Feather mites and birds: an interaction mediated by uropygial gland size?

Feather mites (Arachnida: Acari: Astigmata) feed mainly on secretions of the uropygial gland of birds. Here, we use analyses corrected for phylogeny and body size to show that there is a positive correlation between the size of this gland and mite abundance in passerine birds at an interspecific level during the breeding season, suggesting that the gland mediates interactions between mites and birds. As predicted on the basis of hypothesized waterproofing and antibiotic functions of uropygial gland secretions, riparian/marsh bird species had larger glands and higher mite loads than birds living in less mesic terrestrial environments. An unexpected pattern was a steeper relationship between mite load and gland size in migratory birds than in residents. If moderate mite loads are beneficial to a host but high loads detrimental, this could create complex selection regimes in which gland size influences mite load and vice versa. Mites may exert selective pressures on gland size of their hosts that has resulted in smaller glands among migratory bird species, suggesting that smaller glands may have evolved in these birds to attenuate a possible detrimental effect of feather mites when present in large numbers.     

Mycorrhizae of Entoloma saepium: parasitism or symbiosis?

The mycorrhizae of Entoloma saepium on Rosa sp. are comprehensively described and compared to other mycorrhizae of the genus Entoloma, as well as to similar unidentified mycorrhizae known from the literature. E. saepium appears to be more a parasite than a symbiont, as it invades and almost completely destroys the root meristem and young root cells.Considered as part IL of the series Studies on ectomycorrhizae; part XLVIII: Waller and Agerer (1993)     

Impacts of fever on locust life-history traits: costs or benefits?

Fever, like other mechanisms for defence against pathogens, may have positive and negative consequences for host fitness. In ectotherms, fever can be attained through modified behavioural thermoregulation. Here we examine potential costs of behavioural fever by holding adult, gregarious desert locusts at elevated temperatures simulating a range of fever intensities. We found no effect of fever temperatures on primary fitness correlates of survival and fecundity. However, flight capacity and mate competition were reduced, although there was no relation between time spent at fever temperatures and magnitude of the response. While these effects could indicate a direct cost of fever, they are also consistent with a shift towards the solitaria phase state that, in a field context, could be considered an adaptive life-history response to limit the impact of disease. These conflicting interpretations highlight the importance of considering complex defence mechanisms and trade-offs in an appropriate ecological context.     

Evolution of eusocial behavior: Offspring choice or parental parasitism?

Assuming the subsocial route towards eusociality, I show that selection favors worker habits (under haplodiploidy) provided the mutant workers are able to bias the sex ratio towards sisters and/or selectively substitute their sons for brothers. If the original workers are not able to do this, selection does not favor the habit. However, under these conditions, selection is indifferent as to whether an individual rears offspring or sibs. This makes it easy for a mother to enlist her daughter's services in rearing other offspring, since the daughter cannot evolve to stop the parental parisitism. These results deal with the origin of eusocial behavior. I also look at selection acting on genes (to invest in offspring rather than sibs) in existing eusocial societies. It is shown that selection for laying workers is very strong, even if such workers give up rearing a seemingly advantageous combination of brothers and sisters. This poses distinct problems for the maintenance of eusocial societies.     

Commensalism or mutualism: conditional outcomes in a branchiobdellid–crayfish symbiosis

