DOI: 10.5061/dryad.xpnvx0kds
title: Data and code from: The evolution of developmental thresholds and reaction norms for age and size at maturity

description: Developing organisms typically mature earlier and at larger sizes in
 favorable growth conditions, while in rarer cases maturity is delayed. The
 rarer reaction norm is easily accommodated by general life history models,
 whereas the common pattern is not.  Theory suggests that a
 solution to this paradox lies in the existence of developmental thresholds
 that define the minimum size at which maturation or metamorphosis can
 commence, and in the evolution of these threshold sizes in response to
 environmental variation. For example, more ephemeral environments might
 favor the evolution of smaller thresholds, enabling earlier maturation.
 The threshold model makes two unique and untested predictions; first,
 reaction norms for age and size should steepen, and even change sign, with
 decreases in the threshold size, and second, food reductions at sizes
 below the threshold should delay maturation, while those occurring after
 the threshold should accelerate maturation. We test these predictions by
 performing targeted food manipulations in a suite of five damselfly
 species, which theory suggest should differ in threshold size. The results
 provide strong support for the threshold model’s predictions. In all
 species, early food reductions delayed maturation, while late reductions
 accelerated maturation. Reaction norms were steeper, and the effect of
 food reductions changed from decelerating to accelerating at a much
 smaller size in fast-developing species from ephemeral habitats. These
 results support the view that developmental thresholds can account for the
 widespread observation of negative correlations between age and size at
 maturity. Moreover, evolution of the threshold appears to be both central
 to the observed diversity of reaction norms for age and size at maturity,
 and is predictable.
description: We conducted a common garden experiment using five species of
 damselflies from Eastern North America: <i>Enallagma doubledayi, E.
 pollutum, Ischnura ramburii, I. hastata </i>and<i> I. posita
 </i>(Odonata: Coenagrionidae)<i>.</i> Larvae used in the
 experiment were hatched from eggs laid by field-collected females sampled
 at sites in North Carolina, Georgia and Florida, USA, in June and July,
 2014. We reared multiple family groups of each species from egg to adult
 in the laboratory to test for the effect of food reductions within species
 and for divergence in developmental thresholds and reaction norms among
 species. One subset of larvae from each family experienced high food
 levels during the entire experiment, whereas random subsets of individuals
 from each family were switched from high to low food levels at different
 body sizes.<b> </b>From the beginning of the experiment,
 larvae were fed <i>Artemia</i> nauplii <i>ad
 libitum</i> six days a week (174±34 nauplii per feeding, mean ± 1SD,
 n=43 random food doses), which represents our high food level treatment.
 At set intervals, random subsets of individuals from each family were
 switched from high to low food levels. After the switch, low food
 individuals were fed three days a week (Monday, Wednesday, Friday) for the
 remainder of the experiment using the same amount of nauplii as high food
 larvae.  To estimate larval size, we measured the head
 width (maximum distance between the distal parts of the eyes) of living
 larvae. We converted larval head widths to dry weights by killing ~4
 individuals per week and species, drying them for 48h, and weighing them
 to the nearest 0.001mg. With this data, we established allometric
 relationships between head width and dry weight for each species. Adult
 size was estimated by drying and weighing individuals that had emerged and
 hardened for 24h. For individuals that did not fully emerge
 from their exuvium, we weighed the adults together with their exuvia. To
 enable direct comparisons, we weighed the exuviae of successfully emerged
 individuals and estimated the relationship between the total dry weight
 (adult + exuvium) and adult dry weight (excluding the exuvium) for each
 species. Using these relationships, we converted total dry weights into
 adult dry weights for individuals that were measured with their exuviae.
 The dataset presented here only include data on adult dry weight
 (excluding exuviae) estimated in this way.
description: <strong>R code_The evolution of developmental
 thresholds.R</strong><br> Computer code for replicating the
 analyses and figures in the manuscript. The code is fully annotated to
 allow the user to replicate the analysis.
 <strong>hatch.rds</strong><br> Data on the
 hatching size (hw, head width in mm) of the five species of damselflies
 studied. Each of the five species (species) was represented by individuals
 (ind) from multiple full-sib family groups (fam).
 <strong>morph.rds</strong><br> Data on the head
 width (hw.mm, in mm) and dry weight (drywt, in mg) of individual damselfly
 larvae that were sacrificed during the experiment to establish the
 allometric scaling between head width and dry weight of each species. Due
 to a handling error, one batch of individuals were poorly preserved and
 deemed unsuitable for estimating dry weights. We scored the preservation
 quality of each individual by visual inspection (quality, range 0 to 3).
 The analysis was only performed on individuals with no visible signs
 of deterioration (quality score = 3). For convenience, the natural log of
 dry weights and head widths is also included (logwt, loghw).
 <strong>combined.rds</strong><br> This dataset
 contain data on the head width (hw, in mm) and dry weight (drywt, in mg;
 estimated based on the allometric scaling of head width and dry weight of
 sacrificed individuals) of individual larvae measured at different points
 in time during the experiment. The age of each individual at the time of
 the measurement is given in the 'age' variable (days since
 hatching). Individuals belong to four different treatment groups
 (treatm.group). They were either continously fed high food levels
 (treatm.group=HI) or switched to low food levels early, intermediate or
 late in ontogeny (treatm.group=LO1, LO2 or LO3). Size measurements were
 taken during ontogenetic growth at constant high food levels
 (measure.group = "hi"), at the point in time when individuals
 were switched from high to low food levels (measure.group =
 "lo.start"), during ontogenetic growth after being switched to
 low food (measure.group = "post.lo"), when individuals
 were sacrificed and dried to estimate dry weights (measure.group =
 "essayed") and after completion of metamorphosis (measure.group
 = "adult"). The feeding regimen at the time of each measurement
 is given in the 'treatm' variable (HI or LO). Emergence success
 was scored on a scale from 0 to 4 by counting the groups of body part
 (legs, head, thorax and abdomen) that remained inside the exuvium
 (stuck.score, range = 0 to 4).