Polymorphic color vision in white-faced capuchins (Cebus capucinus): Is there foraging niche divergence among phenotypes?

Many species of platyrrhine primates are characterised by sex-linked color vision polymorphism. This presents an opportunity to study the biology and ecology of individuals with different phenotypes living in the same group. Several evolutionary processes could maintain polymorphic genes in populations. In this study, we evaluate the hypothesis that foraging niche divergence among monkeys explains the presence of multiple color vision phenotypes. Specifically, we test whether dichromats and trichromats differ in foraging time devoted to cryptic vs brightly colored resources. We did not find any differences in foraging time spent on different food types by dichromatic and trichromatic monkeys in two groups of white-faced capuchins (Cebus capucinus) living in a tropical dry forest. We conclude that in so far as these variables are concerned, niche divergence does not likely explain color vision polymorphism in our study population.     

Evolutionary variation in the expression of phenotypically plastic color vision in Caribbean mantis shrimps, genus Neogonodactylus

Many animals have color vision systems that are well suited to their local environments. Changes in color vision can occur over long periods (evolutionary time), or over relatively short periods such as during development. A select few animals, including stomatopod crustaceans, are able to adjust their systems of color vision directly in response to varying environmental stimuli. Recently, it has been shown that juveniles of some stomatopod species that inhabit a range of depths can spectrally tune their color vision to local light conditions through spectral changes in filters contained in specialized photoreceptors. The present study quantifies the potential for spectral tuning in adults of three species of Caribbean Neogonodactylus stomatopods that differ in their depth ranges to assess how ecology and evolutionary history influence the expression of phenotypically plastic color vision in adult stomatopods. After 12 weeks in either a full-spectrum “white” or a narrow-spectrum “blue” light treatment, each of the three species evidenced distinctive tuning abilities with respect to the light environment that could be related to its natural depth range. A molecular phylogeny generated using mitochondrial cytochrome oxidase C subunit 1 (CO-1) was used to determine whether tuning abilities were phylogenetically or ecologically constrained. Although the sister taxa Neogonodactylus wennerae and N. bredini both exhibited spectral tuning, their ecology (i.e. preferred depth range) strongly influenced the expression of the phenotypically plastic color vision trait. Our results indicate that adult stomatopods have evolved the ability to undergo habitat-specific spectral tuning, allowing rapid facultative physiological modification to suit ecological constraints.     

The development of sex differences in ring-tailed lemur feeding ecology

Sex differences in feeding ecology may develop in response to fluctuations in physiological costs to females over their reproductive cycles, or to sexual size dimorphism, or function to minimize feeding competition within a group via resource partitioning. For most mammal species, it is unknown how these factors contribute to sex differences in feeding, or how the development of males and females reflects these intraspecific feeding differences. We show changes in dietary composition, diversity, overlap, and foraging behavior throughout development in ring-tailed lemurs (Lemur catta) and test how the development of sex differences in feeding is related to female costs of reproduction and year-round resource partitioning. Sex differences in dietary composition were only present when females were lactating, but sex differences in other aspects of feeding, including dietary diversity, and relative time spent feeding and foraging, developed at or near the time of weaning. Sex difference in juveniles and subadults, when present, were similar to the differences found in adults. The low year-round dietary overlap and early differences in dietary diversity indicate that some resource partitioning may begin with young individuals and fluctuate throughout development. The major differences between males and females in dietary composition suggest that these larger changes in diet are closely tied to female reproductive state when females must shift their diet to meet energetic and nutritional requirements.     

