Body Size Research Articles

A Comparative Metabolomics Study of the Potential Marker Compounds in Feces from Different Hybrid Offspring of Huainan Pigs

As a notable native Chinese genetic population, the Huainan pig has an exceptional meat quality but a low percentage of lean meat and subpar genetic performance. To better exploit the superior genetic traits of the Huainan pig and address knowledge gaps regarding the optimization of its hybrid offspring, this study used Huainan pigs as the maternal line and bred them with Yorkshire, Landrace, and Berkshire sires. This approach produced three hybrid combinations: Yorkshire × Huainan (YH), Landrace × Huainan (LH), and Berkshire × Huainan (BH). The body size, fat ratio, and average backfat thickness of these hybrid progeny were evaluated under the same feeding management and nutritional circumstances. The results revealed that the average backfat thickness of YH was significantly lower than that of LH and BH. In order to better understand the causes of these variations, fecal samples were taken from three pigs in each group for metabolomic analysis. A total of 2291 metabolites were identified, including benzene derivatives (16.6%), amino acids and their metabolites (14.5%), and organic acids (13.4%), with pyruvaldehyde and norethindrone acetate elevated in YH compared to LH and BH. In addition, the three hybrid pig groups commonly exhibited differences in the “glycerophospholipid metabolism” pathway. This variation may also contribute to differences in their fat ratio and backfat thickness. Our findings provide a novel perspective on the role of hybrid vigor in advancing the genetic population of Huainan pigs, while also revealing the unique metabolic characteristics of the YH with regard to fat deposition. This study is expected to enhance the conservation and effective utilization of genetic resources within the Huainan pig population.

Optimizing ring selection for secondary tricuspid regurgitation: the role of body size

Abstract OBJECTIVES To investigate whether a larger prosthetic ring relative to a patient’s body surface area (BSA) is associated with an increased risk of tricuspid regurgitation (TR) recurrence after tricuspid annuloplasty and adverse effects on long-term outcomes. METHODS We retrospectively enrolled 239 patients who underwent tricuspid ring annuloplasty and mitral valve surgery between 2011 and 2016. The tricuspid annuloplasty ring index (TARI) was calculated by dividing the size of the annuloplasty ring (mm) by the BSA (m2). Risk factors for recurrent TR were determined using multivariate analysis. Long-term clinical outcomes were compared between propensity score-matched large and small TARI groups. RESULTS The annuloplasty ring size unadjusted for BSA did not affect TR recurrence (P = 0.388). TARI (subdistribution hazard ratio 1.34; 95% CI 1.07–1.67, P = 0.009) and right ventricular dimension (P = 0.020) were independent risk factors for recurrent TR in multivariate analyses. The cutoff value for discriminating the small from the large TARI group was 19.0 mm/m2. In the matched cohort, the cumulative TR recurrence at 3 years postoperatively was 0% in the small TARI group and 7.1% (95% CI 0–14.8%) in the large TARI group(P = 0.025). The cumulative incidence of adverse events at 3 years postoperatively was 8.3% (95% CI 5.1–16.2%) in the small TARI group and 13.2% (95% CI 3.3–23.0) in the large TARI group (P = 0.085). CONCLUSIONS The patient’s body size might better be considered when determining the tricuspid ring size.

Genetic and biological insights into Hydatigera taeniaeformis in invasive black rats from southern Chile

IntroductionThis study investigates the genetic variability of Hydatigera taeniaeformis in black rats (Rattus rattus), a common tapeworm that infects cats and rodents worldwide. Despite its widespread presence and zoonotic potential, little is known about the genetic diversity of this parasite in the Americas.MethodsWe conducted DNA barcoding analysis using mitochondrial cox1 gene sequences using samples collected from 171 invasive wild black rats, captured in the temperate rainforest of Southern Chile. We also included two adult parasites isolated from road killed Kodkods (Leopardus guigna), a small felid species native to the Americas.ResultsOur findings revealed only two haplotypes, suggesting low genetic variability in a parasite that arrived in the Americas with the Spanish colonization.DiscussionThese haplotypes are more closely related to parasite populations from Peru, Africa, Australia, and Europe, suggesting an origin linked to the Spanish colonization, possibly from North Africa via the Canary Islands. The study also analyzed infection rates, parasite size, and their correlation with host body size, age, and weight, revealing significant patterns. These results provide new insights into the biogeography and genetic diversity of H. taeniaeformis in a new geographical area, enhancing our understanding of its evolutionary history.

