Long‐distance call evolution in the Felidae: effects of body weight,habitat, and phylogeny

Long‐distance calls used for mate attraction and territorial spacing are distinctive signals in the felid vocal repertoire. Their evolution is subject to natural and sexual selection, as well as various constraints. Body size is an important morphological constraint, with the scaling of the spectral characteristics of a species' vocalizations with its body size being established for several vertebrate groups. Alternatively, the structure of long‐distance calls may have been optimized for transmission in species' habitats (acoustic adaptation hypothesis). The present study assessed whether the mean dominant frequency of long‐distance calls in the Felidae (approximately 70% of all species incorporated) is influenced by the species' body size and/or conforms to the acoustic adaptation hypothesis. After controlling for phylogenetic relationships, we found a significant correlation between mean dominant frequency of a taxon's long‐distance calls and conditions for sound transmission in its habitat type (‘open/heterogeneous’ versus ‘dense’), although no significant influence of body size. Taxa living in more open habitat types have long‐distance calls with significantly lower mean dominant frequencies than those living in dense habitats. The result obtained in the present analysis is fairly robust against random removal of single or few taxa from the data, and also against the use of different branch‐length transformation models in phylogenetic regression. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 487–500.     

The origins of sexual dimorphism in body size in ungulates

Jarman (1974) proposed a series of relationships between habitat use, food dispersion, and social behavior and hypothesized a series of evolutionary steps leading to sexual dimorphism in body size through sexual selection in African antelope species. The hypothesis states that sexual size dimorphism evolved in a three-step process. Initially, ancestral monomorphic and monogamous ungulate species occupying closed habitats radiated into open grassland habitats. Polygynous mating systems then rapidly evolved in response to the aggregation of males and females, perhaps in relation to the clumped distribution of food resources in open habitats. Subsequently, size dimorphism evolved in those species occupying open habitats, but not in species that remained in closed habitats or retained monogamy. This hypothesis has played an important role in explaining the origins of sexual dimorphism in mammals. However, the temporal sequence of the events that Jarman proposed has never been demonstrated. Here we use a phylogeny of extant ungulate species, along with maximum-likelihood statistical techniques, to provide a test of Jarman's hypothesis.     

The effect of body size and habitat on the evolution of alarm vocalizations in rodents

When confronted with a predator, many mammalian species emit vocalizations known as alarm calls. Vocal structure variation results from the interactive effects of different selective pressures and constraints affecting their production, transmission, and detection. Body size is an important morphological constraint influencing the lowest frequencies that an organism can produce. The acoustic environment influences signal degradation; low frequencies should be favoured in dense forests compared to more open habitats (i.e. the ‘acoustic adaptation hypothesis’). Such hypotheses have been mainly examined in birds, whereas the proximate and ultimate factors affecting vocalizations in nonprimate mammals have received less attention. In the present study, we investigated the relationships between the frequency of alarm calls, body mass, and habitat in 65 species of rodents. Although we found the expected negative relationship between call frequency and body mass, we found no significant differences in acoustic characteristics between closed and open‐habitat species. The results of the present study show that the acoustic frequencies of alarm calls can provide reliable information about the size of a sender in this taxonomic group, although they generally do not support the acoustic adaptation hypothesis.     

Sexual segregation by Masai giraffes at two spatial scales

In this paper alternative explanations for observed patterns of sexual segregation by giraffes are examined at two spatial scales: within-habitats and within-landscape. Habitats are defined as recognizable plant associations and the landscape as the collection of all available habitat types. The study was conducted in Mikumi National Park, Tanzania. At the within-landscape level, all sex and age classes of giraffes exhibited high degrees of preference for riverine habitats. Sex differences in habitat selection were mostly due to females with young, who tended to select open floodplain habitats in which their vigilance time was lowest. Males, and females without young, preferred more heavily-wooded habitat. Habitat preferences were not related to observed habitat-specific forage intake rates for either males or females. Within habitats, male and female giraffes selected different feeding heights, males feeding higher in the canopy than females. Females showed a strong tendency to generalize with respect to feeding height. It is suggested that a sexual dimorphism–body size hypothesis provides a parsimonious explanation for the observed feeding height selection patterns, whereas a reproductive strategy hypothesis can explain sex-differences in habitat selection patterns within the landscape.     

