42 Hot and Cold Reptiles-High

Hot and Cold Reptiles FOCUS Question What determines whether some reptiles become male or female?

A Girl or a Boy? Your dog is about to have puppies! Will she have girls or boys? The answer is in the cells. Dogs, like many other animals, carry information in chromosomes (CROW-ma-zohms) that determine whether their offspring will be female or male. The mother has two X chromosomes in her cells. The father has an X and a Y chromosome in his cells. Each offspring will get one of these chromosomes from each parent. If it gets an X from each parent, it will become a female (XX). If it gets an X from the mother and a Y from the father, it will become a male (XY). There is a 50 percent chance that the offspring will be male and a 50 percent chance it will be female.

Check the Temperature Not all animals use chromosomes to determine gender, though. Many reptiles rely on nest temperature instead! A mother reptile lays her eggs. Then there is a period of time when heat in the nest causes changes to the baby reptiles growing in the eggs. Eggs at lower temperatures make one gender. Eggs at higher temperatures usually make the other gender. A reptile will be male or female depending upon the amount of heat at its level in the nest.

Heat in the Nest Nest temperature can determine whether a baby will become male or female in three main ways: In some reptiles, lower-temperature nests produce females. Higher-temperature nests produce males. In other reptiles, nests with lower temperatures produce males, and nests with higher temperatures produce females. Certain reptiles produce females at both the lowest and highest nest temperatures. Males are only produced at the middle temperatures. Study the graph to explore a sample reptile species of each type.

Cool Females, Warm Males Tuataras are often called living fossils because they show us what some animals were like long ago. Their closest relatives are an extinct group of reptiles that were alive when dinosaurs roamed Earth! They live in cooler places than most reptiles do. They are active at night instead of during the day. Nest temperature affects whether tuatara offspring will become male or female. At lower temperatures, the babies become females. At higher temperatures, they become males.

Cool Males, Warm Females Red-eared sliders are a common type of turtle that live near ponds and lakes. The females build nests in soft, sandy soil in a sunny place near water. Nest temperatures affect turtle eggs in the opposite way from tuataras. If the whole nest is below 28°C, all the baby turtles will be males. If the eggs rest in nests at temperatures over 31°C, the baby turtles will all become females. At temperatures in between, about half the turtles will become male and half will become female. Math Moment At about what temperature will red-eared slider nests produce the most even mix of male and female turtles: 28.5°C, 29.5°C, or 30°C? Use the graph on page 4.