Climate stress can impair the romantic gift of male spiders

The blinding of friendship in spiders can lose a bit of vigor in uncertain climates. Men in places with rain and unpredictable temperatures turn into suitors who please mostly with cheap and useless gifts.

Researchers have described gift-giving in courtship in only 15 or 20 of the more than 50,000 known species of spiders in the world, says evolutionary biologist Maria José Albo of Universidad La República in Montevideo, Uruguay (SN: 26.7.16). Since 2015, she and her lab have focused on spiders that flirt mostly in the evenings and nights among the rocks and pebbles of the rivers of Uruguay and southern Brazil.

When the brown male Paratrechalea ornata feels a female to attract, he chooses an object to spin silk for the presentation of friendship. Male bodies are only about the size of peppercorns hanging from the legs, so the gifts are “very, very small,” says Albo.

The best of these gifts are scraps of fresh insect flesh that a female can feed on while a male injects his sperm. The more time she spends opening and eating her gift, the more sperm she can deliver. This abundance can help as sperm in her reproductive tract from different males compete for daddy.

So it was surprising when, while monitoring gifts wrapped in spider silk in six countries in Uruguay and Brazil, Albo and colleagues found some painful foods on offer.

The first Albo opened was … a seed. This is useless for a spider. Their mouth parts function more like milkshake straws, but for meat shakes. Over the years of monitoring friendship gifts, Albo and colleagues have found a host of equally inappropriate gifts: broken pieces of insect exoskeletons, pieces of plant stems, and more.

In two study sites, more than half of the gifts analyzed were inedible garbage wrapped in silk, Albo and colleagues report in The American Naturalist November issue. What these two friendship gift zones have in common, the researchers say, is the low predictability of rainfall and temperatures.

In places with more surprises and stresses, perhaps with more flooding or insect predation cycles, spiders have many other challenges. So maybe what’s wrapped inside competitive gifts just isn’t that important anymore. The gift goes on, but sexual selection based on what’s inside is “relaxed,” as biologists say. Among the many implications, says Albo, the weakening of a friendship signal “shows how climate change can affect something very fundamental—which is reproduction.”

Also, it is “definitely possible” that the mercurial weather could change not only the content of the men’s gifts, but also the important silk wrapping, says behavioral ecologist Michelle Beyer, of the Ludwig-Maximilians-Universität München in Germany. She was not part of the South American study, but works with Europe’s best-known gift-giving counterpart, the baby web spider. Pisaura mirabilis. Male donors of the species on both continents coat their silk with compounds that somehow enhance paternity.

“It’s definitely possible,” she says, that rain or high temperatures could change the composition of the silk, as well as the length of time the men doped in it. Plus, the information silk road goes both ways. Research is already underway on female silk of the European species, she says, to see if the spins from heat-stressed females retain their usual attractiveness to males.