A growing body of evidence suggests that the costs and benefits involved in cleaning interactions can vary over space and time depending on the environmental conditions. However, whether the same cleaners actually induce variable net outcomes in terms of host fitness remains unclear to date. Branchiobdellid annelids are generally regarded as commensals or rarely parasites of their freshwater crayfish hosts, but a recent study suggests that they may also function as cleaning organisms. Under natural conditions, crayfish can experience fouling of the exposed surfaces of their exoskeletons and their gills (e.g., epibiosis) by various epibionts and particles of organic debris, and branchiobdellids graze on these sources of fouling. Here, we examined the extent to which variation in fouling pressure in the environment alters the outcome of the interaction between branchiobdellids and their crayfish host Cambaroide similis. A series of manipulations were performed in artificial environments designed to simulate either high or low fouling pressure. We used crayfish growth rates and mortality as direct measurements of the net costs and benefits of cleaning. Branchiobdellids had no significant effect on crayfish growth or mortality when cultured under low fouling pressure. However, their presence had a significant positive impact on host growth rates when cultured under high fouling pressure. These results suggest that the relationship between crayfish and branchiobdellids can fluctuate between commensalism and mutualism depending mainly on the environmental fouling pressure. We hypothesize that the outcome of cleaning interactions may largely depend on the factors directly related to the need for cleaning, such as parasite loads or fouling pressure.     

Breeding birds on small islands: island biogeography or optimal foraging?

1. We test MacArthur and Wilson's theory about the biogeography of communities on isolated habitat patches using bird breeding records from 16 small islands off the coasts of Britain and Ireland. 2. A traditional examination of patterns of species richness on these islands suggests that area and habitat diversity are important predictors, but that isolation and latitude have a negligible impact in this system. 3. Unlike traditional studies, we directly examine the fundamental processes of colonization and local extinction (cessation of breeding), rather than higher-order phenomena such as species richness. 4. We find that many of MacArthur and Wilson's predictions hold: colonization probability is lower on more isolated islands, and extinction probability is lower on larger islands and those with a greater diversity of habitats. 5. We also find an unexpected pattern: extinction probability is much lower on more isolated islands. This is the strongest relationship in these data, and isolation is the best single predictor of colonization and extinction. 6. Our results show that examination of species richness alone is misleading. Isolation has a strong effect on both of the dynamic processes that underlie richness, and in this system, the reductions in both colonization and extinction probability seen on more distant islands have opposing influences on species richness, and largely cancel each other out. 7. We suggest that an appropriate model for this system might be optimal foraging theory, which predicts that organisms will stay longer in a resource patch if the distance to a neighbouring patch is large. If nest sites and food are the resources in this system, then optimal foraging theory predicts the pattern we observe. 8. We advance the hypothesis that there is a class of spatial systems, defined by their scale and by the taxon under consideration, at which decision-making processes are a key driver of the spatiotemporal dynamics. The appropriate theory for such systems will be a hybrid of concepts from biogeography/metapopulation theory and behavioural ecology.     

Pheromones in birds: myth or reality?

Birds are anosmic or at best microsmatic… This misbelief persisted until very recently and has strongly influenced the outcome of communication studies in birds, with olfaction remaining neglected as compared to acoustic and visual channels. However, there is now clear empirical evidence showing that olfaction is perfectly functional in birds and birds use olfactory information in a variety of ethological contexts. Although the existence of pheromones has never been formally demonstrated in this vertebrate class, different groups of birds, such as petrels, auklets and ducks have been shown to produce specific scents that could play a significant role in within-species social interactions. Behavioral experiments have indeed demonstrated that these odors influence the behavior of conspecifics. Additionally, in quail, deprivation of olfactory inputs decreases neuronal activation induced by sexual interactions with a female. It seems therefore well established that birds enjoy a functional sense of smell and a fast growing body of experimental evidence suggests that they use this channel of olfactory communication to control their social life. The unequivocal identification of an avian pheromone is, however, still ahead of us but there are now many exciting opportunities to unravel the behavioral and physiological particularities of chemical communication in birds.     

Mutualism or parasitism? The variable outcome of cleaning symbioses

The exact nature of many interspecific interactions remains unclear, with some evidence suggesting mutualism and other evidence pointing to parasitism for the same pair of interacting species. Here, we show spatial variation in the outcome of the cleaning relationship between Caribbean cleaning gobies (Elacatinus evelynae) and longfin damselfish (Stegastes diencaeus) over the distribution range of these species, and link this variation to the availability of ectoparasites. Cleaning interactions at sites with more ectoparasites were characterized by greater reductions in ectoparasite loads on damselfish clients and lower rates of removal of scales and mucus (i.e. cheating) by cleaning gobies, whereas the opposite was observed at sites where ectoparasite abundance was lower. For damselfish clients, cleaning was therefore clearly mutualistic in some locations, but sometimes neutral or even parasitic in others. Seasonal variability in ectoparasite abundance may ensure that locally low parasite availability, which promotes cleanerfish cheating, may be a transient condition at any given site. Conflicting conclusions about the nature of cleaning symbioses may, therefore, be explained by variation in ectoparasite abundance.     

