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Energy availability (EA) is defined as the amount of dietary energy available to sustain physiological function after subtracting the energetic cost of exercise. Insufficient EA due to increased exercise, reduced energy intake, or a combination of both, is a potent disruptor of the endocrine milieu. As such, EA is conceived as a key etiological factor underlying a plethora of physiological dysregulations described in the female athlete triad, its male counterpart and the Relative Energy Deficiency in Sport models. Originally developed upon female-specific physiological responses, this concept has recently been extended to males, where experimental evidence is limited. The majority of data for all these models are from cross-sectional or observational studies where hypothesized chronic low energy availability (LEA) is linked to physiological maladaptation. However, the body of evidence determining causal effects of LEA on endocrine, and physiological function through prospective studies manipulating EA is comparatively small, with interventions typically lasting ≤ 5 days. Extending laboratory-based findings to the field requires recognition of the strengths and limitations of current knowledge. To aid this, this review will: (1) provide a brief historical overview of the origin of the concept in mammalian ecology through its evolution of algebraic calculations used in humans today, (2) Outline key differences from the ‘energy balance’ concept, (3) summarise and critically evaluate the effects of LEA on tissues/systems for which we now have evidence, namely: hormonal milieu, reproductive system endocrinology, bone metabolism and skeletal muscle; and finally (4) provide perspectives and suggestions for research upon identified knowledge gaps.

The carbohydrate deficit induced by exercise is thought to play a key role in increased post-exercise insulin action. However, the effects of replacing carbohydrate utilized during exercise on postprandial glycaemia and insulin sensitivity are yet to be determined. This study therefore isolated the extent to which the insulin-sensitizing effects of exercise are dependent on the carbohydrate deficit induced by exercise, relative to other exercise-mediated mechanisms. Fourteen healthy adults performed a 90-min run at 70% V ˙ O 2 max starting at 1600–1700 h before ingesting either a non-caloric artificially-sweetened placebo solution (CHO-DEFICIT) or a 15% carbohydrate solution (CHO-REPLACE; 221.4 ± 59.3 g maltodextrin) to precisely replace the measured quantity of carbohydrate oxidized during exercise. The alternate treatment was then applied one week later in a randomized, placebo-controlled, and double-blinded crossover design. A standardized low-carbohydrate evening meal was consumed in both trials before overnight recovery ahead of a two-hour oral glucose tolerance test (OGTT) the following morning to assess glycemic and insulinemic responses to feeding. Compared to the CHO-DEFICIT condition, CHO-REPLACE increased the incremental area under the plasma glucose curve by a mean difference of 68 mmol·L−1 (95% CI: 4 to 132 mmol·L−1; p = 0.040) and decreased the Matsuda insulin sensitivity index by a mean difference of −2 au (95% CI: −1 to −3 au; p = 0.001). This is the first study to demonstrate that post-exercise feeding to replaceme the carbohydrate expended during exercise can attenuate glucose tolerance and insulin sensitivity the following morning. The mechanism through which exercise improves insulin sensitivity is therefore (at least in part) dependent on carbohydrate availability and so the day-to-day metabolic health benefits of exercise might be best attained by maintaining a carbohydrate deficit overnight.

Meristic morphological variation among samples of A. neomexicana, A. sexlineata viridis, and their hybrids from Ute and Conchas lakes, New Mexico, was assessed to determine if evidence of separate introductions resided in differences in A. neomexicana between the two localities. However, there were no significant differences between samples of A. neomexicana (and between samples of A. sexlineata viridis) based on multivariate statistical analyses of eight meristic characters. Paradoxically, hybrids from the two localities were significantly different in principal components 1 and 2 and in pigmentation. Hybrids from Ute Lake were paler than hybrids from Conchas Lake and A. neomexicana from both localities. All hybrids could be visually distinguished from A. neomexicana by the absence of thin transverse gray bands on the ventral surface of the tail and closer paravertebral stripes.

