Do sharks exhibit heterodonty by tooth position and over ontogeny? A comparison using elliptic Fourier analysis

Tooth morphology is often used to inform the feeding ecology of an organism as these structures are important to procure and process dietary resources. In sharks, differences in morphology may facilitate the capture and handling of prey with different physical properties. However, few studies have investigated differences in tooth morphology over ontogeny, throughout the jaws of a single species, or among species at multiple tooth positions. Bull (Carcharhinus leucas), blacktip (Carcharhinus limbatus), and bonnethead sharks (Sphyrna tiburo) are coastal predators that exhibit ontogenetic dietary shifts, but differ in their feeding ecologies. This study measured tooth morphology at six positions along the upper and lower jaws of each species using elliptic Fourier analysis to make comparisons within and among species over their ontogeny. Significant ontogenetic differences were detected at four of the six tooth positions in bull sharks, but only the posterior position on the lower jaw appeared to exhibit a functionally relevant shift in morphology. No ontogenetic changes in morphology were detected in blacktip or bonnethead sharks. Intraspecific comparisons found that most tooth positions significantly differed from one another across all species, but heterodonty was greatest in bull sharks. Additionally, interspecific comparisons found differences among all species at each tooth position except between bull and blacktip sharks at two positions. These morphological patterns within and among species may have implications for prey handling efficiency, as well as in providing insight for paleoichthyology studies and reevaluating heterodonty in sharks.     

Sex ratio variance and the maintenance of environmental sex determination

Although variation in population sex ratios is predicted to increase the extinction rate of clades with environmental sex determination (ESD), ESD is still seen in a wide array of natural systems. It is unclear how this common sex-determining system has persisted despite this inherent disadvantage associated with ESD. We use simulation modelling to examine the effect of the sex ratio variance caused by ESD on population colonization and establishment. We find that an accelerating function of establishment success on initial population sex ratio favours a system that produces variance in sex ratios over one that consistently produces even sex ratios. This sex ratio variance causes ESD to be favoured over genetic sex determination, even when the mean global sex ratio under both sex-determining systems is the same. Data from ESD populations suggest that the increase in population establishment can more than offset the increased risk of extinction associated with temporal fluctuations in the sex ratio. These findings demonstrate that selection in natural systems can favour increased variance in a trait, irrespective of the mean trait value. Our results indicate that sex ratio variation may provide an advantage to species with ESD, and may help explain the widespread existence of this sex-determining system.     

An analysis of birth sex ratio bias in captive prosimian species

The extent to which sex ratio bias is a common reproductive characteristic of prosimians has not been well established. The present study analyzed reproduction in 13 breeding groups of captive prosimians for evidence of birth sex ratio bias. A substantial male bias was demonstrated in nongregarious, but not gregarious, breeding groups. Analyses of birth sex ratios of individual mothers suggested that the observed bias did not result from the tendency of a few mothers to overproduce males, but rather from a small but reliable excess of male births in general. An examination of infant mortality revealed that male Otolemur garnettii and Microcebus murinus infants were more vulnerable to preweaning mortality, whereas female Eulemur fulvus albifrons infants were more vulnerable. An analysis of birth order by sex found that mothers of one group (O. garnettii) tended to produce males initially and females later. Additionally, a distinct pattern of birth seasonality was noted among Malagasy prosimians that was absent in the African prosimians. Greater length of period of sexual receptivity for nongregarious females as compared to gregarious females is proposed as a possible mechanism of male birth sex ratio bias. © 1996 Wiley-Liss, Inc.     

Phylogeny of sex‐determining mechanisms in squamate reptiles: are sex chromosomes an evolutionary trap?

