The sudden shift from a scurrying arachnid to a completely motionless object is a surprising defensive trick spiders use when they perceive an immediate threat. This behavior, often called “playing dead,” is a refined survival mechanism. It represents a last-ditch effort to avoid being eaten when primary strategies, like running or hiding, have failed. Understanding this performance requires looking at the specific biological state the spider enters and the factors that govern how long it maintains the ruse.
Defining Tonic Immobility
The scientific term for this death-feigning behavior is tonic immobility, or thanatosis. It is a state of profound motor inhibition where the spider adopts a characteristic posture mimicking a deceased state. This usually involves the spider flipping onto its back and drawing its legs inward toward the body in a tight, curled position.
The curled-leg posture is a mechanical consequence of how spiders move. Spiders use hydraulic pressure from their internal fluid (hemolymph) to extend their legs. When they enter tonic immobility, this pressure is reduced, causing the flexor muscles to pull the legs into the signature “death curl.” This display involves a complete, temporary loss of responsiveness to external stimuli.
Duration of the Immobility
The length of time a spider remains in this motionless state is highly variable and depends significantly on the species and the circumstances of the threat. The duration can range from just a few seconds up to several hours, but for most common household species, it is limited to minutes. Smaller, faster-moving spiders like jumping spiders may only maintain the act for about ten seconds before attempting a rapid escape.
Larger species often display a longer period of immobility. Black widow spiders, for example, have been observed remaining curled and unresponsive for as long as forty minutes or even a couple of hours after being disturbed. These extended periods are necessary to ensure that persistent predators lose interest and move away. The variation underscores that thanatosis is a flexible, risk-assessment behavior.
Factors Influencing the Response Time
Several environmental and biological variables determine whether a spider’s motionless response lasts for seconds or extends into a longer period.
The duration of tonic immobility is influenced by several factors:
- Genetic predisposition of the species, as some are naturally wired for a longer reaction.
- The perceived intensity of the threat; a physical attack triggers a more prolonged state than a light touch.
- Environmental temperature, with higher temperatures often leading to a shorter response time.
- Metabolic stress, such as starvation, which decreases the time the spider can remain immobile.
- Habituation, where repeated exposure to the same stimulus causes the spider to shorten its death-feigning time.
The Evolutionary Purpose
Thanatosis is rooted in an anti-predator strategy that exploits the preferences of many hunters. The evolutionary benefit is the concept that most predators prefer to consume live, struggling prey, or they may be hesitant to eat something they perceive as already deceased. By falling silent and still, the spider becomes an unattractive or undetectable meal, prompting the predator to abandon the effort and search for a living target.
This defense mechanism is utilized only when other options for evasion have been exhausted, maximizing the spider’s survival odds in a high-stakes scenario. Some male spiders, such as the nursery web spider Pisaura mirabilis, use thanatosis in a non-predatory context. The males briefly feign death during courtship to avoid being attacked or cannibalized by the larger female, increasing their mating success and ensuring the continuation of the lineage. This tactic allows the spider to manage risk and increase its chances of survival.