Migrating animals respond to seasonal changes in the environment, and therefore they should time migration to coincide with peaks in resource abundance. However, it is unclear if and how frugivorous animals use phenological events to time migration, especially in the tropics. The straw-colored fruit bat (Eidolon helvum), Africa’s most gregarious fruit bat and a key seed disperser, forms large seasonal colonies through much of sub-Saharan Africa. We hypothesized that aggregations of straw-colored fruit bats match the timing of their migration with some landscape phenologies. Using monthly colony counts from seventeen sites across much of their range, we matched peak colony size to peaks in remote sensing measures of enhanced vegetation index (EVI), instantaneous rate of green up (IRG), precipitation (PRP), and the instantaneous rate of change of precipitation (IRP). Peak colony size was closest to peak IRG (60% of peak sizes occur within 1 month of peak IRG), while IRP was a close second. Sites with closer temporal matching by colonies typically had higher maximum EVI, high seasonal variation, or a short growing season, and larger peak colony size. E. helvum seem to time their migrations to move into highly seasonal landscapes and away from their core distributional range in the tropical forest belt to exploit short-lived explosions of food. The link between rapid changes in colony size and phenological match may also imply a potential collective sensing of the environment, which could be threatened by overall decreasing bat numbers along with various threats faced by large colonies.

Food distribution and availability fundamentally shape foraging. Yet spatiotemporal distribution of mobile prey and its proximate effects on animals have rarely been assessed. The neotropical bat, Noctilio albiventris, forages on aquatic swarming insects which peak just one to two hours after dusk. We matched seasonal insect distribution at high spatiotemporal resolution to the foraging behavior of adult female bats. Surprisingly, insect abundance was lower in the wet season, and insect patches dispersed more rapidly. Correspondingly, bats emerged 45% earlier, foraged over 40% longer, and flew almost twice as far compared to the dry season. Wet season bats also spent less time at each patch, suggesting that patches, though the same size, were less dense and depleted more rapidly. Our results highlight the tight link between foraging and sharp seasonal shifts in the spatial unpredictability and temporal ephemerality of resources, shedding light on behavioral adaptations and plasticity in response to resource fluctuation.

1. Global climate change affects many aspects of biology and has been shown to cause body size changes in animals. However, suitable datasets allowing the analysis of long-term relationships between body size and climate are rare. 2. The size of the skull, often used as a proxy for body size, does not change much in fully grown vertebrates, but some soricine shrews shrink their skull and brain in winter and regrow it in spring. This is thought to be a winter adaptation in these high-metabolic, nonhibernating animals, as a smaller brain size reduces energy requirements. 3. Climate could thus affect not only the overall size but also the pattern of the size change, i.e., Dehnel’s Phenomenon, in these shrews. 4. We assessed the impact of the changes in climate on the overall skull size and the different stages of Dehnel’s phenomenon in skulls of the common shrew, Sorex araneus, collected over 50 years in the Białowieża Forest, NE Poland. 5. Overall skull size decreased, along with increasingly mild winters and decreasing soil moisture, which determined the availability of the shrews’ main food source, earthworms. The magnitude of Dehnel’s phenomenon increased over time, indicating an increasing selection pressure on animals in winter. Overall, climate clearly affected the common shrew’s overall size as well as its seasonal size changes. With the current acceleration in climate change, the effects on the distribution range of this cold-adapted species may be quite severe.

1. Some small mammals exhibit Dehnel’s phenomenon, a drastic decline in body mass, braincase and brain size from summer to winter, followed by a regrowth in spring. This is accompanied by a reorganization of the brain and changes in other organs. The evolutionary link between these changes and seasonality remains unclear, although the magnitude of change varies between locations as the phenomenon is thought to lead to energy savings during winter. 2. Here we explored geographic variation of the intensity of Dehnel’s phenomenon in Sorex araneus. We compiled the literature on seasonal changes in braincase size, brain and body mass, supplemented by our own data from Poland, Germany and Czech Republic. 3. We analysed the effect of geographic and climate variables on the magnitude of change and patterns of brain reorganization. 4. From summer to winter the braincase height decreased by 13%, followed by 10% regrowth in spring. For body mass the changes were -21%/+82%, respectively. Changes increased along the north-east axis. Several climate variables were correlated with these transformations, confirming a link of the magnitude of the changes with environmental conditions. This relationship differed for the brain mass decline vs. regrowth, suggesting that they may have evolved under different selective pressures. 5. We found no geographic trends explaining variability in the brain mass changes although they were similar (-21%/+10%) to those of the braincase size. Underlying patterns of change in brain organisation in North-Eastern Poland were almost identical to the pattern observed in Southern Germany. This indicates that local habitat characteristics may play a more important role in determining brain structure than broad scale geographic conditions. 6. We discuss the techniques and criteria used for studying this phenomenon, as well as its potential presence in other taxa and the importance of distinguishing it from other kinds of seasonal variation.