Chimpanzee and bonobo mothers sometimes carry the bodies of their deceased infants for days or even weeks, a behavior known as infant corpse carrying (ICC). A new study published in the journal Biology Letters examined 83 recorded instances of ICC from 15 different research sites. The researchers aimed to understand what factors influence how long a mother carries her dead infant, providing valuable insights into the emotional and social lives of these primates. The study found that the cause of the infant’s death plays a significant role in how long ICC lasts. Infants who died from disease were carried for longer periods compared to those who died from infanticide, which is when another adult kills the infant. The researchers also observed that older infants, who are likely more attached to their mothers, were carried longer. Additionally, ICC durations were longer in areas where the time between births was greater, and during cooler seasons. The researchers tested nine possible explanations for ICC, including the mother’s age, the sex of the infant, and whether the behavior is learned from watching other mothers. They identified four key factors that reliably predicted longer ICC durations: the cause of the infant’s death, the strength of the maternal bond, the mother’s level of investment in the infant, and the temperature of the environment. The study suggests that the deep emotional connection between chimpanzee mothers and their infants is a major driver of ICC. However, the most consistent predictor of how long a mother carried her infant was the cause of death. Importantly, the study does not require the assumption that the mothers understand that their infants are permanently dead. Instead, the patterns can be explained by the continuation of maternal behaviors and environmental conditions. The research was conducted by doctoral candidate Reece Hammond and Dr. Thomas A. Püschel from the University of Oxford. By analyzing data from multiple locations, the researchers were able to draw broader conclusions about ICC in chimpanzees and bonobos, something that would be difficult to achieve by studying a single group. This study may also offer clues about the evolutionary roots of how humans respond to death.