Processes driving male breeding colour and ecomorphological diversification in rainbow skinks: a phylogenetic comparative test

We used a phylogenetic comparative approach to investigate the importance of ecological shifts in the diversification of both signalling traits and ecomorphological traits in a diverse group of Australian skinks (Carlia). First, we tested whether divergence in male breeding coloration is associated with shifts in habitat openness. Second, we examined whether the type or location of male breeding coloration changes predictably with habitat openness. Third, we tested the ecomorphological predictions that body size should vary in relation to habitat openness and that limb length, toe length and head depth should vary with substrate use. Divergence in male breeding coloration was positively associated with shifts in habitat openness. Our results also indicate that species occupying more open habitats tend to use male sexual signals located on lateral body regions and not necessarily on body regions that are potentially more concealed from aerial predators (e.g. chest and throat). With regard to ecomorphological traits, habitat openness appears to have no predictable influence on body size at the inter-specific level, contrary to expectations based on intra-specific studies. However, consistent with functional predictions, we found that preference for rocky habitats is associated with relatively longer hind limb length, presumably due to selection for greater speed and jumping ability on these substrates. Overall, results of this study support the hypothesis that ecological shifts play a central role in promoting morphological diversification.     

Habitat type and density influence vocal signal design in satin bowerbirds

1. This study provided a thorough test of the acoustic adaptation hypothesis using a within-species comparison of call structure involving a wide range of habitat types, an objective measure of habitat density and direct measures of habitat-related attenuation. 2. The structure of the bower advertisement call of the satin bowerbird was measured in 16 populations from throughout the species' range and related to the habitat type and density at each site. Transmission of white noise, pure tones and different bowerbird dialects was measured in five of six habitat types inhabited by satin bowerbirds. 3. Bowerbird advertisement call structure converged in similar habitats but diverged among different habitats; this pattern was apparent at both continent-wide and local geographical scales. Bowerbirds' call structures differed with changes in habitat density, consistent with the acoustic adaptation hypothesis. Lower frequencies and less frequency modulation were utilized in denser habitats such as rainforest and higher frequencies and more frequency modulation were used in the more open eucalypt-dominated habitats. 4. The white noise and pure tone transmission measurements indicated that different habitats varied in their sound transmission properties in a manner consistent with the observed variation in satin bowerbird vocalizations. 5. There was no effect of geographical proximity of recording locations, nor was there the predicted inverse relationship between frequency and body size. 6. These findings indicate that the transmission qualities of different habitats have had a major influence on variation in vocal phenotypes in this species. In addition, previously published molecular data for this species suggest that there is no effect of genetic relatedness on call similarity among satin bowerbird populations.     

The role of fecundity and sexual selection in the evolution of size and sexual size dimorphism in New World and Old World voles (Rodentia: Arvicolinae)

Evolutionary ecologists dating back to Darwin (1871) have sought to understand why males are larger than females in some species, and why females are the larger sex in others. Although the former is widespread in mammals, rodents and other small mammals usually exhibit low levels of sexual size dimorphism (SSD). Here, we investigate patterns of sexual dimorphism in 34 vole species belonging to the subfamily Arvicolinae in a phylogenetic comparative framework. We address the potential role of sexual selection and fecundity selection in creating sex differences in body size. No support was found for hyperallometric scaling of male body size to female body size. We observed a marginally significant relationship between SSD and the ratio of male to female home range size, with the latter being positively related to the level of intrasexual competition for mates. This suggests that sexual selection favours larger males. Interestingly, we also found that habitat type, but not mating system, constitutes a strong predictor of SSD. Species inhabiting open habitats – where males have extensive home ranges in order to gain access to as many females as possible – exhibit a higher mean dimorphism than species inhabiting closed habitats, where females show strong territoriality and an uniform distribution preventing males to adopt a territorial strategy for gaining copulations. Nonetheless, variation in the strength of sexual selection is not the only selective force shaping SSD in voles; we also found a positive association between female size and litter size across lineages. Assuming this relationship also exists within lineages (i.e. fecundity selection on female size), this suggests an additional role for variation in the strength of fecundity selection shaping interspecific differences in female size, and indirectly in SSD. Therefore our results suggest that different selective processes act on the sizes of males and females, but because larger size is favoured in both sexes, SSD is on average relatively small.     

