How to tell males from females
On the iNaturalist forum, I recently came across an older 'feature request' to allow the iNat Computer Vision to suggest annotations, such as for males or females. As part of that discussion it was pointed out that the model would need observations already annotated for training purposes. Hence, in the past few months I have been making a concerted effort to add to the 'sex' annotation for Uta to make that possible when/if that happens (feel free to vote on this feature request if you like). However, it occurs to me that you, dear reader, might be interested in learning how to identify the sexes of Uta, some of which is knowledge applicable to other lizards as well.
Tail Bulge
The most reliable way to distinguish males from females is via morphology. Squamate reptiles (lizards and snakes) have what are called 'hemipenes,' which are paired penile organs that are stored in the base of the tail for use during copulation. Males fill one of these with blood during copulation, which causes the hemipene (left or right) be everted and inserted into the female for sperm transfer (see here for example). The presence of hemipenes in males, but not females, means the shape of the base of the tail is different between the sexes.
In males, the base of the tail is slightly wider where the hemipenes sit before quickly tapering to a more modest width that then gradually tapers the rest of the length of the tail. It's this rapid change in width just behind the hind legs that is diagnostic. Here's a nice close up of this feature as well example observations where this bulge should be readily apparent for each of the primary subspecies of Uta stansburiana (listed alphabetically): U.s.elegans, U.s.nevadensis, U.s.stansburiana, U.s.stejnegeri, U.s.uniformis .
In females, the base of the tail is slightly narrower, but more importantly, it has an even taper along the length of the tail. Here are example observations where this bulge should be readily apparent for each of the primary subspecies of Uta stansburiana (listed alphabetically): U.s.elegans, U.s.nevadensis, U.s.stansburiana, U.s.stejnegeri, U.s.uniformis .
While the bulge in the base of the tail works on many lizards species, not just Uta, the bulge can be very subtle in some taxa like skinks or gecko. There is a second morphological feature, enlarged post-anal scales, which allows the sexes to be distinguished. However, this trait is limited to Family Phrynosomatidae.
Enlarged Post-Anal Scales
On the under side of the tail just behind the cloaca (the term for the single opening for metabolic and nitrogenous waste) are two scales that are noticeably larger than the surrounding scales in males. In females, all the scales behind the cloaca are the same size. Interestingly, this feature is diagnosable at hatching meaning phrynosomatids of any age can be sexed. However, in hatchlings this feature is so tiny that usually a magnifying hand lens is necessary to see it. Here are examples of the enlarged post-anal scales in males (see picture #7) and females (see pictures #4 & 5).
The problem with post-anal scales is one needs a ventral view of the lizard. However, there are situations where post-anal scalation can be all one needs for identifying a lizard's sex. For example, this observation of a dead lizard lying on its back where one of the two enlarged post-anal scales is barely distinguishable.
Color/Pattern
Another feature that is often a useful indicator of anatomical sex is a lizard's color or pattern. However, this is more limited because a) breeding colors can be quite muted out of the breeding season and b) some subspecies, namely U.s.uniformis, are very similar in color and pattern. In fact, this is why they were named uniformis...the sexes are uniform. For the remaining subspecies of Uta stansburiana, there are differences described below and organized by subspecies.
Three of these subspecies (U.s.elegans, U.s.stansburiana, U.s.stejnegeri) are all similar in that males have blue flecks on the back (dorsum) and atop the tail. These flecks are usually composed of small groupings of 3-5 blue scales. Even when the blue is faint, these flecks are light colored on a dark ground color as in this observation. In some populations, such as in Grand Canyon, the blue tail can be a vibrant or electric blue, especially when agitated during courtship or stressed by handling. Females never have the blue as in males. Thus, the blue in undisturbed males can be quite muted, but still present, as in this observation. Check out the difference between undisturbed (picture #4) and handled (picture #5) blue color in this male (yes, that's the same animal).
The only subspecies really different (other than U.s.uniformis) is U.s.nevadensis where the blue on the dorsum is still present, but the tail is instead vibrant orange. Again, the blue becomes more vibrant during interactions with other lizards and during handling. In U.s.nevadensis, males more consistently have orange coloration along their flanks. One thing to note, as one approaches the southern limits for this subspecies in Nevada and California the orange tail and flanks can be much more difficult to distinguish, such as this male from near Hawthorne, NV.
While the pattern can be variable within and between the subspecies as mention above, the blue flecks are fairly consistent. Females on the other hand lack these blue flecks even in their most colorful. So it's really the presence of color that suggests a male, but the absence of much color, other than dark and light flecks on a gray/brown ground color, is more consistent with females. This is true regardless of the chevroned, striped, or mixed morphs present in females.
So there you have it. Three ways to recognize males from females. Keep in mind that Uta are highly variable in color and pattern, which is why I list it third and tried to focus on those hard morphological traits like tail shape that help me in identification of the anatomical sexes. Hopefully these notes help you as well.
—Pete Zani
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