Capital energy drives production of multiple clutches whereas income energy fuels growth in female collared lizards Crotaphytus collaris

Animals invest energy in reproduction that is obtained at two distinct times relative to the reproductive cycle. Energy obtained during egg production is referred to as income energy whereas stored energy acquired prior to reproduction is capital energy. Similar to most ectotherms, squamate reptiles are generally hypothesized to be capital breeders. Nearly all squamates in which income/capital energy investment has been examined thus far produce only one clutch per reproductive season. Although it is likely that squamates producing multiple seasonal clutches fuel first clutches with capital energy, either capital or income energy may be used to produce later clutches. We first monitored female eastern collared lizards over 14 reproductive seasons to confirm that the number of clutches females produce seasonally is a plastic response to variable environmental parameters, and to examine the effects of female body condition at the beginning of the reproductive season on clutch production. Clutch production varied annually and both the size and number of clutches were positively correlated with body condition. We then tested the competing predictions of the income and capital hypotheses experimentally by supplementing the diets of female collared lizards in situ for one season. Diet-supplementation had no effect on the number of clutches produced but increased growth rates of gravid females. We further tested the competing predictions of these two hypotheses by examining variation in maternal energy investment per clutch using preserved specimens collected near our primary field site. Clutch size was highly correlated with female body size. Together, our results suggest that variation in reproductive output by female collared lizards is linked to stored capital energy rather than income energy, similar to most ectotherms.