The research, conducted by Mónica F. Cisternas, an independent researcher based in Stephenville, Texas, and Ernesto Gianoli of the Department of Biological Sciences at Tarleton State University, took advantage of a rare opportunity to measure thermal biology where it actually happens: in the field, under natural sunlight, wind, and fluctuating temperatures, rather than in the controlled conditions of a laboratory. The team compared solitary and grouped caterpillars of the gregarious butterfly species, asking a deceptively simple question: how does the difference between a caterpillar’s body temperature and the surrounding air temperature change as air temperature changes? The answer, it turns out, depends strongly on whether the larva lives alone or in a group.

The core metric the researchers examined is known as the body–air temperature differential, essentially the gap between how hot or cold an animal’s body is and how hot or cold the air is. For ectotherms like caterpillars, this differential is shaped by a tug-of-war between heat gained from solar radiation, heat exchanged with the surrounding air, and heat generated or absorbed through metabolism and evaporative water loss. When the differential is positive, the animal is warmer than its environment, which can be beneficial on a cool morning but dangerous during a heat spike. When it is negative, the animal is cooler than the air, a state that may protect tissues from thermal stress. The researchers measured this differential across a natural gradient of air temperatures for both solitary larvae and members of aggregations, then analyzed how the two groups differed.

The results were striking in their asymmetry. Grouped caterpillars showed an overall relationship between their body–air temperature differential and air temperature, meaning their thermal status relative to the environment shifted systematically as conditions warmed. Solitary caterpillars, by contrast, showed no such overall association. Most notably, gregarious caterpillars registered significantly lower temperatures than the surrounding environment in the warmer regions of the temperature gradient. In other words, when the day heated up, the larvae in groups were literally cooler than the air around them, a relative cooling effect that solitary individuals did not display. The researchers note that the evidence for the opposite pattern, grouped caterpillars exceeding air temperature under cooler conditions, was comparatively weak in their dataset, hinting that the cooling benefit of aggregation may be more pronounced in the field than the warming benefit.

Digging deeper into the structure of the aggregations themselves, the team ran an analysis incorporating the number of larvae in each group, and here the story became even more interesting. The interaction between group size and air temperature was statistically significant even after accounting for population-level differences and the intensity of solar radiation at each microsite. The direction of the effect was clear: as air temperature increased, the thermal excess, the amount by which a caterpillar’s body temperature surpassed air temperature, declined more steeply in larger groups. Bigger aggregations, in other words, shed their relative warmth faster as the environment heated up. This suggests that the collective thermal properties of a group, such as the way many bodies alter airflow, shading, and evaporative surfaces, scale with the number of participants, amplifying the cooling effect precisely when it matters most.

Solar radiation at the microsites the caterpillars occupied also emerged as a significant predictor of thermal excess, a reminder that body temperature in small ectotherms is never determined by air temperature alone. A caterpillar basking in direct sun can be many degrees warmer than the air, while one sheltered in shade may be cooler. By including radiation in their models, the researchers could separate the genuine effect of grouping from the simple fact that groups might occupy sunnier or shadier spots. The persistence of the group-size effect after this adjustment strengthens the case that aggregation itself, not just microsite selection, drives the relative cooling. Still, the authors caution that the mechanistic basis of the pattern remains to be worked out, whether it stems from reduced convective heat exchange within clusters, shared shading, evaporative cooling, or some combination of these physical processes.

Perhaps the most conceptually intriguing result concerns the solitary caterpillars. The analysis showed that solitary individuals, but not grouped ones, behaved as thermoconformers, meaning their body temperatures closely tracked ambient temperature with little deviation. Thermoconformity is often considered the default state for small ectotherms lacking physiological or behavioral means of temperature control, but the gregarious larvae broke free of it. By clustering, they effectively decoupled their body temperatures from the ambient thermal environment, gaining a degree of thermal autonomy that a lone larva cannot achieve. From an adaptive standpoint, this decoupling could matter enormously, because enzyme function, growth rates, feeding behavior, and survival in insects are all tightly temperature dependent, and staying within a favorable thermal window can be the difference between thriving and wilting on a hot afternoon.

The study situates itself within a rich literature on the thermal ecology of social caterpillars. Earlier work on eastern tent caterpillars documented synchronized group foraging and tent-based thermoregulation tied to microclimate, while research on emperor moth caterpillars assessed the thermal and water-relations benefits of aggregation. Studies of forest tent caterpillars and processionary moths have explored how colony size shapes larval performance, and comparative work on nettle-feeding nymphalid butterflies contrasted the thermal ecologies of gregarious and solitary larvae. What sets the new study apart is its explicit field test of the cooling hypothesis. As the authors point out, prior evidence showed that aggregation in caterpillars may enhance heat gain under cool conditions, but whether it also facilitates cooling under warm conditions had remained unknown. Their data now supply that missing half of the picture, and they do so under authentic field conditions rather than in artificial settings.

