White shark comparison reveals a slender body for the extinct megatooth shark, Otodus megalodon (Lamniformes: Otodontidae).

The megatooth shark, †Otodus megalodon, which likely reached at least 15 m in total length, is an iconic extinct shark represented primarily by its gigantic teeth in the Neogene fossil record. As one of the largest marine carnivores to ever exist, understanding the biology, evolution, and extinction of †O. megalodon is important because it had a significant impact on the ecology and evolution of marine ecosystems that shaped the present-day oceans. Some attempts inferring the body form of †O. megalodon have been carried out, but they are all speculative due to the lack of any complete skeleton. Here we highlight the fact that the previous total body length estimated from vertebral diameters of the extant white shark (Carcharodon carcharias) for an †O. megalodon individual represented by an incomplete vertebral column is much shorter than the sum of anteroposterior lengths of those fossil vertebrae. This factual evidence indicates that †O. megalodon had an elongated body relative to the body of the modern white shark. Although its exact body form remains unknown, this proposition represents the most parsimonious empirical evidence, which is a significant step towards deciphering the body form of †O. megalodon.

Endothermic physiology of extinct megatooth sharks.

The evolution of the extinct megatooth shark, Otodus megalodon, and its close phylogenetic relatives remains enigmatic. A central question persists regarding the thermophysiological origins of these large predatory sharks through geologic time, including whether O. megalodon was ectothermic or endothermic (including regional endothermy), and whether its thermophysiology could help to explain the iconic shark's gigantism and eventual demise during the Pliocene. To address these uncertainties, we present unique geochemical evidence for thermoregulation in O. megalodon from both clumped isotope paleothermometry and phosphate oxygen isotopes. Our results show that O. megalodon had an overall warmer body temperature compared with its ambient environment and other coexisting shark species, providing quantitative and experimental support for recent biophysical modeling studies that suggest endothermy was one of the key drivers for gigantism in O. megalodon and other lamniform sharks. The gigantic body size with high metabolic costs of having high body temperatures may have contributed to the vulnerability of Otodus species to extinction when compared to other sympatric sharks that survived the Pliocene epoch.

Cenozoic megatooth sharks occupied extremely high trophic positions.

Trophic position is a fundamental characteristic of animals, yet it is unknown in many extinct species. In this study, we ground-truth the 15N/14N ratio of enameloid-bound organic matter (δ15NEB) as a trophic level proxy by comparison to dentin collagen δ15N and apply this method to the fossil record to reconstruct the trophic level of the megatooth sharks (genus Otodus). These sharks evolved in the Cenozoic, culminating in Otodus megalodon, a shark with a maximum body size of more than 15 m, which went extinct 3.5 million years ago. Very high δ15NEB values (22.9 ± 4.4‰) of O. megalodon from the Miocene and Pliocene show that it occupied a higher trophic level than is known for any marine species, extinct or extant. δ15NEB also indicates a dietary shift in sharks of the megatooth lineage as they evolved toward the gigantic O. megalodon, with the highest trophic level apparently reached earlier than peak size.

Trophic position of Otodus megalodon and great white sharks through time revealed by zinc isotopes.

Diet is a crucial trait of an animal's lifestyle and ecology. The trophic level of an organism indicates its functional position within an ecosystem and holds significance for its ecology and evolution. Here, we demonstrate the use of zinc isotopes (δ66Zn) to geochemically assess the trophic level in diverse extant and extinct sharks, including the Neogene megatooth shark (Otodus megalodon) and the great white shark (Carcharodon carcharias). We reveal that dietary δ66Zn signatures are preserved in fossil shark tooth enameloid over deep geologic time and are robust recorders of each species' trophic level. We observe significant δ66Zn differences among the Otodus and Carcharodon populations implying dietary shifts throughout the Neogene in both genera. Notably, Early Pliocene sympatric C. carcharias and O. megalodon appear to have occupied a similar mean trophic level, a finding that may hold clues to the extinction of the gigantic Neogene megatooth shark.

Helpful or harmful? The comparative value of self-weighing and calorie counting versus intuitive eating on the eating disorder symptomology of college students.

