Activity pattern and fat accumulation strategy of the Natterer’s bat (Vespertilionidae, Chiroptera) swarming population indicate the exact time of male mating effort

Abstract

Studies concerning bat autumn swarming behavior suggest that the main purpose of this phenomenon is mating. However, the process of fat accumulation is crucial for surviving winter, and it seems to be in clear conflict with a need to strive for the opportunity to mate prior to hibernation. Investment in one activity limits the other one creating a trade-off between them. The aim of our study was to describe the activity pattern of each sex-age group (adult males, adult females, subadults) of the Natterer's bat during swarming period and to investigate the fat accumulation process of adult males in the context of their reproductive strategy. Bats were captured by mist nets at the swarming site fortnightly from the early August until the late November. The age, sex, reproductive status, and body condition index (mass to forearm ratio, BCI) were recorded. The activity peak of both sexes, adults, and subadults was observed in the late September. That time in season, BCI of adult males was the lowest, and there was no correlation between the hour of an adult male capture and its BCI value within one night (r s = 0.23; p = 0.157). Such correlation was observed later in the season (early October: r s = 0.44; p = 0.020; late October: r s = 0.48; p = 0.002). A negative correlation between adult males' BCI and proportion of adult females was found (r = 0.44; p = 0.000). We conclude that the activity peak of females is likely to be responsible for the effort of the mating behavior of the males, which is reflected by their low condition index. We suggest that the gleaning foraging strategy of Natterer's bat allows the males to postpone their fat accumulation until just before hibernation.

Differences in seasonal survival suggest species‐specific reactions to climate change in two sympatric bat species

“…The monthly NAO indicated that in 2010 the December was particularly harsh. At this time of the winter, Natterer's bats are typically still active as indicated by a lot of activity at the hibernaculum according to the logger data (own unpublished data), and especially males try to accumulate their fat reserves very late in Natterer's bats (Kohyt, Rozik, Kozakiewicz, Pereswiet‐Soltan, & Gubała, ). In contrast, Daubenton's bats start to hibernate much earlier (own unpublished data) and thus should not have been affected by the cold conditions during that month.…”

Social calls of Myotis nattereri during swarming: Call structure mirrors the different behavioral context

“…Autumn swarming is accompanied by intense social vocalizations that are associated primarily with mating [14, 25]. In most swarming bat species, males outnumber females at autumn swarming sites [14, 24, 26, 27]. Genetic studies revealed that autumn swarming increases genetic diversity within populations, which is especially important for highly philopatric bat species [26, 28, 29].…”

Phenology of reproductive condition varies with age and spring weather conditions in male Myotis daubentonii and M. nattereri (Chiroptera: Vespertilionidae)

“…Nevertheless, despite the temporal separation between spermatogenesis and mating that is possible in the reproductive cycle of temperate bats due to prolonged sperm storage 5,10 it seems likely that a selective advantage does exist for males to be ready to mate soon after females have completed lactation. Mating within day roosts in the summer habitat 16 , and peak autumn swarming activities [25][26][27][28] , both occur within a short period that is post-maternity and pre-hibernation for our two study species. The timing of mating readiness in males and receptivity in females differs between these sympatric congeners but is highly synchronised within each species.…”

“…Mating at swarming sites may maximise their breeding potential through copulation with numerous sexual partners and outbreeding with non-related females from different colonies drawn from a large catchment area 16 . In contrast, M. nattereri adult males typically commence spermatogenesis during July and August then leave their summer habitat (presumably to migrate to autumn swarming sites [23][24][25] during September. These observations fit well with the temporal distribution of swarming activity recorded in these species, with the peak in M. daubentonii activity at swarming sites occurring during August, and the peak in M. nattereri activity at swarming sites occurring during September [25][26][27][28] .…”

“…In contrast, M. nattereri adult males typically commence spermatogenesis during July and August then leave their summer habitat (presumably to migrate to autumn swarming sites [23][24][25] during September. These observations fit well with the temporal distribution of swarming activity recorded in these species, with the peak in M. daubentonii activity at swarming sites occurring during August, and the peak in M. nattereri activity at swarming sites occurring during September [25][26][27][28] . The difference in timing of spermatogenesis between species also corresponds with the breeding phenology of females, with independent juveniles and post-lactating M. daubentonii being recorded from early July onwards, whilst volant juveniles and post-lactating M. nattereri are recorded from late July onwards in our study system 14 .…”