Turtle reproduction differs between the land-dwelling species and the aquatic species.
REPRODUCTION IN AQUATIC TURTLES
Every species has its own particularities in both reproductive physiology and ethology. Here we will take Trachemys spp. as an example, since this is one of the most common aquatic species in Italy. The females are usually larger than the males and sexing can be established easily by observing the tail and the claws of the anterior limbs which in both cases are longer in the male than in the female. Sexual maturity is reached in both the male and the female partly based on age, but more on the basis of size. The minimum size to reach maturity is about 10 cm and 15 cm in the male and female, respectively. The reproductive season is from the start of spring to the middle of summer (on average from March to July) with considerable differences depending on the place and environmental conditions.
Mating takes place in water and is preceded by a courtship in which the male strokes the face of the female with the claws of his anterior limbs. The fecund female lays her eggs on the land in a damp but sunny place and digs the nest with her posterior flippers. From 30 to 50 days after mating the turtle lays a clutch of eggs in a few minutes (at most 10-20 minutes depending on the number of eggs). The number of eggs in a clutch varies and can be as many as 20. The female then covers the nest so that it cannot be discovered. A female that does not find suitable conditions to lay her eggs can retain them in her body for longer. Turtles that live in a terraquarium must be supplied with an area of land where the female can dig a nest and lay her eggs as if in the wild. In optimal conditions the eggs take from 60 to 120 days to hatch, but in low temperatures, for example in clutches laid at the end of the season (up to three clutches may be laid in a season), the hatching may be delayed until after the winter.
At birth the small turtles are already a few centimetres long (Fig. 1) and still have the vitelline sac attached to the plastron. This sac should not be removed; it will be absorbed gradually over the following days. In this species the sex of the unborn turtle is strongly influenced by the temperature of incubation, such that incubation at temperatures below 27°C gives rise to more males (longer incubation) while incubation above 30°C produces more females (shorter incubation); when the temperature is between 27 and 30°C both sexes are born.
REPRODUCTION IN LAND-DWELLING TURTLES
To discuss reproduction in land-dwelling turtles we shall take Testudo hermanni as an example, as this is very widespread in Italy. Sexual dimorphism is fairly evident, with the male being slightly smaller than the female. The sexually distinguishing features are, however, the plastron (more concave in the male and flatter in the female), the tail (longer and with a larger diameter in the male, which also has a more developed horny spur and a cloacal opening further away from the base of the tail than that of the female) and the supracaudal scute (more concave and curved towards the tail in the male) (Fig. 2). As already mentioned for Trachemys spp., it is size rather than age that determines sexual maturity in T. hermanni, and this maturity is reached on average at 9-10 cm for males and 12-14 cm for females.
Mating occurs mainly in the spring and summer and is far from peaceful: the male pursues the female and attacks her, with head-butting and bites, emitting very characteristic sounds from his mouth. The courtship and copulation are, therefore, fairly violent and can lead to the female having several wounds. For this reason, in captivity and restricted environments, every male should have several females available so that his attentions can be divided equally (Fig. 3). In a terrarium, the male must be separated from the female after mating. The period between copulation and laying of the eggs is from 30-60 days although sperm within the female genital apparatus can survive for more than 1 year, so that eggs from several cycles can be fertilised by a single copulation. The female of the Testudo spp. usually lays two clutches of eggs during the reproductive season, with two to eight eggs in each clutch (Fig. 5). The eggs are laid in a nest that the female digs with her hind limbs in a peaceful, sunny place. After having laid the eggs, the nest is covered and hidden by earth and leaves. As for other turtles, the sex of the unborn T. hermanni is influenced by the temperature of incubation. At birth the young turtles are a few centimetres long (Fig. 4) and still have the residue of the vitelline sac as well as a rather soft, fragile carapace.
