Predator exaptations and defensive adaptations in evolutionary balance: No defence is perfect

Feather characteristics of loral zone in an insectivorous passerine: The Iberian gray shrike Lanius meridionalis, in southern France

In the French Mediterranean plain, the northern extreme of its native range, the Iberian gray shrike, Lanius meridionalis, predominantly feeds on arthropods. Its type of loral plumage plays a key role in protecting its eyes while transporting large prey. The aims are to understand the role played by feathers in protecting the animal from various types of defensive prey. I combine an inspection of large insect prey types found on larders with a review of bird specimens found in museum collections to examine the morphometric characteristics of rictal feathers and culmen. In addition, precision photographs are used to observe the posture of the plumage in natura. I could identify four categories of protective feathers: clustered bristles, semi-bristles, semi-plumes distributed in the loral area, and semi-plumes above the eyes. My results suggest that the Iberian gray shrike has a complex structure of loral feathers, specific to its foraging activity and prey types. The presence of these rictal bristles is probably a protection against the movements of larger prey items, which might damage loral zone of Lanius meridionalis.

Pretty Picky for a Generalist: Impacts of Toxicity and Nutritional Quality on Mantid Prey Processing

Prey have evolved a number of defenses against predation, and predators have developed means of countering these protective measures. Although caterpillars of the monarch butterfly, Danaus plexippus L., are defended by cardenolides sequestered from their host plants, the Chinese mantid Tenodera sinensis Saussure guts the caterpillar before consuming the rest of the body. We hypothesized that this gutting behavior might be driven by the heterogeneous quality of prey tissue with respect to toxicity and/or nutrients. We conducted behavioral trials in which mantids were offered cardenolide-containing and cardenolide-free D. plexippus caterpillars and butterflies. In addition, we fed mantids starved and unstarved D. plexippus caterpillars from each cardenolide treatment and nontoxic Ostrinia nubilalis Hübner caterpillars. These trials were coupled with elemental analysis of the gut and body tissues of both D. plexippus caterpillars and corn borers. Cardenolides did not affect mantid behavior: mantids gutted both cardenolide-containing and cardenolide-free caterpillars. In contrast, mantids consumed both O. nubilalis and starved D. plexippus caterpillars entirely. Danaus plexippus body tissue has a lower C:N ratio than their gut contents, while O. nubilalis have similar ratios; gutting may reflect the mantid's ability to regulate nutrient uptake. Our results suggest that post-capture prey processing by mantids is likely driven by a sophisticated assessment of resource quality.

Saving the best for last: Differential usage of impaled prey by red-backed shrike (Lanius collurio) during the breeding season

We compared the prey composition of the red-backed shrike's (Lanius collurio) larders in agricultural habitats in Italy, France and Poland. This species exhibits the behaviour of impaling prey in larders, a behaviour attributed not only to storing food, but also as a social indication for sexual selection and/or demarcation of territories. A total of 426 impaled items were identified in 244 larders. Most common prey were identified for each country: Insecta (Hymenoptera) in Italy, Amphibia, Insecta (Diptera) and Clitellata in Poland, and Insecta (Orthoptera, Lepidoptera) in France. We found no relationship between type of prey impaled and height of impalement, however, we noted a negative relationship between the height of impalement and the distance to the nearest road. Furthermore, impaled toxic prey were found in all three countries, strengthening the possibility that prey are exposed to expedite the degradation of toxins or used as a social signal. Our results showed that the average weight of impaled prey was greater during the last reproductive stage (hatching and feeding young), providing evidence of differential usage of impaled prey during the breeding season. We therefore hypothesize that larger animals provide more energy, then vertebrates are preferred to invertebrates, especially when parents are feeding their nestlings.

Sex differences in impaling behaviour of Great Grey Shrike Lanius excubitor: do males have better impaling skills than females?

Prey impaling in shrikes Laniidae is considered to be a feeding adaptation to dismember and consume large prey and is unique among food-storing animals. However, other exaptations of this behaviour were recorded, including signals in mate choice, where cache size is a sign of male quality. Thus, due to a strong sexual selection, male and female birds might differ in their behavioural patterns of impaling behaviour. We examined sex differences in impaling behaviour of the Great Grey Shrike Lanius excubitor - one of the species where caches are known to be sexual signals. Data were collected in western Poland during breeding seasons in the years 2006-2010. In the studied population, we recorded several sex-specific differences in impaling behaviour. Males impaled prey, invertebrates as well as vertebrates, faster and with fewer attempts per impaling event than females. Sexes differed in the location of impaled prey; males selected more visible places, especially during the mating and courtship phase, whereas females impaled prey in concealed locations. Males also had slightly better impaling success compared to females. We suggest that sex differences in impaling behaviour may be due to different uses of impaled prey, and the better impaling skills of males may be the result of better experience in impaling which is forced by sexual selection in this species. We also discuss other factors which might trigger sex-specific differences in food caching by shrikes.

