Mixed company: a framework for understanding the composition and organization of mixed‐species animal groups

Mixed‐species animal groups (MSGs) are widely acknowledged to increase predator avoidance and foraging efficiency, among other benefits, and thereby increase participants' fitness. Diversity in MSG composition ranges from two to 70 species of very similar or completely different phenotypes. Yet consistency in organization is also observable in that one or a few species usually have disproportionate importance for MSG formation and/or maintenance. We propose a two‐dimensional framework for understanding this diversity and consistency, concentrating on the types of interactions possible between two individuals, usually of different species. One axis represents the similarity of benefit types traded between the individuals, while the second axis expresses asymmetry in the relative amount of benefits/costs accrued. Considering benefit types, one extreme represents the case of single‐species groups wherein all individuals obtain the same supplementary, group‐size‐related benefits, and the other extreme comprises associations of very different, but complementary species (e.g. one partner creates access to food while the other provides vigilance). The relevance of social information and the matching of activities (e.g. speed of movement) are highest for relationships on the supplementary side of this axis, but so is competition; relationships between species will occur at points along this gradient where the benefits outweigh the costs. Considering benefit amounts given or received, extreme asymmetry occurs when one species is exclusively a benefit provider and the other a benefit user. Within this parameter space, some MSG systems are constrained to one kind of interaction, such as shoals of fish of similar species or leader–follower interactions in fish and other taxa. Other MSGs, such as terrestrial bird flocks, can simultaneously include a variety of supplementary and complementary interactions. We review the benefits that species obtain across the diversity of MSG types, and argue that the degree and nature of asymmetry between benefit providers and users should be measured and not just assumed. We then discuss evolutionary shifts in MSG types, focusing on drivers towards similarity in group composition, and selection on benefit providers to enhance the benefits they can receive from other species. Finally, we conclude by considering how individual and collective behaviour in MSGs may influence both the structure and processes of communities.     

Species‐specific communication bars interspecific mating between syntopic species of Zwicknia stoneflies (Plecoptera: Capniidae)

Most Northern Hemisphere stoneflies have species‐specific mating signals that are generally thought to constitute a barrier against interspecific mating. We tested this hypothesis in two species of the genus Zwicknia that have only very recently been recognised as distinct species, and that were found to occur together in a stream in Lower Saxony, Germany. Analyses of molecular markers COI and 28S in combination with wing length (distinguishing males of both species) and mating signals revealed no instance of hybridisation among 23 studied specimens. In addition, eleven further males identified on the basis of morphology alone all produced the expected species‐specific signal. Females and males of both species were presented with played back conspecific and heterospecific signals and duetting sequences, and responded only to conspecific stimuli. This lends support to the hypothesis that the intersexual communication system functions as an important pre‐mating barrier against gene flow, although post‐mating isolation cannot be excluded. Interspecific mating did occur when a mixed pair was confined together in a small container. Males of both species were found to call in response to played back duetting sequences with stereotypic latencies that are clearly longer than the latencies in male‐female duets. We interpret this as an indication of eavesdropping behaviour coupled with attempts to take over the perceived duet. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 969–980.     

Intra‐ and interspecific divergence in the nuclear sequences of the clock gene period in species of the Drosophila buzzatii cluster

We have measured nucleotide variation in the CLOCK/CYCLE heterodimer inhibition domain (CCID) of the clock X‐linked gene period in seven species belonging to the Drosophila buzzatii cluster, namely D. buzzatii, Drosophila koepferae, Drosophila antonietae, Drosophila serido, Drosophila gouveai, Drosophila seriema and Drosophila borborema. We detected that the purifying selection is the main force driving the sequence evolution in period, in agreement with the important role of CCID in clock machinery. Our survey revealed that period provides valuable phylogenetic information that allowed to resolve phylogenetic relationships among D. gouveai, D. borborema and D. seriema, which composed a polytomic clade in preliminary studies. The analysis of patterns of intraspecific variation revealed two different lineages of period in D. koepferae, probably reflecting introgressive hybridization from D. buzzatii, in concordance with previous molecular data.     

