Hormonal and physiological control of clutch size, egg size, and egg shape in side‐blotched lizards (Uta stansburiana): Constraints on the evolution of lizard life histories

Abstract

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.

Maturational costs of reproduction due to clutch size and ontogenetic conflict as revealed in the invisible fraction

“…Such variation would be due to a genotype!environment interaction with density, the salient cue that induces variation in other female life-history traits. Previous studies on side-blotched lizards (Sinervo & Licht 1991;Sinervo 2000) and the manipulations of clutch size used in our study indicate that levels of FSH regulate clutch size. In males, FSH regulates other reproductive activities besides gonadal function and sperm production, such as territorial behaviour and mate attraction (Mills et al in press).…”

“…We increased clutch size and mass by implanting Elvax pellets (Dow Corning) in which we had embedded ovine FSH (Sinervo & DeNardo 1996). Hormone release from implants was designed to increase clutch size by two eggs (1993, NZ30), as previously assessed in laboratory trials (Sinervo & Licht 1991). Clutch mass was also increased with corticosterone implants (1991, NZ18;1992, NZ60;1993, NZ42) that were designed to increase clutch mass by 0.6 g (Sinervo & DeNardo 1996).…”

“…Mothers that produce large clutches also lay small eggs and mothers that produce small clutches lay large eggs (Sinervo & Licht 1991). Female or male progeny might die owing to maternal effects of yolk provisioning that are correlated with clutch size, but not causally related to clutch-size genes that progeny express at maturity.…”

Hormonally mediated maternal effects, individual strategy and global change

“…Treatment males displayed enhanced nuptial coloration, elevated levels of testosterone and increased locomotor performance [117]. In contrast, manipulation of FSH and follicle number in female side-blotched lizards illuminated the endocrine control of the trade-off between egg size and egg number [118]. FSH-manipulated females treated during vitellogenesis resulted in an increased clutch size with a concomitant decrease in egg mass [119].…”

Avian Life History Evolution in Relation to Nest Sites, Nest Predation, and Food

“…Such experiments assume that changes in feeding effort reflect genetic correlations and they may not. Moreover, if reproductive effort is relatively fixed (e.g., Wilbur 1977, Sinervo and Licht 1991, Boggs and Ross 1993, then individuals may not increase effort. For example, Great Tits do not increase their feeding rates with increases in brood size ) and, consequently, survival costs of increased brood size are not manifested (Gustafsson and Sutherland 1988).…”