Temporal changes and sexual differences of impaling behavior in Southern Grey Shrike (Lanius meridionalis)

The Southern Grey Shrike (Lanius meridionalis) is a common resident along the rift valley. During 2007-2008 we studied the impaling behavior of Southern Grey Shrikes at the Shezaf nature reserve by food supplementation. Our findings indicate seasonal shifts in impaling behavior. During the winter, there was no difference between the sexes and shrikes impaled house mice (Mus musculus) close to their territorial boundaries with neighboring conspecifics. All impaled mice disappeared from the tree within a few hours of impalement. During the mating season, males impaled the whole prey and let females eat it, and prey was impaled by both sexes only in order to dismember and feed the nestlings. Shrikes impaled and cached meal worms only after they were satiated. The number of meal worms taken to be hoarded increased during the non-breeding season, but during the breeding season male shrikes preferred to feed the females. During the entire research period we did not find any specific cache sites or trees and no prey remained impaled for extended periods of time. In conclusion, impaling prey changed temporally and between sexes: from being a signal for conspecifics to pair bonding.

Thermoregulation constrains effective warning signal expression

Evolution of conspicuous signals may be constrained if animal coloration has nonsignaling as well as signaling functions. In aposematic wood tiger moth (Parasemia plantaginis) larvae, the size of a warning signal (orange patch on black body) varies phenotypically and genetically. Although a large warning signal is favored as an antipredator defense, we hypothesized that thermoregulation may constrain the signal size in colder habitats. To test this hypothesis, we conducted a factorial rearing experiment with two selection lines for larval coloration (small and large signal) and with two temperature manipulations (high and low temperature environment). Temperature constrained the size and brightness of the warning signal. Larvae with a small signal had an advantage in the colder environment, which was demonstrated by a faster development time and growth rate in the low temperature treatment, compared to larvae with a large signal. Interestingly, the larvae with a small signal were found more often on the plant than the ones with a large signal, suggesting higher basking activity of the melanic (small signal) individuals in the low temperature. We conclude that the expression of aposematic display is not only defined by its efficacy against predators; variation in temperature may constrain evolution of a conspicuous warning signal and maintain variation in it.

Impaling in true shrikes (Laniidae): a behavioral and ontogenetic perspective

The impaling of prey is a behavioral trait restricted to the true shrikes (Laniidae). Here, we suggest the ontogeny of this behavior. We believe impaling originated from wedging behavior that occurs among several other groups of birds, including corvids. Accidental impaling during wedging was likely the behavioral precursor of purposeful impaling. Fidelity to impaling sites lead to the creation of caches, which were eventually used by females for male evaluation. Caching allowed males to increase their fitness by using the caches as a display to attract potential mates. Further, caching is used by shrikes to demarcate territories, store food for inclement weather or periods of stress in the breeding cycle, divide labor between the breeding pair, and for "aging" while deterrent chemicals in prey decompose.

Predator Mixes and the Conspicuousness of Aposematic Signals

Conspicuous warning signals of unprofitable prey are a defense against visually hunting predators. They work because predators learn to associate unprofitability with bright coloration and because strong signals are detectable and memorable. However, many species that can be considered defended are not very conspicuous; they have weak warning signals. This phenomenon has previously been ignored in models and experiments. In addition, there is significant within- and among-species variation among predators in their search behavior, in their visual, cognitive, and learning abilities, and in their resistance to defenses. In this article we explore the effects of variable predators on models that combine positive frequency-dependent, frequency-independent, and negative frequency-dependent predation and show that weak signaling of aposematic species can evolve if predators vary in their tendency to attack defended prey.

Disparate determinants of summer and winter diet selection of a generalist herbivore, Ochotona princeps

The North American pika, Ochotona princeps, is a generalist herbivore that simultaneously selects two distinct diets: one consumed immediately (summer diet), the other harvested, transported, and stored for later consumption (winter diet). I investigated factors influencing diet selection at two sites on the West Knoll of Niwot Ridge, Boulder County, Colorado during 1991 and 1992. The composition of summer and winter diets differed significantly from each other as well as from the relative abundance of food items in the environment. Thus, pikas were not foraging randomly for either diet. To explore winter and summer diet selection, I tested two existing hypotheses: (1) that plant morphology restricts the winter diet breadth to plants that are easily harvested and large, and thereby maximizes the amount collected per foraging effort, or (2) to compensate for nutrients lost during storage, pikas bias their winter diet with high-nutrient species. I also tested the hypothesis that plant secondary compounds may be higher in the winter diet either because they function as preservatives or because pikas delay consumption of these species until the toxins degrade. For individual dietary items, I measured energy, nitrogen, water, fiber, total phenolic, condensed tannin, and astringency contents. There was little evidence to suggest that morphology excluded plants from the winter diet. Plant size was not a good predictor of abundance in the winter diet. Even after harvesting costs had been experimentally removed, cushion plants were still not included in the winter diet. There was weak support for an effect of nutrients on winter diet selection; in three of four cases, the winter diet was significantly lower in water and higher in total energy content as predicted by the nutrient compensation hypothesis. However, other nutrients exhibited no consistent pattern. Nutrients were not reliable predictors of the winter diet in multiple regression analyses. There was strong support for the hypothesis of manipulation of secondary compounds. The winter diet was significantly higher in total phenolics and astringency. Total phenolics were consistent predictors of the winter diet in multiple regression analyses. The winter diets of six additional pika populations contained plant species high in secondary compounds. The results suggest that pikas preferentially select plants with high levels of secondary compounds for their winter diet, possibly because the presence of such compounds promotes preservation of the cache. This behavior may also enable the exploitation of an otherwise unusable food resource, i.e., toxic plants.