Pregnancy is associated with reduced locomotor performance in many types of animals, but we do not know to what degree this correlation is caused by simple physical burdening, versus physiological changes associated with pregnancy, or to confounding variables (such as season or female body size) that simultaneously influence both reproductive investment and locomotor speeds. To identify causal effects of burden on locomotion, we need to experimentally manipulate the size of the load being carried. Injection of sterile fluid into the peritoneal cavities of 84 garden skinks (Lampropholis guichenoti) showed that speeds decreased with increasing burdens. Lizards with a burden equivalent to 25% of their body mass ran about 15% slower, mirroring the situation seen in gravid lizards of this population. Thus, simple physical burdening appears to be the primary causal component of the locomotor cost of reproduction within these animals. A lizard's sex, body size and shape had little effect on its running ability either before or after treatment, but faster lizards showed a greater performance decrement after burdening than did their slower conspecifics.