Squamate reptiles possess two general modes of sex determination: (1) genotypic sex determination (GSD), where the sex of an individual is determined by sex chromosomes, i.e. by sex‐specific differences in genotype; and (2) temperature‐dependent sex determination (TSD), where sex chromosomes are absent and sex is determined by nongenetic factors. After gathering information about sex‐determining mechanisms for more than 400 species, we employed comparative phylogenetic analyses to reconstruct the evolution of sex determination in Squamata. Our results suggest relative uniformity in sex‐determining mechanisms in the majority of the squamate lineages. Well‐documented variability is found only in dragon lizards (Agamidae) and geckos (Gekkota). Polarity of the sex‐determining mechanisms in outgroups identified TSD as the ancestral mode for Squamata. After extensive review of the literature, we concluded that to date there is no known well‐documented transition from GSD to TSD in reptiles, although transitions in the opposite direction are plentiful and well corroborated by cytogenetic evidence. We postulate that the evolution of sex‐determining mechanisms in Squamata was probably restricted to the transitions from ancestral TSD to GSD. In other words, transitions were from the absence of sex chromosomes to the emergence of sex chromosomes, which have never disappeared and constitute an evolutionary trap. This evolutionary trap hypothesis could change the understanding of phylogenetic conservatism of sex‐determining systems in many large clades such as butterflies, snakes, birds, and mammals. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 156 , 168–183.     

黄海南部黄鮟鱇摄食生态

根据对2008年9月至2009年7月在黄海南部所捕获的黄鮟鱇胃含物的分析,对其摄食生态进行了研究。结果表明,黄海南部黄鮟鱇主要摄食小黄鱼(Pseudosciaena polyactis)、矛尾鰕虎鱼(Chaeturichthys stigmatias)、细条天竺鲷(Apogonichthys lineatus)、带鱼(Trichiurus haumela)、龙头鱼(Harpadon nehereus)、鹰爪虾(Trachypenaeus curvirostris)、鳀鱼(Engraulis japonicus)等饵料生物。鱼类是黄鮟鱇最主要的摄食类群,在食物中所占的重量百分比之和为83.44%。黄鮟鱇的摄食强度具有明显的季节变化和体长变化,冬季最高,春季最低;小于100mm体长组最高,大于500mm体长组最低。在4个季节和6个体长组中,小黄鱼都是黄鮟鱇胃中重量百分比最高的饵料生物。矛尾鰕虎鱼和细条天竺鲷在黄鮟鱇幼鱼胃含物中所占比例较高,但在成鱼胃含物中所占比例较低。聚类分析的结果显示,黄鮟鱇在体长为100mm时发生食性转换。黄鮟鱇营养级为3.66,表明其在东、黄海食物网中处于较高的位置。     

Sex ratios observed in 80 species of parrots

A number of potential evolutionary and physiological factors may be involved in avian sex ratio bias so that under certain conditions a sex ratio bias may favour males or females within a population. In addition different factors may be important in manipulating sex ratio bias through the different life stages. In this study sex ratio bias was examined in a total of 16 570 captive parrots, representing 80 species, many of which are endangered in the wild, using database records originating form commercial laboratories that offer genetic sexing. Within the species examined 72% showed a male bias this was significant in three species, when adjusted for multiple comparisons. This preliminary study is limited due to lack of data on the age of the individuals sampled. However, the large dataset do suggest that this phenomenon should be further considered by investigators working at a species level where such data can be collected.     

The ruminant dental grooming apparatus

Correlations between dental morphology and dietary preferences in ruminants explain the similarities but not the differences in relative incisor width encountered within the group. Observations on African browsing antelope have revealed extensive use of the lateral anterior dental elements for grooming purposes using a distinctive upward sweeping movement of the head. Inspection of these dental elements (I₂, I₃₎ and C) reveals a comb-like array remarkably similar to the prosimian tooth-comb. An hypothesis is presented to explain differences in incisor morphology based on the use of the teeth for purposes other than eating. The alternative biological role has implications for the use of dental characteristics in the determination of the feeding ecology of living and extinct ruminants.     

Feeding biology of sunfishes: patterns of variation in the feeding mechanism

Fishes are generally believed to differ in their ability to alter (modulate) their feeding behaviour in response to different prey. We investigated modulation quantitatively in four species of sunfishes (Centrarchidae) by evaluating the variation in 11 electromyographic variables measured from recordings of electrical activity in head muscles during feeding on three prey types. The experimental design used allowed us to partition variation between species, among individuals within species, among prey types, and among feedings. Duration of activity of the sternohyoideus muscle was the only variable significantly different among the four species. All variables showed significant differences among individuals within species. The overall range of activity of each muscle activity variable was about the same for all four sunfishes. However, three species showed a significant ability to modulate most muscle variables while a fourth did not change its feeding response with respect to prey type. The results indicate that: (1) intraspecific variation is an important source of variability in functional attributes that should be accounted for in comparisons between species; (2) the ability of some species to modulate appears to be independent of the potential variability in muscle activity possessed by each species; (3) closely related species can differ considerably in their ability to alter muscle electrical activity patterns during feeding; and (4) a quantitative assessment of variation in electromyographic patterns is a powerful approach for asking questions about differences in feeding behaviour.     