Different modes of acoustic communication in deep‐diving short‐finned pilot whales (Globicephala macrorhynchus)

Toothed whales use a pneumatic sound generator to produce echolocation and communication sounds. Increasing hydrostatic pressure at depth influences the amplitude and duration of calls but not of echolocation clicks. Here we test the hypothesis that information transfer at depth might be facilitated by click‐based communication signals. Wild short‐finned pilot whales (27) instrumented with multisensor DTAGs produced four main types of communication signals: low‐ and medium‐frequency calls (median fundamental frequency: 1.7 and 2.9 kHz), two‐component calls (median frequency of the low and high frequency components: 2 and 9 kHz), and rasps (burst‐pulses with median interclick interval of 21 ms). Rasps can be confused with foraging buzzes, but rasps are shorter and slower, and are not associated with fast changes in body acceleration nor reduced acoustic output of buzzes, characteristic of prey capture attempts. Contrary to calls, the energy flux density of rasps was not significantly affected by depth. This, and a different information content, may explain the observed increase in the relative occurrence of rasps with respect to calls at depth, and supports the hypothesis that click‐based communication signals may facilitate communication under high hydrostatic pressure. However, calls are produced at depth also, indicating that they may carry additional information relevant for deep‐diving animals, including potential communication among whales diving at the same time in this highly social deep‐diving species.     

Lack of habitat segregation and no interspecific territoriality in three syntopic cryptic species of the golden‐spectacled warblers Phylloscopus (Seicercus) burkii complex

It is widely accepted that exploitative competition prevents the coexistence of any two or more closely‐related species unless differences exists in their ecological niches and resource use. In sibling bird species, exploitative competition is reduced mainly by spatial segregation of competing species. Spatial segregation can be achieved in two basic ways: by using different habitats or microhabitats that each species is predominately exploiting, or by interspecific territoriality. To our knowledge, either habitat segregation or interspecific territoriality or both have been found in all dyads and groups of sympartic sibling bird species studied so far. In this study, we investigated the coexistence of three cryptic sibling species of the golden‐spectacled warblers Phylloscopus burkii complex in Hunan Province, China: Ph. tephrocephalus, Ph. omeiensis and Ph. valentini. We analyzed their habitat preferences, and spatial interrelations between the species. In order to analyze either presence or absence of interspecific territoriality, we additionally performed con‐ and heterospecific playback experiments. Contrary to expectations, we found no evidences for either habitat segregation or interspecific territoriality in these three species.     

Electrocommunication behaviour during social interactions in two species of pulse‐type weakly electric fishes (Mormyridae)

This study compares electrocommunication behaviour in groups of freely swimming weakly electric fishes of two species, Marcusenius altisambesi and Mormyrus rume. Animals emitted variable temporal sequences of stereotyped electric organ discharges (EOD) that served as communication signals. While the waveform of individual signals remained constant, the inter‐discharge interval (IDI) patterns conveyed situation‐specific information. Both species showed different types of group behaviour, e.g. they engaged in collective (group) foraging. The results show that in each species, during different behavioural conditions (resting, foraging and agonistic encounters), certain situation‐specific IDI patterns occurred. In both species, neighbouring fishes swimming closely together interacted electrically by going in and out of synchronization episodes, i.e. periods of temporally correlated EOD production. These often resulted in echo responses between neighbours. During group foraging, fishes often signalled in a repetitive fixed order (fixed‐order signalling). During foraging, EOD emission rates of M. altisambesi were higher and more regular than those of M. rume. The two species also differed in the quantity of group behaviours with M. altisambesi being more social than M. rume, which was reflected in the lack of specific agonistic IDI patterns, more fixed‐order signalling and more communal resting behaviour in M. altisambesi.     

Comparative phylogeography of two sibling species of forest‐dwelling rodent (Praomys rostratus and P. tullbergi) in West Africa: different reactions to past forest fragmentation

Two sibling species of the rodent genus Praomys occur in West African forests: P. tullbergi and P. rostratus. By sampling across their geographical ranges (459 individuals from 77 localities), we test the hypothesis that climatic oscillations during the Quaternary made an impact on the observed pattern of cytochrome b sequence variation. We show that, although these two species have parapatric geographical distributions, their phylogeographical histories are dissimilar, which could be related to their distinct ecological requirements. Since the arid phases of the Pleistocene were characterized by isolated forest patches, and intervening wetter periods by forest expansion, these changes in forest cover may be the common mechanism responsible for the observed phylogeographical patterns in both of these species. For example, in both species, most clades had either allopatric or parapatric geographical distributions; however, genetic diversity was much lower in P. tullbergi than in P. rostratus. The genetic pattern of P. tullbergi fits the refuge hypothesis, indicating that a very small number of populations survived in distinct forest blocks during the arid phases, then expanded again with forest recovery. In contrast, a number of populations of P. rostratus appear to have survived during the dry periods in more fragmented forest habitats, with varying levels of gene flow between these patches depending on climatic conditions and forest extent. In addition, historical variations of the West African hydrographic network could also have contributed to the pattern of genetic differentiation observed in both species.