DOI: 10.5061/dryad.6q573n60d
title: Seasonal variation in age, sex, and reproductive status of Mexican free-tailed bats

description: In North America, Mexican free-tailed bats (Tadarida brasiliensis
 mexicana) consume vast numbers of insects contributing to the economic
 well-being of society. Mexican free-tailed bats have declined due
 to historic guano mining, roost destruction, and bioaccumulation of
 organochlorine pesticides. Long-distance migrations and dense
 congregations at roosts exacerbate these declines. Wind energy development
 further threatens bat communities worldwide and presents emerging
 challenges to bat conservation. Effective mitigation of bat mortality at
 wind energy facilities requires baseline data on the biology of affected
 populations. We collected data on age, sex, and reproductive condition of
 Mexican free-tailed bats at a cave roost in eastern Nevada located six km
 from a 152-megawatt industrial wind energy facility. Over five years, we
 captured 46,353 Mexican free-tailed bats. Although just over half of the
 caught individuals were non-reproductive adult males (53.6%), 826
 pregnant, 892 lactating, 10,101 post-lactating, and 4,327 non-reproductive
 adult females were captured. Juveniles comprised 11.5% of captures. Female
 reproductive phenology was delayed relative to conspecific roosts at lower
 latitudes, likely due to cooler temperatures. Roost use by reproductive
 females and juvenile bats demonstrates this site is a maternity roost,
 with significant ecological and conservation value. To our knowledge, no
 other industrial scale wind energy facilities exist in such close
 proximity to a heavily used bat roost in North America. Given the
 susceptibility of Mexican free-tailed bats to wind turbine mortality and
 the proximity of this roost to a wind energy facility, these data provide
 a foundation from which differential impacts on demographic groups can be
 assessed.
description: <strong>Study site</strong> – This study
 was conducted at a cave located in the Snake Range, White Pine County,
 Nevada (2,040 m elevation). The cave is approximately 10 m up a steep
 inclined rock face and is accessible through a 4 by 13 m opening (Figure
 1). The cave opening leads to a large upper first chamber, which descends
 roughly 20 m below the opening, narrows, and continues to a second large
 chamber. The internal structure of the cave allows the second chamber to
 retain heat produced by roosting bats, buffering roosting bats from
 surface conditions, while the first chamber provides an area for bats to
 gain altitude before outflight by flying counterclockwise around the room
 (Figure 1). The cave was mined for guano during the
 1920’s. In 1926, a 66 m long adit was driven from the hillside into the
 lower chamber (Figure 1). This adit facilitated guano removal by providing
 direct access to the greatest accumulations of guano at the base of the
 second chamber. Placement of the adit portal at a lower elevation than the
 natural cave opening facilitated airflow between the two openings, likely
 causing a drop in cave temperature and humidity. In 1996, the adit opening
 was sealed so that the internal climate of the cave could return to
 historical conditions. Historical bat use of this roost
 is largely anecdotal. Bat use is suspected to have increased in response
 to the adit closure, due to an increase in cave temperature and humidity.
 Occasional historical surveys suggested that use was limited to late
 summer and early fall by migrating bachelor males. Beginning in 2010,
 intensive surveys revealed large numbers of bats occupying the cave from
 June to October. <strong>Bat capture and
 assessment</strong> – Bats were captured using a 2 by 2 m
 double-frame harp trap (Austbat, Victoria,
 Australia http://www.faunatech.com.au/products/harptrap.html, Kunz,
 Hodgkison, & Weise, 2009). Foam padding was attached to the frame
 on the outflight side of the harp trap to reduce risk of injury to bats
 that hit the frame. The harp trap was deployed at the mouth of the cave,
 perpendicular to the direction of outflight in one of the main flight
 paths of the emerging column of bats (Figure 1). The trap was opened at
 dusk before outflight and monitored continuously until it was closed at
 the end of primary nightly exodus of bats from the cave, or sooner if more
 bats were caught than could be processed in a timely manner. Although bats
 continued to fly in and out of the cave all night, trapping focused on the
 main outflight and typically ended by midnight. After being captured, bats
 remained in the harp trap bag until removal for processing, which occurred
 continuously throughout the night as bats were captured. Bats were
 released immediately following data collection. For
 each bat captured, we recorded sex, age (adult or juvenile), and
 reproductive condition (female – pregnant, lactating, post-lactating, or
 non-reproductive (parous or nulliparous); male – non-reproductive or
 scrotal; (Racey, 2009). Sex was determined by observing genitalia. Age was
 assessed by observing epiphyseal plates of the metacarpal-phalangeal
 joints (Brunet-Rossinni & Wilkinson, 2009). Bats were classified
 as juvenile when there was a translucent cartilaginous gap in these
 joints. We use juvenile synonomously with flying young or volant young of
 year. Reproductive condition of females was determined by examining
 nipples and palpating the abdomen. Females were classified as pregnant
 when the abdomen was enlarged and a fetus could be felt (Gannon, 2003).
