Discussion
We characterise the reproductive phenology of two free-tailed bat
species in south-eastern Kenya – Mops condylurus and Mops
pumilus – and present quantitative information on the synchronicity,
magnitude, and timing of their birth pulses. Our data are the highest
resolution available for these species across their distributions
(Mutere 1973; Happold and Happold 1989; Vivier and Merwe 1997). This has
allowed us to empirically estimate birth-pulse synchrony for the first
time in M. condylurus and M. pumilus . This provides
ecological information relevant for understanding mechanisms of
transmission and maintenance for ebolaviruses and other zoonotic
pathogens in bat populations (Peel et al. 2014; Hayman 2015).
For M. condylurus , observed patterns in reproduction had
important distinctions to those in existing literature in Kenya and
across their broader range. Notably, our estimate of birth-pulse
synchronicity, based on more detailed data, was considerably longer than
previous “best-guess” estimates (~8.5 weeks, compared
to 3-4 weeks) (Mutere 1973; O’Shea and Vaughan 1980; Happold and Happold
1989; Vivier and Merwe 1997) (Table S1). We also observed proportionally
more females than expected based on previous estimates of male-to-female
ratios (1:1.06 vs 2.6:1), but proportionally fewer reproductively active
females than expected (16.6% non-reproductively active females, vs 0%)
(Happold and Happold 1989). These findings will have countering effects
on the potential magnitude of the birth pulse – a population with
proportionally more females will have greater potential to produce more
susceptible juveniles, but only if reproductively active.
For M. pumilus , the timing of each reproductive stage was wide
and largely non-defined, and birthing spanned nearly the entire study
period (11 of 12 weeks). The absence of a discernible birth pulse in our
data may be consistent with a pattern of continuous breeding, as
described elsewhere for this species (e.g., Uganda) (Marshall and Corbet
1959; Mutere 1973; Happold and Happold 1989) or reflect a mix of two,
tight birth pulses in November and March-April (O’Shea and Vaughan 1980)
(Table S2). The male-to-female ratio of the M. pumilus population
was lower than anticipated. Female-defence polygyny is the mating system
proposed for M. pumilus , with observations of harems including
one male and up to 21 females within a roost (McWilliam 1988; Bouchard
2001). The sex ratios observed in this study would suggest a more
neutral male-to-female roost composition – contradicting what would be
observed for a female-defence polygyny strategy. The proportion of
reproductively active females were in line with previous literature
(~90%), though previous estimates vary widely dependant
on how many birth pulses are observed for that location (e.g., 93% of
females have been observed to birth to 3 litters in Ghana, and 80% in
Malawi. The proportion of females achieving the maximum of 5 births/year
has not been determined) (Marshall and Corbet 1959; Mutere 1973;
McWilliam 1976; Happold and Happold 1989).
Theory suggests that the large
within-pulse asynchrony observed for M. condylurus and M.
pumilus should increase the probability of long-term viral maintenance
within populations and is consistent with an ecological trait of a
reservoir host. Birthing events of African molossid bats have been
linked to human EVD spillover previously (Hranac et al. 2019),
supporting the plausibility of molossid bats as reservoir hosts, and
that seasonal birthing drives epidemic cycles in these species.
The effects of birth pulse
magnitude and timing on infection dynamics are more difficult to predict
from existing theory. The intervals between birthing pulses are uneven
for M. condylurus – assuming peak parturition in March and
November, intervals are roughly April-October (seven months) and
December-February (three months). This contrasts to even intervals used
in prior models of bi-annual birthing (Hayman 2015). The potential
magnitude of the birth pulse will be driven (in part) by the
male-to-female ratio observed in the population, the proportion of
females reproducing per birth pulse, population size, and the maturation
period, which are ecological features of M. condylurus that
differ from generic bat parameters used in Hayman (2015).
Species-specific models are
needed to interpret how, specifically, identified attributes of the
birth pulse may interact with other features of M.
condylurus -ebolavirus ecology to influence infection dynamics.
It is worthwhile noting that we cannot determine whether lactating
non-pregnant females caught early in the M. condylurus birth
pulse (~January/February) were females that had recently
given birth in the recorded birth pulse, as interpreted, or had given
birth in the prior birth pulse and were not yet (or not going to be)
pregnant for the recorded birth pulse. If the latter is true, our
estimate of synchronicity for M. condylurus could be much lower
than 8.5 weeks. However, we observed a wide span of gestational stages
throughout the study period. Moreover, an even wider span of gestational
stages has been observed at this location during this timeframe, from
previous capture events by the authors (Appendix S2).
10, 21 and 56 M.
condylurus at early-, mid-, and late-stage gestation stages,
respectively (here, directly determined by dissection and examination of
foetus size) were captured over a 7-day capture period in early March
2019 (Appendix S2). As the gestation period for M. condylurus is
85-90 days (~3 months), this range of gestation stages
(particularly those observed within a one-week period in 2019) implies
an equally wide period of birthing. These data support a long birth
pulse, as interpreted.
In addition, our estimate of non-reproductive females during the pulse
(16.6%) could be higher than the true estimate if females at
early-gestation were miscategorised as non-reproductive. The observation
of early gestation by dissection of adults in late March 2019
underscores this possibility (Appendix S2). However, we would note that
non-reproductive adult females were observed throughout the study
period, including April, when gestation and/or lactation should be
obvious. As such, it is reasonable to conclude that less than 100% of
females were pregnant immediately prior to the birth pulse, which is
less than previous estimates (Mutere 1973; Happold and Happold 1989;
Vivier and Merwe 1997).