Transcriptomic profiling of backfat and muscle in Lijiang pigs with divergent body size across growth stages

Transcriptomic profiling of backfat and muscle in Lijiang pigs with divergent body size across growth stages

A systematic review of observational studies on the association between diet quality patterns and visceral adipose tissue

Abstract Beyond obesity, visceral adipose tissue (VAT) has emerged as an important predictor of chronic disease, but the role of diet quality patterns (DQP) in VAT development is not well defined. Therefore, we conducted a systematic review of how various DQP are associated with VAT via literature searches in PubMed and EMBASE. We included observational investigations in disease-free adults/adolescents that related DQP to VAT assessed by imaging methods. The studies were evaluated separately for a priori and a posteriori DQP and according to design differences. Study quality was assessed using the Risk of Bias in Non-randomised Studies of Interventions tool. Of the 1807 screened articles, thirty-five studies met the inclusion criteria. The majority of a priori indices, for example, the Healthy Eating Index, showed significant inverse associations with VAT, while only a small proportion of a posteriori patterns were related to VAT. Results did not differ substantially by the method of exposure and outcome assessment or between studies with (n 20) or without (n 15) body-size adjustment, but significant findings were more common in younger v. older individuals, USA v. other populations and investigations with moderate v. serious risk of bias. The heterogeneity of the existing literature limited the ability to quantify the magnitude of the associations across studies. These findings suggest that a high-quality diet, as assessed by a priori DQP, is generally inversely associated with VAT, but results for a posteriori DQP are less consistent. As associations persisted after adjusting for body size, diet quality may beneficially influence VAT beyond its association with obesity.

Using unmanned aerial vehicles to estimate body volume at scale for ecological monitoring

Abstract Demographic data are essential to construct mechanistic models to understand how populations change over time and in response to global threats like climate change. Existing demographic data are either lacking or insufficient for many species, particularly those that are challenging to obtain direct measurements from that can be used to estimate demographic rates, like marine mammals. A method for collecting accurate demographic data to construct robust demographic models at scale would fill this knowledge gap for difficult‐to‐access species. We introduce a novel, non‐invasive method to estimate the 3D body size (volume) of pinnipeds (seals, sea lions and walruses) that will allow monitoring at high spatial and temporal scales. Our method integrates 3D structure‐from‐motion photogrammetry data collected via planned flight missions using off‐the‐shelf, multirotor unmanned aerial vehicles (UAVs). We apply and validate this method on the grey seal Halichoerus grypus, a pinniped species that spends much of its time at sea but is predictably observable during its annual breeding season. We investigate the optimal ground sampling distance (GSD) for surveys by calculating the success rates and accuracy of volume estimates of individuals at different altitudes. Based on current technology, we establish an optimal GSD of at least 0.8 cm px−1 for animals similar in size to UK grey seals (~1.2–2.5 m length), making our method reproducible and applicable to other species. We found volume estimates were accurate and could be successfully estimated for up to 68% of hauled‐out seals in study areas. Our method accurately estimates individual body volume of pinnipeds in a time‐ and cost‐effective manner while minimising disturbance. While the approach is applied to pinnipeds here, the method could be adapted to further taxa that are otherwise challenging to obtain direct measurements from. Our proposed approach therefore has the potential to fill demographic research gaps, which will improve our ability to protect and conserve species into the future.

The relationship between body size and diet breadth in non‐web building spiders

Abstract The handling hypothesis assumes that large animals have broader diets than small animals because small animals are restricted to small prey while large animals can handle both small and large prey. On the other hand, the optimal foraging hypothesis assumes no relationship between predator body size and diet breadth because large predators should shift from small prey to large prey to maximize the net energy intake. Using the diets and body sizes of 89 non‐web building spider species obtained from public databases (World Spider Trait database, Spiders of Europe and World Spider Catalogue), we investigated the relationship between body size and diet breadth in this group of terrestrial arthropod predators. We found that the taxonomic niche diet breadth measured by Hill numbers (i.e., prey richness, Shannon index of diversity and Simpson index of equitability) had no significant relationship with spider body size. The minimum prey size did not show any relationship with spider body size. On the other hand, mean, maximum, range and variability in prey size increased with spider body size. The results provide support for both hypotheses. The pattern in prey taxonomic dimension supports the optimal foraging hypothesis, while the pattern in prey size dimension supports the handling hypothesis. Overall, the observed distinct patterns between the taxonomic and prey size niche axes can be caused by the fact that the non‐web building spiders utilize similar prey types that are however highly size‐structured.

