The evolutionary radiation of modern birds (Neornithes): reconciling molecules, morphology and the fossil record

The pattern, timing and extent of the evolutionary radiation of anatomically modern birds (Neornithes) remains contentious: dramatically different timescales for this major event in vertebrate evolution have been recovered by the 'clock-like' modelling of molecular sequence data and from evidence extracted from the known fossil record. Because current synthesis would lead us to believe that fossil and nonfossil evidence conflict with regard to the neornithine timescale, especially at its base, it is high time that available data are reconciled to determine more exactly the evolutionary radiation of modern birds. In this review we highlight current understanding of the early fossil history of Neornithes in conjunction with available phylogenetic resolution for the major extant clades, as well as recent advancements in genetic methods that have constrained time estimates for major evolutionary divergences. Although the use of molecular approaches for timing the radiation of Neornithes is emphasized, the tenet of this review remains the fossil record of the major neornithine subdivisions and better-preserved taxa. Fossils allowing clear phylogenetic constraint of taxa are central to future work in the production of accurate molecular calibrations of the neornithine evolutionary timescale.  © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society , 2004, 141 , 153–177.     

Incus facet morphology in carnivorous mammals from different ecosystems: Taxonomy vs. habitat

This study is prompted by the discovery of an incus of Hyaenodon, the first known auditory ossicle of this genus and thus of any hyaenodont mammal so far. A large set of incudes of recent Carnivora, including felids, hyaenids, viverrids, herpestids, nandiniid and canids of different ecosystems, was set up for morphological comparison. This study examines especially the incudo-mallear facet. Typically, the incudo-mallear facet is composed of: (1) three articular surfaces in felids, (2) a U-shaped surface in hyaenids and (3) four surfaces in canids. Both taxonomy (on family level) and habitat (open, closed or mixed habitat preference) might have an impact on the morphology of the incus facets, the former having a major impact in our sample. The Hyaenodon incus is small, delicate and possesses an incudo-mallear facet of a general saddle-shape with two articulation facets, a large superior articulation area and a circular, inferior articulation area. Herein, its general morphology and facet shape is most similar to the felid incus morphology.     

Lifetime energy budgets in mammals and birds.

1. Two data sets for standard energy metabolism (351 and 320 species, respectively) and one for maximal lifespan (494 species) in mammals have been assembled from the literature. 2. In addition smaller data sets of active (field) energy metabolism in mammals (36 species) and in birds (25 species) have been drawn on. 3. The products of the respective regression parameters as well as the products of energy metabolism and maximal lifespan in individual species have been computed in order to estimate lifetime energy metabolism in mammals generally and in various mammalian orders. 4. It is found that lifetime energy budgets in mammals generally, whether standard or active, very systematically with body mass with slopes between 0.87 and 0.93, significantly different from unity (P less than 0.001 or P less than 0.01). 5. In birds, lifetime energy budgets, whether standard or active, vary with slopes of 0.94 +/- 0.05 and 0.88 +/- 0.09, which are not significantly different from unity (P greater than 0.1). 6. In carnivores, artiodactyls, primates and bats the slopes for lifetime standard as well as lifetime active energy budgets are not significantly different from one in any of the investigated data sets. 7. In rodents the lifetime standard energy budgets have slope significantly different from one; in marsupials one data set for lifetime standard and the one for lifetime active energy budget lead to slopes significantly different from one. 8. It is concluded from this analysis that current data do not support the hypothesis that lifetime energy budgets, whether standard or active, vary as the first power of body mass in mammals generally.(ABSTRACT TRUNCATED AT 250 WORDS)     

Southern Africa and modern human origins.

