Courtship rituals among spiders may lose their sparkle in unpredictable climates. Males in regions with erratic rainfall and temperature patterns tend to evolve into suitors who primarily offer inexpensive, worthless gifts. Evolutionary biologist Maria José Albo of Universidad La República in Montevideo, Uruguay, notes that researchers have documented gift-giving in courtship in only about 15 to 20 of the world’s over 50,000 known spider species (SN: 7/26/16). Since 2015, Albo and her team have concentrated on spiders that engage in nocturnal courtship among the rocks and pebbles along rivers in Uruguay and southern Brazil.
When the brown male Paratrechalea ornata detects a female, he selects an object to wrap in silk as a courtship offering. Given the males’ tiny size, roughly that of peppercorns, the gifts are “extremely small,” Albo explains. The most valuable presents are fresh insect meat that females can consume while the males deposit sperm. The longer the female spends opening and eating the gift, the more sperm the male can deliver, potentially aiding in sperm competition within her reproductive tract.
However, Albo and her colleagues were surprised to find pitiful offerings among the silk-wrapped gifts they monitored at six locations in Uruguay and Brazil. One of the first gifts Albo examined was a seed—useless for a spider, whose mouthparts are adapted for consuming meat. Over years of studying courtship gifts, Albo and her team have encountered numerous inedible items, such as broken insect exoskeletons and plant stem fragments.
At two study sites, over half of the analyzed gifts were inedible, silk-wrapped debris, as reported in The American Naturalist’s November issue. These two sites shared a commonality: low predictability in rainfall and temperature. In environments with more unpredictable conditions, such as frequent floods or disrupted insect prey cycles, spiders face numerous challenges. Consequently, the contents of competitive gifts may no longer be as crucial. Gift-giving persists, but sexual selection based on the gift’s content has “relaxed,” as biologists describe it.
Albo highlights that this weakening of a courtship signal underscores how climate variability can impact essential aspects of reproduction. Additionally, behavioral ecologist Michelle Beyer of Ludwig-Maximilians-Universität München in Germany suggests that fluctuating weather could alter not only the gifts’ contents but also the silk wrapping. Beyer, who was not involved in the South American study but works with Europe’s well-known gift-giving spider, Pisaura mirabilis, notes that males of both species enhance their silk with compounds that may improve their chances of fatherhood.
Beyer posits that rain or high temperatures could affect the silk’s composition and the duration of the doping compounds. Moreover, the silk’s informational pathway operates in both directions. Research is ongoing to determine if heat-stressed females’ silk retains its usual appeal to males.
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