10.3 Life Cycles & Adaptations

Animal Life Cycles

A life cycle is the series of stages a living thing passes through from the start of one generation to the start of the next. Life cycles of plants and animals appear from the Introductory paper onwards, and the big idea ICAS returns to is that not all animals grow up the same way.

Complete metamorphosis has four stages, and the young look nothing like the adults. A butterfly goes egg -> larva -> pupa -> adult: the caterpillar (larva) hatches from the egg and does nothing but eat and grow, then seals itself into a chrysalis (pupa) where its body is rebuilt, and finally emerges as a winged adult. Beetles, flies, moths and bees follow the same four-stage plan. A frog is the other famous example: eggs become tadpoles with gills and tails, tadpoles grow legs and lose their tails, and the adult frog breathes air on land. That dramatic reshaping is what makes it metamorphosis.

Incomplete metamorphosis has only three stages: egg -> nymph -> adult. A grasshopper hatches as a nymph, which is essentially a miniature adult without wings. The nymph sheds its skin several times as it grows - each shed is called a moult - and develops wings at the final moult. There is no pupa stage and no dramatic rebuild. Cicadas, dragonflies and cockroaches grow this way too.

Humans and other mammals are different again: there is no metamorphosis at all. A baby is born resembling a small adult and simply grows and develops into maturity. If a question asks which animal has a nymph stage, think grasshopper, not butterfly.

Comparing life cycles

Plant Life Cycles

The plant life cycle is a loop: seed -> seedling -> adult plant -> flower -> fruit -> seed. A seed germinates into a seedling, the seedling matures into an adult plant, and the adult produces flowers - its reproductive parts. After pollination (pollen reaching the flower's female part, often carried by wind or insects) comes fertilisation, and the fertilised flower develops into a fruit containing seeds. Those seeds are then dispersed - scattered by wind, water, animals or explosive pods - and the loop begins again. Two dispersal examples worth knowing: a dandelion's parachutes ride the wind, while a burr hooks onto fur and hitchhikes. ICAS questions often start the cycle at a strange point, such as asking what must happen between a flower and a fruit - the answer is pollination, then fertilisation.

Adaptations: Built for Where You Live

An adaptation is any feature of an organism that helps it survive and reproduce in its particular environment. Adaptations come in three kinds, and being able to sort an example into the right kind is a regular ICAS task:

• Structural adaptations are body parts: the echidna's spines for defence, the kangaroo's huge hind legs for efficient hopping across open country, a water lily's broad floating leaves for catching light on the water's surface, or a cactus's spines (modified leaves) that slash water loss.
• Behavioural adaptations are things an organism does: migratory birds flying thousands of kilometres to follow food and warmth, lizards basking in the morning sun, or kangaroos resting in shade through the heat of the day and feeding at dawn and dusk.
• Physiological adaptations are internal processes: a kangaroo rat producing extremely concentrated urine to save water, or camels and many desert Australian mammals tolerating big swings in body temperature rather than sweating away precious water.

Notice how the same environment can be answered in all three ways. A cactus in the desert has a structural answer (spines, waxy skin), a physiological answer (it can open its stomata at night to avoid daytime water loss) and a deep root system or wide shallow roots to catch rare rain. A water lily answers the opposite problem - too much water - with air spaces in its stems and leaves that keep it afloat. The environment sets the exam question; the adaptation is the organism's answer.

Camouflage and mimicry

Camouflage means blending into the background so predators or prey cannot spot you: a stick insect's twig-shaped body, or a tawny frogmouth freezing on a branch so it looks like broken wood. Mimicry is different - one species copies another's warning signals. A harmless hoverfly wears the yellow-and-black stripes of a wasp, so predators that learned to avoid wasps leave it alone. Camouflage says I am not here; mimicry says I am dangerous. Keep those two messages separate in your head, because swapping them is an easy trap to set.

The classic ICAS stimulus move

The hardest questions in this topic show you an animal you have never seen and ask which adaptation explains a feature or behaviour. The routine never changes: read the environment described in the stimulus, name the problem it creates (heat? cold? predators? scarce water? darkness?), then match the feature to that problem. A fish described as nearly transparent in open water - the problem is predators spotting it, and transparency is camouflage. A desert mouse that never drinks - the problem is water scarcity, so look for a physiological answer such as getting all its water from food and producing concentrated urine. An arctic animal with a compact, rounded body and small ears - the problem is heat loss, and a small surface area relative to volume keeps warmth in. Feature, environment, problem, solution: reason through those four steps and even a completely unfamiliar animal becomes a readable question.