Cold Discomfort: A Model to Explain Repetitive Linear Enamel Hypoplasia Among Pan troglodytes and Pan paniscus

A geographically and temporally widespread pattern of repetitive episodes of developmental stress, recorded as furrows of linear enamel hypoplasia (LEH) in most recent and fossil apes, requires explanation. I compared observations of LEH recurrence among museum specimens of Pan troglodytes and Pan paniscus with historical weather records of seasonally recurrent combinations of lower temperature, higher rain and wind (“cold discomfort”). I imaged samples of 34 canine teeth (N = 20 animals, 54 independent LEH) from P. troglodytes from Fongoli, Senegal, and Taï Forest, Côte d’Ivoire and P. paniscus from the Democratic Republic of Congo with a scanning electron microscope and counted perikymata between and within LEH events. I converted counts to time using published Retzius periodicities (the number of days taken to form enamel layers, visible in thin sections) and compared their recurrence and duration to seasonal peaks of incidence and the intensity of cold discomfort. Using the longest Retzius periodicity (9 days), chimpanzees and bonobos show LEH lasting about 7–9 weeks, respectively, recurring annually. Most bonobos also show just-under semiannual recurrence of LEH. “Colder, wetter, windier” weeks recur annually at the P. troglodytes locations and semiannually at P. paniscus sites. When the combination of “below median temperatures, above median rain and winds” peaks in intensity and incidence over a 7-week period, daily “minimum hourly” temperatures average 20–21°C (7–9°C below lower critical body temperature for chimpanzees) with wind 3 times and rainfall 30 times higher than usual. These findings suggest that seasonal cold discomfort may be an important factor in episodic enamel hypoplasia in many nonhuman primates.