The reproductive ecology of two snakes, female Vipera aspis (terrestrial) and Elaphe longissima (semi-arboreal), was compared. Mean clutch sizes were close in the two species; 6.17 ± 2.50 (n = 69) in the asp viper, and 6.59 ± 1.38 (n = 29) in the Aesculapian snake. When controlled for body size, body condition (hence amounts of body reserves), measured at the beginning of vitellogenesis, correlated positively with litter size in the asp viper but with clutch size in the Aesculapian snake. As in most species, maternal body length positively influenced clutch or litter size. Thus, the trade-off between maternal reserves and growth may favour reserves in the asp viper, and growth in the Aesculapian snake. The asp viper is a bi- or triennial breeder (33% of reproductive-females each year), the Ausculapian snake is an annual breeder (77% of reproductive females each year). These differences may be related to their contrasting foraging ecology. The asp viper is a terrestrial ambush predator with a specialized diet (98% Microtus) based on un-predictable prey availability. This species moves slowly (9.07 ± 8.40 m/day during the active season), and has a small home range (3,024 m2); an increasing body mass (large body reserves) should not affect its activity abilities. The Aesculapian snake is a semi-arboreal predator, which feeds on a large range of prey including birds and eggs, and which is often active (118, 11 ± 134,55 m/day during the active season). This species has a large home range (11,400 m2); an increasing body mass (large body reserves) should be a handicap during arboreal displacements. Vitellogenesis depends on body reserves in the asp viper, while it depends on foraging success in the Aesculapian snake.
We studied 36 pregnant females and 70 litters of Vipera aspis. Vipers were either recently caught, living in outdoor terrariums (semi-natural conditions), or living in the laboratory (artificial conditions). Following results were obtained: 1. There were significant correlations between the number of eggs and the weight of the litter and between the length and weight of the females. 2. The weight of hatchings does not depend, or only to a small extent, on the length or on the weight of the female, but is negatively correlated with the number of eggs. Hatching weight is generally higher in semi-natural and artificial conditions than in natural conditions. On the other hand, litter size was independent of the rearing conditions. 3. The weight of a litter at birth represents an average of 44 and 47% of the female's weight, respectively in natural and semi-natural conditions, where vipers breed generally once every two years; and only 34% in artificial conditions, where females breed once or twice a year. In all cases, this proportion is independent of the female's weight.
Blood glucose levels in Vipera aspis show great variations, both between months during the annual cycle, and between males and females. The seasonal variations are clearly related to climatic conditions, and particularly air temperatures and possibilities for thermoregulation. The blood glucose levels of the snakes are very low during the hibernation period and high in summer. Nevertheless, climatic conditions alone cannot explain changes of blood glucose levels observed during the period when the vipers are active, particularly the peaks measured soon after emergence from hibernation (males in February, females in March). Testicular activation, vitellogenesis and digestion need high body temperatures for long periods, thus the blood glucose level in reptiles chould be the result of a balance between climatic and physiological conditions expressed by the thermoregulatory behaviour. Plasma glucose levels are higher when the body temperatures are high and when the vipers spend more time in the sun (e.g. males in February, females in March); however, hormonal factors probably play a role.
Blood samples were taken in Vipera berus during a five year period. Changes in plasma thyroxine and testosterone levels were examined. Plasma testosterone levels show a marked profile. Highest levels are observed a few weeks after the emergence of hibernation from the end of March till the end of April. These values coincide with the mating period and spring spermiogenesis. Plasma thyroxine levels show also seasonal variations. Low values are observed in winter. Plasma thyroxine levels increase in spring and reach a maximal concentration shortly after the peak of plasma testosterone. A secondary peak is observed at the end of summer.
A study of the acute oxygen consumption of 16 young vipers (8 Vipera aspis and 8 V. berus) at temperatures of 10-15-20-25-30°C shows a number of distinctive features common to both species, and also specific differences. The oxygen consumption obviously increases with temperature, more or less irregularly for the average consumption which partly depends on the activities of each animal, while it follows an exponential progression for the minimum consumption and a linear progression for the maximum consumption. In the total consumption the part caused by the rather limited exploratory activities of our animals (routine aerobic scope) decreases as the temperature rises and it even decreases in absolute value between 25 and 30°C. At all temperatures, the oxygen consumption of V. berus is greater than that of V. aspis, the difference being globally significant. However, these differences are important only for the minimum consumption at low temperatures. These specific differences are discussed according to the ecology and the geographical distribution of the two species.