Diel Cycles in Chemosensory Behaviors of Free-Ranging Rattlesnakes Lying in Wait for Prey by Barbour and Clark 2012
Background:
Sensory systems are energetically costly to develop and maintain over a lifetime. Therefore, certain systems are selected for while others are constrained, and is heavily dependent on the environment certain systems to thrive more than others. It is often hard to tell which develops first or whether the trait co-evolves with the environmental pressures. However, it is clear that certain signals propagate through mediums better than others and can only be received by certain receptors. The loss in eyesight in moles is most likely due to years upon years of living underground, and so eyesight served little function (Stevens, Ch.4.) The same is the case for blind snakes, a non-venomous snake that lives under ground. Eventually, the cost in developing eyes and expending energy to use them became traits that were of little benefit to these animals.
We have known for a while know that snakes use chemoreceptors in similar ways as many animals use olfactory bulbs and taste buds, but these are a part of a different sensory system than the latter two. They flick their tongues and aortic body and carotid receptor cells on the tongue pick up chemical changes that they can use for tracking down prey, finding mates, or recognizing conspecifics. It has also be speculated that they can use this tongue flick to determine changes in infrared waves but this paper focuses on its chemosensory function. A behavior called a chemosensory probe, where rattles snakes extend their heads past their coil and tongue flick right before returning to a stationary position has recently been observed with a novel behavior: mouth gapes, which is just like it sounds. This study investigated whether snakes perform mouth gapes and chemosensory probes at higher rates during nocturnal or diurnal hours to understand the chemosensory function of mouth gapes in relation to sensory probes. Finally, the paper will examine how abiotic factors, like light availability, influences sensory ecology in free-ranging predators.
Results:
Like chemosensory probes, snakes performed more mouth gapes during nocturnal hours than diurnal hours. Also, nearly half of all mouth gapes were observed accompanying a probe, which suggests that mouth gaping serves a chemosensory function. The two could serve a similar purpose but could enhance the power of chemical detection when used in tandem.
When snakes tongue flick, they catch chemicals on their tongue and place them on their vomeronasal organ VMO. Based on the order in which these two behaviors were performed, with mouth gaping following the probe, it may serve to clear the VNO in order to ‘reset’ the system for subsequent tongue-flicking. Volatile chemicals can help predators find prey because the stronger an odor: the greater indication that the animal leaving the scent is nearby. However, with such strong chemical detection, snakes must be able to track these chemicals despite misleading scents found throughout their foraging grounds.
When behaviors are observed side by side in this manner, they tend to serve similar functions. The fact that they are in coiled position for much of their foraging time, and uncoil to perform this function at night more often may suggest two things. One, that this system is relied on more heavily when vision is not accessible and two, they take on the risk of revealing their location to predators. Therefore, the benefit gained from reducing uncertainty in prey location must outweigh predation rate. Our knowledge of the probing behavior was originally limited to obtaining chemical cues from immediate surroundings. Mouth gaping may help them evaluate potential profit of staying at an ambush site, similar to the sit-and-wait technique that spiders are known to do with visual cues. By having a detection system that is nearly silent and requires little initial energy, rattlesnakes can lie undetected by prey and save energy for predation and digestion.
This paper reveals much about the compensation for impaired vision in the chemosensory system of snakes. It also suggests that snakes are no longer constrained to remain cryptic to successfully ambush prey or avoid predators. Their detection ability is especially successful when the lack of ambient light favors does not favor visual systems. Sensory systems can explain a lot about environment and its effect on evolution. Form tends to meet back up with function and sensory systems emulate what has been and can be effective given selected pressures like chemical, mechanical, magnetic and electric propagation, sound clutter, ambient light, predator avoidance, available mediums and so forth. A greater understanding of when certain sensory systems are lost and gained between closely related species or even lineages of the same species can help us better understand how phylogeny is reflected by ontogeny, physiology, and current utility. Sensory trade-offs are a gateway into Tinbergen’s 4 questions if observed in the correct scope.
Reference
Barbour, M. A. and Clark, R. W. (2012), Diel Cycles in Chemosensory Behaviors of Free-Ranging Rattlesnakes Lying in Wait for Prey. Ethology, 118: 480–488. doi:10.1111/j.1439-0310.2012.02035.x