This paper argues that southern Africa was a remote part of the Old World in the late Pleistocene (125-10 ka ago). Because of this isolated position there was continuity without significant replacement in the resident population. Isolation and the relatively recent spread of agriculture to the region has allowed a section of this population to survive into the present. They are the Bushmen (San). Studies of geographic patterning in conventional genetic markers and mitochondrial DNA indicate that the Bushman clade has a long evolutionary history in southern Africa. Estimates of more than 100 ka for the continued presence of this population in the region are supported in archaeological investigations of sites with long sequences such as Klasies River main site and Border Cave. Human remains dating to the earlier part of the late Pleistocene have been recovered from these sites and the samples form a morphological series with the Klasies River remains possibly 20 ka older than those from Border Cave. There is no fossil record for the later Pleistocene, however, at a period when selection for a gracile morphology may have been pronounced. The cultural associations in the earlier late Pleistocene are with the Middle Stone Age. Expressions of cultural 'style' and the occurrence of similar artefact design types in the Middle and Later Stone Ages can be interpreted with reference to the ethnographic present. Temporal continuity can be shown in the geographical distribution of stylistic markers and this suggests participation in a shared cognitive system.(ABSTRACT TRUNCATED AT 250 WORDS)     

Behavioral thermoregulation in mammals and birds

Thermoregulation in homoiotherms is achieved by physiological and behavioural adjustments which involve the musculature, skin, sensory capacities, hypothalamus and endocrine glands. Under thermal stress animals exhibit anorexia, body extension, gasping, languor, lethargy, excessive drinking, bathing, decreased locomotor activities, group dispersion, and shade seeking. When exposed to cold, animals show body flexure, huddling, hyperphagia, extra locomotor activities, depressed respiration and nest building. Species and breed differences in the behavioural adjustments to unfavourable climates are related to habitat, morphological characteristics of body covering, degree of physiological adaptability, degree of physiological immaturity at birth or hatching, and the number of young.     

Density-dependent dispersal in birds and mammals

Density‐dependent dispersal can be caused by various mechanisms, from competition inducing individuals to emigrate (positive density‐dependence) to social crowding effects impeding free movement (negative density‐dependence). Various spatial population models have incorporated positively density‐dependent dispersal algorithms, and recent theoretical models have explored the conditions for density‐dependent dispersal (DD) to evolve. However, while the existence of DD is well documented in some taxa such as insects, there is no clear picture on its generality in vertebrates. Here I review the available empirical data on DD in birds and mammals, focusing mainly on variation in dispersal between years and on experimental density manipulations. Surprisingly few studies have explicitly focused on DD, and interpretation of the available data is often hampered by differences in approach, small sample sizes and/or statistical shortcomings. Positive DD was reported in 50 and 33% of the selected mammal and bird studies, respectively, while two studies on mammals (out of eight) reported negative DD. Among bird studies, DD was more often reported for emigration rates or long‐distance recoveries than for average distances within finite study areas. Experimental studies manipulating densities (mainly on mammals) have consistently generated positive DD, typically showing reduced emigration in response to partial population removal. Studies that examined dispersal in relation to seasonal changes in density (small mammals only) have more often reported negative DD. Studies that compared dispersal between sites differing in density, also show a mixture of positive and negative DD. This suggests that dispersal changes in a more complex way with seasonal and spatial density variation than with annual densities, and/or that these results are confounded by other factors differing between seasons and sites, such as habitat quality. I conclude that both correlational and experimental studies support the existence of positive, rather than negative, density‐dependent dispersal in birds and mammals.     

