Scientists have discovered the potent and surprisingly pungent secret behind the black widow spider’s deadly allure. To attract mates from afar, female black widows and their close relatives release a chemical cocktail into the air that carries the distinct aroma of cheesy or sweaty feet. This odorous signal, composed of simple fatty acids, serves as a powerful long-distance beacon, guiding hopeful males through complex environments directly to a potential partner.
The new research decodes how these spiders create their signature scent. The females initially lace their silk webs with large, complex chemical compounds that have no smell. These larger molecules then slowly break down, releasing smaller, lighter molecules that can travel on air currents. Among these are chemicals like butyric acid, isobutyric acid, and isovaleric acid, notorious for their roles in the sharp smells of certain cheeses, body odor, and rancid butter. This chemical strategy resolves a long-standing question about how male spiders, with their relatively simple sensory tools, manage to locate females from a distance.
A Pungent Chemical Portfolio
The core of the spider’s seductive perfume lies in a group of volatile carboxylic acids. Studies on the false black widow, Steatoda grossa, revealed that their webs emit butyric acid, isobutyric acid, and hexanoic acid. Similarly, research on the invasive Australian redback spider, Latrodectus hasseltii, a true widow, confirmed that butyric acid and isovaleric acid were present on the webs of virgin females but absent after mating. These compounds are highly effective, carrying scents that humans find sharp and unpleasant but which are an irresistible call to male spiders.
The identification of these specific molecules provides a chemical vocabulary for the spiders’ communication. While scientists have known for decades that spiders use pheromones, the exact identity of the airborne components remained elusive for many species. By isolating these simple acids, researchers can now precisely study how they influence male behavior and why such a strong, distinctive odor proved to be an effective evolutionary strategy for a spider that relies on remote attraction.
An Elegant Two-Stage System
From Solid to Scent
The method of the scent’s release is as elegant as it is effective. The female spider does not produce the smelly acids directly in a gland. Instead, she synthesizes and deposits larger, more complex molecules called serine esters onto her web. These precursor molecules are odorless and remain stable on the silk, acting as a chemical reservoir. This initial deposit serves as a contact pheromone, which can trigger courtship behavior in a male spider if he is already on the web.
A Controlled Chemical Reaction
The true innovation is the conversion of these contact pheromones into an airborne signal. Over time, the serine esters undergo hydrolysis, a chemical reaction that breaks them down and releases the smaller, pungent acids. This process is believed to be controlled by the female, possibly through enzymes or changes in the web’s pH, allowing her to adjust the signal’s strength. This gives her the ability to manage her web’s attractiveness, potentially “turning up” the scent when she is receptive to mating, ensuring that the powerful signal is only broadcast when it is most advantageous.
Pinpointing the Pheromone
Identifying these elusive compounds required sophisticated analytical techniques. Researchers collected the chemical traces from spider webs using methods like solid-phase microextraction, which uses a special fiber to absorb the volatile molecules. These samples were then analyzed using gas chromatography, a process that separates the different chemicals in the mixture and allows them to be identified. In the case of the Australian redback spider, two major chemical peaks corresponding to isovaleric acid and butyric acid appeared in samples from virgin females but were missing from the webs of mated females, providing strong evidence of their role in mating.
To confirm their findings, scientists conducted behavioral assays. In laboratory tests, male redback spiders were placed in an olfactometer, a device that allows them to choose between two different scent sources. The males were drawn to pure, synthetic butyric acid just as strongly as they were to the silk from a virgin female’s web, confirming the acid is a key component of the attraction.
Evolutionary Edge of a Strong Odor
For a male black widow, the search for a mate is a perilous journey fraught with dangers, including predation and intense competition from rivals. Life for a male is often short, and his reproductive success hinges on his ability to find a receptive female as quickly as possible. A strong, unambiguous chemical signal that can travel long distances is therefore a major advantage. The simple chemical nature of butyric acid and its relatives allows the scent to effectively disseminate in the air, providing a clear path for the male to follow.
This powerful beacon helps minimize the time males spend wandering aimlessly, which in turn reduces their risk of being eaten by predators. A clear signal also helps males avoid wasting time and energy on females who have already mated or are not sexually receptive. By responding only to the specific scent of a virgin, males can focus their efforts where they have the highest chance of success in a competitive mating landscape.
New Insights into Spider Society
This discovery pushes the field of arachnid chemical ecology forward significantly. While many complex contact pheromones have been identified in spiders, knowledge of their long-range, airborne counterparts has been limited. The finding that different widow spiders and their relatives use a shared palette of simple, smelly acids suggests a common and effective communication system. Furthermore, this knowledge has practical applications. The identification of butyric acid as a primary attractant for the invasive Australian redback spider provides a crucial foundation for developing targeted pheromone traps. Such tools could be used to monitor or control populations of this medically significant spider in regions where it has become a pest, such as New Zealand and Japan.