The relationship between bleaching and mortality of common corals

Reef corals are likely to have many subtle but four gross responses to anomalous warm water. These are (1) not bleach and live (mortality <10%), (2) not bleach and die (mortality >20%), (3) bleach and live, and (4) bleach and die. The frequency of these four possible gross responses was determined for 18 common coral taxa over an exceptionally warm 1998 El Niño where intense bleaching was observed, and mortality determined from line transects averaged 41.2±34.7 (±SD). Field studies included (1) recording the loss of color (bleaching) and observing recently dead individuals among 6,803 colonies during five sampling periods and (2) estimating mortality based on 180 m of line-intercept transects completed 4 months before and near the end of the bleaching episode. There was no clear relationship between the loss of color and either direct observation or transect-based estimates of mortality for the 18 taxa. The morphology of the taxa did not influence color loss but branching and encrusting taxa had higher mortality than massive and submassive taxa. Loss of color and mortality are the most common responses to warm water as only Pavona did not lose color or die and only two taxa, Cyphastrea and Millepora, did not significantly lose color but died. Of the 15 taxa that lost color, five taxa, Astreopora, Favia, Favites, Goniopora, and Leptoria, did not die. These taxa are those most likely to have reduced potential mortality by the loss of pigments and associated algal symbionts. Death of the branching taxa was detected reasonably by direct field observation but some taxa were underestimated when compared with mortality estimates based on line transects. Death of encrusting and massive taxa including Echinopora, Galaxea, Hydnophora, Montipora, Platygyra, and massive Porites was poorly detected from direct observations but they proved to have modest to high mortality (20–80%) based on line transects. There was no single response of these common corals to warm water but these data, collected during an extreme warm-water anomaly, indicate that the loss of color is most frequently a sign of morbidity, particularly for branching and encrusting taxa.Communicated by P.W. Sammarco, Chauvin     

Activity budgets in two species of bee flies (Lordotus: Bombyliidae,Diptera): a comparison of species and sexes

Summary This study compares time budgets of males and females of two sympatric species of bee flies, Lordotus pulchrissimus and L. miscellus. These species occur synchronously in sand dune habitat on the north shore of Mono Lake, Mono Co., California, and feed almost exclusively on the flowers of a composite shrub, Chrysothamnus nauseosus. The two closely related species, with similar ecological requirements and in an identical environment, allocate time and energy in very different ways, even when accomplishing the same ecological goals. L. pulchrissimus males and females engage respectively in aggressive interactions and feeding primarily in the morning. In contrast, L. miscellus males and females intersperse brief periods of resting with flying when defending territories and feeding, and they keep up these activities until later in the day. Similarly within species, males are involved in aggressive interactions for a shorter period each day, and females feed over a longer period. Flies in all sex-species classes but male pulchrissimus allocate their time energetically in similar ways. Male pulchrissimus spend more time each day in energetically costly activities; they engage in continuous hovering flight and intense interactions in aerial aggregations. While male miscellus feed little, male pulchrissimus spend a large portion of their time feeding, approximately as much as females, contrary to the expectation that males should be foraging-time minimizers. This study corroborates the conclusions of previous studies on bee-fly communities by showing that nectar and pollen are important resources for adult bee flies, at least for some species: Individuals of these two species spend a large proportion of their adult lifetimes feeding.     

Group histories and offspring sex ratios in ringtailed lemurs (Lemur catta)

Birth sex ratios were examined for ringtailed lemurs (Lemur catta) at the Duke University Primate Center. This population provides a long-term database of births under a variety of demographic and management conditions, including two semi-freeranging groups between which males transfer freely and females defend stable territorial boundaries. We examined three hypotheses usually considered in studies of primate sex ratio bias. The Trivers-Willard hypothesis predicts that dominant females produce males, local resource competition at the population level (LRC-population) predicts that the dispersing sex (males) will be overproduced in dense populations, and local resource competition among individuals (LRC-individual) predicts that dominant females overproduce the philopatric sex (females). We also examined a fourth hypothesis, local resource enhancement (LRE), which is usually subsumed under LRC-individual in studies of primate sex ratio evolution. LRE predicts that under certain conditions, females will produce the sex that provides later cooperative benefits, such as alliance support for within- or between-group competition. Our data provide support for LRE: females overproduce daughters given prospects of new group formation, either through group fission or threatened expulsion of young mothers. Behavioral data from Duke and also wild populations show that daughters serve mothers as important allies in this context and LRE effects also have been documented in other mammals that experience similar group histories. Nonsignificant trends in the data supported the LRC-population hypothesis, and we suggest that LRC interacts with LRE to explain offspring sex ratios in ringtailed lemurs. Received: 27 August 1999 / Received in revised form: 6 March 2000 / Accepted: 18 March 2000     