Heterothermy in Afrotropical mammals and birds: a review

Recent years have seen a rapid increase in the number of Afrotropical endotherms known to avoid mismatches between energy supply and demand by using daily torpor and/or hibernation. Among mammals, heterothermy has been reported in 40 species in six orders, namely Macroscelidea, Afrosoricida, Rodentia, Eulipotyphla, Primates and Chiroptera. These species span a range in body mass of 7-770?g, with minimum heterothermic body temperatures ranging from 1-27°C and bout length varying from 1?h to 70 days. Daily torpor is the most common form of heterothermy, with true hibernation being observed in only seven species, Graphiurus murinus, Graphiurus ocularis, Atelerix frontalis, Cheirogaleus medius, Cheirogaleus major, Microcebus murinus and Microcebus griseorufus. The traditional distinction between daily torpor and hibernation is blurred in some species, with free-ranging individuals exhibiting bouts of > 24?h and body temperatures < 16 °C, but none of the classical behaviours associated with hibernation. Several species bask in the sun during rewarming. Among birds, heterothermy has been reported in 16 species in seven orders, and is more pronounced in phylogenetically older taxa. Both in mammals and birds, patterns of heterothermy can vary dramatically among species occurring at a particular site, and even among individuals of a single species. For instance, patterns of heterothermy among cheirogalid primates in western Madagascar vary from daily torpor to uninterrupted hibernation for up to seven months. Other examples of variation among closely-related species involve small owls, elephant shrews and vespertilionid bats. There may also be variation in terms of the ecological correlates of torpor within a species, as is the case in the Freckled Nightjar Caprimulgus tristigma.     

Object recognition: similar visual strategies of birds and mammals

Behavioral testing has revealed that pigeons may use the same visual information sources as humans to discriminate between three-dimensional shapes.     

The origins of ecotypic variation of rainbow trout: a test of environmental vs. genetically based differences in morphology

Although morphological plasticity has been observed in a variety of taxa, few experimental studies have compared the relative proportion of morphological variability that is accounted for by environmentally induced plasticity, and how much is because of genetically based differences among populations. We compared the morphology of six rainbow trout (Oncorhynchus mykiss) populations from different ecotypic categories that were raised under flowing vs. standing-water conditions. Our data indicate that both environmental conditions and ecotypic differences account for a significant proportion of variation in morphology. Among ecotype effects, however, accounted for a much larger proportion of morphological variability than environmental conditions. Rainbow trout from stream populations had deeper caudal peduncles, and longer fins than lake populations, and rainbow trout from a piscivorous population had larger mouth and head lengths than all other ecotypes. Environmentally induced differences in morphology were primarily related to differences in mouth and head lengths, as well as fin length. Relative to morphometric differences from natural rainbow trout populations, most characteristics deviated in the same direction in our experimental populations. Our data indicate that morphological differences across rainbow trout populations have a genetic basis and may represent locally adaptive characteristics and highlight the role of ecology in promoting phenotypic divergence.     

Variability and size in mammals and birds

Body size, its variability, and their ecological correlates have long been important topics in evolutionary biology. Yet, the question of whether there is a general relationship between size and size-relative variability has not previously been addressed. Through an analysis of body-mass and length measurements from 65 074 individuals from 351 mammalian species, we show that size-relative variability increases significantly with mean species body size. Analysis of mean body mass and standard deviations for 237 species of birds revealed the same pattern. We present three plausible alternatives explanations and eliminate several others. Of these, the hypothesis that the increase in size-relative variability with mean body mass is related to the scaling of body mass components is most strongly supported. In effect, larger mammals and birds are more variable because their body mass is composed to greater relative degree of components with higher intrinsic variability (bone, fat, and muscle). In contrast, smaller mammals and birds have lower body mass variability because they are composed to a greater relative extent of components (viscera and nervous system) in which size variation is more highly constrained by energetic and functional factors.     

