California ground squirrels and rattlesnakes: Irreducibly complex?

CLAIM: When California ground squirrels discover rattlesnakes in their habitat, they will internally pump blood into their tails to create a decoy for the rattlesnake to lunge at, since the snake can see the tail using its infrared vision. The squirrel won't be harmed if bitten because it has proteins in its blood that function as a natural antivenin. Interestingly, the ground squirrels will also use this decoy technique without sending blood to their tails against gopher snakes, which cannot detect heat in the same way that rattlesnakes can. How does the squirrel know when to make its tail larger, or that a rattlesnake can see heat and a gopher snake can't? Obviously, God knew that they would need this defense mechanism and supernaturally designed it to be able to tell the difference. (Taylor & Cadwallader, n.d.) (Von Vett & Malone, 2017, p.112)

RESPONSE: It's important to note that the heated tail response in the ground squirrels is not a decoy mechanism but a behavioral adaptation to deter attacks in general, causing the snake to retreat rather than to bite. (Barbour & Clark, 2012; Minkel, 2007) By pumping blood into their tails, the squirrel can create a larger and more intimidating heat signature, which rattlesnakes interpret as a significant threat via their infrared detection systems. The ability to differentiate when to pump blood and when not to is not consciously decided but is dictated by the squirrels' olfactory and acoustic systems, which recognize distinct chemical and auditory cues associated with each snake. (Minkel, 2007) This ability isn't unique to the squirrels and snakes, either. Numerous species exhibit similar abilities - for example, mice select mates based on olfactory cues linked to immune system compatibility, a process driven by biology rather than conscious decision making. (Ferkin, 2018; Handwerk, 2015)

IRREDUCIBLE COMPLEXITY AND SELECTIVE PRESSURE

Irreducible complexity states that certain biological systems are composed of multiple interdependent parts, all of which are necessary for functionality. The removal of any single component would render the entire system nonfunctional, suggesting that such systems could not have evolved through successive, slight modifications since all parts must be present from the beginning of that system's existence. (Behe, 2006, p.29) Based on this definition, the ground-squirrel defenses in question aren't irreducibly complex. The tail-waving behavior occurs in numerous contexts, including social communication or even when no stimuli is present. (Putnam & Clark, 2015)

Further, the idea that the venom/antivenin relationship between the squirrels and rattlesnakes cannot be explained by evolutionary mechanisms is incorrect. Research suggests that this predator-prey dynamic has co-evolved over approximately 10 to 12 million years, with adaptations developing incrementally in both species. (Eaton, 2003) The model for this process predicted evidence of a dynamic evolutionary arms race, with rattlesnake venom potency and delivery mechanisms evolving alongside the ground squirrels' venom resistance, since rattlesnake venom profiles are known to exhibit diversity based on diet, environment, and available prey. (Gibbs & Mackessy, 2009) In 2016, this model was put to the test in research published by the Royal Society (Holding et al.) where researchers studied rattlesnakes and ground squirrels living in different regions. They measured how effective the snake venom was at killing squirrel cells (venom potency) as well as how the squirrels' blood neutralized the venom (venom resistance). They found that venom and resistance are linked: in areas where rattlesnakes and squirrels frequently interact, the venom of the snakes tends to be more potent, and the squirrels are more resistant to it (i.e., both species are constantly adapting to each other). The snake populations in different areas produced venom tailored to the resistance levels of the local squirrel populations, directly influencing the potency/resistance strength of each other. If the relationship were irreducibly complex and designed to work "perfectly" from the beginning, we would expect the same venom and resistance across the populations. However, the observed differences between populations of snakes and squirrels reveal that the traits are shaped by local interactions and selective pressures, not by a fixed design.

REFERENCES AND FURTHER READING

Barbour, M. A., Clark, R. W. (2012) Ground squirrel tail-flag displays alter both predator strike and ambush site selection behavior of rattlesnakes. Proceedings of the Royal Society B: Biological Sciences, 279(1743), 3827-3833

Behe, M. (2006) Darwin's Black Box: The Biochemical Challenge to Evolution. Free Press (2nd ed.).

Eaton, J. (2003, May 15) The Best of Enemies. Ecology Center.

Ferkin, M. H. (2018) Odor Communication and Mate Choice in Rodents. MDPI Biology, 7(1), 13.

Gibbs, H. L. & Mackessy, S. P. (2009) Functional basis of a molecular adaptation: Prey-specific toxic effects of venom from Sistrurus rattlesnakes. Toxicon, 53, 672-679.

Handwerk, B. (2015, June 4) Mouse Noses Can Bypass the Brain to Make Females Blind to Males. Smithsonian Magazine.

Holding, M. L., Biardi, J. E., Gibbs, H. L. (2016) Coevolution of venom function and venom resistance in a rattlesnake predator and its squirrel prey. Proceedings of the Royal Society B: Biological Sciences, 283(1829), 20152841.

Minkel, J. R. (2007, August 14) Squirrel Has Hot Tail to Tell Snakes. Scientific American.

Putnam, B. J. & Clark, R. W. (2015) The fear of unseen predators: ground squirrel tail flagging in the absence of snakes signals vigilance. Behavioral Ecology, 26(1), 185-193.