DOI: 10.5061/dryad.0k6djh9wt
title: Evidence for rapid downward fecundity selection in an ectoparasite (Philornis downsi) with earlier host mortality in Darwin’s finches

description: Fecundity selection is a critical component of fitness and a major driver
 of adaptive evolution. Trade-offs between parasite mortality and host
 resources are likely to impose a selection pressure on parasite fecundity,
 but this is little studied in natural systems. The ‘fecundity advantage
 hypothesis’ predicts female-biased sexual size dimorphism whereby larger
 females produce more offspring. Parasitic insects are useful for exploring
 the interplay between host resource availability and parasite fecundity,
 because female body size is a reliable proxy for fecundity in insects.
 Here we explore temporal changes in body size in the myiasis-causing
 parasite Philornis downsi (Diptera: Muscidae) on the Galápagos Islands
 under conditions of earlier in-nest host mortality. We aim to investigate
 the effects of decreasing host resources on parasite body size and
 fecundity. Across a 12-year period, we observed a mean of ~17% P. downsi
 mortality in host nests with 55 ± 6.2 % host mortality, and a trend of
 ~66% higher host mortality throughout the study period. Using specimens
 from 116 Darwin’s finch nests (Passeriformes: Thraupidae) and 114 traps,
 we found that over time, P. downsi pupae mass decreased by ~32%, and male
 (~6%) and female adult size (~11%) decreased. Notably, females had ~26%
 smaller abdomens in later years, and female abdomen size was correlated
 with number of eggs. Our findings imply natural selection for faster P.
 downsi pupation and consequently smaller body size and lower parasite
 fecundity in this newly evolving host-parasite system.
description: In this study we use nine years of field data spanning a 12-year
 period to examine changes in body size (an indirect measure of fecundity)
 in the dipteran ectoparasite, <i>P. downsi</i>, in response to
 the increasingly earlier death of its host. We monitored 116 Darwin’s
 finch nests for nesting outcome using our well-established field
 protocols. Upon nesting termination (fledging or death of the last
 nestling), each nest was collected in a sealed plastic bag, and all
 <i>P. downsi</i> larvae, pupae, empty puparia and adult flies
 were counted within 1-24 hours of collection. All <i>P.
 downsi</i> samples were stored in 90% ethanol immediately after
 counting. <i>Philornis downsi</i> intensity in the nest was
 measured as the total number of larvae, pupae, puparia and adult flies
 present upon collection of the nest. The sample size per year and host
 genus (<i>Camarhynchus, Geospiza</i>) is provided in Table
 S1. We placed a total of 114 McPhail Traps in the
 lowlands and highlands of Santa Cruz and Floreana Island to sample adult
 <i>P. downsi</i> flies in the years 2004, 2005, 2012, 2013 and
 2014 (for details see Table S1). The McPhail traps were baited with a
 liquid lure of blended papaya, water and white sugar (following trapping
 protocol developed by P. Lincango and C. Causton) that was replaced every
 7 days. Traps were hung in trees along 4 x 90m transects and flies were
 collected twice per week and stored in ethanol. In 2014 on Floreana
 Island, we placed 28 McPhail traps along four transects, seven traps per
 transect, at heights of 2 to 7 metres. In other years and locations, traps
 were placed <i>ad hoc</i> every 50 m within 100 m x 200 m
 plots spanning a 2 km transect within study sites. We analysed data from
 46 lowland traps and 68 highland traps (Table S1). Mass
 (g), length and width (mm) were measured for each pupa, as these
 measurements are known to be highly correlated with adult fly size and can
 therefore be an indirect indicator of an individuals' fecundity upon
 maturity. Pupae cannot be sexed, therefore these data could not be used
 for sexual dimorphism analysis but are useful when looking at general
 temporal shifts in body size in the <i>P. downsi
 </i>population. All pupae were removed from ethanol and placed on
 filter paper to dry for 30 seconds before taking measurements. We measured
 the total mass of all intact pupae per nest and divided this by the number
 of pupae to calculate average pupa mass. The pupae were weighed to the
 nearest 0.001 g using an A&D HR-200 Digital Analytical Balance.
 The length (mm) and width (mm) of the largest pupa per nest was measured
 using digital callipers. For analysis we used the average mass per nest.
 Pupa mass was measured from the nests of 19 <i>C. parvulus</i>
 (268 pupae), 10 hybrid <i>Camarhynchus</i> tree finch (55
 pupae), 25 <i>C. pauper</i> (332 pupae)<i>, </i>57
 <i>Geospiza fuliginosa</i> (816 pupae) and 5 <i>G.
 fortis</i> (52 pupae) (Table S1).