Latitudinal Variation in Life History Reveals a Reproductive Disadvantage in the Texas Horned Lizard (Phrynosoma cornutum)

Geographically widespread species that occupy many thermal environments provide testable models for understanding the evolution of life-history responses to latitude, yet studies that draw range-wide conclusions using descriptive data from populations in the core of a species’ distribution can overlook meaningful inter-population variation. The phrynosomatid lizard Phrynosoma cornutum spans an extensive latitudinal distribution in North America and has been well-studied in connection with life-history evolution, yet populations occupying the most northern and coldest areas within its range were absent from previous examinations. We tested genus-wide models and challenged species-specific findings on the evolution of the life-history strategy for P. cornutum using populations at the northern edge of its geographic range and comparative material from farther south. Multivariate analyses revealed that egg dimensions decreased with clutch size, suggestive of a previously unrecognized tradeoff between egg size and egg number in this species. Interestingly, reproductive traits of females with shelled eggs did not covary with latitude, yet we found that populations at the highest latitudes typified several traits of the genus and for the species, including a model for Phrynosoma of large clutches and delayed reproduction. A significant deviation from earlier findings is that we detected latitudinal variation in clutch size. This finding, although novel, is unsurprising given the smaller body sizes from northern populations and the positive relationship between clutch size and body size. Intriguing, however, was that the significant reduction in clutch size persisted when female body size was held constant, indicating a reproductive disadvantage to living at higher latitudes. We discuss the possible selective pressures that may have resulted in the diminishing returns on reproductive output at higher latitudes. Our findings highlight the type of insights in the study of life-history evolution that can be gained across Phrynosomatidae from the inclusion of populations representing latitudinal endpoints.