SEARCH · Search PubMed

Results for “Thyroid Hormones”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

85 records · Page 5Linked to original sources

PubMed2026

Cis-regulatory variation in the MdCKX6 promoter is associated with allele-specific expression and fruit size in apple.

Fruit size is a key determinant of apple fruit quality and market value and is strongly influenced by phytohormone-regulated cell proliferation and expansion during early fruit development. Cytokinin oxidase/dehydrogenase (CKX) enzymes regulate cytokinin homeostasis by irreversibly degrading active cytokinins, but the contribution of natural variation in CKX genes to fruit size remains poorly understood. Here, we identified MdCKX6 as a candidate regulator of fruit growth in apple (Malus domestica). MdCKX6 exhibited pronounced allele-specific expression during fruit development in the cultivar 'Royal Gala'. Sequence analysis identified a promoter SNP associated with differential promoter activity and allele-specific expression. Genotyping of diverse apple cultivars and wild Malus accessions revealed a significant association between MdCKX6 promoter genotype and fruit size. Cultivars carrying low-expression alleles produced larger fruits, whereas high-expression alleles were associated with smaller fruits. To investigate gene function, MdCKX6 was overexpressed in tomato, resulting in reduced fruit size. Histological analyses of the transgenic tomato fruit revealed smaller pericarp cells. Transcriptome analysis of transgenic fruits revealed widespread changes in genes associated with cell-cycle regulation, cell wall modification, hormone-related processes, and transcriptional regulation. Together, these results identify MdCKX6 as a potential negative regulator of apple fruit growth and reveal an association between cis-regulatory variants, gene expression, and fruit size. This study provides new insights into the role of cytokinin metabolism in fruit development and highlights regulatory variation in MdCKX6 as a potential target for apple breeding.

Malus↗

PubMed2026

Autophagy activation in granulosa cells as a mechanism of astaxanthin action: evidence from a pilot randomised trial in PMOS-associated infertility.

Astaxanthin (AST) has been reported to influence oxidative stress, endoplasmic reticulum stress, and apoptosis in women with polyendocrine metabolic ovarian syndrome (PMOS), formerly referred to as polycystic ovary syndrome (PCOS), but its effects on granulosa-cell (GC) autophagy remain unclear. Given the central role of autophagy in follicular development, this triple-blind, placebo-controlled pilot randomised trial evaluated whether AST modulates autophagy-related signalling in GCs and how these molecular effects relate to ovarian response. Fifty women with PMOS-related anovulatory infertility were enrolled between November 2023 and September 2024 and received AST (12 mg/day) or placebo for six weeks prior to oocyte retrieval; forty-four completed the study (21 AST, 23 placebo). Primary exploratory endpoints were molecular markers of adenosine monophosphate-activated protein kinase (AMPK)-autophagy signalling, and primary clinical outcomes included ovarian response indicators and cleavage stage embryo quality. AST supplementation increased autophagy-related gene 7 (ATG7) expression, enhanced autophagy flux, reduced apoptosis, and showed a trend toward increased AMPK activation. Before adjustment, AST improved oocyte maturity rate (OMR) and increased mature (metaphase II; MII) oocyte yield. After adjusting for age, body mass index, and anti-mullerian hormone level, total oocyte and MII oocyte yields remained significantly higher with AST, while OMR became non-significant. Among embryology outcomes, both the top-ranking embryo rate and the number of embryos suitable for cryopreservation were significantly higher with AST after adjustment. Pregnancy outcomes were numerically higher but not statistically significant. This pilot trial suggests that AST activates autophagy- and apoptosis-related pathways in GCs and may enhance oocyte competence and embryo quality in PMOS. Larger studies are needed to confirm these mechanistic and clinical effects.

Female↗

PubMed2026

Sex-stratified analyses of femoral intercondylar notch morphology and ACL injury risk: a systematic review and meta-analysis.

