Butterfly species vary in sex-specific sodium accumulation from larval diets

Sodium is essential for animals, and its heterogeneous distribution can
cause a range of phenomena, from sodium-seeking behaviors to impacting
their performance. Although sodium content in soils and plants is
relatively well-documented, data for higher trophic levels are limited.
Knowledge of the variation in sodium in lower trophic levels could have
implications for understanding the behavior and physiology of species at
higher levels. We investigated the variation in tissue sodium
concentration between males and females of four butterfly species.
Puddling behavior of Lepidoptera suggest sodium needs of males are
generally greater than females, thus, we predicted males would accumulate
more sodium than females on a given diet. Larvae were reared on plants
(for Battus philenor, Chlosyne lacinia, and Danaus plexippus) and
an artificial diet (for Pieris rapae) under Low Na (no added sodium) and
High Na (sodium added) conditions. Among species and sexes, we quantified
and compared adult absolute tissue sodium concentrations and
bioconcentration factors, which indicate net sodium accumulation or
excretion relative to individuals’ diets. On average, individuals on
low-sodium diets had higher bioconcentration values across all species.
Male butterflies accumulated significantly higher sodium concentrations
than females in two sodium treatments for B. philenor, and P.
rapae and only in the High Na treatment for C. lacinia. However, in D.
plexippus, individuals accumulate sodium in the High Na treatment, but
males and females responded in the same way. Our study revealed sex- and
species-specific patterns of butterfly sodium accumulation, which could be
linked to variations in behavior and/or performance. Differences in sodium
content across species have implications for variation in predation and
trophic-level interactions, an interesting avenue for future ecological
and evolutionary research.