Seasonal Food Habits of Five Sympatric Forest Microchiropterans in Western Madagascar

Limited Physiological Compensation in Response to an Acute Microclimate Change in a Malagasy Bat

Rapid environmental changes are challenging for endothermic species because they have direct and immediate impacts on their physiology by affecting microclimate and fundamental resource availability. Physiological flexibility can compensate for certain ecological perturbations, but our basic understanding of how species function in a given habitat and the extent of their adaptive scope is limited. Here we studied the effect of acute, experimental microclimate change on the thermal physiology of two populations of the widespread Malagasy bat, Macronycteris commersoni. Populations of this species are found roosting under contrasting conditions, i.e., in a constant hot and humid cave or below foliage unprotected from fluctuations in ambient conditions. We exposed free-ranging individuals of each population to the respective opposite condition and thus to novel microclimate within an ecologically realistic scope while measuring metabolic rate and skin temperature. Cave bats in forest setting had a limited capacity to maintain euthermia to the point that two individuals became hypothermic when ambient temperature dropped below their commonly experienced cave temperature. Forest bats on the other hand, had difficulties to dissipate heat in the humid cave set-up. The response to heat, however, was surprisingly uniform and all bats entered torpor combined with hyperthermia at temperatures exceeding their thermoneutral zone. Thus, while we observed potential for flexible compensation of heat through “hot” torpor, both populations showed patterns suggestive of limited potential to cope with acute microclimate changes deviating from their typically occupied roosts. Our study emphasizes that intraspecific variation among populations could be misleading when assessing species’ adaptive scopes, as variation may arise from genetic adaptation, developmental plasticity or phenotypic flexibility, all of which allow for compensatory responses at differing time scales. Disentangling these mechanisms and identifying the basis of variation is vital to make accurate predictions of species’ chances for persisting in ever rapidly changing habitats and climates.

Disparate roost sites drive intraspecific physiological variation in a Malagasy bat

Many species are widely distributed and individual populations can experience vastly different environmental conditions over seasonal and geographic scales. With such a broad ecological reality, datasets with limited spatial and temporal resolution may not accurately represent a species and could lead to poorly informed management decisions. Because physiological flexibility can help species tolerate environmental variation, we studied the physiological responses of two separate populations of Macronycteris commersoni, a bat widespread across Madagascar, in contrasting seasons. The populations roost under the following dissimilar conditions: either a hot, well-buffered cave or within open foliage, unprotected from the local weather. We found that flexible torpor patterns, used in response to prevailing ambient temperature and relative humidity, were central to keeping energy budgets balanced in both populations. While bats’ metabolic rate during torpor and rest did not differ between roosts, adjusting torpor frequency, duration and timing helped bats maintain body condition. Interestingly, the exposed forest roost induced extensive use of torpor, which exceeded the torpor frequency of overwintering bats that stayed in the cave for months and consequently minimised daytime resting energy expenditure in the forest. Our current understanding of intraspecific physiological variation is limited and physiological traits are often considered to be fixed. The results of our study therefore highlight the need for examining species at broad environmental scales to avoid underestimating a species’ full capacity for withstanding environmental variation, especially in the face of ongoing, disruptive human interference in natural habitats.

Simplified Communities of Seed-Dispersers Limit the Composition and Flow of Seeds in Edge Habitats

Edge effects, driven by human modification of landscapes, can have critical impacts on ecological processes such as species interactions, with cascading impacts on biodiversity as a whole. Characterizing how edges affect vital biotic interactions such as seed dispersal by frugivores is important for better understanding potential mechanisms that drive species coexistence and diversity within a plant community. Here, we investigated how differences between frugivore communities at the forest edge and interior habitats of a diverse tropical rainforest relate to patterns of animal-mediated seed dispersal and early seedling recruitment. We found that the lemur communities across the forest edge-interior gradient in this system showed the highest species richness and variability in body sizes at intermediate distances; the community of birds showed the opposite pattern for species richness. Three large-bodied frugivores, known to be effective dispersers of large seeds, tended to avoid the forest edge. As result, the forest edges received a lower rate of animal-mediated seed dispersal compared to the interior habitats. In addition, we also found that the seeds that were actively dispersed by animals in forest edge habitats were smaller in size than seeds dispersed in the forest interior. This pattern was found despite a similarity in seed size of seasonally fruiting adult trees and shrubs between the two habitats. Despite these differences in dispersal patterns, we did not observe any differences in the rates of seedling recruitment or seed-size distribution of successful recruit species. Our results suggest that a small number of frugivores may act as a potential biotic filter, acting on seed size, for the arrival of certain plant species to edge habitats, but other factors may be more important for driving recruitment patterns, at least in the short term. Further research is needed to better understand the potential long-term impacts of altered dispersal regimes relative to other environmental factors on the successional dynamics of edge communities. Our findings are important for understanding potential ecological drivers of tree community changes in forest edges and have implications for conservation management and restoration of large-seeded tree species in disturbed habitats.