BACKGROUND: Anterior cruciate ligament (ACL) injury is more common in females. Although ACL injury risk is multifactorial, involving neuromuscular, hormonal, and biomechanical factors, femoral intercondylar notch morphology is a proposed non-modifiable anatomical contributor that may help explain this disparity. This review synthesizes evidence on its association with ACL injury risk, analyses stratified by male and female sex. METHODS: A literature search of PubMed, Web of Science, and Scopus was conducted through January 2026. After duplicate removal, 527 unique records were screened and 134 full texts assessed for eligibility. Twenty-four studies met inclusion criteria, with 22 providing quantitative data. Data were extracted on ACL-injured (ACLi) and control (ACLc) knees, stratified by sex. RESULTS: ACLi knees had narrower notch widths than ACLc knees in both females (MD -1.74 mm, 95% CI -2.54 to -0.95) and males (MD -1.31 mm, 95% CI -1.94 to -0.68), with substantial heterogeneity (I2 81-95%). NWI differences were significant in males (MD -0.01, 95% CI -0.02 to 0.00) but not females. Limited notch shape data suggested higher ACL injury odds with A-shaped notches in females (OR 2.85, 95% CI 0.07-110.66) and males (OR 1.67, 95% CI 0.00-3.98 × 105). CONCLUSION: Narrower femoral intercondylar notch width was consistently associated with ACL injury in both sexes, whereas findings for NWI and notch shape were limited by heterogeneity and sparse data. These results provide a structured synthesis of sex-specific anatomical risk factors and support femoral notch narrowing as a shared contributor, while highlighting the need for prospective, standardized studies in the future.

Humans↗

PubMed2026

Screening of Estrogenic and Antiestrogenic Effects of Estradiol, Bisphenol A, and Fulvestrant Using 2D and 3D Breast Cancer Cell Systems With a Luciferase Reporter Gene Assay.

Endocrine-disrupting chemicals (EDCs) like bisphenol A (BPA) pose health risks by interfering with hormones. This study develops and utilizes in vitro 2D and 3D cell models to evaluate the estrogenic and antiestrogenic properties of compounds. Human breast cancer cell lines T47D and MCF7, stably transfected with a luciferase reporter gene (ERE-LUC), were first compared in 2D. Due to the significantly higher sensitivity and responsiveness observed in the T47D line during preliminary 2D screenings, this cell line was exclusively selected for the development of the 3D spheroid model. Cells were treated with 17β-estradiol (E2), BPA, and Fulvestrant (FUL) to assess cell viability and luciferase activity. In 2D models, T47D ERE-LUC cells showed higher responsiveness than MCF7 ERE-LUC, which failed to show significant luciferase induction with E2. In the 3D T47D model, cells exhibited significant and robust changes in luciferase activity in response to E2 and BPA, highlighting the enhanced fidelity of 3D cultures in replicating tissue conditions compared to their 2D counterparts. The study highlights the effectiveness of 3D models over 2D in evaluating estrogenic activity. Specifically, the 3D T47D ERE-LUC system serves as a superior, sensitive, and reliable platform for screening EDCs, offering benefits in cost, data speed, and reduced in vivo reliance.

Humans↗

PubMed2026

Fatty acids and breast cancer: Epidemiology, subtype-specific metabolism, immune regulation, and clinical translation.