Is Alcohol Consumption Pattern Dependent on Prenatal Sex‐Steroids? A Digit Ratio (2D:4D) Study Among University Students

ABSTRACTIntroductionThere is evidence that alcohol consumption is influenced by prenatal sex steroids (as measured by digit ratio [2D:4D]). Here, we clarify the effect size of the relationship in a student (rather than a patient) population.MethodsThere were 258 (169 women) participants. Digit length was measured directly with calipers. Alcohol use was evaluated by the Polish version of Alcohol Use Disorders Identification Test (AUDIT) and operationalized as total AUDIT scores and grams of alcohol/week.ResultsDigit ratios were sexually dimorphic (males < females). There were negative correlations between right 2D:4D and Dr‐l (right 2D:4D minus left 2D:4D) and AUDIT scores and grams of alcohol/week in both sexes. Relationships varied from small (r = −0.29) to large (r = −0.69) and they were stronger in males in comparison to females and for right 2D:4D in comparison to Dr‐l. In males only, there were small (r = 0.21) to moderate (r = 0.31) positive associations with body size (height, weight, and mean right digit length) and alcohol consumption. Multiple regression analyses showed relationships between digit ratios remained significant but those for body size did not.ConclusionAlcohol consumption was negatively related to 2D:4D, suggesting high prenatal testosterone and low prenatal estrogen are linked to its consumption. Correlations varied in strength from small to large with the strongest found for right 2D:4D and for males. Positive relationships between body size and alcohol were small to moderate, confined to males, and were not independent of digit ratios. Prenatal androgenization may influence alcohol drinking patterns in non‐clinical individuals.

Ontogenetic scaling of disc width with total length in west African batoids

AbstractMorphological scaling describes changes in the size or shape of one morphological character (e.g. mass, length, width, area etc.) as another increases in size. Understanding how morphological characters scale with body size can shed light on how natural selection influences morphology, and the nature of ecomorphological relationships through ontogeny. Batoids (Elasmobranchii: Batoidea) are a highly specialised lineage of cartilaginous fishes displaying extreme dorsoventral flattening. Despite this, little is known about morphological scaling in batoids compared to sharks. In this study we test the relationship between disc width and total length in five batoid species (Torpedo torpedo, Mobula tarapacana, Fontitrygon margarita, Raja parva, Rhinobatos irvinei) representing four orders that differ in both ecology and morphology, measured from artisanal fisheries in Western and Central Ghana. Whilst a lack of existing ecological data presents some limitations, our results are broadly consistent with ecomorphological theory previously applied to sharks. Moreover, we find that for some lineages (including some myliobatiform taxa) total length may represent a valid proxy for estimating overall body size. This finding has applications for body size and shape estimation in partially processed batoids obtained from fishing camps, and extinct taxa known only from incomplete or fragmented remains.

Unravelling the diversity of Posthodiplostomum Dubois, 1936 (Trematoda: Diplostomidae) in fish-eating birds from the Neotropical region of Mexico, with the description of a new species

Abstract Adults of the genus Posthodiplostomum, Dubois, 1936 are parasites of fish-eating birds, mainly of the family Ardeidae, and are globally distributed. The genus currently comprises 35 species, although recent molecular evidence has shown that the diversity of the genus is underestimated since several candidate species have been recognized. In the Neotropical region of Mexico, at least 6 Posthodiplostomum lineages have been detected with metacercaria stages recovered from unrelated fish hosts. Here, we obtained adult specimens of Posthodiplostomum from 6 fish-eating birds representing 2 families (Butorides virescens, Ardea herodias, Nycticorax nycticorax, Tigrisoma mexicanum – Ardeidae, and Rynchops niger and Leucophaeus atricilla – Lariidae) from 4 localities in southern Mexico. Specimens were sequenced for 2 nuclear (28S and ITS1–5.8S–ITS2) and 1 mitochondrial (cox1) molecular marker. Phylogenetic analyses allowed us to link metacercariae and adult specimens and recognized a lineage, which was described morphologically. The new species can be distinguished from its congeners by its prosoma morphology and body size; this is the first described species in the Neotropical region of Mexico. Additionally, new host and locality records for P. macrocotyle and P. pricei are presented, expanding their geographical distribution range in the Americas.