Comparative echolocation and foraging ecology of horseshoe bats (Rhinolophidae) and Old World leaf-nosed bats (Hipposideridae)

Horseshoe (Rhinolphidae) and Old World leaf-nosed (Hipposideridae) bats are high duty cycle (HDC) echolocators sharing a suite of adaptations including long duration signals relative to their signal periods, peak energy concentrated in a narrow spectral band dominated by a constant frequency (CF) component, ‘auditory fovea’ (over-representation and sharp tuning of neurons responsible for frequencies at or around the CF) and ability to compensate for Doppler shifts in echoes. HDC bats separate signals from returning echoes in the frequency domain. Rhinolophids are more specialised neurobiologically than hipposiderids, producing longer duration signals at higher duty cycles, and have narrowly tuned auditory fovea and almost full Doppler shift compensation. Here, I examine whether these differences have produced ecological divergence between the families by testing predictions of differences in prey perception, prey capture behaviour, foraging habitat and diet. I found no discernible differences in these variables between the two families. Rhinolophids and hipposiderids both forage close to vegetation, capture prey by aerial hawking and gleaning from surfaces, and consume mostly flying insects with spiders and terrestrial, flightless arthropods taken occasionally. The data presented here show that the two families are similar in foraging ecology despite differences in echolocation and audition.

Phylogeography and population genetics of the endemic Malagasy bat, Macronycteris commersoni s.s. (Chiroptera: Hipposideridae)

Macronycteris commersoni (Hipposideridae), a bat species endemic to Madagascar, is widespread across the island and utilizes a range of habitat types including open woodland, degraded habitats, and forested areas from sea level to 1,325 m. Despite being widely distributed, there is evidence that M. commersoni exhibits morphological and bioacoustic variation across its geographical range. We investigated the fine-scale phylogeographic structure of populations in the western half of the island using extensive spatial sampling and sequence data from two mitochondrial DNA regions. Our results indicated several lineages within M. commersoni. Individuals collected from northern Madagascar formed a single monophyletic clade (clade C). A second clade (clade B) included individuals collected from the south-western portion of the island. This second clade displayed more phylogeographical partitioning with differences in mtDNA haplotypes frequency detected between populations collected in different bioclimatic regions. Lineage dispersal, genetic divergence, and timing of expansion events of M. commersoni were probably associated with Pleistocene climate fluctuations. Our data suggest that the northern and the central western regions of Madagascar may have acted as refugia for this species during periods of cooler and drier climate conditions associated with the Pleistocene.

Short and hyperthermic torpor responses in the Malagasy bat Macronycteris commersoni reveal a broader hypometabolic scope in heterotherms

The energy budgets of animal species are closely linked to their ecology, and balancing energy expenditure with energy acquisition is key for survival. Changes in animals' environments can be challenging, particularly for bats, which are small endotherms with large uninsulated flight membranes. Heterothermy is a powerful response used to cope with changing environmental conditions. Recent research has revealed that many tropical and subtropical species are heterothermic and display torpor with patterns unlike those of "classical" heterotherms from temperate and arctic regions. However, only a handful of studies investigating torpor in bats in their natural environment exist. Therefore, we investigated whether the Malagasy bat Macronycteris commersoni enters torpor in the driest and least predictable region in Madagascar. We examined the energy balance and thermal biology of M. commersoni in the field by relating metabolic rate (MR) and skin temperature (T_skin) measurements to local environmental characteristics in the dry and rainy seasons. Macronycteris commersoni entered torpor and showed extreme variability in torpor patterns, including surprisingly short torpor bouts, lasting on average 20 min, interrupted by MR peaks. Torpid MR was remarkably low (0.13 ml O₂ h^-1 g^-1), even when T_skin exceeded that of normothermia (41 °C). Macronycteris commersoni is thus physiologically capable of (1) entering torpor at high ambient temperature and T_skin and (2) rapidly alternating between torpid and normothermic MR resulting in very short bouts. This suggests that the scope of hypometabolism amongst heterothermic animals is broader than previously assumed and underlines the importance of further investigation into the torpor continuum.

Cryptic diversity in Hipposideros commersoni sensu stricto (Chiroptera: Hipposideridae) in the western portion of Madagascar

Background

The Commerson’s leaf-nosed bat, Hipposideros commersoni sensu stricto, is endemic to Madagascar and is relatively common in the western portion of the island, where it is found in areas, including forested zones, from sea level to 1325 m. A previous study on morphological patterns of geographic variation within the species highlighted the presence of two distinct morphotypes; larger individuals in the north portion of the island and smaller individuals in the south. The main aim of this study was to use a combination of craniodental morphology and molecular data (mitochondrial and nuclear) to test previous hypotheses based on morphology and clarify the evolutionary history of the species group.

Methods

We sequenced mitochondrial and nuclear genes from Hipposideros commersoni obtained from the western portion of Madagascar, and compared them with other African species as outgroups. We analyzed the sequence data using Maximum Likelihood and Bayesian phylogenetic inference. Divergence dates were estimated using Bayesian molecular clock approach. Variation in craniodental variables was also assessed from sequenced individuals.