The most recent taxonomic revision of the Aspidoscelis hyperythra complex of teiid lizards from Baja California referred all peninsular populations to A. hyperythra and recognized five insular populations as species based on their diagnosability and allopatric distributions. We used multivariate analyses of seven quantitative morphological characters to determine relative distinctiveness and divergence among these populations and the four geographically closest peninsular populations of A. hyperythra. The two northernmost insular species, Aspidoscelis picta (Isla Monserrate) and Aspidoscelis carmenensis (Isla Carmen) were significantly more divergent than were Aspidoscelis danheimae (Isla San José), Aspidoscelis franciscensis (Isla San Francisco), Aspidoscelis espiritensis (Islas Espiritu Santo and Partida Sur), and peninsular samples of A. hyperythra. Divergence information from mtDNA sequence variation divided the five insular species into the same high-divergence/low-divergence groups. We present new testable hypotheses relating to relative times of colonization and peninsular location of the source population of island colonists. La revisión taxonómica más reciente del complejo Aspidoscelis hyperythra de lagartijas teidas de Baja California considera a todas las poblaciones peninsulares como A. hyperythra y reconoce cinco poblaciones insulares como especies basándose sobre en características diagnósticas y distribuciones alopátricas. Utilizamos análisis multivariado de siete características morfológicas cuantitativas para determinar la diferencia y divergencia relativas entre estas poblaciones y las cuatros poblaciones peninsulares más cercanas geográficamente de A. hyperythra. Las dos especies insulares más septentrionales, Aspidoscelis picta (Isla Monserrate) y Aspidoscelis carmenensis (Isla Carmen) fueron significativamente más divergentes que Aspidoscelis danheimae (Isla San José), Aspidoscelis franciscensis (Isla San Francisco), Aspidoscelis espiritensis (Islas Espíritu Santo y Partida Sur), y muestras peninsulares de A. hyperythra. Información sobre la divergencia de estas especies a partir de variación en las secuencias de ADN mitocondrial dividió las cinco especies insulares en los mismos grupos de alta-divergencia/baja-divergencia. Presentamos nuevas hipótesis comprobables sobre los tiempos relativos de colonización y ubicación peninsular de las poblaciones fuente que colonizaron las islas.

The parthenogenetic triploid lizard Aspidoscelis neotesselata originated from a hybridization event between a female of diploid parthenogenetic Aspidoscelis tesselata (pattern class C) and a male of Aspidoscelis sexlineata viridis, and A. neotesselata is morphologically more similar to its maternal progenitor, A. tesselata. The geographic distribution of A. neotesselata is characterized by localized arrays of individuals located within a four-county area in southeastern Colorado, and postorigin divergence is visually evident in its four allopatric color pattern classes (A, B, C, and D). A fundamental pattern of morphological divergence was revealed by a multivariate partitioning of its four color pattern classes into two basic groups: an A group (pattern classes A and D) and a B group (pattern classes B and C). A problem introduced by this grouping is the incongruence between the multivariate similarity of pattern classes A and D and the closer geographic proximity of other color pattern classes to each of A and D. All four color-pattern classes of A. neotesselata had a modified triploid karyotype of 69 + 1 chromosomes (3n = 70). Electrophoretic assessment of 31 nuclear gene loci across the four color-pattern classes revealed postorigin genetic variation only at the mannose-6-phosphate isomerase (MPI) locus, but A. neotesselata had a remarkable heterozygosity of 71% based on these loci.

The range of the parthenogenetic lizard Aspidoscelis tesselatus extends from eastern Chihuahua, Mexico, to southeastern Colorado, USA. In Colorado, pattern-class D, source of the neotype of the species, is syntopic with the more widely distributed pattern-class C only in Ninemile Valley of the Purgatoire River, beyond which, in all directions, these pattern classes are allopatric. We identify a recent northward range expansion of pattern-class C to the same northern latitude attained by pattern-class D, thereby establishing a latitudinal baseline for the species. The two northern arrays of pattern-class C, reported herein, occupied open habitats of sparsely distributed shrubs on rocky slopes, whereas the northernmost arrays of pattern-class D were using juniper woodland. Although this allopatric arrangement suggests ecological segregation of pattern classes, we provide an example of pattern-class C in juniper woodland only ca. 11 km south of the new records, which suggests that other factors could be involved.

