Geographically widespread species that occupy varied thermal environments provide testable models for understanding the evolution of life-history responses to latitude. 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 Texas Horned Lizard (Phrynosomatidae: 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 patterns and challenged species-specific findings on the evolution of the life-history strategy of P. cornutum using populations at the northern edge of its geographic range and comparative material from farther south. Traits in populations at the highest latitudes corroborated several patterns for the genus and species, including delayed reproduction and a trend towards smaller adult body size with increasing latitude. Novel in our study, however, was the finding of a comparative increase in clutch size among size-adjusted females in Kansas, indicating a reproductive response for greater fecundity at the northern edge of its geographic range. Furthermore, analyses adjusted for body size revealed that egg dimensions were constant across variation in clutch size, suggesting that there is not a strong relationship between egg size and egg number across latitude. We discuss the selective pressures that may have resulted in the diminution of adult body size coupled with greater fecundity that is unique to the northernmost populations of P. cornutum. Our findings highlight the type of insights into the study of life-history evolution that can be gained across Phrynosomatidae from the inclusion of populations representing latitudinal endpoints.
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Vol. 107 • No. 4