Thursday, September 7, 2017

Why Are These Rattlesnakes Piled On Top of Each Other? ---Guest Post---

I recently received this e-mail from Dan G. in New Mexico and knew I had to pass this to Melissa Amarello to tackle in a guest post.

"...I know what they both are, but I'm curious about what it is they're doing, and why, despite being the same species (or maybe they're not!) they're so different in color and size. (The smaller appears to have only one less rattle than the big guy).  They're in the driveway, presumably gathering the last bit of warmth from the rocks. A bit cooler today (a relative term in the desert, I know) The second picture is the smaller snake from two days ago. It had scooped out all the rocks and made a nice depression in the earth.. this time to stay cool.  Anyway, I've never seen them pile on like this before, so I'm curious.


OH! I'm positive the big one is a Western Diamondback, and since the tail markings are the same on the smaller snake, I presume it is, too, but maybe you, or someone else will know better.

Dan G.
Chihuahuan Desert, New Mexico


    Indeed, these are western diamond-backed rattlesnakes (Crotalus atrox). Like most rattlesnakes, western diamondbacks are sexually dimorphic: males and females look different. Males are bigger, with relatively wider heads and longer tails than females. Adulthood (i.e., sexually maturity) is determined by size, not age, in rattlesnakes. Adult females allocate their resources to reproduction rather than growth, while male rattlesnakes continue to grow and thus reach larger sizes. So the smaller snake in this photo is an adult female and the big guy is an adult male.

Left: a still-growing young Western Diamondback
Right: Old adult that's been same size for many years
 
  
How do we know the smaller snake isn’t a juvenile? 

    Rattlesnakes add a new segment to their rattle each time they shed their skin, which may happen one or more times per year. The width of each segment is correlated to the snake’s body length, so a young snake that is still growing will have a strong taper to their rattle while an adult snake will have segments of similar width. The most recent segments of both snakes’ rattles are fairly similar in width, indicating that they are both adults who are about as big as they’re going to get.

Are male and female western diamondbacks different colors too? 

Rattlesnakes can vary a lot in color and pattern, 
even if they are the same species! 
These are all Arizona Black Rattlesnakes.
    Although I have seen this idea discussed by snake enthusiasts, to my knowledge it has not been quantified (color measured and compared between the sexes). Snakes can vary dramatically in color and pattern, even within the same species in the same area, which is why they can be so hard to identify. Unlike birds, the snake you encounter in the wild is unlikely to look exactly like the one in the field guide. The color differences here are a perfect example of the variety of colors common within a population of western diamondbacks.

What are these two rattlesnakes doing and why? 

    Rattlesnakes are great romantics. Once a male finds a female, he may stay with her for days or even weeks

Why? 

    Females can be hard to find or may not be immediately receptive, and there are incentives to keeping other males from mating with her. This prolonged courtship may include accompaniment (hanging out with and following her if she moves), mate-guarding, male-male combat, chin-rubbing and tail-searching, and finally (if he’s lucky) mating. Mating is not necessarily the end of their time together though. Males will often accompany, guard, and engage in combat with other suitors after they’ve mated. Multiple paternity is apparently common in western diamondbacks, so mating doesn’t guarantee he’ll be the only or even one of the fathers of her litter next year. Litters may have 1-3 fathers, which may not include all males observed mating with the mother. How the father or fathers are determined post-mating is something we know little about, but it is probably beneficial to decrease the competition via guarding and combat.

How many Western Diamondback Rattlesnakes can you see?
    So back to the photo – what’s happening here? The male’s tail is clearly visible and unattached to the female’s so they are not mating. He also isn’t in the right posture for chin-rubbing and tail-searching. Their posture most closely resembles a form of mate-guarding called stacking, which is exactly what you’d guess: a male coiled and stacked on top of a female, sometimes so perfectly that it’s difficult to see that there are two snakes.

    If you’ve observed any of these courtship behaviors in your yard and are worried about being overrun with baby snakes – don’t be. Most rattlesnakes in the southwestern US mate in the late summer to early fall (our summer rainy season or monsoon) and give birth the following year during the same season (some species a bit earlier in the summer). So their nest sites are often far away from and in very different habitat than where they court and mate. Western diamondbacks add a little twist to this story in that they also court and mate at their winter dens soon after emerging in the spring, perhaps to add or replace a father of that year’s litter.

    There are surprisingly few photographs of stacking, so thanks for sharing this with us! Scientists don’t often get funding for natural history research, so we depend on everyone to document and share interesting observations like this one. And thanks for moving these snakes only a short distance, out of harm’s way; backyard courtships often lead to unhappy endings for rattlesnakes.

About the Author: Melissa Amarello, Co-founder, Advocates for Snake Preservation 

    Melissa’s lifelong fascination with snakes led her to work on a variety of projects on natural history and conservation of reptiles in Arizona, California, and Mexico. After witnessing how negative attitudes can stifle conservation efforts, she incorporated education and outreach into her research to foster appreciation for snakes by sharing stories and videos of their behavior in the field. In the spring of 2014 she co-founded Advocates for Snake Preservation (ASP) with Jeff Smith, to change how people view and treat snakes. In 2017 they received the Jarchow Conservation Award for commitment and creativity in studying snake behaviors and tireless and continuous efforts to use scientific knowledge to advocate for snake conservation through outreach and social activism. Melissa received her B.S. in wildlife, watershed, and rangeland resources at the University of Arizona and her M.S. in biology at Arizona State University, where she studied rattlesnake social behavior.

Relevant Scientific References:


Taylor, E. N. and D. F. DeNardo. 2005. Sexual size dimorphism and growth plasticity in snakes: an experiment on the Western Diamond-backed Rattlesnake (Crotalus atrox). Journal of Experimental Zoology 303A:598–607.



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