The Indus Valley Civilization (IVC) was among the earliest known urban cultures, thriving between 5,000 and 3,500 years ago along the Indus River and its tributaries in what is now Pakistan and northwest India. At its height from 4,500 to 3,900 years ago, the society was known for its planned cities, extensive infrastructure, and innovative water management systems. Despite this high level of development, the reasons behind its long, gradual downturn have remained difficult for researchers to fully explain.

Climate Simulations Reveal Temperature Rise and Reduced Rainfall

To investigate past conditions, Vimal Mishra and colleagues reconstructed climate patterns across the region spanning 5,000 to 3,000 years ago. Their analysis combined climate modeling with several indirect indicators of ancient environmental change. These included the chemical signatures preserved in stalactites and stalagmites from two Indian caves and water level histories recorded in five lakes across northwest India. Together, the data point to a temperature increase of about 0.5 degrees Celsius during this interval, along with a 10 to 20 percent reduction in annual rainfall.

The team also identified four extended drought periods occurring between 4,450 and 3,400 years ago. Each drought lasted more than 85 years and affected between 65 percent and 91 percent of the area associated with the IVC, indicating widespread and long-lasting impacts on water availability.

Shifts in Settlement Patterns During Prolonged Dry Periods

According to the authors, these droughts likely influenced where people chose to establish settlements. Between 5,000 and 4,500 years ago, most communities were situated in regions that received higher rainfall. After 4,500 years ago, settlement patterns changed, with populations moving closer to the Indus River. This shift may reflect increasing dependence on a more reliable water source as drought conditions intensified.

One particularly long drought lasting 113 years, identified between 3,531 and 3,418 years ago, aligns with archaeological evidence of widespread deurbanization in the region. Based on these findings, the researchers conclude that the Indus Valley Civilization did not collapse abruptly from a single climate event. Instead, the society likely experienced a prolonged and uneven decline in which repeated droughts became a significant contributing factor.

In many mammals, females are receptive to mating only when ovulating, which gives males a clear window to maximize reproductive success. Bonobos (Pan paniscus) differ from this pattern because females remain sexually receptive for long periods and develop a bright pink genital swelling that persists well beyond the actual fertile stage.

Tracking Wild Bonobos to Understand Fertility Signals

To examine how males respond to this unreliable signal, researchers observed a wild bonobo community at Wamba in the Luo Scientific Reserve in the Democratic Republic of the Congo. The team documented sexual interactions each day and visually assessed the degree of genital swelling in every female. They also collected urine samples on filter paper to measure estrogen and progesterone, allowing them to identify when ovulation occurred.

The data showed that the likelihood of ovulation was highest between 8 and 27 days after a female reached maximum swelling, a range that makes prediction challenging. Even so, male behavior closely followed the true timing of ovulation. Males focused their mating activity on females who had reached maximum swelling earlier and who had older infants, two indicators linked to a greater chance of ovulation.

Flexible Mating Strategies Maintain an Imperfect System

These findings reveal that males improve their reproductive success by combining information about swelling patterns with knowledge of a female’s reproductive history. Because males are able to estimate fertility reasonably well despite the lack of a precise signal, the researchers suggest there has been little evolutionary pressure to make the signal more accurate. This may help explain why the system has persisted over long evolutionary timescales.

The authors add, “In this study, we found that bonobo males, instead of trying to predict precise ovulation timing, use a flexible strategy — paying attention to the end-signal cue of the sexual swelling along with infant age — to fine-tune their mating efforts. This finding reveals that even imprecise signals can remain evolutionarily functional when animals use them flexibly rather than expecting perfect accuracy. Our results help explain how conspicuous but noisy ovulatory signals, like those of bonobos, can persist and shape mating strategies in complex social environments.”

Researchers Reflect on Months of Field Observation

“The male bonobos weren’t the only ones paying close attention to sexual swelling — we spent countless days in the rainforest at Wamba, DRC doing exactly the same thing! All that watching, sweating, and scribbling in our notebooks eventually paid off. By tracking these daily changes, we uncovered just how impressively bonobos can read meaning in a signal that seems noisy and confusing to us.”

This study was supported by the Global Environment Research Fund (D-1007 to TF) of the Japanese Ministry of the Environment, the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (22255007 to TF; and 25304019 to CH;), and the JSPS Asia-Africa Science Platform Program (2012-2014 to TF). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.