Among-population variation in sexual size dimorphism of Nepa cinerea (Heteroptera: Nepomorpha)

AbstractNepa cinerea is a common aquatic predator. It is widely distributed in various aquatic biotopes of the Palaearctic. Body size is an important trait that impacts the fitness and survival of animals and thus their evolution. Like most insects, N. cinerea males are smaller than females, but sexual size dimorphism (SSD) has not been investigated. We investigated the extent of SSD, the validity of Rensch's rule, and morphological differences between nine populations based on 51 morphological parameters. Females were found significantly and marginally significantly larger than males in all but one population. However, no difference in body length was found between the populations. Population had a significant effect on 12 traits for females and only on 5 for males, showing that females are more variable than males. Specifically, differences were found in the length of the second and third leg pairs. We did not find any support for the Rensch's rule. Nevertheless, female body size and SDI (sexual dimorphism index) were positively correlated. We suggest that fecundity selection accounts for female-biased SSD in N. cinerea.

The osteology of Triisodon crassicuspis (Cope, 1882): New insights into the enigmatic “archaic” placental mammal group “Triisodontidae”

Following the end-Cretaceous mass extinction, mammals underwent an increase in body size, taxonomic diversity and ecological specialization throughout the Paleocene, exemplifying their adaptability. One especially enigmatic group is the “Triisodontidae”, medium- to large-sized ungulate-like placentals from the Paleocene which are best known from their teeth that exhibit adaptations towards carnivory. The “triisodontids” were the first large carnivorous mammals and pre-date, and may have given rise to, Mesonychia, a group of more specialized placental carnivores. The “triisodontids” have been well-described from dental material, although very little is known about their postcrania. Here, we describe the postcrania of Triisodon crassicuspis–the most completely represented species of the genus to date–from a specimen (NMMNH P-72096) recovered from basal Torrejonian strata of the Nacimiento Formation in the San Juan Basin, New Mexico. Anatomical comparisons reveal that the forelimb long bones of Tri. crassicuspis are robust relative to its size, more so than other “triisodontids”. Attachment sites on the ulna are evidence of well-developed muscles involved in powerful extension and flexion of the manus. In Tri. crassicuspis, the range of pronation-supination was limited as evident from the humeroradial morphology. Qualitative functional assessment of osteological features of the forelimb of Tri. crassicuspis is suggestive of terrestrial locomotion with at least moderate digging ability. Re-analyses of the dentition confirmed that Tri. crassicuspis had specializations for carnivory, and provide a body mass estimate of ca. 32–44 kg based on dental proxies. In summary, Tri. crassicuspis was a relatively large and powerful terrestrial animal, and one of the first known placentals to fill a largely carnivorous niche.

Body mass and growth rates predict protein intake across animals

Organisms require dietary macronutrients in specific ratios to maximize performance, and variation in macronutrient requirements plays a central role in niche determination. Although it is well recognized that development and body size can have strong and predictable effects on many aspects of organismal function, we lack a predictive understanding of ontogenetic or scaling effects on macronutrient intake. We determined protein and carbohydrate intake throughout development on lab populations of locusts and compared to late instars of field populations. Self-selected protein:carbohydrate targets declined dramatically through ontogeny, due primarily to declines in mass-specific protein consumption rates which were highly correlated with declines in specific growth rates. Lab results for protein consumption rates partly matched results from field-collected locusts. However, field locusts consumed nearly double the carbohydrate, likely due to higher activity and metabolic rates. Combining our results with the available data for animals, both across species and during ontogeny, protein consumption scaled predictably and hypometrically, demonstrating a new scaling rule key for understanding nutritional ecology.

A new species of the genus Scincella (Squamata: Scincidae) from Mount Fansipan, Hoang Lien Son Range, northwestern Vietnam

We describe a new species of the genus Scincella Mittleman, 1950 from northwestern Vietnam, based on a new collection of ground skinks from Mount Fansipan of the Hoang Lien Son Range in Lao Cai Province. Scincella fansipanensis sp. nov. is distinguished from other Scincella species in the Indochina region and southern China by body size (SVL), separation of prefrontals, number of midbody scale rows, paravertebral scale rows, nuchals and subdigital lamellae on toe IV, separation of toe from finger when limbs are adpressed along the body, and dorsal color pattern. The new species is further distinguished from its congeners by uncorrected genetic distances of 14.60–21.41% (COI gene). The new species is currently known only from high elevation areas of Mt. Fansipan in Vietnam.

