1. How Big Is an Ostrich Egg? The Largest Egg of Any Living Bird
Ostrich eggs are introduced as averaging about 15 cm (about 6 inches) in length, 13 cm (about 5 inches) in width, and weighing about 1.4 kg (about 3 pounds). [2] Nogeyama Zoo in Yokohama lists their kept ostrich eggs as about 15 cm long and weighing about 1.2 kg (about 2.6 pounds). [1] Sources give values from about 1.2 to 1.4 kg, so it is safest to say "about 1.2 to 1.4 kg." [1]
In food science reports, the total weight of an ostrich egg is about 1,200 g. The whole egg and the white are about 20 times the weight of a chicken egg, while the yolk is about 15 times. [3] The fact that the yolk multiplier is slightly lower is an interesting point. [3]
Female ostriches lay about 40 eggs per year, with an interval of 5 to 10 days between eggs. It is also noted that they do not lay much from summer to autumn. [1]
2. Why the Ostrich Egg Is Small Compared With the Mother's Body Weight
When viewed as a ratio to body weight, the ostrich egg is small. The female ostrich weighs about 100 kg (about 220 pounds), and the male about 130 kg (about 286 pounds). [9] If the egg is 1.4 kg, that is about 1.4% of the female's body weight. [9] A New Zealand conservation group explains that an ostrich egg is about 2% of the body weight and is among the smallest ratios among birds. [8] Because sources show a difference between 1.4% and 2%, it is safer to write "about 1 to 2%." [8]
In comparison, the kiwi is remarkable. The same group states that a female kiwi's egg reaches about 20% of her body weight. This is about 6 times the size of an egg from a bird of similar size. [8] They compare this to human babies, which are about 5% of the mother's weight at full term. [8] The ratio for kiwis varies between 15 and 25% depending on the source. [8]
3. How Thick Is the Ostrich Egg's Shell Compared With a Chicken Egg?
The thickness of an ostrich egg shell is about 2.0 mm (about 0.08 inches) when measured with calipers, which is about 5 times that of a chicken egg. [3] In another study using X-ray tomography, the average was 1.83 mm. [4] The difference may come from the measurement methods; both values are around 2 mm. [4]
The weight of the shell alone is about 30 times that of a chicken egg. Since this multiplier is larger than the whole egg's multiplier (about 20 times), the proportion of the shell in the whole egg is higher than in a chicken egg. [3]
In a study examining eggs from 463 bird species, shells became thicker as eggs got larger, with thickness roughly proportional to the square root of the egg's weight. [7] An egg of the extinct elephant bird is said to have exceeded 9 kg (about 20 pounds), while an ostrich egg is about 1.5 kg (about 3.3 pounds). [7]
4. How the Embryo Breathes Through About 40,000 Pores in the Shell
The shell has small tunnel-like holes called pores. These are pathways for the embryo (the part that becomes the chick) to breathe, taking in oxygen and releasing carbon dioxide. They are also pathways for water evaporation. [5]
In a study using X-rays, the total number of pores in an ostrich egg shell averaged about 40,600. [4] For chicken eggs, sources list 7,000 to 17,000 pores. [5] Since the sources are different, we avoid stating an exact multiple, but we can say ostrich eggs have more. [5]
Across bird eggs, examples show that larger eggs have more and larger pores. Figures cited are about 300 pores for a 1 g egg and about 30,000 for a 500 g egg. However, these numbers are from a summary, so read them as rough guidelines. [6]
5. How Ostrich Parents Share About 40 Days of Incubation
According to Asahiyama Zoo records, incubation lasts about 40 days. The male and the most dominant female take turns; the female handles the daytime, and the male handles the night. [10] Eggs from several females are gathered in one nest. [10]
The incubation period varies between 35 and 45 days in the literature, and it is on the shorter side among large ratites (large flightless birds). [2]
6. What We Know and Still Don't Know About Why Ostrich Eggs Are So Big
In summaries of zoo breeding records, chicks are introduced as opening their eyes and walking shortly after hatching, starting to peck for food on their own. [11] The trait of being able to walk shortly after hatching is called "precocial." [11] This may be related to the chick developing further inside the large egg, but the cause-and-effect link is not known. [11]
As far as researched, no single definitive answer to "why is it so big?" was found. What is known is that ostriches themselves are large birds, so the absolute size of the egg is large, and that the ratio to the mother is small. Across bird species in general, shells tend to be thicker and pores more numerous and larger as eggs get bigger, but this trend does not by itself establish a relationship for ostrich eggs alone. We cannot yet state the cause definitively. [11]
7. How to Calculate an Egg's Weight Ratio Using Your Own Body Weight
If your weight is 30 kg (about 66 pounds), 1.4% is about 0.42 kg (about 0.9 pounds), and 2% is 0.6 kg (about 1.3 pounds). That is about the weight of 500 mL (about 17 oz) of water, so a full plastic bottle of that size can help you picture it. [8] Using the same ratio, a kiwi's 20% would be 6 kg (about 13 pounds), or three 2 L (about 67 oz) plastic bottles. [8]
Comparing with things you can hold in your hand helps you feel the meaning of the ratio. Be careful not to drop the bottles or bump into anyone while you hold them. [8]
If you want to know more, try reading the "Ostrich Egg" article from Nogeyama Zoo or the Asahiyama Zoo newsletter, or the original papers by Juang and colleagues on shell thickness and by Willoughby and colleagues on pore counts (the latter was checked only from its abstract). [1]