Morphological variation in the Cape Dwarf Chameleon (Bradypodion pumilum) as a consequence of spatially explicit habitat structure differences

An organism's phenotype is to some extent influenced by costs and benefits in terms of natural and sexual selection. The intensity of natural selection can in part be driven by habitat structure, which may result in varying levels of crypsis and/or selection on traits related to maximizing performance in that habitat. This may be countered by sexual selection, which can lead to sexual dimorphism in body size and/or the expression of conspicuous ornamentation relating to maximizing reproductive success. The intensity of these forces can also be different between the sexes, resulting in complex patterns of phenotypic variation. With this in mind, we examined morphological variation within the Cape Dwarf Chameleon, Bradypodion pumilum. The species inhabits two geographically disjunct habitat types and, in the present study, we demonstrate that chameleons from the two habitats show morphological differences. Large, conspicuous individuals inhabit closed vegetation, whereas small, drab individuals inhabit open vegetation. However, when morphological traits are size‐adjusted, the open vegetation morph displays many traits that are larger for its body size than the closed vegetation morph, especially for characters related to locomotion (limbs) and bite force (head width). Sexual dimorphism is also present, although the degree and number of dimorphic characters was very different between the two morphs, with size‐adjusted male‐biased dimorphism much more pronounced in the closed morph. Overall, our findings suggest that natural selection in open habitats limits both body size and conspicuous characters, although sexual selection in closed habitats favours the development of ornamentation related to display. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 878–888.     

Body mass and habitat correlates of song structure in a primitive group of birds

We assessed relationships between acoustic frequency, body mass, and habitat in tinamous. This monophyletic group of primitive birds comprises c. 47 ground dwelling species whose habitats range from dense humid forest to open grasslands. The relationship between frequency and body mass was found to be negative, while the songs of open-habitat species exhibited higher frequencies and a wider bandwidth than the closed-habitat ones. Residual variation in song frequency, after controlling for the effect of body mass and phylogeny, tends to differ among habitats. However, a statistical test of this pattern was not possible because of the existence of only five pairs of sister species differing in habitat. In spite of this, positive contrasts of bandwidth were associated with positive contrasts of habitat, confirming that songs of open-habitat species have a wider bandwidth than those of their more closed habitat relatives.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 423–430.     

Sexual differences in morphology and niche utilization in an aquatic snake,Acrochordus arafurae

Filesnakes (Acrochordus arafurae) are large (to 2 m), heavy-bodied snakes of tropical Australia. Sexual dimorphism is evident in adult body sizes, weight/length ratios, and body proportions (relative head and tail lengths). Dimorphism is present even in neonates. Two hypotheses for the evolution of such dimorphism are (1) sexual selection or (2) adaptation of the sexes to different ecological niches. The hypothesis of sexual selection is consistent with general trends of sexually dimorphic body sizes in snakes, and accurately predicts, for A. arafurae, that the larger sex (female) is the one in which reproductive success increases most strongly with increasing body size. However, the sexual dimorphism in relative head sizes is not explicable by sexual selection.The hypothesis of adaptation to sex-specific niches predicts differences in habitats and/or prey. I observed major differences between male and female A. arafurae in prey types, prey sizes and habitat utilization (shallow versus deep water). Hence, the sexual dimorphism in relative head sizes is attributed to ecological causes rather than sexual selection. Nonetheless, competition between the sexes need not be invoked as the selective advantage of this character divergence. It is more parsimonious to interpret these differences as independent adaptations of each sex to increase foraging success, given pre-existing sexually-selected differences in size, habitat or behavior. Data for three other aquatic snake species, from phylogenetically distant taxa, suggest that sexual dimorphism in food habits, foraging sites and feeding morphology, is widespread in snakes.     

