Researchers Uncover Complexities of Sharks’ ‘Playing Dead’ Behavior

Tonic immobility (TI) has emerged as a captivating behavior observed across various species in the animal kingdom, including sharks and rays. This phenomenon involves a temporary cessation of movement, causing animals to appear frozen in place. While often likened to the behavior of opossums “playing dead” to evade predators, the reasons behind TI can be more complex. A recent study conducted by researchers from James Cook University, namely Joel H. Gayford and Dr. Jodie L. Rummer, sheds light on this intriguing behavior in chondrichthyans, a group that includes sharks, rays, and chimaeras.

The researchers embarked on an extensive investigation, combining hands-on experimentation with a thorough literature review to assess the occurrence and variability of the tonic limp response (TLR) among different shark and ray species. They tested 13 species by gently inverting them to observe if this common technique would induce relaxation, cessation of struggling, and deep rhythmic breathing—hallmarks of TLR.

Of the 13 species studied, seven exhibited the expected tonic limp response when flipped onto their backs. The time taken for the behavior to manifest varied significantly; for instance, the common smooth-hound (Mustelus mustelus) exhibited TLR in as little as seven seconds, while the blacktip reef shark (Carcharhinus melanopterus) took up to 25 seconds. Once immobilized, the duration of stillness also varied, ranging from a brief 12 seconds to over two minutes (131 seconds) in the Atlantic guitarfish (Rhinobatos lentiginosus).

A significant finding from the study is the consistency of TLR within species. Each individual either consistently displayed TLR or did not, indicating that TLR is a species-level trait unaffected by individual differences or short-term environmental factors. The researchers found no correlation between TLR and factors such as body size, habitat depth, or predatory status. This suggests that ecological aspects may not strongly influence the presence of TLR, pointing instead to its evolutionary origins.

The study also examined the presence of TLR in the chimaera species, Callorhinchus milii, commonly known as the elephant fish. Notably, this species showed no signs of TLR when inverted. This raises questions about the evolutionary history of this trait: did it exist in their common ancestor, was it lost in chimaeras, or did it never develop within this group?

According to the researchers’ models, TLR likely existed in the common ancestor of all chondrichthyans but has been lost at least five times across various lineages, with no evidence suggesting it evolved anew. This pattern indicates that TLR may be a plesiomorphic trait, inherited from a distant ancestor and retained by certain species despite its unclear utility today.

Several hypotheses attempt to explain the evolutionary purpose of TLR. One posits that it serves as a mechanism for predator avoidance, akin to playing dead. However, the mechanics of predator attacks on sharks make this explanation tenuous. Another theory suggests that TLR could be linked to mating behaviors, as instances of males inverting females during copulation have been observed. Yet, this theory also faces challenges: no differences in TLR have been noted between genders, and if going limp made females more susceptible to unwanted mating, natural selection would likely eliminate that behavior.

A third hypothesis proposes that TLR might serve as a protective measure against sensory overload, acting as a reset mechanism. Nevertheless, this theory remains untested specifically in sharks.

Particularly noteworthy is that all shark and ray species lacking TLR are smaller-bodied and inhabit shallow, complex environments, such as coral reefs or kelp forests. In these habitats, the risk of becoming stuck or injured if they go limp could have led to the evolution of this trait being suppressed. Conversely, larger sharks or those in open water may experience fewer risks, allowing them to retain TLR without significant cost.

The findings of this study underscore the complexity of evolutionary biology and the importance of avoiding broad assumptions regarding traits across species. While TLR may appear similar among various sharks and rays, its evolutionary significance likely differs from species to species. The continued investigation into such behaviors not only enhances our understanding of these fascinating marine creatures but may also unlock further insights into their evolutionary past.