Why Do Butterflies Gather Around Puddles?

How Mud Helps Butterflies Reproduce

Butterflies on mud puddle.
Southern Small White (Pieris mannii) five adults, 'mud-puddling', drinking minerals from damp ground. Getty Images/Corbis Documentary/FLPA/Bob Gibbons

On sunny days after a rain, you may see butterflies gathering around the edges of mud puddles. What could they be doing?

Mud Puddles Contain Salt and Minerals Butterflies Need

Butterflies get most of their nutrition from flower nectar. Though rich in sugar, nectar lacks some important nutrients the butterflies need for reproduction. For those, butterflies visit puddles.

By sipping moisture from mud puddles, butterflies take in salts and minerals from the soil.

This behavior is called puddling, and is mostly seen in male butterflies. That's because males incorporate those extra salts and minerals into their sperm.

When butterflies mate, the nutrients are transferred to the female through the spermatophore. These extra salts and minerals improve the viability of the female's eggs, increasing the couple's chances of passing on their genes to another generation.

Mud puddling by butterflies catches our attention because they often form large aggregations, with dozens of brilliantly colored butterflies gathered in one location. Puddling aggregations occur frequently among swallowtails and pierids.

Herbivorous Insects Need Sodium

Herbivorous insects like butterflies and moths don't get enough dietary sodium from plants alone, so they actively seek other sources of sodium and other minerals. While mineral-rich mud is a common source for sodium-seeking butterflies, they can also procure salt from animal dung, urine, and sweat, as well as from carcasses.

Butterflies and other insects that get nutrients from dung tend to prefer the dung of carnivores, which contains more sodium than that of herbivores.

Butterflies Lose Sodium During Reproduction

Sodium is important for both male and female butterflies. Females lose sodium when they lay eggs, and males lose sodium in the spermatophore, which they transfer to the female during mating.

Sodium loss is much more severe, it seems, for the males than for the females. The first time it mates, a male butterfly may give away a third of its sodium to its reproductive partner. Since the females receive sodium from their male partners during mating, their sodium procurement needs aren't as great.

Because males need sodium, but give so much of it away during mating, puddling behavior is much more common in males than in females. In one 1982 study of cabbage white butterflies (Pieris rapae), researchers counted only two females among the 983 cabbage whites observed puddling. A 1987 study of European skipper butterflies (Thymelicus lineola) found no females puddling at all, although 143 males were observed at the mud puddle site. The researchers studying European skippers also reported the area population consisted of 20-25% females, so their absence from the mud puddles didn't mean females weren't in the vicinity. They simply didn't engage in puddling behavior the way the males did.

Other Insects That Drink from Puddles

Butterflies aren't the only insects you'll find gathering in mud puddles. Many moths use mud to make up their sodium deficits, too. Mud puddling behavior is common among leafhoppers, too.

Moths and leafhoppers tend to visit mud puddles at night, when we are less likely to observe their behavior.

Sources:

  • "Puddling Behavior by Lepidoptera," by Peter H. Adler, Clemson University. Encyclopedia of Entomology, 2nd edition, edited by John L. Capinera.
  • "Mud puddling by butterflies is not a simple matter," by Carol L. Boggs and Lee Ann Jackson, Ecological Entomology, 1991. Accessed online February 3, 2017.