Alternative reproductive tactics in male eastern gray squirrels: "making the best of a bad job"

Male eastern gray squirrels (Sciurus carolinensis) congregatearound and pursue a female on her single day of estrus. Thetactics of uniquely marked adult males were monitored duringwinter mating bouts from 1986 to 1990 to examine variation inmale copulatory success. Two tactics were chosen by males: activepursuit or satellite. Active-pursuit males were dominant anddefended proximity to females. Satellite males were subordinateand remained dispersed in the female's home range. Active pursuitwas used only by males 2.75 years old. The switch point betweenthe tactics is about 3 years. Copulations were not distributedevenly among males, with about 30% of all adult males failingto copulate during a breeding season. Active pursuit was themost successful strategy, with male success attributed to theability to defend access to the female. However, satellite malessuccessfully copulated due to the escape of females from dominantmales. Females appear to avoid the overt aggression characteristicof the competition among active-pursuit males by running fromthe group of males. Male success after a female's breakawaywas evenly distributed between the two tactics and accountedfor all copulations by satellite males. The activepursuit andsatellite tactics appear to be a conditional evolutionarilystable strategy where young, subordinate males are "making thebest of a bad job".     

Allometric plasticity in a polyphenic beetle

Abstract 1. Environmental conditions, such as variation in nutrition, commonly contribute to morphological variation among individuals by affecting body size and the expression of certain morphological traits; however the scaling relationship between a morphological trait and body size over a range of body sizes is generally assumed not to change in response to environmental fluctuation (allometric plasticity), but instead to be constant and diagnostic for a particular trait and species or population. The work reported here examined diet‐induced allometric plasticity in the polyphenic beetle Onthophagus taurus Schreber (1759) (Coleoptera: Scarabaeidae). 2. Male O. taurus vary in body size depending on larval nutrition. Only males above a critical body size threshold express fully developed horns; males smaller than this threshold develop only rudimentary horns or no horns at all. 3. Field populations that naturally utilise two different resources for feeding larvae (horse dung vs. cow manure) exhibited significant differences in the average scaling relationship between body size and male horn length over the same range of body sizes. Males collected from cow manure populations expressed consistently longer horns for a given body size than males collected from horse dung populations. 4. Males reared in the laboratory on horse dung or cow manure also exhibited significant differences in the average scaling relationship between body size and horn length. Differences between laboratory populations reared on horse dung or cow manure were of the same kind and magnitude as differences between field populations that utilise these different resources naturally. 5. These findings suggest that between‐population differences in scaling relationships between horn length and body size can be the product of differences in the quality of resources available to developing larvae. Results are discussed in the context of onthophagine mating systems and recent insights in the developmental and endocrine control of horn polyphenisms.     

Chick growth and survival in northern lapwings Vanellus vanellus indicate that secondary females do the best of a bad job

The polygyny threshold model predicts that monogamous and secondary females on average settle at the same time and have similar reproductive success. This is not generally found. Incorporating varying female competitive strength into the model, changes the predictions to state that secondary females should breed later and show a reduced success compared to that of monogamous and primary females. We examined if this was the case by investigating growth and survival in chicks of northern lapwings Vanellus vanellus from mothers of monogamous, primary and secondary mating status. Chicks where monitored from hatching to the age of 15–18 d. Growth and survival in secondary chicks was lower than in monogamous and primary chicks. Primary chicks survived significantly better than secondary chicks. Survival of monogamous chicks was comparable to primary chicks and close to significantly higher than in secondary chicks (p = 0.086). Among surviving chicks, daily weight gain in monogamous chicks was significantly higher than in secondary chicks. Growth rates of primary chicks were comparable to monogamous chicks and tended to be higher than in secondary chicks (p = 0.11). Monogamous and primary females both bred significantly earlier than secondary females, and chick survival and body‐mass growth decreased significantly with hatching date. Given the premium on early breeding in lapwings, secondary females appeared to do the best of a bad job, and their later onset of breeding could have been caused by poorer condition and/or lower breeding experience. Additional costs might also have accrued from sharing breeding resources with primary females that presumably were stronger competitors.     

