Evaluating the reproductive cost of migration for females in a partially migrating pond-breeding amphibian

Populations of animals with resident and migratory individuals provide an ideal system for addressing questions concerning the evolution of migration. Partially migratory populations may persist because residents and migrants have equal fitness or because migration is based on conditional asymmetries. Studies measuring the costs and benefits of migration provide empirical data to test hypotheses concerning the maintenance of partial migration within a population. In this study, we measured the reproductive differences between resident and migrant females in a pond-breeding amphibian, the red-spotted newt Notophthalmus viridescens . We used large field enclosures to repeatedly sample egg laying over the prolonged breeding season of this species. Resident females did not lay a greater number of eggs or begin laying eggs earlier, despite beginning the breeding season earlier and having a higher mass than migrant newts. The only difference in reproduction we detected was that the eggs of resident females hatched into larger larvae compared with the larvae of migrant females. We discuss this result in the context of other potential trade-offs between residency and migration. This study illustrates the phenology of egg laying of N. viridescens and our results contribute to understanding the population dynamics of partially migratory species.     

Sex-specific plasticity of growth and maturation size in a spider: implications for sexual size dimorphism

Sex-specific plasticity in body size has been recently proposed to cause intraspecific patterns of variation in sexual size dimorphism (SSD). We reared juvenile male and female Mediterranean tarantulas (Lycosa tarantula) under two feeding regimes and monitored their growth until maturation. Selection gradients calculated across studies show how maturation size is under net stabilizing selection in females and under directional selection in males. This pattern was used to predict that body size should be more canalized in females than in males. As expected, feeding affected male but not female maturation size. The sex-specific response of maturation size was related to a dramatic divergence between subadult male and female growth pathways. These results demonstrate the existence of sex-specific canalization and resource allocation to maturation size in this species, which causes variation in SSD depending on developmental conditions consistent with the differential-plasticity hypothesis explaining Rensch's Rule.     

Plasticity and heritability of morphological variation within and between parapatric stickleback demes

The threespine stickleback (Gasterosteus aculeatus) has emerged as an important model organism in evolutionary ecology, largely due to the repeated, parallel evolution of divergent morphotypes found in populations having colonized freshwater habitats. However, morphological divergence following colonization is not a universal phenomenon. We explore this in a large-scale estuarine ecosystem inhabited by two parapatric stickleback demes, each physiologically adapted to divergent osmoregulatory environments (fresh vs. saline waters). Using geometric morphometric analyses of wild-caught individuals, we detected significant differences between demes, in addition to sexual dimorphism, in body shape. However, rearing full-sib families from each deme under controlled, reciprocal salinity conditions revealed no differences between genotypes and highly significant environmental effects. It is also noteworthy that fish from both demes were fully plated, whether found in the wild or reared under reciprocal salinity conditions. Although we found significant heritability for body shape, we also noted significant direct environmental effects for many latent shape variables. Moreover, we found little evidence for diversifying selection acting on body size and shape (Q(ST) ). Nevertheless, uniform compressive variation did exceed neutral expectations, yet despite evidence of both allometry and genetic correlation with body length, we detected no correlated signatures of selection. Taken together, these results suggest that much of the morphological divergence observed in this system is the result of plastic responses to environmental variation rather than adaptive differentiation.     