Fatty acids (FAs) are bioactive dietary and metabolic molecules that participate in membrane architecture, energy homeostasis, inflammatory signaling, gene regulation and immune function, all of which intersect with breast cancer (BC) risk, progression and treatment response. In this narrative review we integrate epidemiological, clinical, translational and mechanistic evidence on the role of FAs in BC. Saturated, monounsaturated, trans- and polyunsaturated FAs (PUFAs) are treated as distinct biological exposures rather than interchangeable measures of total fat intake. Similarly, evidence from dietary assessment, circulating biomarkers, erythrocyte membrane composition, adipose tissue stores and tumor lipid signatures is interpreted separately, because each captures exposure and biology at a different level. BC subtypes differ in FA synthesis, uptake, oxidation, storage and remodeling: luminal tumors are frequently linked to hormone-regulated lipogenesis, human epidermal growth factor receptor 2 (HER2)-positive tumors to growth-factor-driven lipid metabolism, and triple-negative tumors to exogenous FA uptake, inflammatory lipid mediators and ferroptosis-related vulnerabilities. FA-derived mediators also shape immune-cell polarization, cytokine signaling and the tumor microenvironment, and dietary FAs may reshape the gut microbiota; the fiber-derived short-chain FAs it produces, distinct from dietary FAs, likewise help regulate immune and inflammatory tone. Clinical data suggest possible roles for fat-quality modification and selected n-3 PUFA interventions, but findings are heterogeneous and not yet sufficient to support routine biomarker-guided precision onco-nutrition. Candidate biomarkers, such as erythrocyte n-6:n-3 composition, require prospective validation before clinical implementation. FA biology thus represents a modifiable but complex axis in BC prevention, tumor biology and supportive care.

Humans↗

PubMed2026

Phenotypic, physiological and transcriptomic analysis of graded salt stress responses in Pyrus betulifolia Bunge and functional characterization of the hub gene PbSTY46.

Pyrus betulifolia Bunge is a salt‑tolerant rootstock for pear, but its salt‑tolerance mechanisms remain largely unknown. In this study, P. betulifolia seedlings were subjected to graded NaCl stress at concentrations of 0 (CK), 50 (T1), 100 (T2), and 200 (T3) mM. We integrated phenotypic observation, physiological assessment, transcriptomic profiling, and functional gene validation to systematically elucidate its salt tolerance mechanisms. Salt stress inhibited seedling growth and root traits in a concentration-dependent manner, and T3 caused the most severe damage. Osmotic solutes responded differentially: soluble sugars peaked under T2, while proline peaked under T3. Antioxidant enzymes showed tissue-specific biphasic responses and declined after prolonged T3 stress. Meanwhile, chlorophyll and photosynthesis decreased, whereas anthocyanin increased, indicating a metabolic shift from photosynthesis to photoprotection. Transcriptome analysis revealed distinct responses depending on stress intensity: mild stress induced membrane lipid remodeling, moderate stress activated circadian rhythm and hormone signaling, and severe stress enhanced phenylpropanoid biosynthesis and thiamine metabolism. Gene Set Enrichment Analysis (GSEA) further highlighted progressive enrichment of phenylpropanoid biosynthesis, heme binding, and oxidoreductase activity. Weighted Gene Co‑expression Network Analysis (WGCNA) identified a blue module significantly positively correlated with root traits, from which the hub gene PbSTY46 was identified. Functional validation via overexpression, loss‑of‑function mutants, and pharmacological interventions (MeJA/DIECA) confirmed that PbSTY46 acts through JA signaling to enhance antioxidant enzyme activities and thereby confer salt tolerance. Collectively, P. betulifolia adopts a "survival‑first" strategy that coordinates growth arrest, osmotic homeostasis, and ROS scavenging. These findings establish PbSTY46 as a key regulator that links JA signaling to antioxidant defense. Thus, PbSTY46 represents a promising candidate for marker‑assisted breeding of salt‑tolerant pear cultivars.

Salt Stress↗

PubMed2026

Factors Associated With Menopause Symptoms: A Systematic Review and Meta-Analysis.

