Meeting Abstract
Sodium, a micronutrient essential for muscle and neural function in animals, has historically been limited in availability for many terrestrial herbivores. Humans have recently increased the amount of available sodium by using deicing salts on roads and through agricultural irrigation. Using butterfly larvae as model herbivores that are common along roads, previous research showed that moderate increases in sodium may slightly benefit butterflies by allowing them to develop larger brains or flight muscles, but large increases are toxic. Several questions remain: How much genetic variation exists for selection to act on altered life history traits in moderately salted conditions or survival under high salt? How does stress from additional sodium along roadsides interact with other stressors? To address these, we used ten maternal families of monarchs Danaus plexippus reared on low- and moderate-sodium milkweed (Asclepias syriaca – consistent with concentrations found along roadsides in Minnesota). We measured survival, development time, body size, fecundity, eye size as a proxy for neural investment, amount of protein in flight muscles, and recovery from cold stress as adults. We found moderate genetic variation for survival and other life history traits in response to salt. However, families differed greatly in their life history tradeoffs or correlations among traits – in other words, there was no one genotype that showed universally positive or negative effects of increased sodium. Interestingly, we found that males fed salted milkweed were more tolerant of freezing temperatures than those in the control group. This work not only informs monarch conservation, but also adds to our knowledge of how anthropogenic changes in nutrient availability affect life history evolution more broadly.