Genetics and modern human origins

The past decade has brought considerable debate on the subject of modern human origins. The nature of the transition from Homo erectus to archaic Homo sapiens to modern H. sapiens has been examined primarily in terms of the relative contribution of archaic populations to later moderns, both within and among geographic regions. The recent African origin model proposes that modern humans appeared first in Africa between 100,000 and 200,000 years ago, and then spread through the rest of the Old World, replacing preexisting populations.^1–6 This model has been referred to by a variety of names, including “replacement”, “Garden of Eden”, “Noah's Ark”, and “out of Africa”. The recent African origin model contrasts with the multiregional model, which proposes a species-wide transition to modern humans throughout the Old World during the past million years or more.^7–10 Indeed, some proponents of the multiregional model advocate placing Homo erectus and all subsequent species of Homo in the evolutionary species Homo sapiens.¹¹ This contrasts with the view that there were multiple hominid species during the Middle Pleistocene. The debate continues.^12,13 Although the multiregional model is often portrayed as proposing a simultaneous transition to anatomically modern humans in different geographic regions, it explicitly allows for varying degrees of continuity across time and space.¹⁰ This model, in the broad sense, does not rule out the possibility that modern human morphology appeared first in Africa and then spread through the rest of the Old World through gene flow. However, not all advocates of the multiregional model adhere to this specific subset of the general model.⁹ Comparison of the African and multiregional models is complicated by considering other, less extreme, hypotheses. Some versions of the recent African origin model imply a speciation event associated with the initial origin of modern humans. Another version, which suggests the possibility of some admixture between “moderns” leaving Africa and preexisting “archaics” elsewhere in the Old World,^14,15 is similar to some variants of the multiregional model, which also suggest that modern morphology appeared first in Africa, but involved admixture with other Old World populations.¹⁶ The major difference between these views appears to be the extent of admixture, although the exact level is never specified. A further complication is the possibility that multiple dispersals from Africa produced a more complicated pattern of worldwide variation.¹⁷     

Human evolution: origins of modern humans still look recent.

That modern humans have a relatively ancient origin has been suggested on the basis of fossil and genetic evidence. But DNA sequences from an extinct neanderthal, and phylogenetic analyses of hundreds of human and ape sequences, continue to support a recent origin for modern humans.     

Antibodies to Clostridium botulinum toxins in free-living birds and mammals.

Naturally-occuring antibodies against Clostridium botulinum toxins were found in Cathartes aura (turkey vultures), Canis latrans (coyotes) and Corvus brachyrhynchos (crows) by the passive hemagglutination (PHA) test and verified by the serum neutralization (SN) test. The prevalence of IHA antibodies was 18 of 20 vultures (90%), 5 of 12 crows (42%) and 25 to 110 coyotes (23%). Vultures and coyotes were seropositive by the PHA test against A, B, C, D, and F toxins. The highest antibody titer 1:8192 was in vulture serum against type C. In descending order, the highest antibody levels were against type C, D, F, E, A and B toxins.     

Development of the clavicles in birds and mammals

Clavicles (collar bones) are variably present in mammals. Furculae (wishbones)--which may or may not be homologous with clavicles--are variably present and/or fused in birds and present in theropod dinosaurs. In this overview the development of clavicles and furculae is discussed with special attention to modes of skeletogenesis (whether intramembranous or endochondral), numbers of centres of ossification (one in chick furculae; two in murine clavicles), presence of cartilage (primary in clavicles, secondary in furculae), evidence from experimental analysis and from mutations for dependence of both clavicular and furcular growth on mechanical stimulation, and syndromes and mutations affecting clavicular development leading to both under and over development. J. Exp. Zool. 289:153-161, 2001.     

Sex differentiation in mammals and tempo of growth: probabilities vs. switches

In the conventional model of sex differentiation in placental mammals, a switch is envisaged to steer the indifferent gonad into the path of either testicular or ovarian development. The immediate cause of the switch is thought to be the presence or absence of Sertoli cells, which in turn is controlled by the presence or absence of the testis-determining factor on the Y chromosome (TDF in humans, Tdy in mice). Quantitative investigations indicate, however, that the rate of growth of XY gonads is faster than that of XX gonads before the formation of Sertoli cells, and furthermore, that XY embryos develop faster than XX embryos long before the formation of gonadal ridges. Since the genetic constitution of the sex chromosomes appears to manifest itself from the earliest embryonic stages onwards, the concept of indifferent gonads being switched into alternate pathways becomes inappropriate. A model is proposed in which gonadal differentiation depends on developmental thresholds: the formation of Sertoli cells needs to occur by a particular stage in time in a sufficiently developed gonad, failing which the gonad will enter the ovarian pathway. While TDF is the principal factor enhancing the rate of gonadal growth, other factors which influence development rates can modulate the probability of a gonad becoming either a testis or an ovary.