On the sociobiology of four puerto rican parrotfishes (Scaridae)

Behavioral observations were made during SCUBA excursions out of the PRINUL underwater laboratory, off the west coast of Puerto Rico. Most of the work involved four species of parrotfishes, Sparisoma viride, Sparisoma aurofrenatum, Scarus croicensis and Scarus taeniopterus. The species of Sparisoma are relatively patrilocal, solitary, slowly and steadily feeding fishes; the terminal phase (termphase) males of S. aurofrenatum hold large territories that encompass the home ranges, possibly territories, of a number of females. The two species of Scarus are roving, aggregating fish that periodically descend to the bottom where they feed rapidly. Reproductive males are of two types, either highly dimorphic, brilliantly colored termphase males, or midphase males that resemble the females. At times, termphase males become territorial and are visited by females and midphase males. Females spawn with termphase and with midphase males. Aggression in all four species was directed largely to conspecific individuals. All species show the ability to change color rapidly, sometimes dramatically, in social interactions.     

Identification and validation of novel SNP markers in European populations of marine Mytilus mussels

Mussels of the genus Mytilus are widespread in both northern and southern hemispheres. Mytilus taxa are very important components of marine coastal ecosystems, but are difficult to differentiate morphologically. Sequencing and Sequenom MassARRAY iPLEX genotyping technology was used to identify and verify novel SNP markers in three European taxa of Mytilus: M. edulis, M. galloprovincialis, and M. trossulus. SNPs were localized in coding and non-coding sequences of some functionally important genes. Eight SNPs located in genes of the histone family, hsp70 and p53, were discovered and applied as novel markers for Mytilus taxa on a large European scale. Five of these differentiated the M. trossulus genome, two M. galloprovincialis, and one M. edulis. Other SNPs differentiated populations within taxa. The new SNPs will be a valuable tool for population studies of European Mytilus mussels. The percentage of polymorphic SNPs ranged from 19 to 100 % in 24 samples of mussels studied. Populations from Scotland, Mecklenburg Bight, and Norway had over 90 % polymorphic loci. Most loci were in Hardy–Weinberg equilibrium in all samples except for the one from Scotland. The highest percentages of heterozygotes were observed in the Atlantic (Banyuls and Vigo) and North Sea (Tjarno and Westerschelde) populations. An excess of homozygotes was observed in samples from Scotland, Norway, and the Barents Sea. Correspondence and Structure analysis also demonstrated the great heterogeneity of these three samples.     

Light environment and mating behavior in Trinidadian guppies (<Emphasis Type="Italic">Poecilia reticulata</Emphasis>)

Male guppies, Poecilia reticulata, have color patterns that result from a balance between natural selection for crypsis to avoid predators and sexual selection for bright, complex patterns that attract females. Males use displays to show off these patterns to potential mates, but their conspicuousness also depends on the light environment in which they are viewed. We investigated variation in natural underwater guppy light environments in Trinidad, West Indies, and found that mating behavior is correlated with both the ‘quantity’ (total irradiance) and ‘quality’ (spectral composition) of light: light intensity and the proportion of ultraviolet light were negatively related to display rates. Experimental manipulation of light environment to mimic natural daily changes demonstrated that these relationships are causal and are independent of time of day effects. At lower light levels, when guppies are less detectable by visually hunting predators, females had more opportunity for active mate choice, because males displayed more. However, these light conditions may reduce the ability of females to accurately discriminate between males. Guppy mating behavior is therefore strongly affected by light environment, and this may have important effects on sexual selection.     