Results

The molecular analyses suggest that H. commersoni is not monophyletic, with strong support for the presence of several independently evolving lineages. Two individuals amongst those sequenced from Isalo (south central) and Itampolo (southwest) form a separate clade (Clade A), distinct from other H. commersoni, and sister to continental African H. vittatus and H. gigas. Within the H. commersoni clade, the molecular data support two geographically distributed clades; one from the south (Clade B) and the other from the north (Clade C), which diverged approximately 3.38 million years ago. Morphometric data were consistent with the molecular analyses, suggesting a north–south break within H. commersoni. However, at some localities, animals from both clades occurred in sympatry and these individuals could not be differentiated based on external and craniodental measurements.

Conclusions

Using a combination of molecular and morphological characters, this study presents evidence of cryptic diversity in H. commersoni on Madagascar. Further fine-scale phylogeographic studies are needed to fully resolve the systematics of H. commersoni. This study highlights the utility of the combined approach in employing both morphological and molecular data to provide insights into the evolutionary history of Malagasy population currently assigned to H. commersoni.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0510-2) contains supplementary material, which is available to authorized users.

Coexistence of morphologically similar bats (Vespertilionidae) on Madagascar: stable isotopes reveal fine-grained niche differentiation among cryptic species

Based on niche theory, closely related and morphologically similar species are not predicted to coexist due to overlap in resource and habitat use. Local assemblages of bats often contain cryptic taxa, which co-occur despite notable similarities in morphology and ecology. We measured in two different habitat types on Madagascar levels of stable carbon and nitrogen isotopes in hair (n = 103) and faeces (n = 57) of cryptic Vespertilionidae taxa to indirectly examine whether fine-grained trophic niche differentiation explains their coexistence. In the dry deciduous forest (Kirindy), six sympatric species ranged over 6.0‰ in δ15N, i.e. two trophic levels, and 4.2‰ in δ13C with a community mean of 11.3‰ in δ15N and −21.0‰ in δ13C. In the mesic forest (Antsahabe), three sympatric species ranged over one trophic level (δ15N: 2.4‰, δ13C: 1.0‰) with a community mean of 8.0‰ δ15N and −21.7‰ in δ13C. Multivariate analyses and residual permutation of Euclidian distances in δ13C–δ15N bi-plots revealed in both communities distinct stable isotope signatures and species separation for the hair samples among coexisting Vespertilionidae. Intraspecific variation in faecal and hair stable isotopes did not indicate that seasonal migration might relax competition and thereby facilitate the local co-occurrence of sympatric taxa.

Trophic niche differentiation and microhabitat utilization revealed by stable isotope analyses in a dry-forest bat assemblage at Ankarana, northern Madagascar

Bats are important components in tropical mammal assemblages. Unravelling the mechanisms allowing multiple syntopic bat species to coexist can provide insights into community ecology. However, dietary information on component species of these assemblages is often difficult to obtain. Here we measured stable carbon and nitrogen isotopes in hair samples clipped from the backs of 94 specimens to indirectly examine whether trophic niche differentiation and microhabitat segregation explain the coexistence of 16 bat species at Ankarana, northern Madagascar. The assemblage ranged over 4.4‰ in δ15N and was structured into two trophic levels with phytophagous Pteropodidae as primary consumers (c. 3‰ enriched over plants) and different insectivorous bats as secondary consumers (c. 4‰ enriched over primary consumers). Bat species utilizing different microhabitats formed distinct isotopic clusters (metric analyses of δ13C–δ15N bi-plots), but taxa foraging in the same microhabitat did not show more pronounced trophic differentiation than those occupying different microhabitats. As revealed by multivariate analyses, no discernible feeding competition was found in the local assemblage amongst congeneric species as compared with non-congeners. In contrast to ecological niche theory, but in accordance with studies on New and Old World bat assemblages, competitive interactions appear to be relaxed at Ankarana and not a prevailing structuring force.

Bats of the Western Indian Ocean Islands

The natural colonisation of many remote oceanic islands by bats, including those of the western Indian Ocean, has been facilitated by their unique capability among mammals for powered flight. In the western Indian Ocean region, only the Malagasy islands of Madagascar and the Comoros archipelago have been naturally colonised by non-volant mammals. Despite their greater potential for inter-island dispersal, and thus gene transfer, endemicity of Chiroptera in the western Indian Ocean islands is high. Given their vulnerability to stochastic and anthropogenic disturbances, greater focus needs to be placed on investigating the demographic and ecological history of bats on Western Indian Ocean islands to safeguard not only their future, but also the ecosystem functioning on these islands, for which they are undoubtedly such an integral part. Here, I summarise the taxonomic and life history information available on bats from Western Indian Ocean islands and highlight knowledge gaps and conservation issues that threaten the continued persistence of some species.