Decreased mobility of gravid females is thought to be an important cost of reproduction in lizards. We measured sprint speeds of female western fence lizards (Sceloporus occidentalis Baird and Girard) before and after they had oviposited. Females from two California populations were about 20% slower when gravid, females from an Oregon population were about 30% slower, and females from a Washington population were about 45% slower, compared to their speeds after recovering from reproduction. The decrease in sprint speed persisted for several weeks after oviposition, suggesting that reproduction impairs sprint performance by affecting body condition in addition to the burdening effect of eggs.

Oregon and Washington females carried more mass (both somatic and clutch mass) per unit body length than California females. On the shorter bodies of Oregon and Washington lizards, eggs may interfere with the mechanics of running, in addition to their effect on the total mass of the female. In addition, gravid females from Washington had significantly higher reproductive investment (mass of clutch relative to the mass of the female after oviposition) than Oregon and California populations. Greater reproductive investment by Washington females increases, the burden carried per unit of body length; we suggest this further impairs sprint performance.

Decrements in sprint speed were not significantly correlated with level of reproductive investment (per unit body mass) among females within any of the study populations. However, the burden carried per unit body length was correlated with the sprint speed decrement among gravid females from Oregon. Comparisons within and among populations suggest that differences in morphology among northern and southern populations interact with reproductive investment to produce interpopulation differences in sprint performance for gravid females.

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