Bergmann's rule; a concept cluster?

The science behind ecology has been contested for years, partially because of the misuse and misrepresentation of concepts within ecology. This paper discusses the use of Bergmann's rule, a fundamental rule of biogeography. The rule was proposed by Carl Bergmann in 1847 and was published only in German; therefore, the majority of researchers have relied on a single translation by Mayr suggesting that races from cooler climates tend to be larger in species of warm-blooded vertebrates than races of the same species living in warmer climates. That many scientists cannot go back to the original source of information because it has not been published in English has resulted in relying on others for interpretation and led to several problems, the largest of which is whether the definition of the rule should include the mechanism, which had been proposed by Bergmann. There has been a large field of research on the subject, but few tests of the mechanisms behind the observed phenomenon. We conducted a review of the literature on Bergmann's rule, and from this suggest (1) Bergmann's original rule be maintained (a direct translation is provided), (2) mechanism is inherent in Bergmann's rule and is required for a rule to be of scientific value; patterns should be labelled as trends, not rules, (3) the focus should be on falsifying hypothesized mechanisms rather than simply describing patterns, and (4) to truly evaluate Bergmann's rule in a scientific manner the original German source should be translated and made available to the scientific public.     

Is there a role for amplifiers in sexual selection?

The amplifier hypothesis states that selection could favour the evolution of traits in signallers that improve the ability of receivers to extract honest information from other signals or cues. We provide a formal definition of amplifiers based on the receiver's mechanisms of signal perception and we present a game-theoretical model in which males advertise their quality and females use sequential-sampling tactics to choose among prospective mates. The main effect of an amplifier on the female mating strategy is to increase her mating threshold, making the female more selective as the effectiveness of the amplifier increases. The effects of the amplifier on male advertising strategy depends both on the context and on the types of the amplifier involved. We consider two different contexts for the evolution of amplifiers (when the effect of amplifiers is on signals and when it is on cues) and two types of amplifiers (the ‘neutral amplifier’, when it improves quality assessment without altering male attractiveness, and the ‘attractive amplifier’, when it improves both quality assessment and male attractiveness). The game-theoretical model provides two main results. First, neutral and attractive amplifiers represent, respectively, a conditional and an unconditional signalling strategy. In fact, at the equilibrium, neutral amplifiers are displayed only by males whose advertising level lays above the female acceptance threshold, whereas attractive amplifiers are displayed by all signalling males, independent of their quality. Second, amplifiers of signals increase the differences in advertising levels between amplifying and not-amplifying males, but they decrease the differences within each group, so that the system converges towards an ‘all-or-nothing’ signalling strategy. By applying concepts from information theory, we show that the increase in information transfer at the perception level due to the amplifier of signals is contrasted by a decrease in information transfer at the emitter level due to the increased stereotypy of male advertising strategy.     

Acoustic divergence in two cryptic Hipposideros species: a role for social selection?

We present evidence that a relatively widespread and common bat from South East Asia comprises two morphologically cryptic but acoustically divergent species. A population of the bicoloured leaf-nosed bat (Hipposideros bicolor) from Peninsular Malaysia exhibits a bimodal distribution of echolocation call frequencies, with peaks in the frequency of maximum energy at ca. 131 and 142 kHz. The two phonic types are genetically distinct, with a cytochrome b sequence divergence of just under 7%. We consider the mechanisms by which acoustic divergence in these species might arise. Differences in call frequency are not likely to effect resource partitioning by detectable prey size or functional range. However, ecological segregation may be achieved by differences in microhabitat use; the 131kHz H. bicolor is characterized by significantly longer forearms, lower wing loading, a lower aspect ratio and a more rounded wingtip, features that are associated with greater manoeuvrability in flight that may enable it to forage in more cluttered environments relative to the 142 kHz phonic type. We suggest that acoustic divergence in these species is a consequence of social selection for a clear communication channel, which is mediated by the close link between the acoustic signal and receptor systems imposed by the highly specialized nature of the hipposiderid and rhinolophid echolocation system.     

Do marine phytoplankton follow Bergmann's rule sensu lato?