 Bats were classified as lactating if mammary glands were visible under the
 skin as a yellow disk and/or there was balding around the nipples
 attributable to the suckling of pups. Post-lactating females retained
 enlarged nipples, but no milk could be seen through the skin.
 Additionally, nipples of post-lactating females were dark and hair around
 the nipples was re-growing (Racey, 2009). Female bats were classified as
 non-reproductive if they did not appear to be pregnant, lactating, or
 post-lactating. Non-reproductive females were assessed and recorded as
 parous if there were signs of past reproduction, or nulliparous if there
 were no signs of past reproduction. Parous and nulliparous females were
 combined and classified as non-reproductive for analysis. Enlarged testes
 indicated that males were scrotal and reproductive while absence of
 enlarged testes indicated that males were non-reproductive (Sherman,
 1937). Presence of a visible gular gland, found in the suprasternal neck
 region, was another indicator of reproductive males (Krutzsch, Fleming,
 & Crichton, 2002). All research was completed
 under protocols approved by Christopher Newport University’s Animal Care
 and Use Committee under protocols 2015-9 and 2018-7, and Nevada Department
 of Wildlife scientific collection permit #39645; and followed the
 guidelines of the American Society of Mammalogists (Sikes & The
 Animal Care and Use Committee of the American Society of Mammalogists,
 2016). <strong>Data analysis</strong> – Bat
 captures were modeled using binomial generalized linear mixed models with
 a logit function. The logit function ensures positive fitted values and
 the binomial distribution is typically used for proportional count data.
 Response variables were captures of juvenile, adult female, pregnant,
 lactating, post-lactating, and non-reproductive females relative to total
 captures by night. Juvenile captures were modeled as the juvenile
 proportion of all individuals and adult female captures as the female
 proportion of all adults. Female reproductive condition was modeled as
 captures by reproductive class relative to all adult female captures. Trap
 night (Julian day) was used as the explanatory variable, modeled as
 quadratic and cubic functions to delineate seasonal variation and peaks in
 proportional abundance. When trapping extended past midnight, night was
 defined as the Julian day when trapping started. Year was modeled as an
 intercept-only random effect. Our models followed the general
 form: · Ci ~ Binomial (pi, Ni)                         
                                         (1) · E(Ci) =
 pi                                                                       
             (2) · Logit(pi ) = nightj(i) +(nightj(i)
 ))2 + (nightj(i) ))3 + yeari             (3) · Yeari ~
 N(θ, σ2Year)                                                             
     (4) Where C is the number of bats captured per
 nighti for the group of interest, p is the proportion of that group
 captured per nighti, N is the total number of captures per nighti, night
 is the Julian date of the sampling night, and year is sample year. Night
 ranged from 159-299 and year from 2015-2019. Yeari is the random
 intercept, assumed to be normally distributed with a mean θ, and variance
 σ2. To facilitate model convergence, night and year
 were constrained between -1 and 1 by centering each variable by the mean
 and scaling by the standard deviation. Scaled values were back-transformed
 for interpretation and graphical presentation. Akaike
 Information Criterion (AICc) was used to guide model selection (Burnham
 & Anderson, 2002). Model inference included means and standard
 errors. Graphical results present model fit ± 95% confidence intervals. We
 used an exact binomial test of proportions to test the null hypothesis
 that sex, age, and reproductive condition were drawn from Bernoulli
 distributions with equal group proportions. Analyses were completed with
 Program R (R Core Team, 2019). The R package ‘nlme’ (Pinheiro et al.,
 2019) was used for generalized linear mixed models and the R package
 “MuMIn” (Barton, 2018) was used for AICc.