NO MEAL, NO DEAL
Because the fate of his sperm depends on it.
A new study, published in Proceedings of the Royal Society B: Biological Sciences reported that P. mirabilis females prefer sperm from males with gifts and store more of those gametes. Maria Albo and her colleagues at Aarhus University in Denmark counted sperm retained in newly mated P. mirabilis females and the number of eggs that hatched. Some male partners presented nuptial gifts, while others did not. Accounting for the effect of copulation time, Albo’s team found that females kept about 40 percent less sperm from non-gifting males compared with their gift-bearing counterparts. The researchers also found reduced egg-hatching rates from matings with males that did not present nuptial gifts. This suggested that females manipulated the sperm to skew fertilization success among males using a process called cryptic female choice, which is widespread in nature and is thought to give females an upper hand in sex.
“Cryptic female choice is notoriously difficult to demonstrate, but the authors did that by keeping other influences on sperm transfer constant, such as copulation time,” Marie Herberstein, an associate professor at Australia’s Macquarie University who studies spider behavior but was not involved in the present study, wrote in an e-mail to The Scientist. The only alternative explanation, she said, is that “gift-giving males produce more sperm than non-gift giving males,” but as yet there is no indication that this is the case.
During sexual reproduction, females typically expend more resources than males to produce offspring. In some cases, costly reproduction has made for choosy females: they tend to pick their mates. Some females can even mate with several males, store the received sperm in a bag-like organ called the spermatheca, and then choose whose sperm to use.
“Cryptic female choice only makes sense for females with multiple mates,” Luiz Ernesto Costa-Schmidt, who studies nuptial gifts at Universidad Nacional de Córdoba in Argentina, wrote in an e-mail to The Scientist. Such females can “prioritize sperm of one male over another.”
So males must convince females to mate. One strategy is to tempt the female with an item she cannot resist; for P. mirabilis females, that would be a nutritious snack, such as insect prey. One of several spider species to use nuptial gifts, the P. mirabilis male wraps the food with silk into a sphere. He then offers it to the female and, as she reaches for it, maneuvers into position and transfers his sperm into her. Females can make off with the gift, so his reflexes must be quick and his grip tight. It is a battle of speed and might.
“Copulation time is always controlled by females and depends on gift presence and size. Females consume the gift during mating, so a bigger prey results in a longer copulation time,” Albo told The Scientist in an e-mail. The longer the two are mating, the more sperm the male can transfer. Despite his efforts, the female has the final say—and she most often favors the sperm from males with gifts.
Nuptial gifts may make a male’s sperm more valuable because they reliably indicate a healthy, well-fed male. “Previously, we found that male condition is reflected in their ability to hunt and produce a gift packed in silk,” Albo said. Starved males struggled to wrap prey in silk, and often chose to eat the prey instead, whereas males bearing nuptial gifts were more likely effective hunters and could afford to give up a meal.
However, conclusive evidence of direct correlation between gifts and the male’s hunting ability remains to be found. More importantly, Pavol Prokop, an associate professor at the University of Trnava in Slovakia who has studied nuptial gifts in P. mirabilis, noted in an e-mail, “whether hunting abilities are heritable requires further studies.”
Still, no matter the evolutionary reasons, P. mirabilis males will continue giving nuptial gifts. If sex is a contract, the males are aware of the case made clear by females: no meal, no deal.
Some species of jumping spiders use filters in their eyes to see colors like red and orange—an unexpected discovery.
by Jane J. Lee
(National Geographic, 2015)
The flamboyant colors and flash dance routine of some male jumping spiders are meant to seduce females—which means that these species must be able to see those fire-engine reds and bright greens.
Yet it's remained something of a mystery as to how their eyes perceive certain colors.
Scientists already knew that some jumping spiders, like Habronattus pyrrithrix, could see green and ultraviolet (UV) light through two of their eight eyes. Researchers presumed they could also see reds and oranges—why else would the males bear patches of the fiery colors? But now, based on experiments with H. pyrrithrix scientists know exactly how these tiny spiders do it. They use filters.
The arachnids have red filters that sit in front of cells in their eyes that normally detect green light, researchers report online May 18, 2015, in the journal Current Biology. The effect is similar to the gels placed in front of theater lights to get different colors on a stage. Although, in the spiders' case, they can't remove their red filters, which are made of a type of pigment.
With these built-in filters, H. pyrrithrix gains a third color channel through which to see the world. (Read about other colorful spiders, Sparklemuffin and Skeletorus)
"In principle, they can see an even broader spectrum of colors than we can," says Nate Morehouse, an evolutionary biologist at the University of Pittsburgh in Pennsylvania. This is because H. pyrrithrix "are sensitive not only to the spectrum of colors visible to us, but also to the UV," explained the study co-author.
TRADE-OFFS
Usually, animals the size of H. pyrrithrix have to make tradeoffs between sight and the spatial resolution and sensitivity of their eyes, says Daniel Zurek, a sensory ecologist at the University of Pittsburgh in Pennsylvania.
All the equipment necessary for the kind of sight people enjoy—like specialized cells that pick up more than two colors—just won't fit in eyes the size of a spider's.
(Read about some small spiders whose brains spill into their legs.)
But jumping spiders have found ways around those typical tradeoffs. They've broken up the duties that a single pair of eyes performs in people and distributed them among four pairs of eyes.
"Spiders have eight eyes that are specialized," says Morehouse, some of which detect motion or garner wide-angle views of the surroundings.
Their new study focused on H. pyrrithix's central pair of eyes, which are adapted for seeing patterns and color. The researchers used multiple techniques to study these eyes, including slicing them into thin sections and examining the slices with sensitive microscopes to discover the filters.
Those filters were completely unexpected, says Zurek.
WHAT'S OLD IS NEW
"It's a sweet way of solving the problem," says Gil Menda, a neuroethologist at Cornell University in Ithaca, New York, who was not involved in the study.
"Spiders have eight eyes that are specialized," says Morehouse, some of which detect motion or garner wide-angle views of the surroundings.
Their new study focused on H. pyrrithix's central pair of eyes, which are adapted for seeing patterns and color. The researchers used multiple techniques to study these eyes, including slicing them into thin sections and examining the slices with sensitive microscopes to discover the filters.
Those filters were completely unexpected, says Zurek.
WHAT'S OLD IS NEW
"It's a sweet way of solving the problem," says Gil Menda, a neuroethologist at Cornell University in Ithaca, New York, who was not involved in the study.
It's not unknown to science, he says. Birds and reptiles use filters made out of oil droplets to see colors. Butterflies also employ a filter system.
But as far as Zurek and Morehouse know, this is the first time filters have been seen in spider eyes. (Watch a jumping spider stalk a bee.)
Menda says that this work will open up all kinds of questions for scientists. He looks at how processes in the brain produce certain behaviors in animals. Now that researchers know certain jumping spiders can see red, it could be interesting to see how a spider's brain processes this information.
"We're really at the beginning of explaining" color vision in these spiders, says Zurek.
