Earth’s climate underwent substantial shifts during the late Pleistocene and early Holocene epochs, which spanned from approximately 130,000 to 7,000 years ago. This period marked a dramatic transformation from the cold conditions of the glacial cycles to the warmer and more stable climate conditions that characterize the present-day interglacial period. These shifts in climate had profound effects on ecosystems and the organisms inhabiting them. One significant ecological impact of this transition was the dramatic reduction in the size of many mammal species, particularly those that had survived the harsh conditions of the Ice Age.
One of the areas in which these changes are most evident is in the fossil record of animals from the Azraq Basin in modern-day Jordan. New research, published in the Quaternary Science Reviews, has examined mammalian fossils from this region, focusing specifically on the body size changes observed in species such as the gazelle, hare, and fox between 24,000 and 7,500 years ago. These findings provide valuable insight into the effects of the Pleistocene-Holocene transition on animal populations.
The Azraq Basin: A Natural Laboratory for Studying Climate Effects on Fauna
The Azraq Basin is a semi-arid region in central Jordan, making it an ideal setting for studying how animals responded to climate and environmental changes during the Late Pleistocene and Early Holocene. The area is particularly sensitive to fluctuations in temperature and precipitation, which makes it an excellent natural laboratory for exploring the relationship between climate and animal body size.
Professor Louise Martin, from University College London’s Institute of Archaeology, and her research team have spent years studying archaeological sites in this region. By examining a range of mammal fossils from 19 different archaeological sites within the basin, the team was able to identify shifts in body size that correspond to key climate events, such as the end of the Last Glacial Maximum and the subsequent warming period.
The bones analyzed by the researchers represent a wide spectrum of skeletal remains, ranging from larger bones, such as those of the limbs, to more delicate structures, such as the astragalus (part of the ankle joint) and phalanges (toe and finger bones). These elements were essential for constructing a detailed picture of body size trends during the transition from the Pleistocene to the Holocene.
The choice of the Azraq Basin was significant because it faces less human interference compared to other parts of the region. This allowed the researchers to better isolate the effects of climatic factors from those of human impacts such as hunting, domestication, and other forms of environmental exploitation.
Understanding the Shifts in Mammalian Body Sizes
Through the osteometric analysis of these fossils, the team found evidence of two distinct periods during which body size reduction occurred. These shifts are best understood in the context of fluctuating climate conditions and ecological pressures:
1. Pleistocene Body Size Diminution (Cold Conditions of the Younger Dryas): The first period of size reduction occurred in the final Pleistocene, particularly during the cold snap known as the Younger Dryas (around 13,000 to 11,500 years ago). During this time, the Earth experienced a temporary return to near-glacial conditions that likely placed significant physiological pressures on mammalian species. The cold climate would have made it more difficult for animals to maintain appropriate thermoregulation, particularly for larger-bodied mammals that require greater amounts of food to sustain their metabolic needs. Reduced resource availability could also have played a role in the smaller size of mammals at this time.
2. Holocene Body Size Diminution (Environmental Shifts Post-Glaciation): The second, more significant reduction in body size occurred between 9,000 and 7,500 years ago, as the climate began warming and humans were transitioning from a hunter-gatherer to agricultural society. The warming of the climate led to new environmental challenges. As vegetation changed and resources became more limited due to the evolving climate, the balance of species shifted. Reduced access to food sources and the increase in competitive pressures among herbivores and carnivores might have triggered this second phase of body size reduction.
Competition and Climate Effects: A Complex Web of Influences
The researchers carefully considered the factors that might explain the observed reductions in body size. While human hunting and other anthropogenic factors undoubtedly influenced mammalian populations during this time, the team concluded that climate was the primary driver behind the observed changes in body size. The two main factors thought to be contributing to this shift were temperature and the availability of resources.