Larval salamanders and diel drift patterns of aquatic invertebrates in an Austrian stream

1. Aquatic predators may influence drift periodicity either directly or indirectly (by non‐consumptive effects involving chemical cues). We took drift samples (eight successive 3‐h sampling intervals over a 24‐h period) on five dates (September 2007, March, April, June and August 2008). Samples were taken at three sites (one site with trout throughout the year, two sites without trout but with fire salamander larvae as top predators from April to August, but without vertebrate predators during the rest of the year) in a stream near Vienna, Austria, to examine the effects of predators on drift periodicity. 2. Of 45 331 specimens caught, the most abundant taxa were Ephemeroptera (32.3%; mainly Baetidae), Diptera (21.5%; mainly Chironomidae), Amphipoda (17.4%; all Gammarus fossarum), Plecoptera (5.4%), Coleoptera (3.5%) and Trichoptera (1.2%). For more detailed analyses, we chose Ephemeroptera (Baetidae; n = 13 457) and Amphipoda (G. fossarum; n = 7888), which were numerous on all sampling dates. 3. The number of drifting baetids and amphipods, as well as total drift density, was generally higher at night than by day, although without predators these differences were significant for Gammaridae but not for Baetidae. 4. When broken down to size classes, night–day drift ratios generally were not significantly different from equality in all size classes of baetids when larval fire salamanders and trout were absent. When predators were present, however, baetid drift density was usually higher at night, except in the smallest and largest size classes. In all size classes of G. fossarum, drift density was usually higher at night, whether with or without the top predators. 5. Although we could study predator effects on drift periodicity at three sites on only a single stream, it seems that non‐consumptive effects may affect Baetidae. Salamander larvae, most probably via kairomones, induced a shift towards mainly nocturnal drift, which could be interpreted as predator avoidance.     

Larval salamanders and channel geomorphology are indicators of hydrologic permanence in forested headwater streams

Regulatory agencies need rapid indicators of hydrologic permanence for jurisdictional determinations of headwater streams. The study objective was to assess the utility of larval salamanders and habitat variables for determining stream permanence across a large geographic area. We sampled four core forests (61 sites in IN, KY, and OH) in spring (April–May) and summer (August–September) over a 2-year period. Sites in each forest were selected to cover a gradient of permanence, from perennial to ephemeral. Salamanders were collected by both benthic core sampling and timed visual search on each site visit. Classification and regression tree (CART) models were used to identify indicators of seasonal permanence at core sites that were then tested using data collected from 6 satellite forests (52 sites) located nationwide. Southern two-lined salamanders, Eurycea cirrigera, were numerically dominant and were the only species included in CART models. Salamander diversity declined with distance from the Appalachians and strong longitudinal changes in assemblage composition were evident within streams. Abundance of E. cirrigera was positively correlated with watershed area, whereas dusky salamanders, Desmognathus spp., and spring salamanders, Gyrinophilus porphyriticus, comprised a greater proportion of salamander communities at intermittent sites within their range. Spring and summer CART models incorporated E. cirrigera abundance and measures of channel geomorphology to accurately classify approximately 80% of core sites as either ephemeral, intermittent or perennial. When applied to validation data from national satellite forests, correct classification rates were >85% for intermittent and ephemeral sites, but were only ～20% for perennial sites. These findings suggest that larval plethodontid salamanders and habitat variables can be valuable predictors of headwater stream hydroperiod, but indicators are largely limited to the regional scale.     

The behavioral ecology of threshold evolution in a polyphenic beetle

Facultative expression of alternative male morphologies is thoughtto allow individual males to select the phenotype with the highestfitness gain given their competitive status relative to othermales with which they compete for females. Choice of, or switchingbetween, morphs commonly relies on developmental threshold responses.Evolutionary changes in developmental threshold responses arethought to provide an important avenue for phenotypic diversificationand the evolution of morphological and behavioral novelties.However, the extent to which alternative male phenotypes andtheir underlying threshold responses actually evolve in naturalpopulations is unclear. Likewise, the ecological factors thatshape the evolution of threshold responses in natural populationsare unexplored for most organisms, as are the consequences ofsuch modifications for patterns of morphological diversity.I examined the ecological basis of rapid threshold evolutionin exotic populations of the horn-polyphenic dung beetle Onthophagustaurus. Male O. taurus vary continuously in body size as a functionof larval feeding conditions. Only males that exceed a criticalthreshold body size develop a pair of long horns on their heads,whereas males below this threshold remain hornless. Populationsin two exotic ranges of this species, the eastern United Statesand western Australia, have diverged in the mean threshold bodysize, which has resulted in the evolution of highly divergentand novel horn length–body size allometries in these populations.Populations in a third and previously unstudied exotic rangeof O. taurus in eastern Australia exhibit threshold body sizesroughly intermediate between the eastern U.S. and western Australianpopulations. I tested three hypothesis to explain how differencesin ecological and demographic factors can drive allometric divergencesbetween populations, using data derived from comparative, standardizedsampling of a large number of populations in each exotic range.Results suggest that differences in the intensity of both intra-and interspecific competition have contributed to the evolutionof divergent thresholds in these populations. My results donot support the hypothesis that shifts in threshold body sizesto larger body sizes are a consequence of increases in the meanbody size of competing males. I discuss my results in the contextof Onthophagus mating systems and the evolutionary implicationsof threshold evolution.     

