Naturally, in a group of animals as diverse as the snakes, and with so many

游客2025-04-26 83

问题 Naturally, in a group of animals as diverse as the snakes, and with so many varied enemies, there are numerous defensive reactions and devices. There is, however, one general pattern of hehaviour, in the presence of a suspected enemy the first reaction is to try to escape observation; if this fails, the next resort is flight to some inaccessible retreat, but if this is not possible, or is circumvented, various kinds of intimidatory gestures and warning devices arc brought into play ;in the last resort the snake attacks. This pattern varies with the circumstances; some stages may be omitted or combined unpredictably whilst some notoriously irascible species may dispense with all the preliminaries and attack almost at once, though seldom or never without some provocation. The difficulty is to know what constitutes provocation, a matter that is apt to be debatable in other fields!
Amongst the factors that increase aggressiveness are hunger, the mating season and surprise, with the last mentioned the commonest ; when hunting for food or for mates, activity and the aggressive instinct are both at their peak, but it is when it is caught unawares that the normal chain of re- actions tends to become telescoped. Owing to their poor sense of hearing, snakes are very liable to be, quite literally, caught napping and a similar situation arises during their periods of temporary blindness just before sloughing begins. By far the greatest number of snake-bite accidents result from the unwitting disturbance of resting snakes, and this hazard is much increased with species that are well camouflaged and whose natural instinct is to trust to this concealment as their principal defence. As well as differences in aggressiveness between individuals of the same species according to the circumstances and conditions, there are also notable differences between species ,even closely allied species; and the reports of those who have been attacked may understandably be lacking in objectivity. So it is impossible to forecast, even in outline, how any encounter will develop.
The Hamadryad, for example,is usually credited with being amongst the most aggressive of snakes ,and there are many accounts of unprovoked attacks ;yet on one occasion fourteen men and seven dogs passed and returned within two yards of a nest and no snake was seen although the fe- male, which guards the nest, could not have been far. away.
One very well-known student of snakes once parked his car under some trees near Nairobi and on his return found a small green snake on it. Being preoccupied at the moment he gave it only a cursory glance and, thinking it was a common harmless tree-snake, bundled it unceremoniously into his pocket ;it was, as he later discovered to his horror, a young Green Mamba, but it made no attempt to bite despite the rough treatment it had received.
The proportion of the snakes of the world that have some forms of procrypsis( i. e. resemblance to the background)is very high, but the frequency with which the resemblance results in accidents suggests that it is incorrect to regard it as primarily a "protective" device. There is every advantage to a predatory animal in being unobserved until its prey comes within striking distance, but it is distinctly hazardous to allow an enemy to approach closely with the hope ,but no certainty, of remaining undetected. The commonest type of proerypsis is the result of colours that harmonize with hose of the normal background, associated with patterns that disrupt the animal’ s outline or produce "counter shading" whereby those parts of the body that will be seen in shadow and appear dark, are lighter in colour whilst highlight areas are dark-coloured. [br] The snake’s pattern of defence varies depending on ______.

选项 A、the circumstances
B、the form of provocation
C、the type of snake and the situation
D、what type of snake is involved

答案 C

解析

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