Biological timing (phenology) of Uta juveniles

This year I've been making a more concerted effort to annotate observations of Uta as I identify. Given that the end of summer is upon us, I thought this would be a good opportunity to explore the timing of the appearance of hatchlings/juveniles in the different subspecies of Side-blotched Lizards. The biological timing (phenology) of reproduction and other aspects of life history are being altered rapidly by our changing environment, so documenting patterns through time and their change through time is a major scientific endeavor at the moment. Below (Fig. 1) is a plot of the total number of observations annotated with "life stage" (vertical axis) with blue being adults and orange being juveniles shown for each month of the year (horizontal axis).

Fig. 1: total number of life stage annotations for all Uta stansburiana over time; blue = adult, orange = juveniles.

First, the seasonality of lizard activity is visible in this plot with a large spike in adult observations in the spring (centered around April) and a smaller peak in the fall. One major caveat to a plot like these is that effort is not controlled meaning there is no way to account for the fact that more people are probably out observing in spring and fall, especially in areas with excessive heat, like the desert southwest.

Second, juveniles have a single peak starting to uptick in June and continuing through October. This probably does represent the very real seasonal spike in hatchlings entering the population, growing into juveniles throughout the summer, and being indistinguishable from adults by late summer/fall.

Next, let's take a look at this same pattern sub-species by sub-species, specifically focusing on juveniles. In case you didn't know, Uta stansburiana subspecies are primarily location based as shown in this map I created (Fig. 2)

Fig. 2: sub-specific range map for Uta stansburiana (based on published data and unpublished observations by me).

The reason I split these by subspecies is the expectation that at higher latitudes juveniles should enter the population later (spring starts later) than at lower latitudes. Luckily for us, sub-species like U.s.uniformis and U.s.nevadensis both occur at relatively high latitude/elevation combinations when compared to U.s.elegans or U.s.stejnegeri.

To start, let's consider the southern subspecies where spring comes early, which means hatchlings/juveniles should be observed relatively early. Below are plots for U.s.elegans (Fig. 3) followed by U.s.stejnegeri (Fig. 4). The data are better for U.s.elegans mostly because the community of observers appears to be stronger in places like Los Angeles, San Diego, and Phoenix, which is their core range.

Fig. 3: total number of life stage annotations for Uta stansburiana elegans over time; blue = adult, orange = juveniles.

Fig. 4: total number of life stage annotations for Uta stansburiana stejnegeri over time; blue = adult, orange = juveniles.

Notice the uptick in juveniles starts in June and maintains a relatively stable level through the remainder of summer. Now, let's consider the more northern/high elevation subspecies U.s.uniformis (Fig. 5) and U.s.nevadensis (Fig. 6).

Fig. 5: total number of life stage annotations for Uta stansburiana uniformis over time; blue = adult, orange = juveniles.

Fig. 6: total number of life stage annotations for Uta stansburiana nevadensis over time; blue = adult, orange = juveniles.

In both of the northern high desert subspecies, the uptick in juveniles observed doesn't begin until July. From my own field work over the past two decades in Oregon, I can verify that hatchlings have never been observed by me in June with the earliest being the end of the first week of July. So, the data here appear to capture the very real difference in the timing of hatching across the range of the species. Granted, month-by-month aggregations of data like this are a very coarse look at the pattern. However, the consistency with expectations is encouraging.

Finally, I would say that for U.s.stansburiana, which is limited to the Bonneville Basin, is a bit lacking in data to be able to answer the question at this time (Fig. 7). The same is true for U.s.stejnegeri (Fig. 4).

Fig. 7: total number of life stage annotations for Uta stansburiana stansburiana over time; blue = adult, orange = juveniles.

So what does this all mean?

First, expected patterns of biological timing related to latitude/elevation are present in the data available to us on iNaturalist. These patterns may not be entirely clear where data are scarce, but where data are abundant (e.g., U.s.elegans, Fig. 3) the pattern is apparent.

Second, and this is the real take-home, if environments are being altered due to climate change (and they are) and if organisms are able to take advantage of that change (and they are...to some extent), then we should continue to see this pattern and the pattern should advance (meaning occur earlier in the year). The logic here is simple: earlier springs means earlier reproduction, earlier reproduction means earlier hatching of eggs, earlier hatching of eggs means earlier observations on iNat over time. That is, as this pattern continues and as we continue to collect and analyze such data, at some point hatchling will start to show up in May in the south and in June in the north. It hasn't really happened extensively yet, but that is what you are contributing to...an ongoing observational study of the biological timing of activity and reproduction of these animals. And for that I thank you for your ongoing contributions.

Carry on and keep up the good work. —Pete Zani

Posted on September 15, 2024 03:19 PM by petezani