Energetics of Blainville's Horned Lizards, Phrynosoma blainvillii, in disturbed and undisturbed habitat

Background/Question/Methods

Blainville’s horned lizard, Phrynosoma blainvillii, is considered a taxon of special concern by the State of California and several Multiple Species Conservation Plans.  The distribution of this species in southern California has declined dramatically largely because of habitat development in addition to the invasion of Argentine ants and general pressures concomitant with increasing human populations. Previous work with this species indicated that horned lizards living in disturbed (from historical grazing) habitats had smaller population densities and body sizes, along with larger home ranges.  These findings suggest that horned lizards in anthropogenically disturbed habitats may have higher energy expenditures resulting from increased foraging costs and greater environmental stress.  The present study aimed to determine the energetics of individual Blainville’s horned lizards to help elucidate the physiological mechanisms driving population decline in suboptimal habitats.   Fourteen horned lizards (seven each in undisturbed and disturbed coastal sage scrub/chaparral habitat sites) were captured and fitted with radio-transmitters in late spring in Riverside County, California.  Doubly labeled water was used to study the field metabolic rates (FMR) of the individuals during two, two-week periods representing late spring and early summer. Data on growth, habitat use and movements were also collected.

Results/Conclusions

A total of 12 of the original 14 transmittered lizards were successfully followed over the study period, during which temperatures increased dramatically.  Overall season had a significant effect on lizard FMR (P = 0.006), but there was also a significant season x site interaction (P = 0.002) and nearly a season x sex x site interaction (P = 0.057).  At the undisturbed site all females (N = 3) had FMRs increase 13 to 27% while all male (N = 3) FMRs decreased by 9 to 40%.  At the disturbed site, all lizard FMRs decreased 20 to 76% between the first and second study periods, including the single female followed there.  The decrease in male FMR was not surprising given both previous work and observations during this study indicate male Blainville’s horned lizards become inactive and estivate after breeding is finishing, typically in late June.  Overall FMR at the disturbed site appeared to be lower than at the undisturbed site, especially during the summer, but this tendency was overshadowed by the large proportion of males (all but one) in the disturbed site.  Mean FMR for both sexes combined in the late spring was 0.158 ml CO₂ per gram hour for lizards with a mean body mass of 42.8 g.