Seasonal plasticity in morphology and metabolism differs between migratory North American and resident Costa Rican monarch butterflies

Abstract

Environmental heterogeneity in temperate latitudes is expected to maintain seasonally plastic life-history strategies that include the tuning of morphologies and metabolism that support overwintering. For species that have expanded their ranges into tropical latitudes, it is unclear the extent to which the capacity for plasticity will be maintained or will erode with disuse. The migratory generations of the North American (NA) monarch butterfly Danaus plexippus lead distinctly different lives from their summer generation NA parents and their tropical descendants living in Costa Rica (CR). NA migratory monarchs postpone reproduction, travel thousands of kilometers south to overwinter in Mexico, and subsist on little food for months. Whether recently dispersed populations of monarchs such as those in Costa Rica, which are no longer subject to selection imposed by migration, retain ancestral seasonal plasticity is unclear. To investigate the differences in seasonal plasticity, we reared the NA and CR monarchs in summer and autumn in Illinois, USA, and measured the seasonal reaction norms for aspects of morphology and metabolism related to flight. NA monarchs were seasonally plastic in forewing and thorax size, increasing wing area and thorax to body mass ratio in autumn. While CR monarchs increased thorax mass in autumn, they did not increase the area of the forewing. NA monarchs maintained similar resting and maximal flight metabolic rates across seasons. However, CR monarchs had elevated metabolic rates in autumn. Our findings suggest that the recent expansion of monarchs into habitats that support year-round breeding may be accompanied by (1) the loss of some aspects of morphological plasticity as well as (2) the underlying physiological mechanisms that maintain metabolic homeostasis in the face of temperature heterogeneity.

Modular switches shift monarch butterfly migratory flight behavior at their Mexican overwintering sites

“…Flight metabolism is important for distinguishing migratory and non-migratory monarchs. 56 , 57 , 58 Importantly, these switches show a degree of modularity such that monarchs can adopt different phenotypes that match the conditions and requirements of their migratory phase.…”

Flight behaviour diverges more between seasonal forms than between species inPierisbutterflies

“…Seasonal flight behaviours may be adaptive as, for example, increased manoeuvrability seems to be advantageous when foraging for newly emerging feeding resources (Evans et al, 2020). Furthermore, increasing wing area might enhance dispersal ability, as quantified in migrating generations vs. sedentary populations of Danaus butterflies (Tenger-Trolander et al, 2023). Behavioural variation can also influence flight patterns in seasonal butterflies, as the preference in flight direction of migrating populations of Pieris brassicae seems to depend on the season of emergence (Spieth & Cordes, 2012).…”

“…Behavioural variation can also influence flight patterns in seasonal butterflies, as the preference in flight direction of migrating populations of Pieris brassicae seems to depend on the season of emergence (Spieth & Cordes, 2012). Thus, in the case of polyvoltine seasonal butterflies, the factors modifying intra-specific flight behaviours likely involve both morphological and behavioural adaptations (Spieth & Cordes, 2012; Tenger-Trolander et al, 2023).…”

“…To assess whether seasonal changes in the environment affect the evolution of morphology and flight behaviour, we study two polyvoltine butterfly species, Pieris napi (Linnaeus, 1758) and P. rapae (Linnaeus, 1758) (Pieridae: Pierini). These two species are closely-related (Okamura et al, 2019), co-occur in Europe and are ecologically very similar (Tenger-Trolander et al, 2023). They are known to differ in wing size and shape in spring and summer (Fric et al, 2006; Shkurikhin & Oslina, 2016).…”

“…Thus, in the case of polyvoltine (i.e. having several generations during a year) seasonal butterflies, the factors affecting intraspecific flight behaviours likely involve both morphological and behavioural adaptations (Spieth & Cordes, 2012; Tenger-Trolander et al, 2023).…”