Professor Martin elaborates, stating that changes in temperature could have had an impact on thermoregulation, requiring animals to adapt in size to maintain a balance between energy use and heat retention. Additionally, fluctuations in vegetation and other food sources, often linked to climate changes, might have caused periods of resource scarcity, further driving changes in the physiology of the animals. When resource competition increased due to such fluctuations, smaller body sizes would likely have been advantageous for survival, particularly among herbivores.
The study’s findings suggest that specific shifts in the size of certain species can be attributed to these underlying pressures, particularly in species like gazelles, hairs, and foxes:
Gazelles
The sharp reduction in gazelle size between 9,000 and 7,500 years ago is particularly notable. The researchers hypothesize that this was partly due to competition with the introduction of smaller gazelle species into the region, particularly the Arabian sand gazelle. This species, which was better adapted to hotter, drier climates, likely began migrating north during the early Holocene, potentially pushing larger gazelle species out of the region or forcing them to adapt to increasingly harsher environmental conditions. The competitive pressures between gazelle species for food and space likely played a role in the observed diminishment of body size.
Foxes
Similar to gazelles, the fox population also experienced size reduction during this period. The researchers propose that new fox species may have entered the region, displacing or competing with larger, more established species. These competitive dynamics, combined with changing environmental conditions, may have caused shifts in body size among fox populations, as those best suited to the new ecological conditions survived.
Hares
Interestingly, the changes in the body size of hares track closely with those of the gazelles, but with a time lag of about a millennium. Hares, being smaller in size and having lower dietary needs, are thought to have been buffered somewhat from the pressures faced by the larger herbivores like gazelles. Nonetheless, they too experienced a gradual body size reduction. This reduction is largely attributed to intra-specific competition—competition within the same species for limited resources, which likely triggered interbreeding between individuals of different sizes.
Beyond Human Hunting: A Broader Ecological Process
While the team briefly explored the potential influence of human hunting on these body size reductions—particularly the idea that early humans may have selectively hunted larger individuals (especially males), resulting in smaller animals being left behind—the osteometric evidence collected from the remains of these species provided no solid proof that human intervention was the primary cause. Rather, the patterns suggest that climatic changes and shifts in the distribution of species are likely more significant factors.
As Professor Martin notes, species distribution shifts could explain why certain mammals like gazelles and foxes underwent size reductions. New species from the Arabian Peninsula likely migrated into the Levant, pushing native species into smaller ecological niches. This form of “character displacement” (in which competing species evolve to reduce overlap in their resource needs) appears to be a plausible explanation for the observed trends in mammalian size.
Looking Ahead: Opportunities for Future Research
While the team’s analysis offers valuable insights into how climate changes and environmental pressures affected the animals of the Azraq Basin, future research is needed to further clarify the complex relationships between climate, human activity, and animal evolution. One promising avenue for exploring these shifts is paleogenetics. Although obtaining ancient DNA from arid environments such as the Azraq Basin has proved difficult, advancements in genetic techniques may enable the identification of genetic evidence that supports the hypothesis of migration and species competition during the Holocene.
The research team hopes that paleogenetic analysis may one day confirm the migrations and evolutionary changes discussed in this study. Such investigations would not only enhance our understanding of ancient ecosystems but also help researchers assess how past climate events might serve as analogs for the environmental challenges currently faced by wildlife today in an era of climate change.
Conclusion
The study of mammalian body size reduction in the Azraq Basin during the transition from the Pleistocene to the Holocene provides a fascinating lens through which we can understand how past climate changes impacted life on Earth. Although human activities certainly played a role in shaping wildlife populations, the primary drivers behind these body size shifts appear to be tied to broader ecological processes driven by temperature fluctuations, resource limitations, and the complex interactions between species. Further investigation and technological advancements will undoubtedly continue to illuminate the past, offering crucial insights for how climate change will continue to shape the planet’s ecosystems.
Reference: Louise Martin et al, Late Pleistocene-Holocene mammalian body size change in Jordan’s Azraq Basin: A case for climate driven species distribution shifts, Quaternary Science Reviews (2024). DOI: 10.1016/j.quascirev.2024.109147