Proximate constraints on egg size, number, and total clutch mass in side-blotched lizards were examined by experimentally reducing average clutch size from 4.6 eggs to one, two, and three eggs. Eggs from experimentally altered clutches were larger than those from controls, reflecting the trade-off between egg size and number. Moreover, the increased frequency of females with oviducally bound eggs or eggs that burst at oviposition suggests that egg size in clutches with very few eggs are at a functional upper size limit. These proximate constraints may also limit evolution of egg size in another group of lizards ( Anolis ) that only produces one-egged clutches.

We present comparative data from studies of natural populations and experimental manipulations addressing the control of clutch size, egg size, and egg shape among three populations of side-blotched lizards, Uta stansburiana. On a geographic basis, females from northern populations located in the Coast Range of California produced small clutches of large eggs compared to populations from more southern locations (as little as 60 km farther south). A similar tradeoff between clutch size and egg size was observed among individuals within a population, females that lay large eggs tend to produce small clutches; on a seasonal basis, females laid smaller clutches consisting of larger eggs later in the reproductive season.Injections of ovine follicle-stimulating hormone (FSH) into vitellogenic females increased the mean clutch size (from 4.2 to 6.0 eggs) with a concomitant decrease in egg mass (from 0.43 to 0.33 g). By contrast, a surgical reduction of clutch size (yolkectomy of two follicles) caused an increase in egg mass (from 0.43 to 0.53 g)-egg shape changed (elongation) with egg hypertrophy. This increase in egg mass was due largely to increased yolk; hatchlings from these hypertrophied eggs were proportionately heavier than those from control eggs. Egg size was independent of the time course of follicular development (time to oviposition from the onset of yolking averaged 29 days).These complementary manipulations indicate that (1) the number of eggs in the clutch is under the control of circulating plasma gonadotropin concentrations, and (2) the size of eggs is set by the number of follicles undergoing vitellogenesis and finally ovulated. Moreover, the effect of manipulations was dependent on the stage of vitellogenesis: production of large clutches of small eggs did not occur in females if administration of FSH commenced when follicles were above -4-5 mm in diameter (ovulation normally occurs at 8 mm), and some females undergoing yolkectomy during early vitellogenesis recruited new follicles.

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