Reproduction And Phenotypic Response Of Egg Size-number Trade-offs In Snakes

Viviparity, a reproductive mode that evolves from oviparity via progressive increases in the duration of uterine retention of developing embryos. It is a focus that what significance there is from oviparity to viviparity. We collected 142 snake species and compared their female snout-vent length, offspring number and variability in two traits, offspring mass, offspring size, and female postpartum body conditions. Our data show that:(1) viviparous females produce more offspring and heavier clutches than do oviparous females of the same SVL, eggs laid by oviparous females do not differ from neonates produced by viviparous females of the same SVL in mean mass, viviparous females produce heavier offspring than do oviparous females of the same SVL; (2) CV of clutch size is greater in viviparous than in oviparous females when accounting for differences in CV of SVL, the mean rate at which clutch size increase with maternal SVL is greater in viviparous than in oviparous snakes; (3) postpartum body conditions are better in viviparous than in oviparous snakes, so viviparous snakes produce heavier clutches but have smaller RCM values.We collected gravid king ratsnakes (Elaphe carinata) from three geographically separated populations in Chenzhou (CZ), Lishui (LS) and Dinghai (DH) of China to study geographic variation in female reproductive traits and trade-offs between size and number of eggs. Not all reproductive traits varied among the three populations. Of the traits examined, egg-laying date, post-oviposition body mass, clutch size, egg mass and egg width differed among the three populations. Egg-laying date, overall stretching from late June to early August, varied among populations in a geographically continuous trend, with females at the most northern latitude (DH) laying eggs latest, and females at the most southern latitude (CZ) laying eggs earliest. Such a trend was less evident or even absent in the other traits that differed among the three populations. The egg size-number trade-off was evident in each of the three populations and, at a given level of relative fecundity, egg mass was significantly greater in the DH population than in the LS population. CZ and DH females, although separated by a distance of approximately 1100 km, were similar to each other in most traits examined. LS females were distinguished from CZ and DH females by their laying more but smaller eggs.We collected gravid Chinese cobras (Naja atra) from Zhoushan (ZS), the range from Lishui to Huangshan (ZW), Quanzhou (QZ) and Baise (BS) populations to study geographical variation in female reproductive traits and the trade-off between the size and number of eggs. Oviposition occurred between late June and early August, with females from the lower latitude localities laying eggs earlier than those from the higher latitude localities. Maternal size was a major determinant of the reproductive investment in all populations, with larger females producing not only more but also larger eggs. Clutch size was more variable than egg size within and among populations. The observed geographical variation in clutch size, egg size, and post-oviposition body condition was not a simple consequence of variation in maternal size among populations, because interpopulation differences in these traits were still evident when the influence of maternal size was removed. The upper limit to reproductive investment was more likely to be set by the space availability in ZS and BS population, but by the resource availability in ZW populations. Trade-offs between size and number of eggs were detected in all populations, with females that had larger clutches for their size having smaller eggs. Egg size at any given level of relative fecundity differed among populations, primarily because of interpopulation differences in the resource availability rather than the space availability. The observed geographical variation in the female reproductive traits could be attributed to the consequence of the effects of either proximate or ultimate factors.The relationship between offspring size and offspring number is crucial to life history evolution. To examine how these two life history variables are coupled and whether an altered balance between them will result in changes in maternal fitness, we manipulated clutch size of the Chinese cobra (Naja atra) by using the techniques of hormonal manipulation and follicle ablation. Females receiving exogenous follicle-stimulating hormone produced more but smaller eggs, and females undergoing follicle ablation produced fewer but larger eggs. Neither body size (body mass and snout-vent length) at hatching nor egg mass at oviposition had a role in determining hatchling survival and growth. Female hatchlings were more likely to die in early post-hatching days and grew more slowly than male hatchlings. Our data show that:(1) there is a nonlinear continuum of egg size-number trade-offs in N. atra within which there is a single inflexion where the rate at which egg size decreases with increasing clutch size, or clutch size increases with decreasing egg size, is maximized; (2) there is a fixed upper limit to egg size for a given-sized female, and the limit is not determined by her body volume; (3) the inflexion variation of the relationship between offspring number and offspring size in two different populations induced by either proximate or ultimate factors could be important for cobras living in the wild; (4) egg size has no role in determining hatchling survival and growth; and (5) the extent to which females may enjoy reproductive benefits in a given reproductive episode depends on how well egg size and egg number are balanced.We kept 48 gravid short-tailed pit vipers (Gloydius brevicaudus) under four laboratory thermal conditions during gestation and collected 10 females from the field soon before they gave birth to test whether Shine’s (1995) maternal manipulation hypothesis applies to temperate reptiles. Females thermoregulated more precisely but did not shift their selected body temperatures during pregnancy, with females at high body temperatures giving birth early in the breeding season. The lowest (22℃) and highest (32℃) temperature treatments increased maternal mortality and resulted in production of offspring with smaller body dimensions. More deformed offspring were produced at 32℃, and more poorly performing offspring were produced at 22℃. In the field, air temperatures lower than 22℃and higher than 32℃accounted for about 9% and 33% of total temperature readings, respectively. However, offspring produced by field-caught females did not differ from those produced by laboratory-kept females with body temperatures optimal for embryonic development in nearly all traits examined. This suggests that in nature, gravid females avoid exposure of their embryos to temperature extremes through thermoregulation. Our study validates the key prediction of the maternal manipulation hypothesis that maternal thermoregulation should enhance fitness-related offspring traits, and demonstrates that viviparity evolves in temperate reptiles because internal development shields offspring from temperature extremes.Fluctuating temperatures influence hatchling phenotypes differently from constant temperatures in some reptiles, but not in others. This inconsistency raises a question of whether thermal fluctuations during incubation always play an important role in shaping the phenotype of hatchlings. To answer this question, we incubated eggs of Naja atra under one constant temperature (28.5℃), eight temperature-shift (23,28,33 and 28℃; incubation temperature change every 3x hours from 3 to 24 hours). In two sexes hatchlings were not difference in morphologic traits but females had smaller than males hatchlings in tail length from the same-sized egg. The constant temperatur treatments resulted in shorter incubation lengths. All examined hatchling traits did not differ among treatments. Our results therefore support the hypothesis that hatchling phenotype is altered by thermal fluctuation in species with no phenotypic response to incubation temperature over some thresholds.Yolkectomy has become a standard tool for manipulating the amount of yolk available to an embryo or neonate organism. Yolkectomy minimizes offspring size but has no role in influencing offspring phenotype One reason may be the lack of residual yolk contribution to growth could be caused by a lack of anabolic substrate left over after embryogenesis, specifically crude protein. Another cause could be hatchlings size has no role in determining hatchling survival and growth. So hatchlings were not inclined to change the way of energy distribution.