 percent of the whale’s total body mass.

  9. Fruit Fly
  10. As mentioned, fruit flies have the longest known sperm of any animal at nearly 2.3 inches.

  11. African Elephant
  12. Elephants have large testicles (weighing about 10 pounds combined), and African elephants have one of the largest numbers of sperm per ejaculate (more than 200 billion).

  13. Wild Boar
  14. The wild boar Sus scrofa may ejaculate 50 billion sperm at a time, which is one of the largest relative to body size (and more than 100 times a typical human ejaculate).

  15. Norway Rat
  16. Norway rats are polygynandrous, which means two or more of one sex mate exclusively with two or more members of the opposite sex. In order to reproduce successfully, the rats have very long sperm (189 micrometers, or about 0.19 millimeter).

  17. Golden Hamsters
  18. Golden hamsters have very long sperm similar to that of rats (187 micrometers). They also have very large testes relative to body size; their testicles account for 3 percent of their body mass.

  19. Lions
  20. Lions may mate up to 60 times a day.

[+]Sources and References [-]Sources and References

  • 1 Proceedings of the Royal Society B November 22, 2015
  • 2, 3, 4 Phys.org November 18, 2015
  • 5 UPI November 19, 2015
  • 6 Discovery News November 17, 2015