Mark R. Forbes

Fluctuating Asymmetry as a Bioindicator of Stress: Comparing Efficacy of Analyses Involving Multiple Traits

Researchers have suggested fluctuating asymmetry (FA) as an indicator of environmental stress and have usually tested this assertion by examining relations between FA of single traits and stress. Fluctuating asymmetry stress relations are real but are typically weak and difficult to detect. Researchers would like to maximize the probability of detecting FA‐stress relations when they exist. We assert that analyses based on the FA of multiple traits may provide better methods for detecting stress. In this article, we used computer simulations to compare the ability of six analyses to detect differences in FA between stressed and unstressed populations. We show that the optimal analysis depends upon the underlying form of the FA distributions. We also show that two of the analyses had inflated Type I errors in some situations. Finally, we quantify the advantage of our preferred analysis over those of single‐trait FA in detecting stress.

Male Biases in Parasitism of Mammals: Effects of Study Type, Host Age, and Parasite Taxon

Sex biases in parasitism may be expected in mammals because estrogens stimulate immunity, whereas androgens depress immunity. Parasites should, therefore, become more readily established in male hosts, leading to higher levels of parasitism for males than for females. We tested this general hypothesis using 145 tests based on mammal hosts taken from 38 published studies. Male biases in parasitism existed overall and for tests restricted to arthropod (but not helminth) parasites. We then controlled for potential dependence by choosing single parasite species based on their likelihood of eliciting immune responses. For these 48 tests, male biases still existed for arthropod, but not for helminth, parasites. As predicted, the average degree of male bias was much higher for studies involving an experimental infection than for field studies. In experimental tests, researchers controlled for differences between the sexes in exposure to parasites. Any biases in parasitism, therefore, should have been due primarily to gender differences in immune response. There also were no male biases in parasitism among juveniles, presumably because juvenile males and females differ less in their hormone profiles or stress levels than do adult males and females. In , our results suggested that hormonally mediated sex differences in susceptibility to parasitism exist for mammals, even though our tests were extremely conservative. Our results also indicate that, on average, differences in parasitism between the sexes are small, and that statistically significant male biases in parasitism are not a general rule.

Parasitism and host reproductive effort

Adaptive changes in reproductive effort (RE) of parasitized hosts may account for both inverse and positive relationships between host reproductive output and incidence or degree of parasitism. This hypothesis can be made more general (adaptive changes in patterns of allocation to reproductive functions), but has been largely ignored in the fields of behavioral and evolutionary ecology. In fact, definitive examples of such adaptive responses are lacking. Yet theory on life-history trade-offs predicts that hosts may minimize the impact of parasites by altering their own RE. Such alterations may occur with or without induction of defenses against parasites

Fluctuating asymmetry in relation to stress and fitness: Effects of trait type as revealed by meta-analysis

Abstract:Recently, there has been much controversy over whether fluctuating asymmetry (FA) is related to stress during development, or to fitness of organisms following development. Many tests have found the predicted relations, whereas others have not. To account for these inconsistencies, some researchers have argued that FA of some traits (e.g., sexually selected traits) are more sensitive measures of stress, or are more strongly related to fitness, and that such relations will differ between poikilotherms and homeotherms. Using meta-analysis, we found that FA-stress and FA-fitness relations were non-spurious, despite the large number of relations tested. However, FA-stress and FA-fitness relations were fairly weak and highly heterogenous. Furthermore, our results suggested that trait type was not predictive of the presence or strength of FA-stress or FA-fitness relations and that relations were not stronger for poikilotherms, as one hypothesis suggested.

Host range and local parasite adaptation

Parasites may be expected to become locally adapted to their hosts. However, while many empirical studies have demonstrated local parasite adaptation, others have failed to demonstrate it, or have shown local parasite maladaptation. Researchers have suggested that gene flow can swamp local parasite–host dynamics and produce local adaptation only at certain geographical scales; others have argued that evolutionary lags can account for both null and maladaptive results. In this paper, we use item response theory (IRT) to test whether host range influences the likelihood of parasites locally adapting to their hosts. We collated 32 independent experiments testing for local adaptation, where parasites could be assigned as having either broad or narrow host ranges (BHR and NHR, respectively). Twenty‐five tests based on BHR parasites had a significantly lower average effect size than seven NHR tests, indicating that studies based on BHR parasites are less likely to demonstrate local parasite adaptation. We argue that this may relate to evolutionary lags during diffuse coevolution of BHR parasites with their hosts, rather than differences in experimental approaches or other confounds between BHR and NHR studies.

Sex-biased parasitism of avian hosts : relations to blood parasite taxon and mating system

Immunosuppressive effects of testosterone lead to a rediction of male-biased parasitism. To test this prediction, prevalences of blood parasites were compared between male and female birds using statistically correct vote counts of data from 33 studies. We found no overall difference in prevalence between males and females, in either breeding or non-breeding birds. However, infections by Haemoproteus (the most common genus of blood parasite found) were significantly more common among breeding females than breeding males, Restricting the analysis to breeding birds of polygynous species, females again were more likely than males to be infected by blood parasites: this result held for an intra-family comparison that controlled for phylogenetic effects. In comparison, measures of sexual size dimorphism did not relate to sex biases in parasitism as predicted, after controlling for phylogeny using independent comparisons. Because testosterone is often implicated in suppressing the immune system, female biases in parasitism are unexpected. Female biases in parasitism by blood parasites could result from differential exposure of the sexes to vectors, or from oestrogen-based effects on immunity.

