Use of water sources by bats in arid environments is an under-investigated topic in ecology and behaviour. Previous studies have shown that for bats, water sources may have structuring effects on bat populations and communities. For example, visitation patterns are temporally orchestrated among bat species that congregate at spatially restricted water sources to drink, thereby increasing efficiency of resource use (Commissaris 1961; Cockrum & Cross 1964; Adams & Simmons 2002; Adams & Thibault 2006), and female bats visit water sources higher in dissolved calcium (Adams et al. 2003). In the present study we used a novel method to provide unique insight into the visitation patterns of female bats as related to their reproductive status.
The availability of free water to reproductive females has been suggested in previous studies, but none of these studies have measured directly the apparent importance of water sources to bats living in natural populations. With our use of a novel PIT-tag system, we were able to gather data on visitation patterns to an artificial water source of individual bats of known reproductive condition. Our results are consistent with our predictions based on bat ecophysiology concerning evaporative water loss, but we show the added effect of lactation on water-seeking behaviours in reproductive females. Moreover, we mist-netted at all other potentially available water sources (n = 7) within 4 km of the Geer Canyon site, and found only one other water source where female M. thysanodes were captured consistently and that was at a quarry approximately 3 km and several mountain ridges away. However, none of our PIT-tagged females from Geer Canyon were captured at that quarry and none of the female M. thysanodes PIT-tagged at the quarry have been acquired by the reader placed in Geer Canyon. Our telemetry data from radio-tagged M. thysanodes captured at the quarry showed that those individuals forage between the water source and their roost site located on a rock wall approximately 1 km due west of the quarry, about 4 km from the Geer Canyon roost site.
Our water visitation data make it possible to formulate a predictive model for bat reproduction in the western United States based upon expected loss of water resources, as projected from current climate-change models. Predicted changes in bat population distributions relative to hibernation and climate change have been modelled previously (Humphries, Thomas & Speakman 2002).
decay model estimating loss of bat lactation rates with climate warming
Bats, marsupials and primates, as a group, have lactation lengths 50% longer (on a log10 scale) than other therians of comparable body masses (Hayssen, van Tienhoven & van Tienhoven 1993). However, the median length of lactation for bats (60 days) is more than twice that of the Insectivora (28 days) (Hayssen, van Tienhoven & van Tienhoven 1993). For vespertilionid bats, the average lactation length is 40·9 days (Barclay & Harder 2003). Combining our data on visits to water sources by lactating female fringed myotis (M. thysanodes) with data from the literature, we have derived the following model to predict affects of expected climate-induced changes on water storage capacity at a local level (Christensen et al. 2004).
We calculated the average volume (length × width × depth) of water across four natural water sources near roost sties in our study area to be 200 000 cm3. The volume of 1 L of water is 1000 cm3, thus on average these water sources held about 200 L. From our calculation above, we estimate that each lactating female will require about 0·113 L of water over a 21-day lactation period. Thus, each water source can support approximately 1770 lactating female M. thysanodes. For simplicity, we assume that stream inflow, local precipitation and evaporation are negligible effects during the lactation period.
Using the scenario presented by Christensen et al. (2004), where 1 °C climate warming will result in 36% less regional water storage, and beginning with 200 L of available water in support of 1770 female lactating M. thysanodes over a 21-day lactation period (O’Farrell & Studier 1973), we generate two decay curves, one that tracks water loss per degree increase in regional climate and the other a decay curve representing predictive declines in lactation ability in female bats (Fig. 4). As it stands, a 1 °C increase in warming would result in reduced water availability, thereby supporting only 1128 of the 1770 (64%) lactating females over the 21-day lactation period. A 2 °C warming results in only 40·8% (722) of females supported during the expected 21-day lactation period. Extrapolating the model to a 5 °C increase in regional average temperature predicts that only 10·7% (189) of the original 1770 lactating females would be supported by water availability (Fig. 4).
Figure 4. Mathematical model showing the relationships between climate warming, loss of storage in natural water sources, and declines in bat lactation ability. See text for further explanation.
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Although the anticipated decline in reproductive ability appears to be clearly significant, in reality our model data may be conservative. For example, our estimation of a 21-day lactation period for M. thysanodes was based upon data concerning flight ability of juveniles from a single, but detailed, study conducted by O’Farrell & Studier (1973). Tuttle (1976) showed that length of lactation period in M. grisescens varied between 58 and 76 days and was dependent upon growth rates of juveniles which varied by the distance travelled to foraging areas and water sources; thus, the longer lactation period occurred in the most stressed colony. These data suggest that lactation periods may vary widely between populations of a single species (Tuttle & Stevenson 1982). Because of this, unless length of lactation is known for a given population, we suggest that the average lactation length of 40·9 days for Myotis species (Barclay & Harder 2003) be used in the model as a conservative measure of water needs for local bat populations. Furthermore, other variables consistent with loss of water from natural systems, such as significant increase in regional human population and its need for more water, may well come into play in determining local resource availability to bats and other wildlife.
To our knowledge, no data exist on consumption of water by free-flying lactating female bats, either in captivity or the wild, but Webb et al. (1993) suggested that consumption of drinking water was 1·76 g day−1 ± 0·42 g under ambient temperature conditions of 14·9 ± 4·1 °C and a relative humidity of 33·8 ± 11·2 for non-reproductive, free-flying, captive M. daubentonii. Hence, the average estimated nightly water consumption of 5·40 mL (g) day−1 for free-ranging lactating female M. thysanodes used in our model seems reasonable under arid conditions.
One obvious question concerning bats and water availability concerns whether bat species living in arid environments are adapted, or are becoming adapted, to living in water-limited environments. Investigations into urine-concentrating ability and renal structure (Carpenter 1969; Geluso 1978, 1980) showed that many of the bat species living in arid habitats, including E. fuscus, M. lucifugus, M. auriculus, M. volans, M. yumanensis and Corynorhinus townsendii, had no specialized adaptations of the kidney for greater water retention than average. Relative to the present study, M. thysanodes, of which many populations are distributed throughout desert and semidesert habitats, had relatively poor urine-concentrating ability (Carpenter 1969; Geluso 1978, 1980). Thus, survival of many of the bat species inhabiting arid environments is conditional on the availability of permanent water sources nearby roost sites (Findley et al. 1975; Adams & Thibault 2006), and thus continued residency and/or survival of such populations under conditions of reduced water availability is questionable.
Year-to-year variation in winter snowpack as translated into spring and summer runoff timing and rates, as well as local timing and rates of precipitation, are variables that may decelerate or accelerate predicted declines in local bat population numbers with increased climate warming. However, long-term predictions from our model suggest overall declines in bat populations in the western United States. We emphasize that this model provides a provisional baseline for understanding the complex relationships between predicted climate-induced declines in water availability as being related to foreseeable consequences for bats and other wildlife populations. We encourage more research in this regard. We also anticipate that the PIT-tag system that we describe will be useful in estimating survival rates as well as monitoring the behaviour and movement patterns of forest bats. The implications of our findings on visits to water sources by lactating female M. thysanodes has far-reaching implications across many species of bats and for the continued biodiversity of arid regions in the western United States and elsewhere.