Rainforest habitat resistance to the migration of Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) in south-eastern Queensland

Abstract  This paper tests the hypothesis that habitat differences affect the migratory ability of the Chilean predatory mite, Phytoseiulus persimilis , an introduced biological control agent of the spider mite, Tetranychus urticae . It is suggested that habitat resistance accounts for the species' inability to invade rainforests in south-eastern Queensland, Australia. Like its prey, P. persimilis migrates to distant plants on air currents. To test our hypothesis, populations of the Chilean predatory mite were established on potted bean plants in both remnant rainforest and adjacent open fields, and their migration monitored using sticky traps. Overall it was found that prey populations on leaves were similar in both habitats, but those of predators were about 20% lower in rainforest. However, the numbers of both predators and prey caught on sticky traps in rainforest were about 6% and 25%, respectively, of those caught in open fields, indicating a strongly reduced rate of aerial migration in the forest. The number of P. persimilis caught on the sticky traps increased with increasing populations of predators on foliage. Thus, dense vegetation inhibits the movement of air currents and inhibits colonisation by both predators and, to a lesser extent, spider mites. These results suggest that the inhibition of aerial migration is one reason for lower numbers of P. persimilis in forest habitats, both because its own vagility is restricted, and because its prey is less able to disperse.     

Relating habitat and climatic niches in birds

Predicting species' responses to the combined effects of habitat and climate changes has become a major challenge in ecology and conservation biology. However, the effects of climatic and habitat gradients on species distributions have generally been considered separately. Here, we explore the relationships between the habitat and thermal dimensions of the ecological niche in European common birds. Using data from the French Breeding Bird Survey, a large-scale bird monitoring program, we correlated the habitat and thermal positions and breadths of 74 bird species, controlling for life history traits and phylogeny. We found that cold climate species tend to have niche positions in closed habitats, as expected by the conjunction of the biogeographic history of birds' habitats, and their current continent-scale gradients. We also report a positive correlation between thermal and habitat niche breadths, a pattern consistent with macroecological predictions concerning the processes shaping species' distributions. Our results suggest that the relationships between the climatic and habitat components of the niche have to be taken into account to understand and predict changes in species' distributions.     

Correlates of extinction risk of birds from two Indonesian islands

Size of distributional range, position in the range, body size and diet are some of the ecological traits that may correlate with local abundance. Evolutionary phenomena such as taxon cycles, acting over much greater time periods, may also influence abundance and promote species extinction. This paper assesses which of a wide range of ecological and historic traits best predict the variation in abundance of tropical forest birds on Sumba and Buru islands in Wallacea (Indonesia). In addition we seek to determine which traits predict species' ability to adapt to secondary or logged forest. The most important correlates of both abundance and ability to transfer were those related to the evolutionary history of the species within the Wallacean Archipelago and not the traits that were more directly related to species ecology. These relationships are maintained when allowance is made for phylogenetic relationships. Our interpretation of the results is that recent colonists to an island are initially rare in the indigenous forest habitat but concomitant with an adaptation to local conditions they gradually become more abundant and taxonomically distinct from other populations of the same species. These results apparently contradict the taxon cycle hypothesis but this may be a result of our focus on indigenous forest habitats rather than on a wider range dominated by anthropogenic ones.     

Sex differences in habitat use by African elephants (Loxodonta africana) in the Okavango Delta,Botswana: is size really the deciding factor?

Many sexually dimorphic mammals show sexual segregation of habitat use. We studied habitat selection and use by male and female elephants in the Okavango Delta, and males of different ages and therefore sizes, to assess whether size was a driving force behind any sex differences in habitat selectivity. There was variation in habitat choices, with males preferring island vegetation and mopane woodland and avoiding grassland/floodplain and Terminalia woodland; females showed no selection for or against most habitats other than selecting for mopane woodland in the rainy season. Male habitat choice changed dramatically during the dry season, when resources were most limited and there was the least sexual difference in habitat selection. Females were more selective in the flood season, when access to resources was restricted. Contrary to other studies and the forage selection hypothesis, males showed greater habitat selectivity than females, but age, and therefore size, did not affect their habitat selection. Whilst our data suggested that males were superior competitors, we could not discount that females excluded males from preferred habitats. In the Okavango Delta, habitat selection by male elephants may be more dependent on social groupings, and hence the decisions of others, than individual size and energetic requirements.     