Modelling fluctuating asymmetry in relation to stress and fitness

Fluctuating asymmetry (or FA) refers to directionally random deviations from bilateral symmetry for a character pair of an organism. Character FA has been related to stress imposed during development of organisms and quality or fitness of individuals. However, FA-stress and FA-quality relations often are absent or variable depending on the characters assessed. Some researchers have hypothesized that FA of sexually selected traits may relate to stress and fitness more reliably than other traits, and that genetic predispositions towards asymmetry may confound relations between asymmetry and either stress or quality. Others have suggested that because character FA is generated randomly, it may not relate to fitness or to FAs of other characters within samples of individuals. In this paper, we modelled development of character FA in relation to random developmental noise or perturbations, stress, developmental stability, and individual quality. We concluded that FA of sexually selected traits need not relate to stress or individual quality more than FA of other characters. We also determined that, in the absence of confounding factors such as genetic predispositions toward asymmetry, measurement error, and character FA-size relations, FA of some characters need not relate to stress or quality. Interestingly, leptokurtic asymmetry distributions could be generated using models expected to generate true FA; thus, leptokurtic asymmetry distributions should be assayed for relations with stress and fitness. We also found that unsigned FA could reliably relate to organism-wide developmental stability and quality, even in the absence of organism-wide FA relations among traits. Finally, conclusions about the utility of FA measures will depend strongly on representation of high FA individuals in samples, because FA-quality relations were typically triangular in distribution.

Host race formation in the Acari

Host race formation generates diversity within species and may even lead to speciation. This phenomenon could be particularly prevalent in the Acari due to the often intimate interaction these species have with their hosts. In this review, we explore the process of host race formation, whether it is likely to occur in this group and what features may favour its evolution. Although few studies are currently available and tend to be biased toward two model species, results suggest that host races are indeed common in this group, and more likely to occur when hosts are long-lived. We discuss future directions for research on host-associated adaptations in this group of organisms and the potential relevance of host race formation for the biodiversity of mites and ticks.

Risk-taking by female ducks: intra- and interspecific tests of nest defense theory

We tested several predictions of nest defense theory by observing variation in flushing distance and probability of nest abandonment within and between six species of waterfowl. In these species, only the females incubate eggs and attend offspring. First, we examined whether flushing distance by females varied in relation to clutch size, stage of incubation, and time of season, after controlling for the number of visits made to nests by observers. Revisits by observers appeared to affect flushing distance by females for reasons unrelated to the relative value of the current clutch. We found that as incubation progressed, females allowed observers to approach more closely before flushing from the nest. In some species, females with larger clutches allowed closer approaches to nests before flushing which was also consistent with nest defense theory. In contrast, time of season (Julian date) did not relate to flushing distance for any species. When species were compared, we found that species with moderate to high yearly mortality and high reproductive output per breeding attempt (e.g., northern shoveler and blue-winged teal) were less likely to abandon nesting attempts and exhibited “riskier” behavior (remained at nests when approached closely by observers) than species that had lower yearly mortality (e.g., mallard). Our results show that flushing distance and patterns of nest abandonment by female ducks conform to several predictions of nest defense theory.

Sex ratios, mating behavior and sexual size dimorphism of the northern water snake, Nerodia sipedon

Competition among males to mate is generally associated with male-biased size dimorphism. In this study we examine mating behavior in the northern water snake (Nerodia sipedon), a species in which males are much smaller than females despite substantial competition among males to mate. Competition among males was a consequence of a male-biased operational sex ratio due to slightly higher female mortality from a birth sex ratio of 1 : 1, and, in 1 year, more synchronous and longer mating activity by males. Approximately one-third of both males and females appeared not to mate in a given year. Larger males were generally more likely to attempt mating, but size did not explain the variance in the number of aggregations in which individual males participated. Within aggregations, males that were successful at achieving intromission were larger than unsuccessful males in 1 of 2 years. Variation in condition (mass relative to length) and relative tail length were not generally useful predictors of either mating effort or success in males. Because large size was often advantageous to males, sexual size dimorphism appeared not to be a consequence of sexual selection favoring smaller males. Because sexual dimorphism was evident at birth, and both males and females matured sexually at about 4 years, sexual dimorphism was not simply a consequence of one sex growing at the maximum rate for longer. Female fecundity increased with size, and sex differences in size-fecundity relations may underly the pattern of sexual size dimorphism. However, because multiple mating by females is common, sperm competition is likely to be important in determining male reproductive success. Therefore, allocation of energy to sperm rather than growth may also prove to be an important influence on male growth rates and sexual size dimorphism.