OBJECTIVE: The current study evaluated the comparative implications of self-weighing and calorie counting versus intuitive eating (IE) on the eating disorder (ED) severity of college students. METHODS: In this cross-sectional study, college students in the US [N = 902; 68% female; mean body mass index (BMI) = 24.3] completed the web-based Healthy Bodies Study in 2015. RESULTS: A hierarchical multiple regression analysis revealed that elevated BMI, more frequent self-weighing and calorie counting, and lower IE scores predicted increased ED severity. The results of Kruskal-Wallis H tests indicated that participants with elevated weight statuses engaged in self-weighing and calorie counting more frequently, and possessed lower IE scores, than their lower weight counterparts. CONCLUSION: Engaging in self-weighing and calorie counting was adversely associated with ED severity among the present sample of college students. Cultivating IE within health promotion efforts may, instead, lead to favorable eating-related outcomes that may translate to the holistic health of this population. LEVEL OF EVIDENCE: V cross-sectional descriptive study.

Bridging the Gap: Connecting Resident Assistants and Suicidal Residents Through Gatekeeper Training.

One hundred sixty-two resident assistants (RAs) at a large southeastern university were randomly assigned to attend either a specialized 1-hr training program in suicide prevention (intervention group) or a stress and time management skills training program (control group). The results failed to show the suicide prevention training program had any impact on RA intervention behaviors, resident help-seeking behaviors, or RA perceptions of resident distress and suicidality 4 months following training. Results are interpreted and discussed in relation to strengthening suicide prevention training programs on college campuses.

Differences between winter oilseed rape (Brassica napus L.) cultivars in nitrogen starvation-induced leaf senescence are governed by leaf-inherent rather than root-derived signals.

Nitrogen (N) efficiency of winter oilseed rape (Brassica napus L.) line-cultivars (cvs.), defined as high grain yield under N limitation, has been primarily attributed to maintained N uptake during reproductive growth (N uptake efficiency) in combination with delayed senescence of the older leaves accompanied with maintained photosynthetic capacity (functional stay-green). However, it is not clear whether genotypic variation in N starvation-induced leaf senescence is due to leaf-inherent factors and/or governed by root-mediated signals. Therefore, the N-efficient and stay-green cvs. NPZ-1 and Apex were reciprocally grafted with the N-inefficient and early-senescing cvs. NPZ-2 and Capitol, respectively and grown in hydroponics. The senescence status of older leaves after 12 days of N starvation assessed by SPAD, photosynthesis and the expression of the senescence-specific cysteine protease gene SAG12-1 revealed that the stay-green phenotype of the cvs. NPZ-1 and Apex under N starvation was primarily under the control of leaf-inherent factors. The same four cultivars were submitted to N starvation for up to 12 days in a time-course experiment. The specific leaf contents of biologically active and inactive cytokinins (CKs) and the expression of genes involved in CK homeostasis revealed that under N starvation leaves of early-senescing cultivars were characterized by inactivation of biologically active CKs, whereas in stay-green cultivars synthesis, activation, binding of and response to biologically active CKs were favoured. These results suggest that the homeostasis of biologically active CKs was the predominant leaf-inherent factor for cultivar differences in N starvation-induced leaf senescence and thus N efficiency.

Transcriptomic analysis of nitrogen starvation- and cultivar-specific leaf senescence in winter oilseed rape (Brassica napus L.).

High nitrogen (N) efficiency, characterized by high grain yield under N limitation, is an important agricultural trait in Brassica napus L. cultivars related to delayed senescence of older leaves during reproductive growth (a syndrome called stay-green). The aim of this study was thus to identify genes whose expression is specifically altered during N starvation-induced leaf senescence and that can be used as markers to distinguish cultivars at early stages of senescence prior to chlorophyll loss. To this end, the transcriptomes of leaves of two B. napus cultivars differing in stay-green characteristics and N efficiency were analyzed 4 days after the induction of senescence by either N starvation, leaf shading or detaching. In addition to N metabolism genes, N starvation mostly (and specifically) repressed genes related to photosynthesis, photorespiration and cell-wall structure, while genes related to mitochondrial electron transport and flavonoid biosynthesis were predominately up-regulated. A kinetic study over a period of 12 days with four B. napus cultivars differing in their stay-green characteristics confirmed the cultivar-specific regulation of six genes in agreement with their senescence behavior: the senescence regulator ANAC029, the anthocyanin synthesis-related genes ANS and DFR-like1, the ammonium transporter AMT1;4, the ureide transporter UPS5, and SPS1 involved in sucrose biosynthesis. The identified genes represent markers for the detection of cultivar-specific differences in N starvation-induced leaf senescence and can thus be employed as valuable tools in B. napus breeding.