Orangutans’ Record-Breaking Lactation

Written by: Lauren Milligan Newmark, Ph.D. | Issue # 134 | 2026

And the gold medal for the longest lactation of any mammal goes to…orangutans! 

Most of the great apes, including chimpanzees, bonobos, and gorillas, nurse their offspring for three to five years, longer than other mammals of their size. Orangutans take this trend to the extreme, nursing their offspring for years longer than their great ape cousins [1,2]. But precisely how many years longer is unclear, with estimates ranging from six to ten years [1-4]. This variability around weaning age is not surprising; observing milk transfer in the wild is notoriously difficult, particularly in arboreal mammals like orangutans that spend most of their time high in the forest canopy [1-4]. 

A new study [1] got around this challenge and identified milk intake without ever needing to observe a single feeding. Trading binoculars for a novel analytical tool called fecal proteomics, a team of Malaysian and Japanese researchers focused on what came out of baby orangutans rather than what went in [1]. 

This pioneering technique not only revealed the timeline for orangutan nursing but also how their prolonged milk intake may promote lower mortality by supercharging the infant and juvenile gut microbiome and immune defenses. The slow-paced life histories for which orangutans are famous may only be possible because of milk. 

Data From the Forest Floor

What some see as waste, others see as a goldmine of dietary data. Feces contain proteins and peptides (fragments of protein chains) from the host, the host’s diet, and the bacteria that line the digestive tract [1, 5]. Fecal proteomics uses mass spectrometry to measure and identify these proteins, which can then be used to reconstruct diet and intestinal health [5].

There are numerous proteins unique to milk, deriving mainly from milk’s casein fraction, and their presence in stool can be used as biomarkers of milk consumption. The efficacy of fecal proteomics for identifying milk intake was validated in a 2020 study, which found that over a hundred milk proteins and peptides survive digestion and end up in infant stool [6]. A year later, researchers demonstrated that fecal proteomics could distinguish between nursing infants and weaned juveniles in a population of captive-living Japanese macaques [5]. 

To establish a timeline for orangutan lactation using fecal proteomics, stool samples (n=27) were collected from the forest floor over a 31 month span from ten wild-living Bornean orangutan individuals from the Danum Valley Conservation Area in Malaysia (Juveniles, age 2.5 – 6.9 years: n=5; adolescents, 7.0 – 15.4 years: n=2, adults, (≥15.5 years: n=3). Twenty samples were from juveniles, three were from adolescents, and four were from adults [1]. This orangutan population has been studied since 2005, and all individuals have been identified and named. Ages of individuals were calculated based on the date of first appearance. All the juveniles and all but one adolescent in this study were first seen as neonates, so ages are unlikely to be underestimated [1]. 

Researchers discovered that orangutan juveniles continuously consumed milk from 2.7 years to at least 6.5 years after birth based on the presence of milk-specific proteins (alpha-S1-casein, beta-casein, kappa-casein, and alpha lactalbumin) in fecal samples [1]. One or more milk-specific proteins were identified in all 20 fecal samples obtained from the juvenile orangutans. In contrast, none of these proteins were identified from the seven fecal samples from two adolescents and three adults [1]. One milk-specific protein and lactase, the enzyme that digests the milk sugar lactose, were present in a fecal sample from the second-oldest juvenile (6.5 years of age). In contrast, a fecal sample from a 7.1 year old adolescent had lactase but not any milk-specific proteins, suggesting recent weaning [1].

Importantly, nursing orangutans were also consuming foods other than milk for those 6.5 years [1]. Like other primates, weaning in orangutans is a process whereby non-milk foods are incorporated in the infant’s diet and increase in quantity over time until the offspring is fully weaned. A previous study on barium levels in teeth suggested that weaning in juvenile orangutans was cyclical, with nursing stopping and then starting again several times until 8 years of age [3]. Seasonal fluctuations in food availability in Asian rainforests were proposed to explain these nursing patterns; juveniles would rely on non-milk food sources during periods of high fruit availability, then mothers would resume lactation during periods of low fruit availability [3]. 

One reason for the discrepancy between the fecal proteomics results, which identified continuous nursing, and those from teeth, which identified cyclical nursing, could be that the presumed signal of milk intake in teeth (calcium-normalized barium concentrations) was instead signaling nutritional stress [1]. The Danum study identified non-milk food proteins in juvenile stool samples and observed that the quantity of these proteins fluctuated over time, suggestive of resource fluctuations [1]. It is possible that the cycles that the researchers measured via barium levels in teeth were indicative of this ecological variation rather than nursing behavior. However, it is also possible that the extreme variation in ecological conditions known to characterize orangutan habitats has favored adaptability in nursing patterns with respect to food availability. Fecal proteomics on stool samples from the same individuals across lactation would be essential in addressing this hypothesis and validating other methods, like barium levels in teeth, for identifying milk intake. 