BACKGROUND: Menopause, marked by hormonal decline and menstrual cessation, is associated with various symptoms. Socio-demographic and behavioural factors may influence symptom type and severity. Understanding these associations can inform better symptom management. OBJECTIVES: To identify factors associated with the presence and severity of menopausal symptoms through systematic review and meta-analysis. SEARCH STRATEGY: We searched Medline, Embase, CINAHL and Cochrane for studies on demographic, behavioural, or health factors linked to vasomotor, vaginal dryness and joint symptoms in women aged 40-60. SELECTION CRITERIA: Studies reporting odds ratios or raw numbers for symptom presence or severity were included. DATA COLLECTION AND ANALYSIS: Studies were combined for meta-analysis, reporting odds ratios and 95% confidence intervals. Quality assessment was performed to quantify the risk of bias. RESULTS: Of 9228 screened articles, 61 were meta-analysed. Compared with White women, Black women had higher odds of vasomotor symptom presence (OR 1.65, 1.41-1.94) and severity (OR 1.91, 1.10-3.29), and vaginal dryness presence (OR 1.27, 1.10-1.47), while Asian had lower vasomotor symptom presence and severity (OR 0.40, 0.22-0.72; OR 0.55, 0.53-0.56). Higher education (OR 1.31, 1.09-1.56), high income (OR 1.41, 1.01-1.97) and depression (OR 2.36, 1.51-3.70) were associated with increased presence of vasomotor symptoms. Smoking and obesity were associated with both presence (OR 1.63, 1.30-2.04 and 1.35, 1.02-1.78) and severity (OR 1.56, 1.07-2.27 and 1.42, 1.11-1.83) of vasomotor symptoms. CONCLUSION: Socio-demographic and behavioural factors, including ethnicity, education, income, smoking, obesity and depression, influence menopausal symptoms, highlighting the need for personalised care. TRIAL REGISTRATION: PROSPERO number: CRD42023459154.

Humans↗

PubMed2026

Microbial allies in a cotton pest: A descriptive account of associated microbiota dynamics in Dysdercus cingulatus across development.

BACKGROUND: Hemipteran insects harbour several symbiotic partners, mainly bacteria, which play pivotal roles for hosts like dietary provision, support overall physiology, xenobiotic degradation and manipulate/regulate behaviour. Most of these symbionts usually reside and operate from the digestive tracts of the animals. Cotton is one of the major cash crops in India and Dysdercus cingulatus (D. cingulatus) though a secondary pest, is causing significant destruction of cotton bolls, poor lint quality and reduce oil content of seeds. Premature opening of cotton bolls often leads to bacterial and fungal infections, thus resulting in extensive economic loss worldwide. D. cingulatus is a hemimetabolous insect that comprises of developmental stages like egg, nymph (5 instar stages), and adult. The present work explored the ontogeny specific diversity in the associated microbiota and predicted their probable functional inputs in D. cingulatus. RESULTS: The data obtained using 16S rRNA gene sequencing (NovaSeq 6000) revealed presence of members of Proteobacteria (65.83%), Firmicutes (24%), Actinobacteria (10%) phyla throughout the ontogeny of D. cingulatus. Highest alpha diversity of these symbiotic bacteria was recorded in the third instar nymphs in contrast to rest of the developmental stages. Among all the observed genera, Stenotrophomonas, Hungatella and Glutamicibacter were predominant from egg to adult stages. MicFunPred, a tool used for predicting the probable functional inputs of these symbionts, hinted at their probable stage specific contribution in crucial biochemical pathways such as polyketide biosynthesis, ascorbate/aldarate metabolism, pentose phosphate and glyoxylate cycles, steroid hormone and peptidoglycan biosynthesis, and glycolysis/pyruvate metabolism. CONCLUSIONS: The primary investigations on the ontogenetic composition and diversity of associated microbiota, suggest dynamic shifts in D. cingulatus, concurrent with their probable functions/roles in the host development and metabolism. To the best of our knowledge, this is the first report on symbiotic microbiota variation across the developmental stages of D. cingulatus that provides preliminary descriptive observations that may guide future functional and experimental investigations into microbiota-based pest management.

Animals↗

PubMed2026

Social support and cognitive function in postmenopausal women.

