Help Track Non-native Sonoran Spotted Whiptails in Southern California

On August 7, 2020, iNaturalist user jjfish posted a photo of a baby whiptail lizard that he caught in his northern San Diego County yard two days earlier. As soon as I saw this observation, I started to worry. While the native Orange-throated Whiptail lives in the general area, I did not expect this species to be in a suburban neighborhood. Plus, this little whiptail lacked the bright blue tail typical of baby Orange-throated Whiptails. This appeared to be the non-native Sonoran Spotted Whiptail that has been rapidly spreading across Orange County. Had this non-native species jumped southward into San Diego County?

I shared jjfish’s observation with Dr. Robert Fisher (alazon on iNaturalist), a colleague at the United States Geological Survey’s Western Ecological Research Center in San Diego, and we started examining other whiptail observations posted to iNaturalist from the area. We discovered that deborah20 had also likely photographed a Sonoran Spotted Whiptail 26 months earlier and 3.3 km west of jjfish’s observation. This earlier observation had been identified by multiple people as the native Orange-throated Whiptail, but we were skeptical and asked deborah20 for any additional photos. Despite the two-year gap since the lizard was observed, she found and uploaded a second photo, and it was now clear this was a Sonoran Spotted Whiptail. With these two observations, it seemed likely that Sonoran Spotted Whiptails were now established in the city of Oceanside, more than 50 km south of the nearest known Sonoran Spotted Whiptails in Orange County.

These two observations kicked off a research project that culminated in a scientific paper published in the journal Frontiers in Conservation Science. The paper is open-access, which means that anyone can download and read it for free. Through multiple surveys of the Oceanside area with first author Sam Fisher (samfisher on iNaturalist), we documented the ongoing expansion of this non-native species, and more generally used this situation as an example of how to improve detection of non-native species when they look very similar to native species. Our goal with this post is to increase awareness about this species so that iNaturalist users can be on the lookout for it across Southern California, both when they are in the field and when viewing others' whiptail observations on iNaturalist.

Before the story continues, a big thank you to jjfish and deborah20 for posting lizard observations to iNaturalist. Without their observations, this non-native species would likely still be unknown from Oceanside.

The native and non-native range of Sonoran Spotted Whiptails

Sonoran Spotted Whiptails are native to Arizona, New Mexico, and adjacent northern Mexico. Since at least 2010, they have been in Orange County, CA where they are expanding extremely rapidly in urban and suburban areas. As mentioned above, the newly discovered population in Oceanside, San Diego County is approximately 50 km south of the nearest known individuals in Orange County. This past spring (on April 20, 2022), Dr. Jim Parham of CSU Fullerton posted an observation of a Sonoran Spotted Whiptail in Fullerton, a little more than 20 km north of their known range in Orange County. 



The appearance of Sonoran Spotted Whiptails in Oceanside and Fullerton highlight that human-assisted jump dispersal is ongoing. This could be from within-region movements of this species, or new translocations of individuals from their native range. This species has also shown up in Northern California, and we know from genetic analyses that the Northern CA and Orange County populations are from separate genetic stocks (meaning separate introduction events). We have observed spotted whiptails at several tree nurseries in Orange County, and we suspect that movement of larger plants in the nursery and landscaping trade is an important source of human-assisted translocations (by movement of eggs in the soil around the plants and by movement of juveniles and adults in larger shipments). 
 

Potential threats to native species

Sonoran Spotted Whiptails are a parthenogenetic species. This means that all individuals are females and can reproduce without mating. As a result, a single individual can establish a new population. Although they can reproduce without mating, individual lizards will very rarely mate with males of other species. For example, Sonoran Spotted Whiptails can hybridize with the Western Whiptail, which, along with the Orange-throated Whiptail, is native to coastal Southern California. There are no known hybrids between Sonoran Spotted Whiptails and Orange-throated Whiptails, but they have never before had overlapping ranges. Now that these two species increasingly occur together in Orange County, and will likely soon occur together in San Diego County, we advise that people be on the lookout for possible Sonoran x Orange-throat hybrids. In addition to hybridization, competition with native Western Whiptails, Orange-throated Whiptails, and other native lizards seems likely given their overlapping diets. Hopefully our work will encourage others to undertake more detailed dietary studies to understand whether the non-native Sonoran Spotted Whiptails compete with native species.

How to differentiate Sonoran Spotted Whiptails from native whiptails in Southern California

Unfortunately, the Sonoran Spotted Whiptail is difficult to tell apart from our two native whiptail species, and especially from the Orange-throated Whiptail. Juvenile lizards are the easiest to identify. In the two native species, the latter half of a juvenile’s tail will have a blue coloration. Even subadults will have a blue tint to their tails. In juvenile Sonoran Spotted Whiptails, the tail will have a similar coloration to the body or have a red or pink tint. You can see this in jjfish’s photos.

In older individuals, the best character for identification (assuming the whiptail will hold still for a few seconds) is to look at the stripes on either side of the vertebral column. These are called the paravertebral stripes. In Sonoran Spotted Whiptails, these stripes remain parallel and then fade as they reach the tail. In Orange-throats, the paravertebral stipes merge usually in the hind-limb region forming a single vertebral stripe that continues onto the tail (note that in Baja California, the position of this merge varies, but the above description works well in Southern California). There are also several differences in the head scales of these whiptail species, but whiptails rarely let you get close enough to see these characters. If you want to learn more about these characters, check out Table 2 in the manuscript or the identification guide in the Supplementary Materials.

Final Thoughts

iNat user dlbowls takes lots of nice photos, including these photos of a Sonoran Spotted Whiptail in Oceanside.

And again, my sincere thanks to jjfish and to deborah20, both of whom have now uploaded multiple observations of Sonoran Spotted Whiptails in Oceanside. Note that they are also mentioned in the first line of the acknowledgements of the published paper. The second line of that paper thanks other iNat users who are contributing photos and/or identifications of whiptails in Southern California. We are grateful to everyone who is helping to document the reptiles of Southern California.

Posted on September 20, 2022 12:12 AM by gregpauly