More baby rattlesnake info

Just when you think you are beginning to understand rattlesnake behavior, they do something completely unexpected. But, of course, that’s exactly why we study them!

Compared to my previous observations of Northern Pacific Rattlesnakes, this year’s mothers and neonates are behaving very differently. For starters, during multiple seasons at my El Dorado Hills study site and last year at Effie Yeaw NC, newborn rattlesnakes basked in the entrance to their birth shelter, usually in the morning, with mom laying just behind them. With a stealthy approach, they were not difficult to photograph.

Six-day-old babies basking at their El Dorado Hills birth site in September 2011. Mom is out of sight just behind them.
Six-day-old babies basking at their El Dorado Hills birth site in September 2011. Mom is out of sight just behind them.
Female 47 with a couple of her 2014 brood on 15 September. Original RAW IMG_2670.CR2
Female 47 with a couple of her 2014 brood at Effie Yeaw Nature Center on 15 September last year.

As of yesterday (6 September), 5 of 6 monitored females had had their babies but I have yet to actually lay my eyes on a youngster. The only images of kids so far this year are from videos made underground with the BurrowCam. In one case, Female 54 has been so far into a void under a large log that even the four-foot probe of the BurrowCam just reveals empty tunnel as far as the light illuminates. Yet, yesterday morning, she was outside for the first time in weeks and clearly no longer pregnant (photo below). Her kids are nowhere to be seen.

Fem 54 06Sep15

Two weeks ago, the radio signal from our Female 53, who had been stationary with two other pregnant females for over a month, disappeared. After searching for her for several days, I finally caught a faint signal and followed it more than 200 m (220 yards) to a small burrow in the soil where the BurrowCam revealed that she was alive and still pregnant. As of yesterday, she was still there and still appeared pregnant (photo below). Such a long move so late in her pregnancy is quite unusual.

Fem 53 06Sep15

Finally, we also had a litter born in a holding bucket; a less than ideal situation but it provided an opportunity to collect some data not otherwise possible to get. Female 41’s transmitter was due for replacement early in July (transmitters function for about 12 months) but she had already gone into her gestation refuge and remained inaccessible ever since. I had resigned myself to her transmitter probably failing any day and having to search for her after she left her babies, along with Males 36 and 37 (who currently carry prematurely-failed transmitters). Then, last Monday, I was surprised to find her outside of her refuge, so I recaptured her. I don’t do transmitter surgeries during late-term pregnancy, plus it was apparent that she was likely to give birth very soon, so I planned to replace her transmitter as soon as she delivered her kids.

I didn’t have long to wait. She delivered nine healthy babies very early Friday morning (photo below). Her transmitter was replaced on Saturday and she, along with her brood, were released into her gestation refuge yesterday morning.

OK, you have to say, "Awww. aren't they cute!" even if you're not fond of rattlesnakes. This is Female 41 reunited with her kids after her transmitter surgery on Saturday. They were released together the next day and should remain together for another 10 days or so..
OK, you have to say, “Awww, aren’t they cute!” even if you’re not fond of rattlesnakes. This is Female 41 reunited with her kids after her transmitter surgery on Saturday. They were released together the next day and should remain together for another 10 days or so.

Here’s the interesting data that resulted from this captive birth: Subtracting mom’s body mass a few hours after birth from her body mass the day before revealed that she had lost 37% of her pre-parturition body weight. Average body mass of the kids was just under 14 g (about 1/2 ounce). Total body mass of her brood was 88% of her lost body mass, meaning that about 12% of her lost weight is attributed to amniotic membranes, fluid, etc. It is important to note that these live births are more akin to the egg-laying process than to mammalian births. That is, there is no placenta; the female rattlesnake secretes a yolk for each embryo that nourishes that embryo as it grows without further contribution from mom. Each embryo is contained in a thin transparent sac, rather than a thick egg shell (see the third photo in my Rattle Growth post from 14 July – click here). In addition to the embryo, the sac is filled with amniotic fluid and membranes enclosing what’s left of the yolk and the embryo’s waste.

The lesson from Female 41’s transmitter running out of time while she was not accessible is this: in the future, I will replace transmitters in females in May, regardless of the remaining battery life. For fifteen years, I have simply replaced transmitters at 12 months but Mohave Rattlesnakes in the desert were not reclusive during pregnancy and I just happened to have avoided summer anniversary dates for transmitters in females during my El Dorado Hills work.

So there it is… now you can amaze your friends with more than they ever wanted to know about rattlesnake reproduction!

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