Eggs are nature’s care-packages from mothers. Males contribute sperm to an egg in order to form an embryo, but the rest of the egg contents - including proteins, fats, sugars, steroids and immune enhancing compounds - are from the mother. Since mothers regulate exactly how many nutrients goes into a particular egg, not all eggs created equal.
Sometimes the identity of the father affects the quality of the eggs she lays. Sperm from a healthy male is more desirable than from an unhealthy male. Female blue footed boobies looking to procreate generally select males based on the quality of their foot-color. Bright blue feet are the most desirable, whereas gray dull feet are less so. And that’s because this coloring is a symbol of their health.
The blue color of these fabulous feet is derived from a pigment called biliverdin, which is also responsible for the beautiful blue hues of many bird-eggs and also for the blue-green colors of bruises on the human body. Biliverdin is directly involved in important immune and antioxidant functions, and so a booby male’s foot color is a great way for a female to decide whether he’s in good health, and will have enough energy to provide for their potential offspring.
After each copulation, females lay two eggs. The first is laid soon after the sperm is received, and the second comes along some 72 hours later. A clever biological experiment shed some light on how a blue footed female influences her egg health.
After copulating with a high-quality, bright blue footed male, a female laid her first egg with large volume, abundant sugars and fats and a significant level of antioxidants and antimicrobial compounds. However, during the 72 hour waiting phase before the second egg came along, biologists painted the sperm-giving male’s feet a dull grey color, making him appear unhealthy and undesirable.
When the second egg was birthed - after mom had been tricked into thinking that the sperm was unhealthy - it was smaller than the first. The intricate biological way this happens is unknown. Survival prospects of the second egg is greatly reduced, but the mother saves on energy by not raising unhealthy young.