Awesome or ordinary? Global diversity patterns of oribatid mites

Diversity of most above-ground organisms increases with decreasing latitude, but the biogeographical and macroecological diversity patterns of below-ground animals have been poorly studied. We investigated the latitudinal diversity gradient in a primarily below-ground living soil taxon, oribatid mites. Furthermore, oribatid mite species richness from islands and mainlainds was tested for correlation with the size of the respective area (island or mainland) to evaluate if their species–area relationships are similar to those of above-ground taxa. The results suggest that for oribatid mites 1) diversity increases from the boreal to the warm temperate region but not further to the tropics, and 2) species–area relationships for islands and mainlands are similar to those of above-ground taxa, but this is mainly caused by very small islands, such as Cocos islands, and very large islands, such as Madagascar. When these islands are excluded the species–area relationship strongly differs from those of typical islands. The results support the view that below-ground animal taxa are generalists that inhabit wide niches. Most small islands have relatively rich oribatid mite faunas, supporting the observation that a large number of species can coexist in a small area (high α-diversity).     

Assimilation efficiency in birds: A function of taxon or food type?

• 1.1. We review the available measurements of assimilation efficiency in birds and find a strong dependence upon food ingested, regardless of bird taxa.
• 2.2. Assimilation efficiency in birds is about 75% for most food types, except plants (36.9%) and fruits (41%).

     

The paradox of invasion in birds: competitive superiority or ecological opportunism?

Why can alien species succeed in environments to which they have had no opportunity to adapt and even become more abundant than many native species? Ecological theory suggests two main possible answers for this paradox: competitive superiority of exotic species over native species and opportunistic use of ecological opportunities derived from human activities. We tested these hypotheses in birds combining field observations and experiments along gradients of urbanization in New South Wales (Australia). Five exotic species attained densities in the study area comparable to those of the most abundant native species, and hence provided a case for the invasion paradox. The success of these alien birds was not primarily associated with a competitive superiority over native species: the most successful invaders were smaller and less aggressive than their main native competitors, and were generally excluded from artificially created food patches where competition was high. More importantly, exotic birds were primarily restricted to urban environments, where the diversity and abundance of native species were low. This finding agrees with previous studies and indicates that exotic and native species rarely interact in nature. Observations and experiments in the field revealed that the few native species that exploit the most urbanized environments tended to be opportunistic foragers, adaptations that should facilitate survival in places where disturbances by humans are frequent and natural vegetation has been replaced by man-made structures. Successful invaders also shared these features, suggesting that their success is not a paradox but can be explained by their capacity to exploit ecological opportunities that most native species rarely use.     

Bacterial virulence factors targeting Rho GTPases: parasitism or symbiosis?

In the past few years, an important question in microbiology has arisen from reports indicating that several pathogenic bacteria have evolved virulence factors directed towards a Ras subfamily of GTPases, namely the Rho GTPases. Progress made in studying both the virulence factors and the signaling pathways involving Rho GTPases has shed light on this crosstalk. One central question is raised by the findings that both activating and inactivating virulence factors that target Rho GTPases coexist in some pathogenic bacteria. Further studies on this peculiar aspect of the bacteria-host cell interactions, which leads to the outbreak of infectious diseases, might clarify whether this aspect of Rho GTPase activation or inactivation represents a finely adapted response of the pathogen for its own benefit or might lead to a reaction of the host against the bacteria.     