Accounting for Salmon Body Size Declines in Fishery Management Can Reduce Conservation Risks

ABSTRACTChanges in population demographic structure can have tangible but unknown effects on management effectiveness. Fishery management of Pacific salmon is often informed by estimates of the number of spawners expected to produce maximum sustainable yield (SMSY), implicitly assuming that reproductive output per spawner does not change over time. However, many salmon populations have experienced long‐term trends in age, sex and length compositions that have resulted in smaller body sizes of mature fish. We present an empirically based simulation approach for evaluating management implications of declining reproductive output resulting from shifting demographics. We simulated populations with or without demographic trends, selective or unselective harvests, and harvest policies based on assessment methods that did or did not account explicitly for demographic trends when estimating SMSY. A management strategy evaluation showed reduced expected harvests and run sizes when populations exhibited negative demographic trends. Reduced abundances and increased conservation risks (higher probability of falling below an abundance threshold) could be partially mitigated by using stock‐recruitment analyses based on total egg mass instead of spawner abundance, or via precautionary management where target escapements were higher than SMSY, especially in fisheries that selectively removed large fish. Explicit accounting of demographic trends in stock‐recruit analyses resulted in up to 25% higher run sizes and up to 20% lower conservation risks compared to traditional methods when trends toward smaller, younger and male‐biased runs were present in the population. Conservation of population demographic structure may be critical for sustaining productive fish populations and their benefits to ecosystems and people.

Sex-dependent shifts in body size and condition along replicated elevational gradients in a montane colonising ectotherm, the common wall lizard (Podarcis muralis)

Sex-dependent shifts in body size and condition along replicated elevational gradients in a montane colonising ectotherm, the common wall lizard (Podarcis muralis)

A Novel and Comprehensive Wellness Assessment for Lifestyle‐Based Interventions

ABSTRACTObjectiveWhile weight status and clinical laboratory measures are important in assessing obesity‐related disease severity and chronic disease risk, including a broader range of emotional, psychosocial, and behavioral factors would provide greater context of an individual's overall state of wellness and could be used to better guide treatment decisions. The purpose of this research was to develop a comprehensive Lifestyle Wellness assessment for use in lifestyle‐based wellness interventions and programs.MethodsA cross‐sectional exploratory factor analysis (EFA) was conducted using baseline data from N = 138 adults participating in behavioral weight loss trials. An unweighted least squares extraction method with oblique rotation was used. Twenty‐one putative measures spanning constructs related to metabolic health, emotional health/wellbeing, body weight and composition, diet quality, and fitness were analyzed for retention.ResultsMean body mass index (BMI) was 38.0 ± 6.6 kg/m2, mean age was 57.3 ± 11.1 years, and 77.5% of participants were female. The EFA produced a five‐factor model with 13 items that explained 80.3% of the variance. The retained factors included: (1) Psychosocial State: mindfulness, resilience, quality of life, and happiness; (2) Blood Pressure State: systolic and diastolic blood pressure; (3) Lipid State: total cholesterol and LDL‐cholesterol; (4) Fitness State: grip strength, jump height, and percent body fat; and (5) Body State: BMI and waist circumference.ConclusionsLifestyle Wellness is a comprehensive assessment that enables innovative wellness‐related research such as metabolically healthy obese phenotypes and weight‐neutral interventions. Future research should include investigations in additional populations with greater age, sex/gender, and body size diversity.

Associations of linear growth with body composition of perinatally HIV-infected African adolescents

Abstract The prevalence of poor linear growth among African children with perinatally acquired HIV remains high. There is concern that poor linear growth may to lead to later total and central fat deposition and associated non-communicable disease risks. We investigated associations between height-for-age Z score (HAZ) and total and regional fat and lean mass measured by dual-energy X-ray absorptiometry, expressed as internal population Z scores, among 839 Zimbabwean and Zambian perinatally HIV-infected male and female adolescents aged 11–19 years. Stunting (HAZ < –2) was present in 37 % of males and 23 % of females. HAZ was strongly positively associated with total, trunk, arm and leg fat mass and lean mass Z scores, in analyses controlling for pubertal stage, socio-economic status and HIV viral load. Associations of linear growth with lean mass were stronger than those with fat outcomes; associations with total and regional fat were similar, indicating no preferential central fat deposition. There was no evidence that age of starting antiretroviral therapy was associated with HAZ or body composition. Non-suppressed HIV viral load was associated with lower lean but not fat mass. The results do not support the hypothesis that poor linear growth or stunting are risk factors for later total or central fat deposition. Rather, increased linear growth primarily benefits lean mass but also promotes fat mass, both consistent with larger body size. Nutritional and/or HIV infection control programmes need to address the high prevalence of stunting among perinatally HIV-infected children in order to mitigate constraints on the accretion of lean and fat mass.

