Efectos de la temperatura de incubación en el periodo termosensitivo de determinación sexual y evaluación de morfometría geométrica como técnica de sexaje en la tortuga del río Magdalena

Abstract

RESUMEN: El período termosensitivo (PTS) durante el cual se define el sexo en los embriones de los reptiles ha sido estudiado principalmente a temperaturas constantes, sin embargo los efectos de temperaturas variables antes y durante el PTS mismo han sido poco evaluados. Igualmente es necesario conocer el efecto que la temperatura de incubación tiene sobre el sexo y la forma, particularmente en reptiles amenazados de extinción cuyos huevos son incubados bajo condiciones artificiales. Este estudio evaluó el efecto de la temperatura de incubación sobre el sexo, el período termosensitivo y la forma del caparazón y del plastrón de la tortuga del río Magdalena, Podocnemis lewyana, que habita en el río Claro Cocorná Sur, en Puerto Triunfo, Antioquia. Los objetivos específicos fueron: evaluar las proporciones sexuales producidas en huevos incubados a temperaturas promedio masculinizantes (29.2 +- 0.8 y 31.2 +- 0.4 ºC) y feminizantes (34.6 +- 1.2 ºC); y determinar el efecto de dos velocidades de desarrollo embrionarias en el inicio y la duración del PTS usando dos temperaturas masculinizantes antes del mismo. Se observó que la temperatura masculinizante baja (29.4 +- 1.6 ºC) indujo el inicio del PTS a partir del día 20 del periodo de incubación con una duración aproximada de 20 días, mientras que una temperatura dos grados más alta (masculinizante alta, 31.3 +- 1.3 ºC) indujo el inicio del PTS también en el día 20 pero con una duración de 30 días. Adicionalmente en este trabajo se evaluó el efecto de la temperatura de incubación sobre la forma del caparazón y plastrón de los recién nacidos. Los resultados sugieren que las temperaturas de incubación más bajas (29.2 ºC) se asocian a neonatos con un cuerpo más ancho y una muesca anal más profunda que los neonatos de temperaturas más altas (34.6 ºC).

ABSTRACT: The thermosensitive period (TSP) during which the sex of reptile embryos is defined has been studied mainly at constant temperatures. However, the effects of varying temperatures before and during this TSP have not been assessed in detail and therefore need to be studied. Likewise, it is important to know the effects of the incubation temperature on the sex and shape of the animals, particularly for threatened reptile species whose eggs are being incubated under artificial conditions to increase their survival. This study assessed the effect of hatching temperature on the sex, the thermosensitive period, and the carapace and plastron shapes of the Magdalena River Turtle, Podocnemis lewyana, inhabiting the Claro Cocorná Sur river, located in Puerto Triunfo, Antioquia, Colombia. The specific objectives were: to evaluate the sex ratios produced in eggs incubated at masculinizing (29.2 ? 0.8 y 31.2 ? 0.4 ºC) and feminizing (34.6 ? 1.2 ºC) average temperatures, and determine the effect of those two embryonic development speeds (under 29.4?1.6 y 31.3?1.3 ºC) on the onset and duration of the TSP. It was observed that the slower masculinizing temperature (29.4 ºC) induced the TSP onset from day 20 of the incubation period and lasted approximately 20 days, while the faster masculinizing temperature (31.3 ºC), a temperature 2 ºC higher induced the TSP onset also on day 20 but lasted 30 days. Additionally, the effect of incubation temperature on the carapace and plastron shape of hatchlings was evaluated. The results suggest that lower incubation temperatures (29.2 ºC) are associated with hatchlings with a wider body and a deeper anal notch than hatchlings of higher temperatures (34.6 ºC). These body forms were similar to those reported in other congeneric species, Podocnemis expansa, suggesting a similar pattern of effects of incubation temperature on body shape, at least for the clade Podocnemis. Finally, as part of the methodology used in this study, I report the effectiveness of geometric morphometrics as a non-lethal sexing technique in hatchlings of P. lewyana. This sexing technique had an effectiveness of 91% and 85% using the plastron shape at 7 days and 3.5 months of age respectively. It was observed that females have a more elongated body and a less marked anal notch than males, similar to the body shapes found under high and low incubation temperatures, respectively. Given the conservation threat degree of this species I suggest the use of this method as a sexing technique for P. lewyana hatchlings or juveniles with no evident sexual dimorphism.