19 April: Things heat up and more courtship

The little male courting Female 47 in the video I posted on 4 April continued to court her until at least 11 April, staying with her a total of 9 days that I observed. During that time, the female made several short moves of just a few meters and I never observed any indication that she did anything but ignore the male. Presuming that she was trying to hunt (these rattlesnakes are predominantly sit-and-wait ambush predators), it is hard to imagine that she could have had much success with his nearly constant movement. Despite my best efforts, the little male evaded several attempts to catch him. On two occasions, I actually had him but was unable to get him into a bag before he wiggled loose and disappeared into thick grass. On both occasions, I was sure I had spooked him and he would abandon the female but he didn’t. He only became more wary and would vanish instantly the moment he detected my approach. Since I have enough photographs to identify him in the future, I have given him the next identification number: CROR51.

Then on 13 April, I found that Female 47 had made a 97 m (318 feet) move out into the meadow. When I located her, she was under a mat of old dry grass beneath the living grass. I did not find Male 51 with her but, because I had to dig around in the grass to find her, he certainly had time to flee unobserved. She was in the same spot with no other snake observed on 15 April, too. On 17 April, she had moved 58 m southeast, back into the edge of the forest. She was coiled in the grass, apparently hunting and alone.

Today, 19 April, she had moved only a couple of meters. But as I got close to her signal, a rattlesnake shot out of the grass near my feet and under a nearby log – Male51! When I actually found Female 47, she was about 5 m from where I had flushed Male 51 and I saw why Male 51 was not with her. She was being vigorously courted by Male 49, a much larger male that was processed and released in early March without a transmitter. In fact, Male 49 had courted this female between 22–26 March at another location 150 m away. You can see the value of marking the rattles with paint in the photo from today (below), Female 47’s rattle is marked with yellow/green paint and Male 49 is marked with white/green. Since Male 49 is not telemetered, he would not be identifiable without the paint in his rattle.

CROR47 19Apr15B

Sadly, Female 39’s radio signal disappeared on 3 April. Of the six animals implanted with that batch of refurbished transmitters, five failed early. I have found and replaced transmitters in three of them, Males 35, 38 and 40. At present, Males 36 and 37 and Female 39 are carrying transmitters with prematurely dead batteries. I am still hoping to either find them courting or being courted by telemetered snakes or to have them turn up around the buildings or Maidu Village where staff can capture them.

Now that we have accumulated some data this spring with three females that are not incubating late-term embryos, I can demonstrate more effectively a behavior I mentioned last year. During the last 2–3 months of pregnancy (generally about mid-July to October), females find shelters where they can thermoregulate to stay within a narrow body temperature range. That is, they need a shelter that keeps them warm at night but where they are protected from the midday heat. I have repeated the chart from last year below on the left, showing the body temperatures of two late-term pregnant females compared to the males. The chart on the right is corresponding data from this spring, comparing body temperatures of non-gestating females with the males.

temps_late summer 2014temps_spring 2015In these charts, “frequency” is the percentage of observations where I  recorded each body temperature for that sex during that period. In 2014, the average male body temp was 24C (75F) while the female average was 30C (86F). This spring, males have averaged 24C (75F) and females 22C (72F). Of course, the weather is cooler in the spring (even this spring) than in summer but there is ample opportunity for these snakes to get their body temperatures up above 30C (by late morning, ground surface temperatures are often above 40C in direct sunlight, even when air temps are relatively cool). But my point is that there is no real difference between average male and female body temperatures before the pregnant females go into their late-term thermoregulatory behavior… which is evident in the late summer data above where body temps of the pregnant females narrowly cluster around 30C (86F).

(For you statisticians that may read this, I admit that these data are not publishable in this form as they contain some significant pseudoreplication. Nonetheless, they serve to illustrate my point that gestating vs non-gestating female body temps are very different.)

Hopefully, we will get a few more females telemetered in the next month or so and, by mid summer, we should know if any are going to reproduce this season.

Stay tuned!

Courtship video

Yesterday, 3 April, I found an unmarked rattlesnake with a very short broken rattle on top of our Female 47. The unmarked snake fled immediately but I was able to see that he was a small adult with only two rattle segments (the live proximal segment plus one hollow one). As there is no reason for another female to be accompanying Female 47, I almost certainly interrupted a courting male.

When I returned today, Female 47 had moved about 6 m and the same unmarked snake was on top of her. Since I approached with more caution today, I was able to watch and video tape the courtship. The female appears to have been in a typical round “pancake” coil and looks to be completely unresponsive; she does not move except for being pushed around by the male. The male, on the other hand, goes through a cycle of 30-90 seconds of rest followed by a short period of head and body jerking, short rapid tongue-flicks and chin-rubbing on the female, culminating with pushing his cloaca around under her. During this process, you will see the male wag his tail. The male then goes back into a rest phase before repeating the effort.

This is very typical rattlesnake courtship behavior. Since the cloaca (on both sexes) is sealed by a flat tight-fitting belly scale, courtship by the male is almost certainly wasted effort unless the female is receptive. What makes her receptive? Can he woo her into being receptive? We are not sure. Head-jerking, tongue-flicking and chin-rubbing is energetically expensive for the male and the movement exposes both snakes to potential predation, so it must be productive or the behavior would not endure. And the female is almost certainly selective, maybe selecting mates based on size, health, courtship performance, or other criteria. How long the male can keep up the courtship may be an indication to the female of stamina and health.

Female health, particularly replenishing body fat since delivering her last litter, may affect the female’s receptivity but fertilization of ova does not necessarily coincide with copulation. Long-term sperm storage is well established in many pit vipers, including rattlesnakes. Thus, it is common for a female rattlesnake to store viable sperm for multiple seasons, allowing her to fertilize ova without having recently mated. But does that mean that a female will mate when she is not physically ready to produce young? At this point, we just don’t know how all of this works.

In the video, the male’s head is at the top of the frame and his tail is in the lower left. The female’s head is in the lower right. So, click here for a 45-second clip of today’s courtship.