Larval growth history determines juvenile growth and survival in a tropical marine fish

Processes that occur around the transition between larval and juvenile life‐stages can have a major effect on the population dynamics of organisms with complex life cycles. We explore the roles of larval history and selective post‐settlement mortality in determining the growth and survival of newly settled individuals of the damselfish, Pomacentrus amboinensis (Pomacentridae). Specifically, we determine whether the direction and intensity of selection on the recruits differs among various size‐classes of predators. A mark‐recapture study showed that individuals who survived 9 or more days were significantly larger at settlement than those that died within the first day (12.3 vs 11.9 mm SL), when mortality was highest (25% d^?1). Censuses revealed that the species and size composition of piscivores differed markedly between two reef habitats where P. amboinensis was common. A cage experiment, conducted in both habitats, manipulated the sizes of predators that could access newly settled P. amboinensis to determine whether the resulting mortality of the recently settled fish was influenced by larval growth history or size at settlement. Ten days after the start of the experiment individuals that grew slowly in the second half of their larval life had been lost from most of the experimental treatments. Small fish were also selectively lost from the coarse‐mesh cage on the reef base. Significant positive relationships between pre‐ and post‐settlement growth rates were found in both habitats for the fine mesh cages, cage controls and open patch reefs. This relationship was reversed in the coarse mesh cages in both habitats. This growth compensation was facilitated through the action of a particular size range of predators, whose impact was disrupted or masked in the open treatments by the action of a diverse predator pool. The present study underscores the complexity of the processes that influence the early post‐transition growth and survival in organisms with complex life‐histories.     

Reproductive strategies in a partially protandrous shrimp,Athanas kominatoensis (decapoda: alpheidae): Sex change as the best of a bad situation for subordinates

An alpheid shrimp,Athanas kominatoensis, inhabiting a Japanese purple sea urchin was shown to be a partially protandrous hermaphrodite. They settled mainly in summer and spent the first reproductive season as males. Smaller males changed into females the following spring, but larger ones remained males throughout their lives. Sex change was socially controlled, as has been known in some fishes. In contrast to the fishes, however, subordinates changed their sex as the best of a bad situation in this shrimp. Most larger individuals lived singly or in sexual pairs, not tolerating others of the same sex on a host. The larger a male was, the more likely he was to be found with a female. On the contrary, males smaller than 3 mm were tolerated by larger males to some extent and had chances to cohabit and copulate with larger females. This made it advantageous to be functional as a male at first. Though the secondary sex change from female to male is theoretically expected, it is hardly realized because of the short life span. Hermaphroditism in this shrimp can be explained by the size-advantage model as a special case.     

Larval development and growth ratios of Odonata of the Azores

To fully understand odonate life cycles, it is vital to analyse the patterns of larval growth, which are tightly associated with variations in environmental factors. However, the identification of larval instars is often difficult, especially for early development stages. We hypothesise that pond hydroperiod influences odonate larval growth, and test this idea with novel information about the environmental characteristics of 12 study ponds in Terceira Island (Azores). This study, which represents the first analysis of larval development of Odonata in the Azores, involved determining the instars of development and growth ratios. We measured the morphological characteristics of 898 Odonata larvae, and found that they significantly differed between temporary and permanent ponds. To estimate the larval instars, we used two methods: frequency distribution methods and correlation diagrams between body characteristics. The first method was the most effective, allowing the estimation of 17 instars for Anax imperator, 13 for Sympetrum fonscolombii and 10 for Ischnura hastata, as well as growth ratios between instars for head width, total length, abdomen length, wings and antenna length. Our results also suggest that a combination of the two methods is the most appropriate strategy for estimating the number of instars and growth ratios during larval development.     

Increased life span in a polyphenic butterfly artificially selected for starvation resistance

Starvation resistance is closely associated with fitness in natural populations of many organisms. It often co-varies with longevity and is a relevant target for understanding the evolution of aging. We selected for increased starvation resistance in the seasonally polyphenic butterfly Bicyclus anynana in a warm, wet-seasonal environment over 17 generations. We measured the response to selection for two selected lines compared to that of an unselected stock. Results show an increase in survival under adult starvation of 50%-100%. In addition, selection lines showed an increase in life span under normal adult feeding of 30%-50%. Female reproduction was changed toward laying fewer but larger eggs. The results indicate a sex-specific response to selection: females reallocated resources toward a more durable body, whereas males appeared to increase starvation resistance through changed metabolic rate. The phenotype produced by artificial selection resembles the form that occurs in the cool, dry-season environment, which suggests that selection has targeted the regulatory mechanisms for survival that are also involved in the suite of traits (including starvation resistance) central to the adaptive plastic response of this butterfly to seasonal conditions. In general, these results imply that the regulation of life span involves mechanisms of phenotypic plasticity.     