The implications extend beyond one butterfly species. Larval gregariousness is widespread in the swallowtail family, Papilionidae, and previous work by the same team has shown that this lifestyle is more prevalent outside the tropics and influences traits such as larval melanism and defensive reactions. If grouping provides a two-way thermal buffer, warming cold bodies and cooling warm ones, it could help explain why gregarious living has evolved repeatedly across Lepidoptera and why it persists even when it carries costs, such as making clusters more conspicuous to predators. In an era of increasing climatic variability, a behavior that moderates thermal extremes could carry real fitness consequences, a possibility the authors flag as a priority for future research, alongside the need to evaluate how the observed cooling translates into caterpillar performance and survival.

For now, the study stands as a vivid reminder that even the humblest animals employ sophisticated physical strategies to manage their body temperatures, and that some of those strategies only make sense when many bodies act together. A single caterpillar sweltering on a leaf has few options beyond conforming to the heat. A cluster of them, by contrast, builds a collective thermal microenvironment, one that grows more effective at shedding excess heat as it grows larger. The next time a cluster of striped larvae pulses gently on a sunlit stem, consider that they may not be huddling for warmth at all. They may be air-conditioning themselves, together, one body at a time.

Subject of Research: Field evidence that larval aggregation provides relative cooling under warm conditions in gregarious caterpillars.

Article Title: Field evidence of relative cooling under warm conditions in grouped caterpillars

Article References: Field evidence of relative cooling under warm conditions in grouped caterpillars. (n.d.). https://doi.org/10.1007/s00114-026-02169-1

Image Credits: AI Generated

DOI: 10.1007/s00114-026-02169-1

Keywords: caterpillars, thermoregulation, aggregation, gregariousness, Lepidoptera, body temperature, ectotherms, thermal ecology, group living, swallowtail butterfly, solar radiation, field study

The problem begins with a paradox inherent to phages themselves. These viruses are essentially nucleic acids wrapped in protein coats, decorated with tail fibers and baseplate structures that must remain precisely folded to recognize and inject their genetic payload into bacterial hosts. Unlike small-molecule drugs, which can often tolerate considerable environmental variation, phage infectivity depends on conformational integrity at the nanometer scale. A tail fiber that unfolds, a capsid that cracks, or a protein subunit that detaches can render a particle noninfectious even though the genetic material remains fully intact. Liquid formulations, which are preferable for many clinical settings because they avoid the stresses and costs of lyophilization, expose particles continuously to the surfaces of their containers, to each other, and to solution conditions that shift subtly over time.

Surface adsorption is the first and often the fastest loss mechanism. Phage particles, like many proteins, carry a mosaic of charged and hydrophobic patches that attract them to interfaces. Glass vials, plastic syringes, rubber stoppers and tubing all present surfaces to which virions can adhere, sometimes irreversibly. For low-concentration preparations, intended for pediatric dosing or for phages that are difficult to propagate to high titers, even a small fraction of adsorbed particles can translate into a clinically significant loss of dose. The study underscores that adsorption is not uniform across container materials: hydrophobic polymers tend to sequester particles more aggressively than borosilicate glass, but glass introduces its own risks through leached metal ions and alkaline surface chemistry. The authors emphasize that adsorption must be evaluated for every combination of phage, container and buffer, because a formulation that works beautifully for one phage type may fail entirely for another.

The second threat, aggregation, arises when particles collide and stick together, forming dimers, larger clusters and eventually visible precipitates. Aggregation is driven by electrostatic attraction between oppositely charged regions of adjacent particles, by hydrophobic interactions between exposed protein patches, and by the removal of stabilizing hydration shells during freezing and thawing or during shifts in ionic strength. Aggregated phages are problematic on multiple levels. Aggregates sediment during storage, creating heterogeneous dosing; they are cleared more rapidly from the bloodstream after injection; and they can trigger immune responses or exceed particulate limits set by pharmacopoeial standards for injectable products. Crucially, aggregation and infectivity loss are not always correlated in simple ways, which means that turbidity measurements or particle counts alone cannot substitute for plaque assays when assessing formulation quality.