Temporal adaptations in visual systems of deep-sea crustaceans

The temporal characteristics of the visual systems of deep-sea crustaceans were examined at varying light levels. Experimental organisms were collected off Hawaii and southern California in 1991 and 1992, and continually maintained in the dark. At the University of California Santa Barbara Marine Laboratory, the temporal components of both visual interneuron activity and electroretinograms (ERGs) of adult Gnathophausia ingens were tested at threshold and with background light levels up to 6 log units above threshold. To compare these responses with those of mid-water organisms inhabiting shallower depths, the temporal characteristics of the ERGs of Oplophorus spinosus and juvenile G. ingens were assessed. Adults of both species showed little or no change in the time components of their responses when light-adapted, except for the response latency. There was also no evidence of a circadian rhythm in visual sensitivity. O. spinosus, a vertical migrator, and juvenile G. ingens, which inhabit a shallower depth than the adults, exhibited more rapid responses than adult G. ingens. These varying responses are considered adaptations to the differences in light levels at the characteristic depths of the organisms.     

The trophic status of herbivorous fishes on coral reefs

Estimates of feeding rates, alimentary tract structure and temporal patterns of food processing obtained from twelve species of nominally herbivorous fishes on the northern Great Barrier Reef were compared. These included members of the families Acanthuridae, Scaridae and Kyphosidae. Based on an analysis of diet and short-chain fatty acid (SCFA) profiles from a previous study we initially partitioned the twelve species into four dietary categories, as follows: (a) Category 1: herbivores with a diet of macroscopic brown algae and high SCFA profiles in the hindgut region (Naso unicornis, Kyphosus vaigiensis); (b) Category 2: herbivores feeding on turfing and filamentous red and green algae with moderate SCFA profiles in the hindgut region (N. tonganus, K. cinerascens, Zebrasoma scopas, Acanthurus lineatus); (c) Category 3: zooplankton feeders with moderate SCFA profiles (N. vlamingii, N. brevirostris); (d) Category 4: species feeding on detrital and sedimentary materials with low levels of SCFA (Chlorurus microrhinos, Scarus schlegeli, Ctenochaetus striatus, A. olivaceus). The purpose of this comparison was to determine whether measures of feeding activity, alimentary tract structure, and food processing were concordant with diet. A dichotomy in feeding rates was observed. Species with a diet of algae and zooplankton (categories 1–3) had slower feeding rates than those feeding on detrital aggregates and sediment (category 4). The pattern of food processing also followed the same dichotomy with species of categories 1–3 retaining food in the alimentary tract overnight and commencing the feeding day with substantial amounts of food in the intestine and hindgut. Category-4 species commenced the feeding day with empty alimentary tracts suggesting a rapid turnover of gut contents. Within the herbivorous and zooplankton-feeding species neither alimentary tract structure nor food processing mode were predicted by diet or SCFA profiles. A hindgut fermentation chamber was present in K. vaigiensis but not in N. unicornis, a species with high levels of SCFA in the hindgut region and a diet of brown macroscopic algae. In contrast N. vlamingii, with a diet dominated by animal matter, retained large amounts of food material in a hindgut chamber over the entire feeding cycle. In tropical perciform fishes, herbivory and fermentation are not associated with the alimentary tract structures that characterise herbivorous terrestrial vertebrates. Estimates of the abundance of the different groupings of nominally herbivorous fishes indicated that the dominant elements in the reef grazing and browsing fauna were consumers of detrital and sedimentary materials. These could not be classified as herbivores. Members of this group were dominant in all habitats investigated. Explicitly herbivorous taxa were a minority component in all habitats investigated.Communicated by G.F. Humphrey, Sydney     

Hunting in unfamiliar space: echolocation in the Indian false vampire bat, Megaderma lyra, when gleaning prey

