A second egg in a penguin nest is rarely a promise of twins. It is more like a biological insurance policy that evolution expects to cash in only when the first policy fails.
In several penguin species, parents lay a pair of eggs but reliably raise a single chick. Natural selection has tuned their energy budget and basic metabolic rate so tightly that feeding two large, fast growing offspring would push adults beyond safe physiological limits. The two eggs are not equal, either. In many cases the first or second egg is consistently smaller, with lower yolk reserves and a poorer survival record, a pattern known as egg dimorphism.
The logic is brutally efficient. Producing a second, cheaper egg slightly raises the probability that at least one chick survives predation, storms, or incubation failures, while avoiding the high marginal cost of rearing a second fledgling through to independence. Field studies show that when food is scarce, parents let the weaker hatchling starve or allow it to die of exposure, rather than dilute care and risk losing both. When feeding conditions are unusually rich, the spare chick may survive, but that outcome is a rare side effect, not the trait that was favored by natural selection. The system is optimized around scarcity, not abundance.
A nest holding one thriving chick and one fading body captures the tension in this strategy, where a tiny gain in survival probability justifies a life primed, from the moment of laying, to be expendable.