Migratory decisions in birds: extent of genetic versus environmental control

Migration is one of the most spectacular of animal behaviors and is prevalent across a broad array of taxa. In birds, we know much about the physiological basis of how birds migrate, but less about the relative contribution of genetic versus environmental factors in controlling migratory tendency. To evaluate the extent to which migratory decisions are genetically determined, we examined whether individual western burrowing owls (Athene cunicularia hypugaea) change their migratory tendency from one year to the next at two sites in southern Arizona. We also evaluated the heritability of migratory decisions by using logistic regression to examine the association between the migratory tendency of burrowing owl parents and their offspring. The probability of migrating decreased with age in both sexes and adult males were less migratory than females. Individual owls sometimes changed their migratory tendency from one year to the next, but changes were one-directional: adults that were residents during winter 2004–2005 remained residents the following winter, but 47% of adults that were migrants in winter 2004–2005 became residents the following winter. We found no evidence for an association between the migratory tendency of hatch-year owls and their male or female parents. Migratory tendency of hatch-year owls did not differ between years, study sites or sexes or vary by hatching date. Experimental provision of supplemental food did not affect these relationships. All of our results suggest that heritability of migratory tendency in burrowing owls is low, and that intraspecific variation in migratory tendency is likely due to: (1) environmental factors, or (2) a combination of environmental factors and non-additive genetic variation. The fact that an individual’s migratory tendency can change across years implies that widespread anthropogenic changes (i.e., climate change or changes in land use) could potentially cause widespread changes in the migratory tendency of birds.     

Female choice, male interference, and sperm precedence in the red-spotted newt

Darwin first identified female choice and male—male competitionas forms of sexual selection resulting in the evolution of conspicuoussexual dimorphism, but it has proven challenging to separatetheir effects. Their effects on sexual selection become evenmore complicated when sperm competition occurs because spermprecedence may be either a form of cryptic female choice ora form of male—male competition. We examined the effectsof tail height on male—male competition and female choiceusing the sexually dimorphic red-spotted newt (Notophthalmusviridescens viridescens). Experiment 1 examined whether maletail height influenced male mating success. Males with deeptails were more successful at mating with females than thosewith shallow tails. Successful, deep-tailed males also were bigger(snout-vent length; SVL) than unsuccessful, shallow-tailed males,but they did not vary in tail length or body condition. Of these,only tail height and tail length are sexually dimorphic traits.Experiment 2 tested the hypothesis that the differential successof males with deeper tails was due to female choice by examiningboth simultaneous female preference for association and sequentialfemale choice. We found no evidence of female choice. When maleswere not competing to mate with females, tail height did notinfluence male mating success. Successful males did not havedifferent SVL and tail lengths than unsuccessful males. Thus,tail height in male red-spotted newts appears to be an intrasexuallyselected secondary sexual characteristic. Experiment 3 usedpaternity exclusion analyses based on molecular genetic markersto examine the effect of sperm precedence on sperm competitionin doubly-mated females. Sperm precedence likely does not havea pervasive and consistent effect on fertilization success becausewe found evidence of first, last, and mixed sperm usage.     

Seasonal butterfly design: morphological plasticity among three developmental pathways relative to sex,flight and thermoregulation

The temperate‐zone butterfly Pararge aegeria can use three developmental pathways corresponding to different seasonal cohorts: (1) development with a pupal winter diapause resulting in early spring adults; (2) development with a larval winter diapause resulting in late‐spring adults and (3) direct development resulting in summer or second generation adults. In order to test adaptive predictions, we compared variation in flight‐ and thermoregulation‐related morphology among adult males and females from the three pathways using both field data (i.e. wild‐caught butterflies) and experimental breeding data (i.e. reared under different photoperiod regimes). Morphological patterns among the pathways were largely similar in the field and rearing data. Seasonal patterns differed between the sexes for most traits, including (relative) size measures and wing colour. Our results suggest sex‐related, adaptive seasonal plasticity for morphological traits related to flight behaviour in a multivoltine insect.     

Green swordtails alter their age at maturation in response to the population level of male ornamentation

Effects of the social environment on age at sexual maturation are assumed to require direct interactions, such as suppression of subordinates through aggression from dominants. Using green swordtails (Xiphophorus helleri), we demonstrate for the first time that females and males adjust their age at maturation in response to visual cues of male sexual ornamentation in the current environment: females matured earlier, whereas males matured later if all the mature males seen had large ornaments. Thus, age at maturation shifted in accordance with the perceived quality of mates (females) or mating competitors (males), demonstrating a capability to use visual cues from the environment to strategically adjust rates of sexual development.