14 April, 2024

It has been a couple weeks, so I thought I would summarize the observations of Uta over the past few weeks. Since the start of April, there have been 643 observations of Side-blotched Lizards. As expected as spring develops, there continue to be a number of observations of females that appear to be gravid. While exact stage of gravidity is nearly impossible to tell visually (I rely on physical palpation to score gravidity), the expansion of the lower abdomen as follicles develop give females a characteristic bulge just in front of the hind limbs. Once females oviposit their abdomens are not nearly as distended, such as this or this female (by @todd-black and @tsirtalis, respectively). However, size is an important determinant of when females lay eggs for the first time each spring, so while the presence of the bulge is indicative of gravidity, the absence of the bulge cannot be interpreted without information on the body size of the female.

Females with the telltale bulge ranged from southern California, Texas/Mexico, Nevada, Utah, and Arizona (submitted by @zeevng, @joseph1008, @cesarponce17, @skylermckinnon, @chrisjensen1, and @nolinad, respectively). As of right now, these observations are limited to the more southern populations/subspecies (U.s.elegans and U.s.stejnegeri), but will expand northward as the spring continues. While copulations began back in late March, as observed by @naturephotosuze, it is this observation (by @festuca59) of a female that appears to be nesting that I have selected for observation of the week.

Like most lizards, female Side-blotched Lizards excavate their own burrows for egg laying and this particular female looks both gravid and like she's burrowing. I did a lab experiment years ago to see how deep females would dig given soil of unlimited depth and noted that they went about three body lengths down before laying eggs...so about 6" (15 cm). Burrow construction can take several days and oviposition often happens late in the heat of a nice sunny day. However, it can be really difficult to spot female burrowing because they often start the burrow next to a rock or other cover object, but they are usually out in the open and not under bushes. This behavior will continue as females oviposit at which time we should start seeing more females with the post-reproductive skinny look before they produce a second, third, and even fourth clutch of eggs. A study years ago near Las Vegas estimated that females there had enough time to lay perhaps seven (7!) clutches of eggs and in Oregon I have observed females producing at least four clutches. The breeding season is long and productive in good years and with all the recent rains, this appears to be a good year for many populations of Uta.

—Pete Zani

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Posted on April 14, 2024 01:26 PM by petezani