Relative facial width, and its association with canine size and body mass among chimpanzees and bonobos: Implications for understanding facial width‐to‐height ratio expression among human populations

AbstractObjectivesFacial width‐to‐height ratio (fWHR) has been widely investigated in the context of its role in visual communication, though there is a lack of consensus about how fWHR serves as a social signal. To better understand fWHR variation in a comparative context, we investigate the associations between fWHR and canine crown height (CCH) and body mass, respectively, among two chimpanzee subspecies (Pan troglodytes schweinfurthii, Pan troglodytes troglodytes) and bonobos (Pan paniscus).Materials and MethodsWe collected landmark data from 3D surface models of 86 Pan cranial specimens to quantify fWHR and upper CCH, and to estimate body mass. We used Spearman's r and Kruskal‐Wallis tests to test for significant relationships among variables, and to assess sexual dimorphism.ResultsThere is an inverse relationship between fWHR and CCH in both sexes of Pan, however there are interpopulation differences in the relationship between fWHR and CCH among Pan taxa. Pan paniscus have relatively wide faces and small canine crowns, and wide faces in Pan t. schweinfurthii males may be driven by body size constraints. Pan troglodytes and Pan paniscus show fWHR dimorphism, and Pan paniscus have significantly higher fWHRs than do either Pan troglodytes subspecies.DiscussionOur findings indicate that CCH and facial breadth may serve subtly different signaling functions among Pan taxa. Further research into the circumstances in which wide faces evolved among chimpanzees and bonobos will likely afford deeper insights into the function of relatively wide faces in the context of visual signaling among humans and our extinct hominin relatives.

The underlying causes of differential migration: assumptions, hypotheses, and predictions

ABSTRACTMechanisms governing the migratory decisions of birds have long fascinated ecologists and sparked considerable debate. Identifying factors responsible for variation in migration distance, also known as differential migration, has been a popular approach to understanding the mechanisms underlying migratory behaviour more generally. However, research progress has been slowed by the continued testing of overlapping, non‐mechanistic, and circular predictions among a small set of historically entrenched hypotheses. We highlight the body size hypothesis and suggest that the predictions commonly tested have impeded progress because body size relationships with migration distance are predictions made by several distinct hypotheses with contrasting mechanisms. The cost of migration itself has not been adequately accounted for in most hypotheses, and we propose two flight efficiency hypotheses with time‐ and energy‐minimizing mechanisms that allow individuals to mitigate the risks inherent to longer migrations. We also advance two conceptual versions of the social dominance hypothesis based on two distinct underlying mechanisms related to distance minimization and food maximization that will help clarify the role of competition in driving migratory decisions. Overall, we describe and refine 12 mechanistic hypotheses proposed to explain differential migration (along with several other special‐case hypotheses), seven of which have underlying mechanisms related to food limitation as past research has identified this to be an important driver of differential migration. We also thoroughly reviewed 145 publications to assess the amount of support for 10 critical assumptions underlying alternative hypotheses for differential migration in birds. Our review reveals that surprisingly few studies explicitly evaluate assumptions within a differential migration context. Generating and testing strong predictions and critical assumptions underlying mechanisms of alternative hypotheses will improve our ability to differentiate among these explanations of differential migration. Additionally, future intraspecific progress will be greatest if investigators continue to focus on mechanisms underlying variation in migration distance within rather than among demographic classes, as previous research has found differing mechanisms to be responsible for differential migration among demographic classes. Interspecifically, a thorough comparative analysis that seeks to explain variation in migration distance among species would broaden both our understanding of the mechanisms regulating current differential migration patterns and those that led to the evolution of migration more generally. Collectively, we provide a framework that, together with advances in animal‐borne tracking and other technology, can be used to advance our understanding of the causes of differential migration distance, and migratory decisions more generally.