Plasticity in body composition in breeding birds: what drives the metabolic costs of egg production?

Body composition in vertebrates is known to show phenotypic plasticity, and changes in organ masses are usually rapid and reversible. One of the most rapid and reversible changes is the transformation of the female avian reproductive organs before breeding. This provides an excellent system to investigate the effects of plasticity in organ size on basal metabolic rate (BMR) through relationships between organ masses and BMR. We compared body composition of female European starlings (Sturnus vulgaris) during various reproductive stages over 3 yr and investigated the pattern of changes in reproductive and nonreproductive organ mass during follicular development and ovulation. Furthermore, we analyzed the relationship between organ mass and resting metabolic rate (RMR) in nonbreeding, laying, and chick-rearing females. Our analysis revealed marked variation in organ masses between breeding stages but no consistent pattern among years except for kidney and pectoralis muscle. Furthermore, changes in nonreproductive organs did not parallel the cycle of growth and regression of the reproductive organs. The oviduct gained 62% of its 22-fold increase in mass in only 3 d, and oviduct regression was just as rapid and began even before the final egg of the clutch was laid, with 42% of the oviduct mass lost before laying of the final egg. In laying females, 18% of variation in mass-corrected RMR was explained by the mass of the oviduct (r2=0.18, n=80, P<0.0005), while pectoralis muscle mass in nonbreeding individuals and liver and gizzard mass in chick-rearing females were the only organs significantly related to RMR (r2=0.31-0.44). We suggest that the nonreproductive organs are affected more by changes in local ecological conditions than the reproductive state itself and that the activity and maintenance cost of the oviduct is high enough that selection has led to a very tight size-function relationship for this organ.     

The effect of unripe fruits on ripe fruit removal by birds in Pistacia terebinthus: flag or handicap?

The shrub Pistacia terebinthus produces crowded infructescences with up to several hundred fruits, which are bright red when unripe and turn green when ripe. Most fruits contain an empty seed and never reach maturity. More ripe fruits were removed by birds from experimental bicolored fruit displays (consisting of infructescences with ten ripe fruits and stripped of unripe fruits, paired with infructescences with only unripe fruits) than from monocolored ones (single infructescences with ten ripe fruits and stripped of unripe fruits). Thus, the presence of unripe fruits seems to increase the conspicuousness or attractiveness of fruit displays to fruit-eating birds. A second experiment compared ripe fruit removal from experimental infructescences having only ripe fruits, with that from control infructescences containing both ripe fruits and natural numbers of unripe fruits, all on P. terebinthus plants. Unlike the first experiment, each bicolored display in this case consisted of a single infructescence with both unripe and ripe fruits. A higher proportion of ripe fruits was removed by birds from infructescences free of unripe fruits. This result suggests that the presence of unripe fruits reduces the accessibility of ripe fruits for fruit-eating birds. This is further supported by field observations of bird foraging behavior.     

The role of floods in the lives of fish-eating birds: predator loss or benefit?

Floods accompanied by high flow and high water turbidity are usually believed to cause problems to fish-eating birds and mammals searching visually for their prey. In the present study the diets of breeding kingfishers were studied during the normal river situation and during a long-lasting flood event with respect to diet composition, size of fish prey and food diversity index. During the normal situation (flow 1.75 m³ s^?1, Secchi disc depth 0.5–1 m), the diet of a kingfisher was dominated by benthic fish species (52.9% by numbers, 63.9% by weight), the average size of fish taken was 6.5 cm L _T and 3.0 g and the food diversity index reached its lowest value (1.57). In contrast, during the long-lasting flood event (flow 5–28 m³ s^?1, Secchi disc depth 0.03–0.4 m) the diet of the kingfisher was dominated by sub-surface fish species (72.4% by numbers, 76.1% by weight) and both the average size of fish taken (7.4 cm L _T and 3.7 g) and the food diversity index (1.83) increased significantly. The birds provided their nestlings with lower numbers of fish of larger sizes, which resulted in very similar weights of the young birds prior to fledging when the flood and normal situations were compared. This study provides evidence that in different foraging conditions the kingfishers adopt different foraging strategies to maintain their high breeding success.     

