Unlocking the mystery of mountain horned lizard reproduction

It is easy to imagine how living things communicate with each other. However, what about communication inside our own body? We can't necessarily see or hear what is going on inside; nevertheless, hormones are constantly sharing information throughout the body, speeding between organs to keep everything running smoothly. Hormones are important for lots of different processes, such as sleep and reproductive cycles. In reproduction, hormones can control when some animals breed, get pregnant and give birth. Scientists can figure out when these reproductive events might happen by measuring the reproductive hormones in an animal's blood. This is particularly important for endangered species, such as the mountain horned lizard (Phrynosoma orbiculare), found in the Parque de la Ciencia Sierra Morelos, Mexico. These lizards are vulnerable to future habitat damage, so knowing more about their reproduction is critical in helping save them from extinction. Laura Elena Hernández-Hernández at the Universidad Autónoma del Estado de México, Mexico, worked with colleagues from Mexico and the USA to determine how the lizards’ hormones changed throughout the year and how these hormones were related to temperature and day length.The researchers collected blood samples from the tails of 31 female horned lizards every month for 12 months, and then measured three hormones related to reproduction (progesterone, estradiol and testosterone). The researchers also felt the stomach of the lizards to see whether they were pregnant. Hernández-Hernández and colleagues found that progesterone increased during the horned lizards’ pregnancy period between September and January. The lizards remained pregnant until April, but their progesterone levels started to decrease in February and kept decreasing until they gave birth. Hernández-Hernández suggests that progesterone helps keep pregnancy going ­– as it does in other animals – and is more important at the beginning, but other hormones might be more important towards the end of the lizard’s pregnancy.One of these other hormones – estradiol – showed the opposite pattern to progesterone and was higher towards the end of the lizard’s pregnancy, but decreased in the mother lizards after birth. Estrodiol is important for egg growth in other reptiles and the researchers believe that, in horned lizards, estradiol helps in moving nutrients into eggs, as well as helping female lizards become ready to breed again. Additionally, the team speculated that the pattern between progesterone and estradiol levels shows that these hormones might interact with each other throughout the year.The final hormone that the researchers tested was testosterone. The testosterone in the lizards’ blood was higher during the breeding season but decreased in later pregnancy. Hernández-Hernández and colleagues also found that testosterone quickly increased before birth and remained high through the next breeding season. The researchers suggest that, like estradiol, testosterone might help female lizards get ready to breed. Additionally, the scientists compared the hormone levels with environmental conditions (temperature, rainfall and daylight hours) to see whether they were related. The team found that, as the yearly temperature increased, progesterone decreased while estradiol increased. The warmest temperatures occurred during the breeding season, meaning that the lizards had low progesterone and high estradiol levels. These patterns were also the same for day length, where longer days were during the breeding season, which makes sense as temperature and day length are connected (longer days during warmer summer months).Ultimately, Hernández-Hernández and colleagues showed that blood samples and hormones can be used to figure out when the lizards are pregnant, breeding or preparing nutrients for the next year's offspring. This is very important in a species that is at risk from habitat damage and could help scientists learn about reproduction in other endangered reptile species.

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