Seed Beetles Are Locked in a Brutal ‘Sexual Arms Race’

By Carl Engelking | May 24, 2017 3:07 pm
This is going to hurt. (Credit: Shutterstock)

This is going to hurt. (Credit: Shutterstock)

Cowpea seed beetle sex is complicated.

During copulation, the male seed beetle, Callosobruchus maculatus, uses his sharp, spiky penis to damage females’ reproductive tract while depositing sperm. All the while, the female vigorously kicks at her suitor—it hurts! As studies have shown, males with longer, harmful penis spikes enjoy more reproductive success, to the detriment of their partner’s health.

But the process of evolution has a way of balancing the scales. In a new study, Liam Dougherty of the University of Western Australia and colleagues closely monitored isolated populations of seed beetles in their lab, and they’ve concluded that males and females of this species are locked in a violent, multigenerational, “sexual arms race.” Sure, males wield destructive penises, but females, in turn, are adapting with their own defenses.


An up-close look at a male seed beetle’s destructive genital spines. (Credit: Johanna Ronn)

Wild Sex

The reproductive strategies of males and females in a particular species don’t always align. Males might develop reproductive behaviors that benefit their health but at the cost of females’, and vice versa. The animal kingdom is rife with examples of this sexual conflict: Certain species of female ducks have vaginas that are twisting labyrinths, so males sport long, corkscrew penises to compensate.

Female fruit flies store sperm in a special pouches, but subsequent male suitors deposit far more sperm than is actually needed in order to flush out their competitor’s inventory. Seed beetles partake in an extreme form of this reproductive tit-for-tat, or what scientists call traumatic mating. It’s actually quite common among invertebrates and other insects.

This Means War

Dougherty’s team monitored 13 isolated populations of seed beetles, collected from various regions, that have been evolving in the lab for over a decade—well over 100 seed beetle generations. Then, they took a close look at their baby-making habits, measuring things like the volume of female reproductive tracts (using micro-CT scanners), penis spine length and damage inflicted by the spines, as well as changes to females’ immune systems.

Within their seed beetle populations, scientists noted a strong correlation between the length of penis spines and the volume of tissue in female reproductive tracts—the larger the spikes, the more voluminous the reproductive tract. While their work only shows a correlation between two traits, it makes sense: As male penises grow more threatening over time, females up their defenses to protect their own fitness.

“It’s probably the harm caused by males that is causing the females to evolve. Because it’s so harmful, you get rapid changes,” Dougherty told New Scientist. “And females that are less harmed have more offspring.”

Why Can’t They Get Along ?

So why are seed beetle penises so brutal in the first place? They may be attempting to deliver a biological message. In addition to the ingredients for life, seminal fluid also contains signaling compounds that alter female physiology. By piercing the reproductive tract, males are perhaps introducing chemicals that cause the female to lay more eggs or discourage mating with other males. Therefore, the longer the spikes, the better the outcome.

Indeed, in 2012 scientists used lasers to shave seed beetle penis spines, and it reduced their reproductive success. They also found that roughly 40 percent of their sperm ends up outside of the reproductive tract—the larger the spines, the greater the leakage, and the more chemicals that are introduced into the female’s body. A male’s longer spines might also harm females enough that they die shortly after laying fertilized eggs, ensuring only his genes are passed down.

But as generations of female seed beetles evolve better defenses, they can mate with more males and produce more offspring—advantage, ladies.

It’s a Cold War-esque arms race, and neither sex will likely emerge victorious. Rather, in the chaotic world of seed beetle sex, males and females are probably co-evolving toward some sort of hellish, sexual equilibrium.

CATEGORIZED UNDER: Living World, top posts
  • Uncle Al

    scientists used lasers to shave seed beetle penis spines” All I got to do was reflux benzaldehyde with potassium cyanide in ethanol. All that other stuff was across campus at the Training Table.

  • Jeddy Khan

    The message is clear, sex and violence go together. Black Widow spiders behead the male when he shoots his sperm. A lion will devour cubs because he wants the lioness should be the mother of his cubs.Penguins resort to all kinds of deviant behaviour.

    • Uncle Al

      The Clintons: Jeffrey Epstein’s “Lolita Express” plus Vince Fostercare.

      • Hibernia86

        Are you a troll account? Stop using a Vince’s suicide to push crazy right wing conspiracy theories.

        If you want real-life perversion, try the case of Trump raping a 13 year old. (because the filter won’t let me post websites, simply remove the spaces after the “www” and before the “com”.)

        http://www .huffingtonpost. com/entry/donald-trump-rape-case_us_581a31a5e4b0c43e6c1d9834

        • Uncle Al

          “Vince’s suicide” Arkancide. “His” suicide note was found three days later in his otherwise empty briefcase.

          • Hibernia86

            Where is your evidence for that?

            Face it, as much as Conservatives hate the Clintons, in reality all they are is a couple who cares about Progressive causes and likes to be elected to office. All of the conspiracy theories about them have ended up being tall tales.

          • OWilson

            Plus an inhuman lust for money and power!

            As for Foster.

            That case would be solved in 15 minutes on “Forensic Files” or “Investigation Discovery”. They’d have to find another case to fill up the time slot :)

            Before her recent billions in looted State Department blackmail, her law partners went to jail, her friend and law partner and Associate Attorney General (and Chelsea’s bio father) went to fail, their White house council committed “suicide”, her husband was Impeached, fined, dis-barred, paid out over a million in today’s money, to settle the Paula Jones case, where he dropped his pants and ordered a State Employee to “Kiss it”, and they both had their way with young White House Interns, she chose Huma, and he chose Monica!

            You mean THOSE Clintons?

    • Maia

      Not universally true, fortunately.

    • OWilson

      Even in the Oval Office!

      “Better put something on that lip”. :)


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