Instinct

“An action, which we ourselves should require experience to enable us to perform, when performed by an animal, more especially by a very young one, without any experience, and when performed by many individuals in the same way, without their knowing for what purpose it is performed, is usually said to be instinctive.” — Charles Darwin

If complex physical structures can evolve, what about complex behaviors? Darwin tackles instincts—the inherited behavioral patterns that animals perform without learning. Can behaviors as intricate as the honeybee’s hexagonal comb or the cuckoo’s parasitism be explained by natural selection? Darwin argues yes, and provides some of his most fascinating examples.

What Is Instinct?

Darwin acknowledges the difficulty of defining instinct precisely, but focuses on its key characteristic: instinctive behaviors are inherited, not learned.

Instinct

An inherited behavioral pattern that animals perform correctly the first time, without instruction or practice. Instincts can be modified by experience but don’t depend on it. They are to behavior what complex organs are to anatomy.

Instincts Can Vary

The first step is showing that instincts, like physical structures, can vary:

Variation in Instincts

• Migratory birds sometimes take unusual routes
• Dogs show varying degrees of pointing and retrieving instinct
• Birds of the same species build slightly different nests
• Fear of humans varies among populations of the same species

If instincts can vary, and if variations are heritable, then natural selection can act on behavior just as it acts on anatomy.

The Cuckoo: A Parasitic Instinct

The European cuckoo lays its eggs in the nests of other birds, leaving foster parents to raise its young. This remarkable behavior troubled naturalists—how could such a complex parasitic strategy evolve?

The Cuckoo’s Instincts

• Female cuckoos lay eggs in nests of specific host species
• Cuckoo chicks hatch early and eject host eggs/chicks
• Host parents feed the cuckoo chick despite its different appearance
• Young cuckoos migrate to correct wintering grounds without parental guidance

Gradual Evolution of Parasitism

Darwin notes that some birds occasionally lay eggs in others’ nests. If this accidentally proved advantageous—if foster-raised chicks survived well—the behavior could be selected for. Each component of the cuckoo’s instincts could evolve incrementally:

• Occasional egg-dumping → regular parasitism
• Random hosts → specialized on hosts that rear chicks successfully
• Chick ejection behavior → ensures all parental care goes to cuckoo

Slave-Making Ants

Some ant species raid the nests of other species, capturing pupae and raising them as workers. Darwin studied this extensively:

Gradation in Slave-Making

• Formica fusca: Enslaved species; does all the work
• Formica sanguinea: Masters can still work; slaves supplement
• Polyergus (Amazon ants): Masters are helpless without slaves; mandibles only suited for fighting

The gradation shows how an extreme instinct could evolve. Early stages involve occasional slave-taking; later stages show increasing dependence until the master species can no longer survive without slaves.

The Honeybee’s Cell: A Marvel of Instinct

Darwin considered the hexagonal cells of honeybee combs the most wonderful instinct of all—and potentially the most difficult to explain:

The Challenge

Honeybee cells are mathematically perfect hexagonal prisms with pyramidal bases—the shape that encloses maximum volume with minimum wax. How could “blind” instinct produce what seems to require advanced geometry?

Darwin’s explanation is a masterpiece of gradualist reasoning:

Darwin’s Solution

Darwin shows gradation among bee species:

• Humble-bees (bumblebees): Build rough, irregular cells
• Melipona (stingless bees): Build cylindrical cells packed together, with flat walls where they meet
• Honeybees: Start by building curved walls, then flatten them where cells adjoin

The Geometry Emerges Automatically

Darwin showed that if bees simply excavate spherical cells at equal distances, the intersecting walls automatically form the optimal hexagonal pattern. No geometric “knowledge” is needed—just consistent spacing and wall-thinning behavior. The mathematics is a byproduct, not a goal.

Objections to Instinct Evolution

Darwin addresses key objections:

Objection: Sterile Worker Castes

How can natural selection act on worker ants and bees, which don’t reproduce? Their instincts can’t be passed to offspring because they have no offspring.

Darwin’s Response: Family Selection

Selection acts on the reproductive members of the colony. If producing sterile workers with particular instincts benefits the colony’s reproductive success, those genes spread. The unit of selection is the family or colony, not the individual worker. Darwin anticipates the concept of kin selection developed fully by Hamilton a century later.

Key Takeaways

• Instincts are inherited behavioral patterns, analogous to complex organs
• Like physical traits, instincts vary and can be selected
• The cuckoo’s parasitism shows how complex behavioral strategies can evolve gradually
• Slave-making ants show gradation from occasional to obligate parasitism
• The honeybee’s hexagonal cells emerge from simple behaviors, not geometric knowledge
• Sterile worker castes evolve through selection on family groups (anticipating kin selection)