Sodium and the courtship of butterflies

Courtship is defined as specific behavioural patterns that induce sexual receptivity, i.e. it facilitates recognition between species and also serves as a mechanism to prevent mating between different species. It can be done by visual, tactile and/or chemical stimulation.

A curious mechanism is that of some species of butterflies, called “puddling“, which consists of the suction of mud, decaying fruit liquids and even faeces. This behavior is mainly performed by males, and they do so because they obtain minerals, especially sodium (Na), an element that is not abundant in their diet. This is what happens when we see many butterflies perched on the ground.

Process by which males that have absorbed sodium from the medium transmit it to their offspring. Personal archive

What does this have to do with courtship? When males mate with females, they transfer that sodium to them by incorporating it into their spermatophores (capsules containing sperm). The females then transfer this sodium to the egg vitellus, so that the caterpillars that hatch from those eggs will have higher amounts of sodium.

It has also been proven that the eggs laid by females that mate with males that do not perform this behavior have a lower amount of sodium. What is even more surprising is that females are able to discriminate between males that have more or less sodium in their spermatophores, so males would increase their reproductive success with this strategy.

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