Life in the Slow Lane

Whether continuous or cyclical, a lactation period lasting for 6.5 or more years is clearly an outlier among mammals. From an evolutionary perspective, such an extended period of offspring dependence is a risky strategy — time and energy spent nursing is time and energy not available for investment in additional offspring. In the Danum Valley study, none of the mothers of juveniles that were still nursing were pregnant with their next offspring, supporting behavioral observations of interbirth intervals between 7 and 9 years [1, 2, 4]. This extended investment strategy could pay off, however, if infant and juvenile mortality were low. 

Orangutans do, in fact, have one of the lowest infant mortality rates among mammals [2, 4]. Growing slowly means a lower daily energetic burden on the nursing mother and a lower daily cost of growth for the offspring, which may act as a buffer against ecological variability. This lower daily cost is demonstrated by orangutan milk composition, which is relatively dilute and lower in energy than that of more rapidly growing nonhuman primates like macaques or squirrel monkeys [7]. 

But extended lactation may provide another survival benefit to orangutan infants and juveniles. Stool samples with milk-specific proteins were associated with higher counts of defense proteins and probiotic bacterial proteins from lactic acid and Bifidobacterium [1]. Although nursing juveniles were likely getting most of their energy from foods other than milk, continued nursing provided them with a daily immune boost of maternal antibodies and healthy gut microbes. Juveniles could allocate energy to body growth and exploration of their environment because milk intake provided critical immunological factors that enhanced survival. 

The Cost of Slow Living 

The life history strategy of orangutans to extend lactation contrasts starkly with the human strategy. Instead of extending the length of lactation from the great ape pattern, humans shortened it and inserted a novel evolutionary stage of development called childhood. Although human offspring are dependent for a duration that equals or exceeds that of orangutan offspring, childhood allows adults other than the mother to provide care [8]. As a result, humans have a higher population growth rate than orangutans and are profoundly more gregarious. Human population growth directly impacts orangutan habitats, for example the deforestation caused by the palm oil industry. 

The same lactation strategy that enhances infant and juvenile survival leaves orangutans highly vulnerable to population decline. Conservation initiatives should prioritize protection of adult breeding females—mothers and their extended nursing of offspring are the linchpin for saving the species from extinction. 

References

1. Makbul NS, Tajima T, Kanamori T, Kuze N, Nishiuchi T, Wong A, Kumar V, Tsutaya T. Continuous and prolonged breastfeeding in wild Bornean orangutans verified with fecal proteomics. Comm Biol. In press.
2. van Noordwijk MA, Atmoko SS, Knott CD, Kuze N, Morrogh-Bernard HC, Oram F, Schuppli C, van Schaik CP, Willems EP. The slow ape: high infant survival and long interbirth intervals in wild orangutans. J Hum Evol. 2018;125:38-49.
3. Smith TM, Austin C, Hinde K, Vogel ER, Arora M. Cyclical nursing patterns in wild orangutans. Sci Adv. 2017: 3(5):e1601517.
4. Wich SA, de Vries H, Ancrenaz M, Perkins L, Shumaker RW, Suzuki A, van Schaik CP. Orangutan life history variation. In: Wich SA, editors. Orangutans: Geographic Variation in Behavioral Ecology and Conservation. Oxford, Oxford Academic. 2008. p 65-76
5. Tsutaya T, Mackie M, Sawafuji R, Miyabe-Nishiwaki T, Olsen JV, Cappellini E. Faecal proteomics as a novel method to study mammalian behaviour and physiology. Mol Ecol Resour. 2021;21(6):1808-1819.
6. Beverly RL, Huston RK, Markell AM, McCulley EA, Martin RL, Dallas DC. Milk peptides survive in vivo gastrointestinal digestion and are excreted in the stool of infants. J Nutr. 2020;150(4):712-21.
7. Hinde K, Milligan LA. Primate milk: proximate mechanisms and ultimate perspectives. Evol Anth. 2011;20(1):9-23.
8. Bogin B. Evolutionary hypotheses for human childhood. Am J Phys Anth. 1997;104(S25):63-89.

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