Humans aren't the only creatures that vocalize during sex.

While mating, female Physocylus globosus spiders emit high-frequency squeaks to let males know what they should be doing, a new study finds.

Multimedia
Audio and Video
of the spiders mating

Called stridulations, the shrill cries sound like squeaky leather and are made in response to the rhythmic squeezing actions of the male's genitalia from inside the female during sex.

Upping the odds

Female spiders are able to store sperm from different males inside their bodies and can choose which lucky male spider gets to fertilize her eggs. Squeezing stimulates the females and raises a male's chances that his sperm will be selected.

"Males that squeezed females more often during copulation sired more offspring than males that squeezed less often," said study team member William Eberhard of the Universidad de Costa Rica and the Smithsonian Tropical Research Institute in Panama.

However, if done too forcefully, the action can physically damage the female. If a male squeezes too hard or too long, the female squeaks to let him know to pick up the pace but to take it easy.

The finding will be detailed in an upcoming issue of the journal Animal Behavior.

How it works

To produce the squeaking sounds, females scrape a part of their "pedipalp"—a leg-like appendage located near their mouths—against the file-like surface of their fangs, or "chelicera."

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A male spider's genitalia are located at the end of its pedipalp. During sex, he inserts this tip into the female. Muscles near the base of the pedipalp flex during sex, creating the rhythmic squeezing motions that cause a female to cry out. [Images]

The researchers mated 68 virgin P. globosus females with two males. They found that the number of squeezes the males made were associated with the number of times the females cried out during sex. Stridulations became more frequent if males failed to loosen a squeeze in response to a previous plea.

Obedient males that consistently followed the female's directions ended up siring more offspring. It's thought that the squeezing motion propels the male's sperm deeper into the female's body, where it is more likely to fertilize her eggs.

"Females presumably favored the paternity of males that could stimulate them, thereby obtaining sons that were better stimulators and thus more able to induce females to fertilize their eggs with their sperm," Eberhard told LiveScience.

Multimedia: Audio and video of the spiders mating.

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