Global warming has revitalized interest in the relationship between body size and temperature, proposed by Bergmann's rule 150 years ago, one of the oldest manifestations of a ‘biogeography of traits’. We review biogeographic evidence, results from clonal cultures and recent micro‐ and mesocosm experiments with naturally mixed phytoplankton communities regarding the response of phytoplankton body size to temperature, either as a single factor or in combination with other factors such as grazing, nutrient limitation, and ocean acidification. Where possible, we also focus on the comparison between intraspecific size shifts and size shifts resulting from changes in species composition. Taken together, biogeographic evidence, community‐level experiments and single‐species experiments indicate that phytoplankton average cell sizes tend to become smaller in warmer waters, although temperature is not necessarily the proximate environmental factor driving size shifts. Indirect effects via nutrient supply and grazing are important and often dominate. In a substantial proportion of field studies, resource availability is seen as the only factor of relevance. Interspecific size effects are greater than intraspecific effects. Direct temperature effects tend to be exacerbated by indirect ones, if warming leads to intensified nutrient limitation or copepod grazing while ocean acidification tends to counteract the temperature effect on cell size in non‐calcifying phytoplankton. We discuss the implications of the temperature‐related size trends in a global‐warming context, based on known functional traits associated with phytoplankton size. These are a higher affinity for nutrients of smaller cells, highest maximal growth rates of moderately small phytoplankton (ca. 10² µm³), size‐related sensitivities for different types of grazers, and impacts on sinking rates. For a phytoplankton community increasingly dominated by smaller algae we predict that: (i) a higher proportion of primary production will be respired within the microbial food web; (ii) a smaller share of primary production will be channeled to the classic phytoplankton – crustacean zooplankton – fish food chain, thus leading to decreased ecological efficiency from a fish‐production point of view; (iii) a smaller share of primary production will be exported through sedimentation, thus leading to decreased efficiency of the biological carbon pump.     

Patterns of size variation in bees at a continental scale: does Bergmann's rule apply?

Body size latitudinal clines have been widley explained by the Bergmann's rule in homeothermic vertebrates. However, there is no general consensus in poikilotherms organisms in particular in insects that represent the large majority of wildlife. Among them, bees are a highly diverse pollinators group with high economic and ecological value. Nevertheless, no comprehensive studies of species assemblages at a phylogenetically larger scale have been carried out even if they could identify the traits and the ecological conditions that generate different patterns of latitudinal size variation. We aimed to test Bergmann's rule for wild bees by assessing relationships between body size and latitude at continental and community levels. We tested our hypotheses for bees showing different life history traits (i.e. sociality and nesting behaviour). We used 142 008 distribution records of 615 bee species at 50 × 50 km (CGRS) grids across the West Palearctic. We then applied generalized least squares fitted linear model (GLS) to assess the relationship between latitude and mean body size of bees, taking into account spatial autocorrelation. For all bee species grouped, mean body size increased with higher latitudes, and so followed Bergmann's rule. However, considering bee genera separately, four genera were consistent with Bergmann's rule, while three showed a converse trend, and three showed no significant cline. All life history traits used here (i.e. solitary, social and parasitic behaviour; ground and stem nesting behaviour) displayed a Bergmann's cline. In general there is a main trend for larger bees in colder habitats, which is likely to be related to their thermoregulatory abilities and partial endothermy, even if a ‘season length effect’ (i.e. shorter foraging season) is a potential driver of the converse Bergmann's cline particularly in bumblebees.     

Extrapair mating between relatives in the barn swallow: a role for kin selection?

Why do females of many species mate with more than one male? One of the main hypotheses suggests that female promiscuity is an insurance mechanism against the potential detrimental effects of inbreeding. Accordingly, females should preferably mate with less related males in multiple or extrapair mating. Here we analyse paternity, relatedness among mating partners, and relatedness between parents and offspring, in the socially monogamous North American barn swallow (Hirundo rustica erythrogaster). In contrast to the inbreeding avoidance hypothesis, we found that extrapair mating partners were more related than expected by random choice, and tended to be more related than social partners. Furthermore, extrapair mating resulted in genetic parents being more related to their extrapair young than to their withinpair young. We propose a new hypothesis for extrapair mating based on kin selection theory as a possible explanation to these findings.     

The evolution of preference strength under sensory bias: a role for indirect selection?

Evidence suggests that female preferences may sometimes arise through sensory bias, and that males may subsequently evolve traits that increase their conspicuousness to females. Here, we ask whether indirect selection, arising through genetic associations (linkage disequilibrium) during the sexual selection that sensory bias imposes, can itself influence the evolution of preference strength. Specifically, we use population genetic models to consider whether or not modifiers of preference strength can spread under different ecological conditions when female mate choice is driven by sensory bias. We focus on male traits that make a male more conspicuous in certain habitats-and thus both more visible to predators and more attractive to females-and examine modifiers of the strength of preference for conspicuous males. We first solve for the rate of spread of a modifier that strengthens preference within an environmentally uniform population; we illustrate that this spread will be extremely slow. Second, we used a series of simulations to consider the role of habitat structure and movement on the evolution of a modifier of preference strength, using male color polymorphisms as a case study. We find that in most cases, indirect selection does not allow the evolution of stronger or weaker preferences for sensory bias. Only in a "two-island" model, where there is restricted migration between different patches that favor different male phenotypes, did we find that preference strength could evolve. The role of indirect selection in the evolution of sensory bias is of particular interest because of ongoing speculation regarding the role of sensory bias in the evolution of reproductive isolation.     