The Aspidoscelis cozumela complex of parthenogenetic lizards of the Yucatán Peninsula originated from hybridization between individuals of Aspidoscelis angusticeps (maternal) and Aspidoscelis deppii deppii (paternal) followed by postorigin clonal divergence. A previous report of histocompatibility between two members of the complex, Aspidoscelis maslini and Aspidoscelis cozumela, is reliable evidence that they share a single hybridization event. However, evidence from mitochondrial DNA and histoincompatibility has been used to conclude that another member of the complex, Aspidoscelis rodecki, originated from a separate hybridization event. Although future evidence might tip the balance in favor of a two-hybridization model, we provide examples where evidence from mtDNA and histoincompatibility probably led to incorrect predictions of the number of hybridization events in parthenogenetic Aspidoscelis. In this study, we compared correspondence between patterns of morphological and karyotypic divergence among A. cozumela, A. rodecki, and northern populations of A. maslini and progenitor species to two-hybridization and one-hybridization models. Although morphological and karyotypic patterns can be explained by either model, the most parsimonious alternative is a single hybridization event followed by postformational divergence. El complejo Aspidoscelis cozumela de lagartijas partenogenéticas de la Península de Yucatán se originó de la hibridación entre individuos de Aspidoscelis angusticeps (especie materna) y Aspidoscelis deppii deppii (especie paterna) seguida por una divergencia clonal postorigen. Un reporte previo de histocompatibilidad entre dos miembros de este complejo, Aspidoscelis maslini y Aspidoscelis cozumela, es una evidencia confiable de que ellos comparten un evento de hibridación único. Sin embargo, evidencia de ADN mitocondrial e histoincompatibilidad ha sido utilizada para concluir que otro miembro de este complejo, Aspidoscelis rodecki, se originó de un evento de hibridación separado. Aunque evidencias futuras podrían inclinar la balanza en favor de un modelo de dos-hibridaciones, proporcionamos ejemplos donde evidencias de ADN mitocondrial e histoincompatibilidad probablemente conducen a predicciones incorrectas del número de eventos de hibridación en Aspidoscelis partenogenéticos. En este estudio, comparamos correspondencia entre patrones de divergencia morfológica y cariotípica entre A. cozumela, A. rodecki, y poblaciones del norte de A. maslini y especies progenitoras de modelos de dos-hibridaciones y una-hibridación. Aunque patrones morfológicos y cariotípicos pueden ser explicados por cualquiera de estos dos modelos, la alternativa más parsimoniosa es un evento único de hibridación seguido de divergencia postformativa.

Multivariate analyses of seven meristic characters from 257 specimens of Aspidoscelis hyperythra from Baja California Peninsula, Mexico, revealed pan-peninsular patterns of morphological variation. We observed a significant morphological shift between southern and northern populations at the Isthmus of La Paz, the location of a Pliocene trans-peninsular seaway that could have separated the two groups for a substantial length of time. However, we did not observe fixed character states, such as number of pale dorsal stripes, diagnostic of southern and northern populations. The co-occurrence in local populations of variable numbers of individuals with five, six, or seven stripes makes peninsular A. hyperythra an unusual member of the A. deppii species-group, where number of stripes is consistent in local populations. Samples from the Cape Region were statistically homogeneous based on the characters analyzed, but samples from the latitudinally expansive northern area revealed a pattern of similarities and significant differences unrelated to distances between localities.

Parthenogenetic Aspidoscelis tesselata (pattern class C) reaches the northern limit of its distribution in Ninemile Valley, Otero County, Colorado. Its coexistence with gonochoristic A. sexlineata permitted comparison of diets between species of different sizes, reproductive modes, and evolutionary histories. Based on numbers of prey in stomachs, A. sexlineata had a broader diet than A. tesselata C in June and July; however, breadth of diets calculated from volumes of prey were nearly the same for the two species. Despite the larger size of A. tesselata C, dietary resources were not partitioned by size and foods present exclusively in one species were rare in its diet. Overlap of diets in June could be explained by chance, but this was not true for July. Remarkably high dietary overlap in July resulted from both species taking advantage of an annual surge in abundance of grasshoppers. There was no evidence that either species was affected adversely by presence of the other.

The most recent taxonomic revision of the Aspidoscelis hyperythra complex of teiid lizards from Baja California referred all peninsular populations to A. hyperythra and recognized five insular populations as species based on their diagnosability and allopatric distributions. We used multivariate analyses of seven quantitative morphological characters to determine relative distinctiveness and divergence among these populations and the four geographically closest peninsular populations of A. hyperythra. The two northernmost insular species, Aspidoscelis picta (Isla Monserrate) and Aspidoscelis carmenensis (Isla Carmen) were significantly more divergent than were Aspidoscelis danheimae (Isla San José), Aspidoscelis franciscensis (Isla San Francisco), Aspidoscelis espiritensis (Islas Espiritu Santo and Partida Sur), and peninsular samples of A. hyperythra. Divergence information from mtDNA sequence variation divided the five insular species into the same high-divergence/low-divergence groups. We present new testable hypotheses relating to relative times of colonization and peninsular location of the source population of island colonists.