OBJECTIVE: An active social lifestyle may protect against cognitive decline in older adults, but few studies have examined specific social support dimensions in relation to specific cognitive domains. This study examines associations between social support dimensions and cognitive function in older women. METHODS: The Early versus Late Intervention Trial with Estradiol (ELITE) was a randomized, double-blinded, placebo-controlled trial of oral 17β-estradiol versus placebo in postmenopausal women, designed primarily to evaluate the timing hypothesis on atherosclerosis progression and cognitive decline; the present analysis is a secondary, hypothesis-generating examination of the psychosocial substudy data (2009-2012). A total of 448 women completed psychosocial assessments every 6 months. Cognitive function was assessed at baseline, 2.5, and 5 years using composite scores for executive function, verbal memory, visual memory, and global cognition derived from 14 standardized tests. Perceived social support was assessed using the Medical Outcomes Study-Social Support Survey (MOS-SSS; 0-100 scale). Paired cognitive-psychosocial assessments were analyzed using linear mixed-effects models adjusted for age, marital status, blood pressure medication, BMI, education, income, race, and randomized treatment (hormone therapy or placebo). RESULTS: A total of 298 women (mean age 60.3 y) provided 322 paired visits. Positive associations were observed between instrumental social support and visual memory (β(SE)=0.0044(0.0022), 95% CI: -0.00002 to 0.0088, P=0.051) and between positive social interaction and global cognition (β(SE)=0.0092(0.0046), 95% CI: 0.0001-0.0182, P=0.047). In age-stratified analyses, the positive social interaction-global cognition association was significant among women <65 years (P=0.042) but not women ≥65 years (P=0.34). No association survived false discovery rate (FDR) correction for multiple comparisons. CONCLUSIONS: In this sample of healthy middle- to older-aged women, no association between specific dimensions of social support and cognitive domains was statistically significant after correction for multiple comparisons. These hypothesis-generating findings suggest any association is modest at best and warrant confirmation in studies designed specifically for this question.

Cognitive function↗

PubMed2026

[A case of bilateral open-lip schizencephaly with West syndrome due to variant of PAFAH1B1 gene and literature review].

OBJECTIVE: To report the clinical manifestations, genetic features, diagnosis, treatment, and prognosis of a child with bilateral open-lip schizencephaly complicated by West syndrome due to a variant of PAFAH1B1 gene, and review the relevant literature. METHODS: Clinical data of a 4-month-old boy were retrospectively analyzed, and 35 previously reported cases were systematically reviewed. This study was approved by the Medical Ethics Committee of Gansu Provincial People's Hospital (Ethics No.: 2025-871). RESULTS: The 4-month-old boy presented with clustered flexor spasms, hypsarrhythmia on electroencephalography, and developmental regression. Brain magnetic resonance imaging revealed bilateral pachygyria, schizencephaly, and dysgenesis of the corpus callosum. Trio whole-exome sequencing identified a de novo heterozygous NM_000430.4: c.703_704delAG (p.Glu235Metfs*20) frameshift variant in the PAFAH1B1 gene. Sanger sequencing confirmed that neither parent carried this variant. Based on the guidelines from American College of Medical Genetics and Genomics (ACMG), the variant has met the criteria for PVS1+PS2_Moderate+PM2_Supporting, and was classified as pathogenic. After treatment with adrenocorticotropic hormone combined with vigabatrin and other antiseizure medications, the epileptic spasms were controlled and the electroencephalographic abnormalities had improved. However, global developmental delay persisted at the 12-month follow-up. Analysis of the present case and 35 previously reported cases showed that PAFAH1B1-related phenotypes were highly consistent, mainly including infantile spasms (28/36), developmental delay/intellectual disability (29/36), and abnormal brain MRI findings (30/36), predominantly cortical malformations. The reported variant types included deletions, frameshift variants, nonsense variants, missense variants, and splice-site variants. CONCLUSION: PAFAH1B1 gene variants are an important cause for cortical malformations, including schizencephaly, and may lead to secondary West syndrome. This study has systematically summarized the genotypic and phenotypic features of bilateral open-lip schizencephaly complicated by West syndrome associated with a frameshift variant of the PAFAH1B1 gene. For infants with epileptic spasms or early-onset epilepsy accompanied by structural brain abnormalities, early genetic evaluation should be performed, and antiseizure treatment should be integrated with neurodevelopmental rehabilitation to facilitate long-term management.