The literature suggests that in familiar laboratory settings, Indian false vampire bats (Megaderma lyra, family Megadermatidae) locate terrestrial prey with and without emitting echolocation calls in the dark and cease echolocating when simulated moonlit conditions presumably allow the use of vision. More recent laboratory-based research suggests that M. lyra uses echolocation throughout attacks but at emission rates much lower than those of other gleaning bats. We present data from wild-caught bats hunting for and capturing prey in unfamiliar conditions mimicking natural situations. By varying light level and substrate complexity we demonstrated that hunting M. lyra always emit echolocation calls and that emission patterns are the same regardless of light/substrate condition and similar to those of other wild-caught gleaning bats. Therefore, echoic information appears necessary for this species when hunting in unfamiliar situations, while, in the context of past research, echolocation may be supplanted by vision, spatial memory or both in familiar spaces.Communicated by T. Czeschlik     

Chemical egg defense in Photuris firefly “femmes fatales”

Summary. Female Photuris fireflies contain defensive chemicals of two types. They sequester steroidal pyrones (lucibufagins) from male fireflies of the genus Photinus that they eat, and themselves produce the defensive betaine N-methylquinolinium 2-carboxylate. Chemical analyses of Photuris eggs showed that females that fed on Photinus males endow their eggs with both lucibufagin and the betaine, while those that did not feed on Photinus lay eggs that contain betaine, but virtually no lucibufagin. Photuris females collected in the field during the Photinus flight season laid eggs that invariably contained betaine, but lucibufagin only at times. Predation experiments showed that Photuris eggs are essentially unacceptable to larvae of a coccinellid beetle (Harmonia axyridis) and an ant (Leptothorax longispinosus), but moderately acceptable to an earwig (Forficula auricularia). When applied experimentally to palatable insect eggs, lucibufagin proved deterrent to these three predators, while the betaine proved deterrent to the ant and coccinellid larva only. Both types of defensive compound decreased egg predation in the field. By endowing their eggs with both exogenous and endogenous chemicals, Photuris females are essentially “maximizing their options”– when feeding on Photinus, their eggs are doubly protected, but they are not entirely defenseless when the females are unable to procure lucibufagin. Received 12 August 1999; accepted 27 August 1999     

Herbivory by the myctophid fish Ceratoscopelus warmingii

During a study of midwater fish feeding in the oligotrophic North Pacific gyre in August 1978, nine specimens of a tropical-subtropical myctophid, Ceratoscopelus warmingii, were found to have been feeding on mats of diatoms, Rhizosolenia spp. Chemical analyses of the intestinal contents of several additional specimens showed significantly elevated levels of diatom degradation products. Therefore, C. warmingii, although known to feed on zooplankton, appears to be adapted for occasional herbivory. Such an adaptation is exceptional within a major ecological group of oceanic fishes that was previously thought to be exclusively carnivorous. Like omnivorous and herbivorous fishes in general, C. warmingii has a longer intestine than other myctophid species. As an adaptive response to competition from a diverse fish fauna in a low-productivity environment, occasional herbivory by C. warmingii runs counter to the theoretical expectation by expanding rather than narrowing the range of potential food types.     

Twilight migrations and foraging activities of the copper sweeper Pempheris schomburgki (Teleostei: Pempheridae)

During 1976 and 1977, movements and foraging activities of Pempheris schomburgki were studied on the shallow coral reefs of northeastern St. Croix, U.S. Virgin Islands. P. schomburgki emerge from daytime refuges on the backreef at about local sunset. Fifteen to 20 min after sunset, local milling groups move beyond the confines of the reef and assemble into larger groups. Twenty-five to 30 min after sunset they move in several waves along complex routes through the reef to forereef feeding grounds. Migrations of up to 1 km occur along the forereef. The reverse sequence occurs in the morning, but is earlier with respect to ambient light levels. The principal stages of these activities appear to be triggered by a combination of absolute light level, rate of change of light and state of adaptation of the eye. Migrating aggregations gradually split up into small, well-dispersed feeding groups, relatively evenly spaced along the forereef. Few individuals feed on the backreef. The principal available food consists of meroplanktonic crustaceans not available during the daytime. P. schomburgki mainly select the larger-sized individuals (mean length 5 to 6 mm), although some particles less than 1 mm are taken. These events probably represent adaptations to optimize diurnal sheltering sites, feeding grounds and the avoidance of predator activity.     