Body mass explains characteristic scales of habitat selection in terrestrial mammals

Niche theory in its various forms is based on those environmental factors that permit species persistence, but less work has focused on defining the extent, or size, of a species' environment: the area that explains a species' presence at a point in space. We proposed that this habitat extent is identifiable from a characteristic scale of habitat selection, the spatial scale at which habitat best explains species' occurrence. We hypothesized that this scale is predicted by body size. We tested this hypothesis on 12 sympatric terrestrial mammal species in the Canadian Rocky Mountains. For each species, habitat models varied across the 20 spatial scales tested. For six species, we found a characteristic scale; this scale was explained by species' body mass in a quadratic relationship. Habitat measured at large scales best-predicted habitat selection in both large and small species, and small scales predict habitat extent in medium-sized species. The relationship between body size and habitat selection scale implies evolutionary adaptation to landscape heterogeneity as the driver of scale-dependent habitat selection.     

Body size distributions of large Costa Rican dry forest moths and the underlying relationship between plant and pollinator morphology

There has been much recent interest in explaining patterns of body size variation within species assemblages. One observation is that frequency distributions of species' body size commonly exhibit a right-skew, even on a logarithmic scale. Here we examine the species' body size distributions in two assemblages of large Costa Rican moths. We find that neither adult Sphingidae or Saturniidae exhibit the classic log right-skewed pattern. Furthermore, the species' body size distributions in these two groups are markedly different, which we suggest is a result of differential selective pressures related to resource and mate acquisition. For Sphingidae, we show (1) that body size is positively correlated with tongue length, and (2) that the distribution of sphingid body sizes/tongue lengths closely matches the distribution of flower corolla tube depths in sphingid-pollinated plants. Thus, morphological fitting between plants and pollinators seems to underlie the species' body size distribution of this sphingid assemblage. We discuss the significance of these results in the context of current theory on mechanisms driving species' body size distributions. Finally, we present an evolutionary hypothesis for the diversity of body sizes seen in this sphingid assemblage related to reciprocal interactions between plants and pollinators. This hypothesis can be tested within a rigorous phylogenetic framework, although a systematic phylogenetic analysis of Neotropical Sphingidae does not currently exist.     

Abiotic noise,call frequency and stream-breeding anuran assemblages

Environmental noise can be an important selective force modulating signal evolution in species with acoustic communication. Many anuran species breed alongside streams; hence, the sound produced by the flowing water is an important source of noise for acoustic communication. Since calling is physiologically very expensive in anurans, and communication is essential for reproduction, we expected adaptations that reduce environmental masking effects and allow acoustic communication in streamside breeders. This basic assumption of the acoustic adaptation hypothesis has not been yet evaluated at a large phylogenetic scale. We combined ahistorical and phylogenetic methods to test whether anuran species that breed alongside streams call at higher frequencies than species that breed away from streams. We compiled primary and secondary data on body size, breeding habitat, and the dominant frequency of the advertisement call for 110 species; 40 of them breed alongside streams and 70 away from streams. Call frequency was slightly higher and body size was significantly smaller in streamside breeding species. After controlling for the effects of body size and phylogenetic signal, only differences in body size persisted between species breeding at both kinds of habitats. Our data suggest that habitat filtering rather than acoustic adaptation explains the high call frequency of stream breeders. Species with large body size, pleiotropically constrained to utter low-frequency calls, would have succeeded less often in establishing viable populations alongside streams, due to the masking effect of low-frequency noise. Thus, small species calling at relatively high frequencies would be more common there. Although our data do not preclude adaptations to noisy habitats in some anuran species, they do not provide support for the acoustic adaptation hypothesis at a wider phylogenetic scale.     