Humans↗

PubMed2026

Gene-environment interaction between perinatal oxytocin exposure and Pten mutation shapes epigenetic reprogramming of oxytocin signaling and behavior in mice.

Synthetic oxytocin (Pitocin) is the most commonly used pharmacologic agent for induction and augmentation of labor. Beyond its uterotonic effects, oxytocin plays a critical role in neurodevelopment and social behavior. Dysregulated oxytocin signaling has been implicated in autism spectrum disorder (ASD), raising concern that perinatal exposure to exogenous oxytocin may have lasting neurodevelopmental consequences. This study aimed to determine whether offspring harboring a genetic predisposition for ASD are differentially impacted by perinatal oxytocin exposures, with a focus on long-term oxytocin signaling and autism-like behavior. Pregnant mice carrying offspring with heterozygous mutations in phosphatase and tensin homolog deleted on chromosome ten (Pten), a well-established monogenic risk factor for ASD, received continuous oxytocin versus phosphate-buffered saline (PBS) control via micro-osmotic pumps during late gestation. Wild-type (WT) offspring exposed to each treatment served as a secondary control. Adult offspring were assessed for oxytocin receptor (Oxtr) methylation in the frontal cortex and hippocampus, oxytocin expression in the hypothalamus, serum oxytocin levels, and were subject to a battery of social and anxiety-related behavior tests. Perinatal oxytocin exposure produced genotype-dependent effects in offspring. Epigenetic analyses revealed bidirectional remodeling of Oxtr methylation in the frontal cortex and hippocampus, with increased exon 1 methylation in WT mice and decreased methylation in Pten-mutant mice, resulting in significant genotype-treatment interactions. Hypothalamic oxytocin expression increased following treatment regardless of genotype, though baseline levels were higher in Pten-mutant mice. Neither oxytocin treatment nor genotype impacted long-term serum oxytocin levels. Behavioral outcomes were modest but context-specific: repetitive behaviors and cognition performance were unchanged, but oxytocin-treated Pten-mutant mice exhibited increased anxiety-like behavior alongside improved social memory. In contrast, oxytocin-treated WT mice showed reduced social novelty preference. Exploratory analyses suggested potential sex-dependent trends. Our findings support a model in which genetic susceptibility shapes the epigenetic encoding of early-life hormonal signals, thereby recalibrating oxytocin system function and downstream behavioral outcomes. Together, these data highlight the context-dependent effects of perinatal oxytocin exposure and argue against uniformly beneficial or detrimental effects, emphasizing the importance of gene-environment interactions in neurodevelopmental trajectories.

Animals↗

PubMed2026

Factors associated with periprosthetic joint infection following total knee arthroplasty: an updated systematic review and meta-analysis.

