Shell we talk about candling? How the Two Oceans Aquarium monitors penguin egg health

The Two Oceans Aquarium recently welcomed two new additions to our African penguin family - the first time in 10 years we've had new African penguin chicks!

The chicks are progressing well and growing faster than ever, but what about their journey before they hatched? Let’s hop into a crash course on candling, a method that helps our penguin keepers monitor egg health and embryo development.

What is candling?

"Candling" the process is where our penguin keepers shine a bright light through an egg to see the inside. This is a crucial way for our team to safely monitor the health of a developing penguin embryo - think of it as the equivalent of a human getting an ultrasound checkup.

The candling method was actually devised long before electric light bulbs were invented. Poultry farmers would place a candle inside a box with a hole cut into the top to backlight an egg, thus assessing it for cracks or identifying fertility. This is where the term “candling” originated. Nowadays, our penguin keepers use an LED flashlight in a dark room or covered box to monitor the eggs.

For our two newest penguin chicks, candling took place weekly, during which the team carefully removed each egg, quickly assessed and documented its development, and quickly returned the egg to the waiting parents.

What were our penguin keepers looking for in the developing eggs?

The main thing that the penguin keepers looked out for was embryo growth – the bigger the penguin embryo grew inside the egg, the less light shone through the egg. An African penguin egg needs to be incubated for up to 40 days, and through candling, penguin keepers can closely monitor development during this time.

The first few days after the egg is laid are the most crucial for the developing embryo, so our penguin keepers did not disturb the expecting parents until a week later. At around day seven after hatching, our penguin keepers could see a clear air sac, tiny embryo, yolk ring, and veins on each of the eggs - a positive sign!.

Between days 14 and 20, the inside of the eggshells are about half covered with veins, and the embryos are visibly moving! Veins transport crucial nutrients and oxygen from the yolk to the developing embryo, so this is an especially exciting stage for our penguin keepers. At this point, the team could breathe a small sigh of relief that the embryos were progressing well.

From this point, the embryo grows continuously, slowly taking over most of the egg space. By day 37, most of the egg will be dark, with just the air sac showing up against the flashlight. This means that the chick is almost ready to hatch.

Thanks to the candling method, our penguin keepers could narrow down the exact date of hatching! Indeed, Ember hatched on 10 February, with Echo following on 13 February, just as estimated.

We are so excited to witness our baby penguins experience the world for the first time! From candling to hatching, from their first weigh-ins to their first steps – we want you to follow along, too. Watch this space for updates!