Drivers of variation in nocturnal insect prey mass and quality

The sky at night is not empty; it is a bustling highway of insects, a living resource that feeds a diverse cast of birds, bats, and other aerial hunters. For these predators, catching food is a high-stakes game of physics and biology. They must balance the energy they gain from a meal against the effort required to catch and eat it. A tiny insect might be easy to grab but offers little fuel, while a massive one might be packed with energy but too large to swallow or too difficult to maneuver with. This concept of "profitability" is central to understanding how these animals survive. When the populations of these flying predators begin to shrink, as they have in recent years, scientists look to the food web to find the cause. They ask whether the insects themselves have changed, becoming less nutritious or harder to catch, effectively starving the predators from the top down. To answer this, researchers must look beyond simple counts of how many insects are in the air and instead examine what those insects are made of and where they come from.

A new study turns its attention to the nocturnal prey that fuels these aerial hunters, seeking to understand what makes one insect a better meal than another. The researchers focused on two main questions: how much energy an insect holds in the form of fat, and how heavy it is. They knew that not all insects are created equal. Some are built differently based on their family tree, while others vary depending on how they grew up. To find the answers, the team collected insects from the night sky and measured their body mass and lipid content, which is the scientific term for the fat that stores energy. They then sorted these insects by their type, how they fed as adults, and where they lived as larvae. This allowed them to see if the insect's identity or its childhood home determined how much fuel it offered to a hungry predator.

The investigation revealed that the quality of this nighttime food supply is driven by three specific factors. First, the family an insect belongs to matters greatly. The researchers found that for mayflies, the amount of fat they carry does not increase in direct proportion to their size; as they get bigger, their fat stores do not keep pace. For the other groups of insects they studied, however, fat content grew steadily alongside body mass. This means that a large mayfly is not necessarily a more energy-dense meal than a small one, whereas for other insects, size is a reliable indicator of a rich meal.

Second, the way an insect feeds as an adult shapes its energy reserves. The study distinguished between insects that continue to eat as adults and those that do not. Those that feed throughout their adult lives, known as life-time-feeders, carried less body mass and fat. In contrast, insects that stop eating as adults, having stored all their energy during their larval stage, were heavier and packed with more fat. These "larval-feeders" represent a more concentrated source of energy for a predator, having banked their resources before taking flight.

Third, the environment where the insect grew up influences its weight, though not its fat concentration. Insects that developed on land were heavier than those that emerged from water. Yet, despite this difference in total weight, the proportion of fat within their bodies remained the same. A heavy terrestrial insect and a lighter aquatic insect offered the same percentage of energy-rich fat, even if the land-dweller provided a larger total payload.

However, the study concludes that having a heavy, fat-rich insect is not the whole story. The true value of a meal is strictly limited by the predator's ability to handle it. A bird or bat has a physical limit on how large a mouthful it can take, a constraint known as gape limitation. An insect might be incredibly nutritious, but if it is too large to swallow or too difficult to manage in flight, it becomes useless to the hunter. The most valuable prey exists within a specific size range where high energy content meets the physical capabilities of the predator. By mapping out these drivers of prey quality, the research shows that the nutritional landscape for aerial predators is not a simple matter of abundance. Instead, it is a complex picture defined by the mix of insect types, their feeding habits, and their developmental history, all filtered through the physical limits of the animals trying to eat them.