Low frequency of observed cowbird parasitism on eastern kingbirds: host rejection, effective nest defense, or parasite avoidance?

Eastern kingbird (Tyrannus tyrannus) nests rarely are parasitizedby brown-headed cowbirds (Molotrus ater). Kingbirds are oneof a dozen or so species known to eject cowbird eggs from theirnests. We hypothesized that either kingbirds eject cowbird eggsso quickly that researchers normally do not detect the eggsduring daily nest inspections, or that cowbirds avoid parasitizingkingbirds. We tested these alternative hypotheses by experimentallyintroducing real cowbird eggs into eastern kingbird nests duringthe pre-egg, early laying, late laying, and incubation stages.We recorded the interval between "parasitism" and ejection ofthe cowbird eggs. Although kingbirds ejected 87 of 88 cowbirdeggs placed in their nests, about 40% of the eggs remained innests for more than 24 h. Thus, during daily nest inspectionswe should have observed cowbird eggs if nests were parasitizedat all. In fact, we detected only one parasitized nest amongthe 402 inspected daily. The time for ejection was longest atnests parasitized early in laying, and shorter at nests parasitizedbefore and after. This variation in ejection times may reflectthe time kingbirds require to learn to recognize their own eggs.Although kingbirds defend their nests aggressively, they donot respond to female cowbirds as unique threats and do notguard their nests before sunrise when cowbirds lay. We concludethat cowbirds avoid parasitizing eastern kingbirds because theireggs most likely will be wasted. The rejection behavior persistspossibly because it is almost cost-free (a maximum of 0.07 kingbirdegg lost or damaged per cowbird egg ejected), or it evolvedin response to conspecific rather than cowbird parasitism. Foreignkingbird eggs introduced into nests at different nest stageswere ejected only during the pre-egg stage. This result supportsthe hypothesis that rejection behavior in eastern kingbirdsevolved in response to cowbird parasitism.     

CMA restricted to mammals and birds: myth or reality?

Chaperone-mediated autophagy (CMA) is a major pathway of lysosomal proteolysis essential for the control of intermediary metabolism. So far, the absence of any identifiable LAMP2A – a necessary and limiting protein for CMA – outside of the tetrapod clade, led to the paradigm that this cellular function was (presumably) restricted to mammals and birds. However, after we identified expressed sequences displaying high sequence homology with the mammalian LAMP2A in several fish species, our findings challenge that view and suggest that CMA likely appeared much earlier during evolution than initially thought. Hence, our results do not only shed an entirely new light on the evolution of CMA, but also bring new perspectives on the possible use of complementary genetic models, such as zebrafish or medaka for studying CMA function from a comparative angle/view.     

Two estimates of the metabolic costs of antibody production in migratory shorebirds: low costs, internal reallocation, or both?

We measured the costs of mounting a humoral immune response using two novel antigens (tetanus and diphtheria) in two shorebird species (Scolopacidae): Red Knot (Calidris canutus, measured in autumn) and Ruff (Philomachus pugnax, measured in spring). Metabolic rate was measured during the preinjection phase, at the building phase of the primary immune response, and at peak secondary immune response by determining the oxygen consumption of the postabsorptive birds at rest. Confirming earlier studies, Red Knots and Ruffs responded with lower antibody titers to the diphtheria than to the tetanus antigen. Although Red Knots and Ruffs produced the same amounts of antibodies, Red Knots showed a significant 13% increase in basal metabolic rate (BMR) during the secondary antibody response, whereas Ruffs showed a 15%, but only marginally significant, reduction in BMR. The results from this study suggest that the energetic costs of an immune response may be small, but the "negative cost" in Ruffs hints at the possibility of resource reallocation and the concomitant difficulty of measuring such costs during "basal" metabolic rate measurements.