The third mechanism, structural degradation, is the slowest but perhaps most insidious. Over weeks and months in liquid storage, capsid proteins can undergo deamidation, oxidation and backbone cleavage, while tail structures can gradually lose the precise geometry required for host recognition. Temperature accelerates these processes dramatically, which is why cold-chain dependence has been such a burden on phage therapy programs. The research highlights that structural degradation is often detected only through functional assays: a formulation may look perfectly clear and show normal particle counts by electron microscopy, yet exhibit a steady decline in plaque-forming units as more and more particles lose their ability to infect. This decoupling of appearance from activity is a central argument for rigorous, infectivity-based stability testing.

Against this backdrop, the study evaluates the protective roles of specific classes of excipients, the pharmaceutically accepted, nonactive ingredients that surround the phage in the final product. Sugars and sugar alcohols, including sucrose, trehalose and mannitol, emerge as foundational stabilizers. These molecules act through preferential exclusion, a thermodynamic effect in which the sugar is excluded from the surface of the protein, effectively making the folded state energetically favorable and discouraging both unfolding and aggregation. Trehalose in particular has a long history in the stabilization of biological drugs and vaccines, and the findings reinforce its value for phages, where it buffers particles against osmotic shocks and reduces interfacial stress. Polysorbates and other nonionic surfactants occupy a complementary niche: by adsorbing competitively to container surfaces and air-liquid interfaces, they deny phage particles access to the very interfaces where adsorption and surface-induced denaturation occur. Even low concentrations of surfactant can dramatically reduce the fraction of virions lost to a plastic syringe barrel.

Amino acids and buffering agents form the third pillar of the protection strategy. Arginine, glycine and histidine appear repeatedly in biologics formulations because they disrupt electrostatic attractions that drive aggregation, while histidine buffer offers mild pH control near neutrality, where most phages are comfortable. Plasma proteins and serum albumin, historically used as catch-all stabilizers in early phage work, are discussed in the context of modern pharmaceutical expectations: although effective, animal-derived proteins raise regulatory and supply concerns, pushing formulators toward chemically defined alternatives. Salts deserve careful handling, the authors note, because ionic strength simultaneously affects electrostatic repulsion between particles, adsorption to charged surfaces and the osmotic environment of the capsid; there is no universally optimal salt concentration, and each phage-buffer-container system must be characterized empirically.

Perhaps the most practically important message of the work is that excipient effects are context-dependent and phage-specific. A tailed, contractile Myoviridae-like phage with a large, complex capsid responds differently to stress than a small, robust Podoviridae-like particle. The study therefore advocates a structured development workflow: characterize the phage’s isoelectric point and hydrodynamic behavior, screen a panel of excipient candidates across relevant pH ranges, stress-test the leading candidates with accelerated temperature challenges, and validate the winners under real storage conditions in the intended container closure system. Analytical tools discussed in the article include plaque assays for infectivity, dynamic light scattering and nanoparticle tracking for aggregation, electron microscopy for morphological integrity, and differential scanning fluorimetry or similar methods for probing thermal stability. Combining these orthogonal readouts, rather than relying on any single metric, gives formulators the confidence that a liquid product will remain potent throughout its shelf life.

The implications for the phage therapy field are considerable. Liquid formulations stored at standard refrigeration temperatures, or even at room temperature in optimized compositions, would remove one of the most burdensome logistics constraints on clinical deployment, particularly in regions where deep-freeze distribution is unreliable. Standardized, excipient-protected formulations would also simplify regulatory pathways, since regulators can evaluate excipient safety profiles that are already well established for other biologics. As phage products progress from bespoke compassionate-use preparations toward licensed medicines, the kind of systematic formulation science presented in this study will be indispensable. It transforms the stabilization of phages from a craft of trial and error into a rational engineering discipline, giving the next generation of antiviral antibacterials a fighting chance to arrive intact, potent and ready at the patient’s bedside.

Subject of Research: Use of pharmaceutical excipients to stabilize bacteriophages in liquid formulations by preventing surface adsorption, aggregation and structural degradation.

Article Title: Investigating the role of excipients in mitigating surface adsorption, aggregation and structural degradation in liquid phage formulations

Article References: Siafakas, E., Duong, H. T. T., & Iredell, J. R. (2026). Investigating the role of excipients in mitigating surface adsorption, aggregation and structural degradation in liquid phage formulations. npj Viruses. https://doi.org/10.1038/s44298-026-00233-1

Image Credits: AI Generated

DOI: 10.1038/s44298-026-00233-1

Keywords: bacteriophage, phage therapy, liquid formulation, excipients, trehalose, polysorbate, surface adsorption, aggregation, capsid stability, biologics formulation, antibiotic resistance, npj Viruses