You may be familiar with X/Y chromosomes in humans. Because of this, we tend to think of XX being female and XY being male. While this is true for most mammals, the diversity in the animal kingdom means nothing is ever quite so simple. We might understand the mammalian method, but birds and reptiles have their own way of revealing gender.
For example, what does ZW mean? ZZ?
How about a lack of sex chromosomes altogether? How does a developing sea turtle egg, with none of these X, Y, Z, or W chromosomes determine whether the hatchling will be male or female?
So... Boy or Girl?
Note: This is a general overview of how biology defines male and female in animals.
This topic is a lot more complex than we can get into here, but for the purpose of this lesson, gender determination will be solely based on male or female, boy or girl.
The biology term used for this is actually sex, whereas gender is used more by sociology.
Genetic (Chromosomes)
X/Y in Mammals
In humans, there are 23 pairs of chromosomes, and all of these chromosomes are inside the nucleus of every cell in your body. That skin cell doesn’t have much use for the X/Y chromosomes, but it still has them. Once a cell knows what type of cell it is, it will use the genetic information that it needs and “ignore” the rest.
In females, all of them are matching pairs, but in males, the XY pair don't look like copies of each other. They are the only non-matching pair of chromosomes in what we consider to be a normal karyotype (the image of all 23 pairs of chromosomes lined up in order of size). Karyotypes are like a chromosome map. Below, for example, is the karyotype of a rock hyrax, a small rodent-like critter that you would never expect to be related to elephants and manatees.
Note that the hyrax has 27 pairs of chromosomes, 26 regular pairs plus XY. Because the hyrax is a mammal, the presence here of an X and a Y means that the animal who lent us this DNA is a male. The Y is much smaller than the X. Otherwise, all the other paired chromosomes look alike.
By the way, this is a hyrax.
Doesn't look much
like a manatee, does it?
Z/W in Birds
Birds have actual chromosomes that determine their gender just like mammals, but they follow a different system. In birds, females are the ones who have the different chromosomes and males have two of the same. Also, we don't use X and Y for the labels, we use Z and W.
So male birds are ZZ and females are ZW.
What's really weird, though, is that the mother bird can control whether the egg she lays will be male or female. In mammals, all eggs have an X and the zygote (fertilized egg) will receive either another X or a Y from the father. It's not a decision, and boy or girl is based on completely random 50/50 chance. But because father bird will only ever contribute a Z, mama bird's eggs are either going to have a Z or a W. If it worked like mammals, it should be random whether the chick is male or female. But scientists have realized that mama bird's hormones actually decide what baby will be.
Most bird species will generally have more males than females overall, but in some, the amount of resources available (food, water, homes, etc.) will influence the outcome. Zebra finches are known to have more females when "times are tough"(1).
Environmental (Temperature-Dependent)
Many fish and reptiles also follow either the X/Y or Z/W format, but the groups are so varied that if you ask how reptiles determine male or female, the answer is "all of the above." Same goes for fish. The closer you look at these animal domains, the more varied and complicated it gets.
In some lizards, the tuatara, all crocodilians, some turtles (including all sea turtles), and some fish, we see what is called Temperature-Dependent Sex Determination (TSD). The temperature of the eggs during incubation determines whether the baby will be male or female. There are many different ways this occurs, but we will look at the two extremes: crocodilians and sea turtles.
Crocodilians
The group crocodilians refers to all crocodiles, alligators, caimans, and gharials.
In these species, it can be different depending on the species, but for the most part, the default gender of each egg is female. From there, the nest temperature is the most important factor in what gender the baby will be when the egg hatches. And the temperature of each egg individually matters. If the temperature is high enough, the egg will start producing male hormones and become male. If it stays below a certain temperature, it remains female. When a nest has dozens of eggs piled atop each other, this means that the upper layer (where temperatures are usually warmest because of the sun and air temperature) are more likely to be male than the eggs down at the bottom. Therefore, where the mother digs her nest can have a huge influence on the babies' genders. Choosing a sunny or shady place can be very important for the ratio of male and female offspring.
Sea Turtles
In sea turtles, the scheme is the opposite. Whereas in crocodilians high temperatures mean males, in sea turtles, higher temperatures produce females and lower temperatures lead to males. Turtle researchers like to use the phrase "Hot chicks, cool dudes" to remember this. Each species has its own pivotal temperature, the point at which an equal distribution of male and female turtles are hatched, and is approximately 29 °C (84 °F) for sea turtles (2) If the egg spends most of its time above 29 °C (84 °F), it's a girl, if it spends most of its time below this temperature, it's a boy!
Variability in incubation temperatures is critical to the long-term survival of sea turtle species, because obviously both males and females need to be around for the species to stick around.Sea turtles have been around for a very long time, and this has worked pretty well for them.
Unfortunately, as the global climate warms, so too does the likelihood of exceeding the pivotal incubation temperature of sea turtle nests. In Australia, it is already being seen. Green turtles born near the Great Barrier Reef were 99% female in natural nests. And because sea turtles tend to return to the same beach to lay their own eggs, all of these female hatchling will likely return to the same overly warm sand, where their own babies will be predominantly female.
Even worse, there exists a nest temperature where the egg just cannot develop at all. It is past the temperature for female babies and into temperatures that are lethal. This is called the thermal threshold. Studies have indicated that by 2070 the upper thermal threshold (over 33 °C or 91 °F) may be reached in nesting grounds near the equator (3).
That said, it takes 20-30 years for a baby turtle to reach the age where she will return to lay her own eggs. We still have time, and there is still hope that when the babies return as mothers, their nesting beach will be better suited to their survival. Protecting the planet means protecting all the species that call it home, whether they are humans, the pets we interact with everyday, or the baby sea turtles popping out of the sand on a far-off beach. We may not ever meet them, but the choices we make today will be really important for the sea turtle that is born tomorrow.