Ontogeny of diet shifts by a microcarnivorous fish, Cheilodactylus spectabilis : relationship between feeding mechanics, microhabitat selection and growth

The mechanisms leading to ontogenetic shifts in prey selectivity are examined for the temperate microcarnivore Cheilodactylus spectabilis (Cheilodactylidae) in north-eastern New Zealand. These fish prey on invertebrates associated with benthic turf and foliose algae, using a suctorial feeding mode combined with oral sorting. All sizes of fish feed in the same shallow-water habitat using the same feeding mode. Dietary analysis revealed that while all sizes of fish consumed similar taxa, the relative proportion of taxa consumed reflected fish size. Juveniles consumed mainly gammarid amphipods while large adults targeted ophiuroids, with an abrupt shift from feeding predominantly on amphipods at 250 mm standard length. This dietary shift loosely coincided with the onset of sexual maturity and a change in growth trajectory, although dietary trends did not differ between sexes. Both juveniles and adults were found to select particular taxa from the available turf micro-fauna, with juveniles consuming smaller sizes of amphipods than adults. Microhabitat use was also found to change ontogenetically. Detailed observations on feeding mechanics suggested that size-related changes in suctorial force allowed the exploitation of a broader range of microhabitats with increasing fish size. Received: 20 June 1997 / Accepted: 1 April 1998     

Diet and food preferences of the adult horseshoe crab Limulus polyphemus in Delaware Bay,New Jersey,USA

Adult horseshoe crabs Limulus polyphemus (L.) feed on a wide variety of infaunal and epifaunal invertebrates during their spring spawning migration in Delaware Bay, New Jersey, USA. Comparison of the gut contents with estimates of available prey showed that the most abundant potential prey item, the bivalve Gemma gemma, was avoided. The thinner shelled but comparatively scarce clam Mulinia lateralis was a preferred prey item. In the laboratory, crabs fed on G. gemma when it was the only available item but not when M. lateralis or soft-shell clams, Mya arenaria, were offered in conjunction. Large M. lateralis (>10mm) were preferred to small M. lateralis; there was no discrimination between M. lateralis and M. arenaria of the same size. Male and female horseshoe crabs had similar gut contents and laboratory feeding preferences, despite the fact that females are larger than males. Crabs spawning later in the summer contained more food than did crabs collected at the peak of spawning activity.     

The neglected scent: on the marking function of urine in Lemur catta

In ring-tailed lemurs (Lemur catta) urine deposition can be combined with two different tail configurations: (i) tail held up in an evident display (urinate tail-up, UT-up); (ii) tail slightly raised to avoid its impregnation with urine (urinate tail-down, UT-down). We used both signaller- and receiver-based approaches to search for functional differences between these two kinds of urine deposition. We predicted that UT-up might be a complex signal combining olfactory and visual cues. We carried out observations and scent tests on four captive groups of ring-tailed lemurs. Group members sniffed/licked UT-up scents more frequently than UT-down ones. Moreover, UT-up showed peak levels during the mating season whereas UT-down did not. These findings suggest that urine can play a role in intra-group reproductive communication. Lemurs more frequently performed UT-up in a few drops and UT-down in streams. Recognition experiments clearly showed that individuals can discriminate between urine of their own group and urine from a foreign group (a necessary prerequisite for the use of urine in inter-group communication). The possible function of UT-up in inter-troop communication was supported by the higher frequency of this pattern along a fence separating two of the study groups. Moreover, in the presence of a dummy, the frequency of UT-up increased significantly. In conclusion, UT-up is a complex signal with multiple characteristics. By using different sensory channels, UT-up provides different types of information (location and signaller quality) and contains multiple messages directed both at group-members and neighbouring groups....to conceal this letter, the Minister had resorted to the comprehensive and sagacious expedient of not attempting to conceal it at all (from the Purloined letter E. A. Poe, 1845)     

Intake and coversion of food in the fish Limanda limanda exposed to different temperatures

