Read the following account and then state which strategy from Table 1.1 it best fits.
While freezing is lethal for most organisms, one group of organisms is unaffected by it. Water-bears or tardigrades (phylum Tardigrada) are tiny creatures, measuring only 0.05-1.5 mm in length (Figure 1). They live in very small water bodies that are liable to dry up in summer and winter. In dry conditions, tardigrades lose most of their water, shrivel up and transform into barrel-shaped 'tuns', in which state they can survive for ten years or more. In experiments, tardigrade tuns have survived being chilled to −272°C for eight hours and being heated up to 150°C.
This response agrees with the 'opt out' strategy (2) in which organisms maintain an inactive existence for the duration of the hostile period.
A crucial question that arises in relation to photoperiodism is whether it is the duration of the light period, the dark period, or both to which organisms respond. Consider the following experiments. Many plants and animals have been kept under artificial L : D regimes in which one or other part of the cycle is briefly interrupted. For example, the dark period can be interrupted by turning on the lights for a short period. Typically, short light periods during the dark period, in some instances as short as one minute, can completely change the flowering of plants or the behaviour of animals, but comparable short dark periods during the light period have no effect.
Do these experiments demonstrate that organisms are sensitive to the duration of the light period, or the dark period or both?
This experiment provides clear evidence that organisms respond to the duration of the dark period, not to the duration of the light period.
Why should the fact that coniferous needles are shaped like cylinders be advantageous in reducing water loss?
A needle, which is a long, thin cylinder, has a smaller surface area relative to its volume than a flat blade, so its rate of water loss per unit mass is correspondingly lower.
The kestrel (Falco tinnunculus) is a solitary hunter all year round. In Britain, it feeds primarily on the common vole (Microtus arvalis), which is also active in winter. Because their prey are relatively large and very nutritious, kestrels do not need many meals each day and the shortness of winter days does not limit their hunting success. One change of behaviour that kestrels do show in winter is to switch from flight-hunting, in which they look for prey while hovering above the ground, to perch-hunting. The latter is much less energetically expensive and involves sitting patiently on a branch until an active vole is spotted on the ground. Why should perch-hunting be favoured in winter?
Perch-hunting is favoured in winter because vegetation is lower and many trees are leafless, so small mammals are more easily seen from a perch.
Which of the four responses to winter is observed in each of the following: bats, edible dormouse, caribou, deciduous trees, annual plants, swallows and conifers?
Conifers remain active during winter (strategy 1). Bats, dormice and deciduous trees become inactive during winter (strategy 2). Most annual plants survive the winter only as seeds (strategy 3, although some species germinate in autumn and overwinter as juvenile plants). Swallows and caribou migrate (strategy 4).