Social and ecological drivers of reproductive seasonality in geladas

Abstract

Many nonseasonally breeding mammals demonstrate some degree of synchrony in births, which is generally associated with ecological factors that mediate fecundity. However, disruptive social events, such as alpha male replacements, also have the potential to affect the timing of female reproduction. Here, we examined reproductive seasonality in a wild population of geladas () living at high altitudes in an afro-alpine ecosystem in Ethiopia. Using 9 years of demographic data (2006-2014), we determined that, while females gave birth year-round, a seasonal peak in births coincided with peak green grass availability (their staple food source). This post-rainy season "ecological peak" in births meant that estimated conceptions for these births occurred when temperatures were at their highest and mean female fecal glucocorticoid concentrations were at their lowest. In addition to this ecological birth peak, we also found a separate birth peak that occurred only for females in groups that had experienced a recent replacement of the dominant male (i.e., a takeover). Because new dominant males cause abortions in pregnant females and kill the infants of lactating females, takeovers effectively "reset" the reproductive cycles of females in the group. This "social birth peak" was distinct from the ecological peak and was associated with higher rates of cycling and conceptions overall and higher glucocorticoid levels immediately following a takeover as compared to females that did not experience a takeover. These data demonstrate that social factors (in this case, male takeovers) can contribute to population-level reproductive seasonality above and beyond group-level reproductive synchrony.

The Goldilocks Effect: Female geladas in mid-sized groups have higher fitness

“…Although takeovers of small units were less frequent, when they did occur, immigrant males killed disproportionately more infants than when they took over mid-sized units. In this population, male takeovers dramatically alter female reproductive patterns ( 9 , 22 ); here we show that females in mid-sized units have a selective advantage over females in smaller or larger units when it comes to these male-mediated birth patterns.…”

“…67 +/− 0.44 SE, p -value = 0.13). Previous analysis in this population have indicated that temperature, and not the availability of green grass, most strongly predicts glucocorticoid levels ( 9 , 26 ). Therefore, thermoregulatory demands, which presumably affect all individuals equally across units of different size, may constrain metabolic needs more than food does.…”

“…For the majority of infants born during the study period (n = 243 out of 272 total), the date of birth was known within a day or two. For those infants where we did not observe their day of birth (n = 29), we were able to assign the date of birth within 1 month of the actual birth date based on established criteria (i.e., the size of the infant, infant motor skills, presence/absence of the umbilical cord; for more details, see 9 ). From these birth dates we were able to calculate conception dates based on the mean gestation length (n = 183 days; for more details, see: 12 ).…”

“…The disappearance of any infant prior to weaning was assumed to be a case of infant mortality. The cause of mortality was assigned based on the following characteristics: if the infant’s mother died at the same time, the cause of death was recorded as “maternal death.” If the mother did not die at the same time, and the infant death occurred within 9 months of a takeover, the cause of death was recorded as “infanticide” ( 9 , 10 ). All other causes of infant deaths were recorded as “unknown.”…”

“…Geladas are long-lived, folivorous primates that live in social groups (hereafter, “units”) that vary in size from 1 to 13 adult females – variation that is present even within the same population ( 8 ). This variability may be due to ecological pressures (e.g., due to the seasonal environment: 9 ) as well as social pressures. For instance, infanticide is the leading cause of infant mortality: immigrant males kill over half of the dependent infants in a unit ( 10 ).…”