Speaking generally, birds are accomplished architects. Certain of them are, however, content with a rudimentary cavern. There is no question here of those who retire to clefts in the rock or in trunks of trees, for in these cases the cavity is only the support of the true house, and it is in the construction of this that the artist reveals his talent. I wish to speak of animals which remain in a burrow without making a nest there. A Parroquet of New Zealand called the Kakapo (Strigops habroptilus) thus dwells in natural or hollowed excavations. It is only found in a restricted portion of the island and leads a miserable life there, habitually staying in the earth and pursued by numerous enemies, especially half-wild dogs. It tries to hold its own, but its wings and beak do not suffice to protect it, and the race would have completely disappeared if these birds were not able to resist, owing to the prudence with which they stay within their dwellings. They profit by a natural retreat, or one constructed in rocks or beneath roots of trees; they only come out when impelled by hunger, and return as soon as they can in case of danger.
A large number of animals also hollow out shelters for their eggs, with the double object of maintaining them at a constant temperature and of concealing them. Most reptiles act in this manner. The way in which a Tortoise, the Cistudo lunaria, prepares its nest is extremely curious. When the time for this labour arrives, the tortoise chooses a site. It commences by boring in the earth with the end of its tail, the muscles of which are held firmly contracted; it turns the tail like a gimlet and succeeds in making a conical hole. Gradually the depth of the hole becomes equal to the length of the tail, and the tool then becomes useless. The Cistudo enlarges the cavity with the help of its posterior legs. Using them alternately it withdraws the earth and kicks it away, then piles up this rubbish on the edge of the hole, arranging it so as to form a circular rampart. Soon the posterior members can take nothing more from the too distant bottom. The moment for laying has now come. As soon as the egg arrives at the cloaca one of the feet seizes it and lowers it gently into the nest, while the second foot seizes another egg, which during this time had appeared at the orifice. This manipulation lasts until the end of the operation, when the tortoise buries all its family, and to flatten the prominence which results she strikes it repeatedly with her plastron, raising herself on her legs.
It is not only land animals which adopt this custom of living in the earth, and there sheltering their offspring. Fish also make retreats on the bank or at the bottom. To mention only one case, the Bullhead (Cottus gobio) of our rivers, which spawns in the Seine in May, June, and July, acts in this manner. Beneath a rock in the sand it prepares a cavity; then seeks females and brings them to lay eggs in its little lodging. During the four or five weeks before they come out it watches the eggs, keeping away as far as possible every danger which threatens them. It only leaves its position when pressed by hunger, and as soon as the hunt is concluded, returns to the post of duty.
Other animals when digging have a double object; they wish to shelter themselves, and at the same time to find the water which they need for themselves or for the development of their young.
It is well known that Frogs and Toads generally go in the spring to lay their eggs in streams and ponds. A Batrachian of Brazil and the hot regions of South America, the Cystignathus ocellatus, no doubt fearing too many dangers for the spawn if deposited in the open water, employs the artifice of hollowing, not far from the bank, a hole the bottom of which is filled by infiltration. It there places its eggs, and the little ones on their birth can lead an aquatic life while being guaranteed against its risks.
A terrestrial Crab, the Cardisoma carnifex, found in Bengal and the Antilles, acts in the same manner; but in this case it has in view its own convenience and not care for its offspring. Its habitat is especially in low-lying spots near the shore, where water may be found at a trifling depth beneath the soil. To establish its dwelling, the Crustacean first buries itself until it reaches the liquid level. Arrived at this point, it makes a large lair in the soft soil, and effects communication with the outside by various openings. It can thus easily come and go and retire into its cave, where it finds security and a humidity favourable for branchial respiration. From time to time it cleans out the dirt and rubbish which accumulate in the hole. It makes a little pile of all the refuse which it finds, and, seizing it between its claws and abdomen, carries it outside. Executing several journeys very rapidly, it soon clears out its dwelling.
The dipnoid Protopterus, which inhabits the marshes of Senegal and Gambia, is curious in more than one respect. Firstly, it can breathe oxygen, whether, like other fish, it finds it dissolved in water or in the atmospheric air. When during the summer the marshes in which it lives dry up, it takes refuge in the mud at the bottom, which hardens and imprisons it, and it thus remains curled up until the time when the water after the rainy season has softened the earth which surrounds it. This fact had been known for some time; travellers had brought back lumps of dried earth of varied size, the largest about as big as two fists. On opening them the same fish was always found within, and the chamber in which it is contained was lined with a sort of cocoon, having the appearance of dry gelatine. Duméril was able to observe one of these animals in captivity. At the period corresponding to the dry period of its own country, the Protopterus buried itself in the mud which had been placed at the bottom of the aquarium. In order to realise the conditions found in nature, the water which covered it was gradually withdrawn. The earth hardened in drying, and when broken the recluse was seen surrounded by hardened mucus, exactly like those which came from Senegal.
Carefully-disposed burrows. — All the cases which we have considered show us the industry of the hollowed dwelling in its primitive state; but other animals know how to furnish it with greater luxury. I will continue in the same order of increasing complication. Many beings live permanently in a burrow; Reptiles — Snakes or Lizards — are to be placed among these. Among others, the Lacerta stirpium arranges a narrow and deep hole, well hidden beneath a thicket, and retires into it for the winter, when cold renders it incapable of movement and at the mercy of its enemies. Before giving itself up to its hybernal sleep, it is careful to close hermetically the opening of the dwelling with a little earth and dried leaves. When spring returns and the heat awakens the reptile, it comes out to warm itself and to hunt, but never abandons its dwelling, always retiring into it in case of alarm and to pass there cold days and nights.
Darwin has observed and described[82] how a little Lacertilian, the Conolophus subcristatus, conducts its work of mining and digging. It establishes its burrow in a soft tufa, and directs it almost horizontally, hollowing it out in such a way that the axis of the hole makes a very small angle with the soil. This reptile does not foolishly expend its strength in this troublesome labour. It only works with one side of its body at a time, allowing the other side to rest. For instance, the right anterior leg sets to work digging, while the posterior leg on the same side throws out the earth. When fatigued, the left legs come into play, allowing the others to repose.
Other animals, without building their cavern with remarkable skill, show much sagacity in the choice of a site calculated to obtain certain determined advantages. In Egypt there are dogs which have become wild. Having shaken off the yoke of man, which in the East affords them little or no support, they lead an independent life. During the day they remain quiescent in desert spots or ruins, and at night they prowl about like jackals, hunting living prey or feeding on abandoned carcasses. There are hills which have in a manner become the property of these animals. They have founded villages there, and allow no one to approach. These hills have an orientation from north to south, so that one slope is exposed to the sun from morning to mid-day and the other from mid-day to evening. Now, dogs have a great horror of heat. They fear the torrid heat of the south as much as in our climate they like to lie warmed by gentle rays; there is no shadow too deep for their siesta. Therefore, on these Egyptian hills every dog hollows out a lair on both slopes. One of these dwellings is thus turned towards the east, the other towards the west. In the morning, when he returns from his nocturnal expeditions, the animal takes refuge in the second, and remains there until mid-day, sunk in refreshing sleep. At that hour the sun begins to reach him, and to escape it he passes over to the opposite slope; it is a curious sight to see them all, with pendent heads and sleepy air, advance with trailing steps to their eastern retreat, settle down in it, and continue their dream and their digestion till evening, when they again set forth to prowl. We never grow tired of admiring the intelligence of their domesticated fellows, but this trait seems to me worthy of remark; it proves a very developed power of observation and reflection.