How Do Mother Macaques Respond to the Loss of Their Infants?

When humans lose a close relative, the event is often accompanied by a cascade of emotional and behavioural changes. Appetite may decline, sleep patterns become disrupted, anxiety increases, and social interactions may diminish. These responses affect both physiological and psychological states, making it difficult to maintain ordinary daily routines. From an evolutionary perspective, such reactions may arise from the deep emotional investment created by highly social relationships. When an important social bond is suddenly severed, the behavioural system can temporarily fall out of balance.

By comparison, research on mourning behaviour in animals remains relatively limited. One of the most commonly examined situations involves mothers losing their offspring, because the mother–infant bond is among the strongest social connections in mammals. This relationship therefore provides a useful context for examining whether animals show behavioural responses comparable to human grief.

The study underlying this discussion was conducted on Cayo Santiago in Puerto Rico, an island that hosts a well-studied population of rhesus macaques (Macaca mulatta). Although this species is naturally distributed across South Asia and parts of East Asia, individuals were historically introduced from India to this Caribbean island. Over time, several free-ranging populations developed there, leading to the island's informal name “Monkey Island.” The macaques on Cayo Santiago form complex social groups with clear hierarchies, interaction patterns, and reproductive behaviour, and because they live within a relatively confined environment, their behaviour can be observed and tracked over long periods.

A rhesus macaque photographed on Cayo Santiago Island（圖片來源：Jami430，採用 CC BY-SA 4.0 授權）

During the course of the research, eleven mother macaques experienced the death of an infant. These cases provided an opportunity to examine behavioural changes following such a loss. Each mother was observed for up to nearly a month after the infant's death. The circumstances of infant mortality varied. Some infants died shortly after birth, while others disappeared a few days later. The researchers focused on four main categories of behaviour: resting, feeding, grooming interactions with other monkeys, and self-directed displacement behaviours. These displacement behaviours include actions such as scratching oneself, touching the body, shaking, pacing, or agitated movement, which are often interpreted as indicators of stress or anxiety.

The results surprised the researchers. Based on patterns known from human bereavement, they had expected that mothers who lost an infant would spend more time resting and appear less active. Instead, the opposite pattern emerged. During the first two weeks after the infant's death, bereaved mothers actually spent less time resting than non-bereaved females used as controls. In contrast, feeding time, grooming behaviour, and displacement behaviours showed no meaningful differences between bereaved and non-bereaved individuals. Even after losing their infants, the mothers continued to forage for natural vegetation on the island in the same way as other members of the troop, and their overall daily routines remained largely intact.

From these observations, the researchers concluded that the behavioural response to infant loss in these macaques appears relatively brief. Any detectable differences were largely confined to the first two weeks following the event. As time passed, even these subtle behavioural changes faded, and the mothers' activity patterns converged again with those of other troop members.

Another notable point is that the macaque mothers did not display behaviours resembling the prolonged depressive grief sometimes observed in humans. Rather than appearing lethargic or withdrawn, they became slightly more active and rested less. The researchers proposed that this pattern may resemble what is known in primate behaviour studies as the “protest stage.” When infant primates are separated from their mothers, they often enter a phase characterized by increased movement, vocalization, agitation, and active searching. Humans also show a similar early stage of protest in response to separation or loss. In human infants, however, this protest phase is typically followed by a second stage often described as despair, marked by reduced activity and withdrawal. Many studies of non-human primate mothers suggest that this second stage may not occur in the same way.

It is important to emphasize that the findings do not demonstrate that macaque mothers experience no grief. Their behavioural responses may simply differ from the typical human patterns used as reference points. Another factor may involve the age of the infants that died. In this study, most infants died very shortly after birth. Because the mother–infant bond may still be developing during this early period, the loss might not trigger strong or long-lasting behavioural changes.

In any case, the observations suggest that although mother macaques may temporarily adjust their activity after losing an infant, their daily routines remain largely embedded within the social structure of the troop. Rather than withdrawing for extended periods, they quickly reintegrate into the ongoing dynamics of group life. This ability to return to normal behavioural patterns may be essential for individuals living within complex primate societies, where maintaining social engagement remains critical for survival and reproduction.

Author: Shui-Ye You

Reference:

Johnson EA et al. (2025). Macaque mothers' responses to the deaths of their infants. Biol. Lett.