Flow cytometric sexing of spider sperm reveals an equal sperm production ratio in a female-biased species

Producing equal amounts of male and female offspring has long been considered an evolutionarily stable strategy. Nevertheless, exceptions to this general rule (i.e. male and female biases) are documented in many taxa, making sex allocation an important domain in current evolutionary biology research. Pinpointing the underlying mechanism of sex ratio bias is challenging owing to the multitude of potential sex ratio-biasing factors. In the dwarf spider, Oedothorax gibbosus, infection with the bacterial endosymbiont Wolbachia results in a female bias. However, pedigree analysis reveals that other factors influence sex ratio variation. In this paper, we investigate whether this additional variation can be explained by the unequal production of male- and female-determining sperm cells during sperm production. Using flow cytometry, we show that males produce equal amounts of male- and female-determining sperm cells; thus bias in sperm production does not contribute to the sex ratio bias observed in this species. This demonstrates that other factors such as parental genes suppressing endosymbiont effects and cryptic female choice might play a role in sex allocation in this species.     

Dietary and Feeding Differences Between Sympatric Propithecus diadema diadema and Indri indri

We analyzed the dietary profiles and feeding behaviors of Propithecus diadema diadema and Indri indri in a community of animals that reside in midaltitude rain forest within Madagascar's Mantadia National Park. Propithecus diadema diadema ate a diverse mixture of fruits, seeds, flowers and young leaves, while the bulk of the diet of Indri indri consisted of young leaves, which resulted in significantly higher levels of fat and water-soluble carbohydrates in foods eaten by Propithecus diadema diadema. Fiber values of items eaten are high (54% NDF) for both species, though not significantly different between them. The preference for immature foliage by Indri indri suggests that their overall intake of fiber is greater than that of Propithecus diadema diadema, which had a high proportion of non-leaf material in their diet. We propose that differences in gut morphology between the two indriids contribute to their disparate diets. Levels of secondary compounds were high in certain food items, but overall they are also not significantly different between the two indriid diets. Propithecus diadema diadema exhibited a strong preference for 2 alkaloid-containing seed species, while no food of Indri indri contained alkaloids. In addition, Propithecus diadema diadema consumed a higher diversity of plant species on a daily/yearly basis, exhibited more feeding bouts on a daily basis, and their feeding bouts (on young leaves, fruit and flowers) are significantly shorter in duration than those of Indri indri. Furthermore, Propithecus diadema diadema had twice the number of geophagic episodes of Indri indri.     

Biased sex ratios in the dioecious annual Croton texensis (Euphorbiaceae) are not due to environmental sex determination

At Arapaho Prairie, in the sandhills of western Nebraska, the dioecious annual Croton texensis (Euphorbiaceae) exhibits biased sex ratios. Moreover, the direction of bias changes from year to year: in 1994 the study population was significantly female biased, in 1995 and 1996 it was significantly male biased, and in 1997 and 1998 the sex ratio did not differ from 1 : 1. Such variation in the observed sex ratio in plants is frequently attributed to environmental sex determination (ESD), which is favored by natural selection if the rate of fitness gain across an environmental gradient is greater for one sex than the other. We performed experiments to determine: (1) whether variation in the sex ratio is correlated with environmental conditions, as would be expected if ESD is operating, and (2) whether ESD, if present, would be favored by natural selection. In a common garden experiment in which water and fertilizer were manipulated the sex ratio was marginally male biased in treatments in which water was added, but not different from 1 : 1 in other treatments. In field plots into which seeds were planted none of several soil characteristics, nor overall plot quality for C. texensis (measured as average plant biomass) were correlated with plot sex ratio. However, plots in which a large number of planted seeds emerged tended to be female biased. These results provide very weak evidence for sex ratio bias across an environmental gradient, and thus provide little evidence for ESD. Moreover, sex-by-environment interactions for fitness, which are required for the evolution of ESD, were absent for all measured variables. Thus, ESD does not appear to be favored by natural selection in this population. Instead, these biases may have been caused by differences between the sexes in germination and/or early mortality.