BACKGROUND: This study aimed to systematically evaluate factors associated with periprosthetic joint infection (PJI) following total knee arthroplasty (TKA), and thereby to provide evidence-based references for clinical prevention and perioperative risk stratification. METHODS: Computerized searches were conducted in the following databases from their inception until May 26, 2025: PubMed, Web of Science, Embase, the Cochrane Library, CINAHL, China National Knowledge Infrastructure, Wanfang Database, Chinese Scientific Journal Database, and Chinese Biomedical Literature Database. Two researchers independently screened the literature, extracted data, and assessed study quality. The methodological quality was assessed using the Newcastle-Ottawa Scale. Quantitative synthesis was performed when at least two studies reported comparable exposure definitions and sufficient comparator information; otherwise, narrative synthesis was used. Review Manager 5.4 software was used for the primary analysis. This study is registered on PROSPERO (CRD420251079339). RESULTS: A total of 25 observational studies were included in the qualitative synthesis, of which 24 contributed to the primary quantitative synthesis. Quantitatively pooled factors associated with PJI included male sex (OR = 1.39, 95% CI 1.27-1.51), BMI ≥30 kg/m2 (OR = 5.72, 95% CI 2.65-12.36), prolonged operative time, type 2 diabetes mellitus (OR = 2.09, 95% CI 1.45-3.01), rheumatoid arthritis (OR = 2.64, 95% CI 1.38-5.02), smoking (OR = 1.65, 95% CI 1.34-2.04), blood transfusion (OR = 2.27, 95% CI 1.59-3.25), American Society of Anesthesiologists score ≥3 (OR = 2.73, 95% CI 1.02-7.32), history of hormone therapy (OR = 4.88, 95% CI 2.90-8.22), postoperative urinary tract infection (OR = 3.59, 95% CI 1.15-11.21), intraoperative blood loss >200 ml (OR = 1.51, 95% CI 1.03-2.22), postoperative drainage tube placement duration ≥24 h (OR = 2.23, 95% CI 1.50-3.32), preoperative anemia (OR = 1.82, 95% CI 1.67-1.99), and combined pulmonary disease (OR = 5.54, 95% CI 1.93-15.96). Age was narratively summarized because its classification differed substantially across studies. CONCLUSION: Multiple demographic, comorbidity-related, and perioperative factors were associated with PJI after TKA. However, because the included studies were observational and clinically heterogeneous, these findings should be interpreted as associations rather than causal effects. Optimization of modifiable factors may help reduce the burden of PJI, but further prospective studies using standardized PJI definitions and adjusted effect estimates are required.

Humans↗

PubMed2026

Trade-offs in avian parental care: a review of theory and meta-analysis of brood size manipulations.

The selective forces shaping parental care have been studied for over 50 years. While theoretical and experimental work has yielded qualitative progress, the large body of empirical work testing predictions about parental investment based on life-history trade-offs has yet to be synthesized. We first provide an overview of the core life-history theory exploring how selection might shape parental care. We then conduct a systematic review and meta-analysis on studies that experimentally manipulated brood size in birds, a widely used experimental approach to manipulate parental investment. We extracted 313 estimates from 62 studies representing 31 species of birds from 19 different families and tested key predictions on trade-offs in parental care derived from theory. Our analysis provides strong support for some predictions about life-history trade-offs in parental care, but weak or equivocal support for others. Specifically, we found that overall, avian parents respond to brood size manipulations as predicted by life-history theory: they increased care in response to brood enlargement, and decreased care in response to brood reductions. Furthermore, for the same relative manipulation size, responses to brood reductions were greater than responses to brood enlargements. This finding is consistent with predictions derived from life-history theory based on some types of non-linear utility curves. However, many predictions derived from theory are not well supported by our comparative analysis. Species' life-history traits such as clutch size (a measure of current reproduction), adult survival, and broods per year (two measures of future reproduction), explained little, if any, among-species variation in response to brood size manipulations. Several factors may explain this. We highlight that brood size manipulations may affect more than just perception of the value of current reproduction, such as altering parents' perception of predation risk. Importantly, these unintended consequences could lead to asymmetric responses like those we observed. Other common experimental approaches - such as hormone manipulations, altering a partner's effort, and food supplementation - often affect multiple traits or fitness components simultaneously, or may involve cues that poorly match the evolved mechanisms guiding parental behaviour. Our review of both theory and experimental approaches suggests that there are multiple opportunities for more precise experiments. We offer several recommendations for effective designs. One is improved understanding of the biology underlying the functions relating to costs and benefits, with careful consideration of not only how the manipulation will affect only one of those, but also the mechanisms that might alter how parents perceive the manipulation. We also emphasize general principles, such as assessing alternative hypotheses and devising multiple independent tests. Armed with these recommendations, we believe there are new opportunities to increase the strength of inference achieved from studies aimed at understanding the trade-offs affecting the evolution of parental care.

Animals↗