KEEPING EGGS WARM
(For cold-blooded reptiles, and some birds too, maintaining incubation temperature calls for special measures.)
1 The American alligator, (a relative of the American crocodile), like all reptiles, is cold-blooded, so the female is poorly equipped to incubate her eggs. Instead, she builds an incubator using her immensely strong tail to scrape together a mound of vegetation in which she buries her eggs. Just like a garden compost heap, the pile begins to decay and warms up. As it does so, the temperature rises to the ideal thirty-seven degrees Celcius.
2 This is a useful solution to the problem of incubation. However, it is not entirely satisfactory. The temperature within the heap varies by only a few degrees, but even a small change can have a major impact on the size and vitality of the new-born. Eggs that incubate at lower than ideal temperatures will take longer to develop, and more of the food content will be used up by the embryo in keeping itself alive. When they therefore hatch, the young offspring are smaller than those incubated at the ideal temperature.
3 In a shrewd evolutionary development, the alligator has adapted to this unpredictability. A small female produces proportionately fewer eggs than a large one, but at least she will produce some. However, for males, size is crucial. Large males win most females and the ferocious fights that happen at the start of the mating season. Small males may never mate at all.
4 To ensure her young ones get the best chance to breed successfully and so pass on her genes, a mother alligator needs her large offspring to be male and her smaller offspring to be female. In fact, sex is determined by the temperature at which an egg is incubated. Eggs that develop in ideal incubation conditions produce large males, but those that are inadvertently kept cooler will turn out smaller females. For every male produced, there are five females.
5 As with alligators, the sex of tortoises is determined by egg temperature. However, the tortoise differs entirely from the alligator in that tortoise eggs that develop at the right temperature produce large females and those incubated at lower temperatures produce smaller males.
6 For male tortoises, there is little advantage in being large, since tortoise fights are relatively rare and most males have a chance to mate. For females, however, there are definite advantages in being big. The number of eggs that a female can carry inside her is strictly limited by the size of her shell. Large females lay more eggs than small ones. Again, the determination of sex by the temperature ensures that females produce young offspring with the best prospects of continuing the species.
7 Another nursery gardener, the Australian mallee fowl, incubates its eggs by burying them in a heap of vegetation or compost. It builds a mound up to one metre high and about five metres wide, collecting together four tons of leaves and soil. The female lays up to thirty eggs in the centre of the heap of vegetation, then she departs and the male covers the eggs and takes up his vigil. The male constantly probes the nest with his beak to gauge temperature, which must be about thirty-three degrees Celcius. The eggs only take seven weeks to develop and hatch, but the whole process keeps the mallee fowl busily occupied for eleven months of the year.
8 Parental concern for eggs is unusual in most snakes. Most are satisfied with burying their eggs in a safe place, then abandon them to take their chances. However, there are some pythons that not only protect their eggs, but incubate them too.
9 After laying a clutch of about a hundred eggs, the female python coils herself round them. She then begins to shiver, making rhythmic contractions of her muscles that can raise her body temperature by as much as seven degrees Celcius above that of her surroundings. She gently shuttles the eggs in and out of the sun.
10 There are cold-blooded snakes which cannot use their body heat to incubate their eggs. Although several other species of snakes guard their eggs, so far as is known, only the python incubates them. This may be because other species lack the massive muscle power required to generate the necessary warmth.
11 For reptiles, the ideal environment is a warm climate whose even temperatures permit the eggs to hatch unaided. Reptiles are also found in places ranging from tropical rain forests to chilly moorlands. In some of these, a number of species have found different solutions to the problem of incubating eggs. The European grass snake, for example, lays its eggs in piles of rotting vegetables. The heat generated by the chemical breakdown of the plant material is just right for the eggs.
12 There are dangers, too, in such an environment. Fungi and bacteria abound and some protection from them is essential if the developing young ones are to survive. The protection is provided by the tough, leathery shell of the grass snake's eggs. To minimise exposing its eggs to predators, the grass snake also lays its eggs in an environment whose background colour is identical to the eggs' colour.
13 The ostrich, the largest living bird, is unable to fly and must therefore make its nest in a scrape on the ground. Ostrich eggs weigh about one and half kilogrammes each and make a meal not to be despised by even such large predators as jackals, hyenas and the occasional lion. Major hazards to survival call for special measures to combat them. The ostrich's response has been to evolve a highly unusual pattern of breeding and incubating.
14 A male ostrich forms a pair bond with only one bird, the 'major hen', although he will also mate with as many other females as he can. These 'secondary females' are allowed to lay their eggs into the nest, but take no part in incubating them. This task, the male shares with his major hen. He sits on the eggs during the night when the danger is greater and the major hen takes over in the day.
15 It is rare for an animal to look after the offspring of others, since helping them will decrease the food and other resources available for its own. The major hen is no more unselfish than any other female bird. When the nest is full and she can no longer effectively incubate all the eggs, she pushes out some of those that do not belong to her. These make a circle of discarded eggs round the nest.
16 Lying neglected in the sun, these discarded eggs will never hatch, but they are the first eggs that any predator will find and may well be sufficient to satisfy its hunger. They will, at any rate, therefore, divert attention from the major hen's eggs which remain in relative security beneath the parent in the centre of the nest.
Adapted from: Exploring the Secrets of Nature, Readers Digest, 1994.