Synchrony: a neuronal mechanism for attentional selection?

Attentional selection involves brain processes that select and control the flow of information into the mechanisms that underlie perception and consciousness. One theory proposes that the neural activity that represents the stimuli or events to be attended to is selected through modification of its synchrony. Recent experimental evidence supports this theory, by showing that changes in attentional focus increase the synchrony of neural firing in some neuron pairs and decrease it in others.     

Cortical maps: a role for astrocytes?

Astrocytes are a multifunctional cell type in the nervous system that can influence neurons and synapses in numerous ways. Astrocytes have been suggested to play important roles in synapse formation during development, as well as in multiple forms of synaptic plasticity in the developing and adult brain. Astrocytes respond to nearby neural activity with elevations in cytosolic calcium concentration, and in sensory cortex these calcium responses have been shown to be topographically aligned to neuronal sensory maps. Here, we review recent evidence for astrocyte interactions with neural circuits, with particular emphasis on how these interactions may shape the development, arrangement and plasticity of cortical sensory maps.     

Trace fear conditioning: a role for context?

Fear conditioning can be rapidly obtained over long trace intervals, but its specificity with respect to both time and stimulus is uncertain. Long-trace fear conditioning often parallels contextual conditioning, and it is sensitive to hippocampal lesions. These properties of trace conditioning are not directly addressed by timing models and multiple-time-scale models of conditioning. It is proposed that during early stages of conditioning, a joint representation of the context and the stimulus trace may underlie conditioned responses, and that discriminative processes allow the emergence of specific responses in a later stage.     

Photoentrainment in mammals: a role for cryptochrome?

There is growing evidence in support of the hypothesis that, in mammals, photoreceptive tasks are segregated into those associated with creating a detailed visual image of the environment and those involved in the photic regulation of temporal biology. The hypothesis that this segregation extends to the use of different photoreceptors remains unproven, but published reports from several mammalian species that circadian photoentrainment survives a degree of retinal degeneration sufficient to induce visual blindness suggest that this may be so. This has lead to speculation that mammals might employ a dedicated 'circadian photoreceptor' distinct from the rod and cone cells of the visual system. The location and nature of this putative circadian photoreceptor has become a matter of conjecture. The latest candidates to be put forward as potential circadian photopigments are the mammalian cryptochrome proteins (CRY1 and 2), putative vitamin-B2 based photopigments. To date, published experimental evidence falls short of a definitive demonstration that these proteins form the basis of circadian photoreception in mammals. Consequently, this review aims to assess their suitability for this task in light of what we know regarding the biology of the cyrptochromes and the nature of mammalian photoentrainment.     

Fouling of gastropods: a role for parasites?

A sample of mud snails Hydrobia ulvae (Prosobranchia) from an intertidal population revealed that the shells of trematode-infected specimens were especially likely to be fouled with epibionts. Experimentally trematode-infected Biomphalaria glabrata (Pulmonata) appeared to be especially prone to develop epigrowth in comparison with uninfected conspecifics as well. These findings suggest an interaction between trematode infections and epibiosis in aquatic gastropods. The two most likely explanations for this are (1) that trematode infections weakens the snails' natural defences against epibionts, or (2) that the defences against epibionts also are effective against invading trematodes, causing snail specimens with a particularly good fouling defence to be less likely to become infected.     

Maternal-age effect in aneuploidy: Does altered embryonic selection play a role?

The age of mothers of children with trisomy 21 (47,+21) is elevated no matter if the extra chromosome is of maternal or paternal origin, and it has been postulated that decreasing maternal selection against affected conceptuses with advancing age might explain this observation. Since the absence of sufficient data on 47,+21 abortuses precludes a direct test of this hypothesis, we have taken an indirect approach. Pooled data from spontaneous abortions and live births with autosomal trisomies, XXY and XXX, were examined to determine the natural history of these aneuploid conceptuses and its relation to maternal age. The results are consistent with decreasing embryonic selection in older women.     

Recurrent cutaneous leishmaniasis: a role for persistent parasites?

Leishmaniosis is, with increasing frequency, reported as an opportunistic infection of immunosuppressed individuals. Re-activation of persistent parasites may be responsible for the disease in a number of these patients. Here, Toni Aebischer reviews some of the evidence for the implication of persistent Leishmania infections in recurrent disease with the emphasis on cutaneous leishmoniasis in humans and in the mouse model. The data suggest that parasite persistence is a common feature in Leishmania infections. The availability of on excellent laboratory model provides on opportunity to study this phenomenon in detail. The findings of these analyses are likely to be important for the identification of people at risk of developing recurrent disease and for the assessment of new therapies for relapsing leishmaniasis and might also have implications for the design of a future anti-Leishmania vaccine.