Larval salamander growth responds to enrichment of a nutrient poor headwater stream

While many studies have measured effects of nutrient enrichment on higher trophic levels in grazing food webs, few such studies exist for detritus-based systems. We measured effects of nitrogen and phosphorus addition on growth of larval Eurycea wilderae in a heterotrophic headwater stream using a repeated mark-recapture design. Growth estimates for 208 recaptured larvae (control stream n = 92; treatment stream n = 116) resulted in a growth rate of 0.0027 d⁻¹ in each stream prior to enrichment, whereas during enrichment treatment growth rates (g = 0.0069 d⁻¹ [±0.0019, 95% C.I.]) were significantly higher than control (g = 0.0043 d⁻¹ [±0.0007, 95% C.I.]). Results indicate that E. wilderae growth is tightly linked to the detrital resource and that growth may be indirectly affected by both quantity and quality of detritus. This study provides some of the first evidence that nutrient enrichment of detritus-based systems can influence multiple trophic levels in ways similar to autotrophic systems.     

Distinguishing between anticipatory and responsive plasticity in a seasonally polyphenic butterfly

Seasonal generations of short-lived organisms often differ in their morphological, behavioural and life history traits, including body size. These differences may be either due to immediate effects of seasonally variable environment on organisms (responsive plasticity) or rely on presumably adaptive responses of organisms to cues signalizing forthcoming seasonal changes (anticipatory plasticity). When directly developing individuals of insects are larger than their overwintering conspecifics, the between-generation differences are typically ascribed to responsive plasticity in larval growth. We tested this hypothesis using the papilionid butterly Iphiclides podalirius as a model species. In laboratory experiments, we demonstrated that seasonal differences in food quality could not explain the observed size difference. Similarly, the size differences are not likely to be explained by the immediate effects of ambient temperature and photoperiod on larval growth. The qualitative pattern of natural size differences between the directly developing and diapausing butterflies could be reproduced in the laboratory as a response to photoperiod, indicating anticipatory character of the response. Directly developing and diapausing individuals followed an identical growth trajectory until the end of the last larval instar, with size differences appearing just a few days before pupation. Taken together, various lines of evidence suggest that between-generation size differences in I. podalirius are not caused by immediate effects of environmental factors on larval growth. Instead, these differences rather represent anticipatory plasticity and are thus likely to have an adaptive explanation. It remains currently unclear, whether the seasonal differences in adult size per se are adaptive, or if they constitute co-product of processes related to the diapause. Our study shows that it may be feasible to distinguish between different types of plasticity on the basis of empirical data even if fitness cannot be directly measured, and contributes to the emerging view about the predominantly adaptive nature of seasonal polyphenisms in insects.     

Recovering from a bad start: rapid adaptation and tradeoffs to growth below a threshold density

ABSTRACT: BACKGROUND: Bacterial growth in well-mixed culture is often assumed to be an autonomous process only depending upon the external conditions under control of the investigator. However, increasingly there is awareness that interactions between cells in culture can lead to surprising phenomena such as density-dependence in the initiation of growth. RESULTS: Here I report the unexpected discovery of a density threshold for growth of a strain of Methylobacterium extorquens AM1 used to inoculate eight replicate populations that were evolved in methanol. Six of these populations failed to grow to the expected full density during the first couple transfers. Remarkably, the final cell number of six populations crashed to levels 60- to 400-fold smaller than their cohorts. Five of these populations recovered to full density soon after, but one population remained an order of magnitude smaller for over one hundred generations. These variable dynamics appeared to be due to a density threshold for growth that was specific to both this particular ancestral strain and to growth on methanol: when tested at full density, this population had become less fit than its ancestor. Simply increasing the initial dilution just 16-fold reversed this result, revealing that this population had more than a 3-fold advantage when tested at this lower density. As this population evolved and ultimately recovered to the same final density range as the other populations this low-density advantage waned. CONCLUSIONS: These results demonstrate surprisingly strong tradeoffs for adaptation to growth at low absolute densities that manifest over just a 16-fold change in density. Capturing laboratory examples of transitions to and from growth at low density may help us understand the physiological and evolutionary forces that have led to the unusual properties of natural bacteria that have specialized to low-density environments such as the open ocean.