Why Does the Egg Refuse to Hatch in Warm Weather? It Is Waiting for Something Else

An animal that becomes inactive because conditions are bad, and resumes as soon as they improve, is doing something straightforward. This is different in two ways that matter. The pause begins before conditions deteriorate, triggered by a cue that predicts the coming problem rather than by the problem itself, and once it has begun the animal will not resume even if conditions become ideal, because a further requirement must be met first. That makes the state programmed rather than responsive, which is what allows an animal to prepare in advance and prevents it from being fooled by a warm spell in autumn.

The cues used are chosen for reliability rather than for directness:

• •Day length is the main trigger, since it never varies between years
• •Shortening days in late summer initiate the state well before frost
• •Temperature and food quality modify the response
• •A period of cold is usually required before it can end
• •Lengthening days then permit development to resume
• •The whole sequence can be induced in the parent generation

Requiring a substantial period of low temperature before development can resume is an elegant safeguard, because it guarantees that a warm autumn cannot be mistaken for spring. Only a real winter supplies enough accumulated cold, so the animal cannot be tricked into emerging in November. The same logic governs seeds and buds in plants, where a chilling requirement prevents germination and bud burst during a mild spell. It also creates a vulnerability under warming, since a winter too mild to satisfy the requirement leaves the animal or the plant unable to resume properly, with delayed or uneven emergence, which is already being recorded in fruit crops.

The internal machinery has been worked out in reasonable detail for several insects and the outline is consistent. Day length is measured by a clock in the brain, and the resulting signal suppresses or permits the release of hormones that drive development, so the pause is maintained by the absence of a growth signal rather than by any active brake. Metabolic rate falls substantially, water content drops, and stores of fat and of protective sugars and proteins accumulate, which is why animals in the state survive freezing and drying far better than active ones. Ending it requires the hormone release to resume, which the accumulated cold permits.

The mechanism solves problems other than cold, which is where the most striking examples come from. Insects in seasonally dry regions use it to pass the dry season rather than the winter. Some species remain in the state for several years, so that one bad year cannot destroy a whole population, and desert species have emerged after more than a decade. Mammals including roe deer, badgers, seals and bears use a version in which a fertilised embryo halts before implanting, which separates mating from birth and lets both happen at the best time of year, so a species can mate in summer and give birth in spring without a pregnancy lasting nine months.

Development halts at a fixed stage before conditions worsen, triggered by day length rather than by cold, and cannot resume until a requirement is met, which is usually a long period of low temperature. That makes a warm autumn unable to trick an animal into emerging, and makes a mild winter a problem. Mammals use a version that halts an embryo before implanting, separating mating from birth.