Using size distributions to detect nutrient and sediment effects within and between habitats

We studied the use of size distributions as a response variable in limnological experiments. Previous quantifications of size distributions were incomplete or difficult to use in experimental settings, and we developed a multivariate approach that more fully describes the shape and biomass of planktonic and benthic size distributions. We re-evaluate the hypothesis that fish affect the shape and nutrients affect the biomass of size distributions, and show that the multivariate approach is more responsive to detecting treatment effects. In a mesocosm experiment, we use this new quantification and analysis of size distributions to detect the main and interactive effects of nutrient addition and sediment type on both benthic and pelagic size distributions. Size distributions in both habitats responded to the nutrient and sediment treatments, indicating linkage since a treatment applied in one habitat affected the size distribution in the opposite habitat. Since size distributions reduce each habitat into a common currency, we were able to examine the nature of the linkage. The relative response of each habitat to the nutrient treatment was different with regard to the shape of the distributions, while the relative response to the sediment treatment was different with regard to the biomass in the distributions.     

Habitat complexity drives experimental evolution of a conditionally expressed secondary sexual trait

The conditional expression of alternative phenotypes underlies the production of almost all life history decisions and many dichotomous traits, including male alternative reproductive morphs and behavioral tactics. Changes in tactic fitness should lead to evolutionary shifts in developmental switch points that underlie tactic expression. We used experimental evolution to directly test this hypothesis by rearing ten generations of the male-dimorphic mite Rhizoglyphus echinopus in either simple or three-dimensionally complex habitats that differed in their effects on morph fitness. In R. echinopus, fighter males develop weapons used for killing rivals, whereas scrambler males do not. Populations evolving in complex 3D habitats, where fighters had reduced fitness, produced fewer fighters because the switch point for fighter development evolved to a larger critical body size. Both the reduced mobility of fighter males and the altered spatial distribution of potential mates and rivals in the complex habitat were implicated in the evolutionary divergence of switch point between the habitats. Our results demonstrate how abiotic factors like habitat complexity can have a profound effect on evolution through sexual selection.     

Sexual segregation in ungulates: a comparative test of three hypotheses

In most social ungulate species, males are larger than females and the sexes live in separate groups outside the breeding season. It is important for our understanding of the evolution of sociality to find out why sexual segregation is so widespread not only in ungulates but also in other mammals. Sexual body size dimorphism was proposed as a central factor in the evolution of sexual segregation in ungulates. We tested three hypotheses put forward to explain sexual segregation: the predation-risk, the forage-selection, and the activity budget hypothesis. We included in our analyses ungulate species ranging from non-dimorphic to extremely dimorphic in body size. We observed oryx, zebra, bighorn sheep and ibex in the field and relied on literature data for 31 additional species. The predation-risk hypothesis predicts that females will use relatively predator-safe habitats, while males are predicted to use habitats with higher predation risk but better food quality. Out of 24 studies on different species of ungulates, females and their offspring chose poorer quality but safer habitat in only eight cases. The forage-selection hypothesis predicts that females would select habitat based on food quality, while males should prefer high forage biomass. In fact, females selected higher quality food in only six out of 18 studies where males and females segregated, in eight studies there was no difference in forage quality and in four studies males were in better quality habitat. The activity budget hypothesis predicts that with increasing dimorphism in body size males and females will increasingly differ in the time spent in different activities. Differences in activity budgets would make it difficult for males and females to stay in mixed-sex groups due to increased costs of synchrony to maintain group cohesion. The predictions of the activity budget hypothesis were confirmed in most cases (22 out of 23 studies). The heavier males were compared to females, the more time females spent foraging compared to males. The bigger the dimorphism in body mass, the more males spent time walking compared to females. Lactating females spent more time foraging than did non-lactating females or males. Whether species were mainly bulk or intermediate feeders did not affect sexual differences in time spent foraging. We conclude that sexual differences in activity budgets are most likely driving sexual segregation and that sexual differences in predation risk or forage selection are additive factors.