Geographic differences in seasonal brain mass changes
Literature on seasonal changes in mammalian brain size is scarce (see Supporting Information), but average brain mass of S. araneusdecreased by 20.9 ± 5.6% from summer juveniles to winter subadults and regrew by 10.0 ± 4.2% to adult size. This is the most remarkable aspect of Dehnel’s phenomenon. The size of the mammalian brain, once fully grown, is usually more or less fixed and changes of this magnitude are unparalleled. Only the song brain of some birds reversibly changes by similar magnitudes (Nottebohm, 1981; Tramontin et al. , 1998). Experimentally induced changes in brain size, by starving or other environmental manipulations, result in changes of less than 5% (Bedi & Bhide, 1988). In humans, brain size increases during ontogeny, reaching a peak at the age of 20; then, after 45-50 years of age it undergoes a progressive, unidirectional decline of 11% over the next 40 years, as a result of ageing (Dekaban & Sadowsky, 1978).
When we analyzed the compiled shrew literature data with regards to geographic variation in the intensity of seasonal size change we found only little variation between populations. However, in contrast to braincase height, we found no correlation between any geographic variable and the intensity of both decrease and regrowth of brain mass (Tables 1 and 2). When we looked at our own data from Gugny and Radolfzell in more detail, we again found no significant effect of sex on the variation of corrected brain mass (AIC(M1) = -509.6, AIC(M2) = -509.7; ANOVA, P > 0.1) and excluded it from the models. We did find significant variation between seasons at the factor level in the final model M2, but not between locations and their interaction (d.f . = 50, adj. R2 = 0.45,F (seas.) = 22.5, F (loc.) = 0.5, F (seas.:loc.) = 2.1, P (seas.) < 0.001, P (loc.) > 0.5, P (seas.:loc.) > 0.1). However, there was no difference in absolute brain mass at any of the age stages that could be explained by geographical variables either (P > 0.1 for all models). At both locations there was a significant but similar decrease from summer juvenile to winter subadult (Tukey test, P< 0.001). Surprisingly we found no significant regrowth from winter subadult to adult (P > 0.1). Thus, corrected brain mass was similar at every stage in both populations (Table 3, Fig. 4). This lack of spring regrowth in brain mass contradicts previous research, where a significant regrowth in brain mass from winter to summer was found in all studied populations of S. araneus (Bielak & Pucek, 1960; Pucek, 1970; Yaskin, 1994; Lázaro et al. , 2018a). In fact, in our own previous study of the Radolfzell population we found a significant regrowth of brain mass, with a maximum during July-August (Lázaro et al. , 2018a). The adult brains used for the present analyses were collected earlier (May-July). Especially in Gugny, we collected most adults in May when body mass regrowth peaks and which is when the regrowth peak is commonly measured. Thus, the adults we included here probably had not completed brain regrowth yet. This emphasizes the importance of timing data collection correctly and defining the size stages for studies of Dehnel’s Phenomenon carefully. An accurate estimation of the maxima and minimum of the cycle is critical for the quantification of the change. Probably most studies of Dehnel’s Phenomenon do not report exact maxima and minima, and consequently all values on change intensity presented here are timed with spring body mass maxima and thus underestimates of the actual change in brain mass. This must be considered when studying Dehnel’s Phenomenon intensity as well as the cycle phenology.
Striking in our combined results is the discrepancy between variables. Changes in braincase height did not reflect changes in brain mass, unlike in previous studies (Bielak & Pucek, 1960; Lázaro et al. , 2018a). The geographical patterns we found in braincase height decrease, were not reflected by patterns in brain mass changes. This is probably due to the small sample size of brain mass data. Only few studies (10 publications about the common shrew, the most intensively studied species) have investigated the seasonal changes in brain mass.