In the flat fish Limanda limanda L., feeding rate and conversion efficiency were studied as functions of body weight, sex, temperature and food quality. When offered herring meat at 13 °C (series I), females (live weights 1 to 150 g) consume more food than males; the magnitude of this difference is body weight-dependent. With increasing wieght, both females and males consume less food per unit body weight per day. Variations in daily ration are considerable; the range of deviation from mean feeding rate is about 60% for males and 40% for females. The range of deviation does not vary significantly among females and males of different body weights. At the same temperature level (13 °C; series II), females consume almost the same, or even less, cod meat than males. Among individuals of series I and II, there is a little difference in the feeding rate; however, herring-fed individuals obtain about 2 times more energy than cod-fed individuals. Each gram wet weight of herring meat yields 2001, each gram cod meat 1137, calories. Small individuals completely cease to feed at 3°C; they feed little at 8 °C. Larger females consume maximum amounts at 8 °C. Small individuals consume maximum amounts at higher temperatures. Thus, with increasing body weight (age), the temperature for maximum feeding shifts downwards. Feeding with cod or herring meat results in considerable changes in composition and calorific content of L. Limanda. The magnitude of these changes depends both on temperature and food quality. Food conversion efficiency values of herring-fed individuals are about 1 1/2 times higher than of cod-fed individuals. In series I and II, females are more efficient converters than males. In individuals weighing more than 50 g, conversion efficiency decreases in the order: 8°, 13°, 18° C; in smaller individuals this order is 13°, 18°, 8 °C. Conversion rate is about 2 to 5 times faster in individuals fed herring meat than those receiving cod meat. Conversion rate decreases in the order 13°, 8°, 18 °C in males, and in the order 18°, 13°, 8 °C in females; females of more than 80 g are exceptional in that they reach the maximum at 8 °C. From the data on food intake and food conversion, the biologically useful energy available for metabolism has been calculated for each test individual kept at 13° and 18 °C. At these temperature levels, the weight exponents are about 0.6; the a value or metabolic level for the 18 °C series is about 2 times higher than that at 13 °C. Thus, temperature affects metabolic rate but not the exponential value. The exponential value for the body weight-metabolism relation at 13 °C is for dab fed herring meat 0.9; the a value amounts to about half that for dab fed cod meat. Food quality, unlike temperature, alters not only the exponential value but also metabolic rate.     

Replacement males in the western bluebird: opportunity for paternity, chick-feeding rules, and fitness consequences of male parental care

In western bluebirds (Sialia mexicana), most pairs remain together for life and share equally in post-hatching parental care. We removed resident males of socially monogamous pairs during laying and after clutch completion to examine chick-feeding rules used by replacement males and current and future fitness consequences of paternal care. Replacement males were not infanticidal and 7 (47%) fed nestlings. Feeding replacement males and the females they joined fed at rates similar to controls. Females without a feeding replacement male compensated by feeding more themselves so that overall feeding rates were not compromised, but they reduced their brooding time. Unlike assisted females, unassisted females exhibited reduced nesting success and their 14-day-old chicks weighed less than controls. Field metabolic rates of unassisted females were 17% higher than those of control females, but the difference was not statistically significant. Older females were better able to raise young without the male's help than were yearling females. Female condition was not affected by male provisioning and unassisted females were as likely to survive to the next breeding season as assisted females. We found no future benefits of provisioning by replacement males; those that fed were no more likely to breed with the female on her subsequent attempt than were males that did not feed, and subsequent clutch sizes were not reduced for females rearing young without the male's help. These experiments suggest that male parental care increases nesting success in western bluebirds and that replacement males use an all-or-none rule to determine whether or not to feed chicks: if they are present during the fertile period they feed at typical rates; if they are not, they usually do not feed at all. Because chick-feeding by males is tied to opportunity for paternity, influences success in the current nest, and does not affect the male's future breeding success, it appears to be parental rather than mating effort. Received: 8 May 1998 / Accepted after revision: 23 July 1998