Structure and Reproductive Patterns in the Summertime Forest-Bat Community of Southeastern Texas

Southern forests extend through 13 southeastern states from Virginia to Texas (Wear and Greis 2012, 2013). In some regions, forest densities reach >80%. These southern forests are diverse and consist of pines (34%), hardwood types (55%), and oak–pine mixture (11%). Southern forests are divided into five large regions (Wear and Greis 2012, 2013): Coastal Plain, Piedmont, Appalachian-Cumberland, Mississippi Alluvial Valley, and Mid-South. The Coastal Plains are divided into two subsections by the Mississippi Alluvial Valley. Texas pine forests are situated in the Coastal Plains west of the Mississippi River. The Coastal Plains have the highest biodiversity of these forested regions, due to their large geographic extent and the high diversity of habitats. Modeling predicts the Coastal Plains to be the most impacted segment of southern forest by four key factors: population growth; climate change; timber markets; and invasive insects, plants, and diseases (Wear and Greis 2012, 2013). All predictive models (Wear and Greis 2012, 2013) suggest that the Coastal Plains will experience in the next few decades an increase in average annual temperatures of 2.5–3.5°C; some models suggest a decrease in precipitation while others predict increased precipitation. This area also is expected to experience increases in human population (urbanization), increased timber production (causing replacement of natural growth pine forests with pine plantations), and increased vulnerability to invasive species and diseases. Together these factors will cause an inevitable decline in biodiversity of this region (Wear and Greis 2012, 2013). Conservation of the biota of this region requires study of the flora and fauna (both surveys and natural history) as predicted changes occur. These studies should provide better understanding of each affected species and should aid in development of effective conservation plans to prevent loss of these species.

The bat fauna of Texas is rich, including all four bat families (Mormoopidae, Phyllostomidae, Vespertilionidae, and Molossidae) and all but 14 of the 47 bat species occurring in United States (Ammerman et al. 2012). Of the 33 bat species recorded in Texas, 22 use forestland for roosting and foraging (Ammerman et al. 2012). Forests of eastern Texas represent the westernmost extent of the southern pine belt of the forested Gulf Coastal Plain (Wear and Greis 2012, 2013). The bat communities of southern pine forests have been studied previously in at least five states. Morris et al. (2010; May–August) surveyed the managed pine forest in North Carolina using both acoustic sampling and netting. Kilgore (2008; May 2002–May 2004) documented the bat community in managed bottomlands in Alabama. Ford et al. (2006; May–July) conducted acoustic surveys in South Carolina in pine forest managed for red-cockaded woodpeckers Picoides borealis. Both Miller (2003; late April–early September) and Welch (2003; June–August) conducted their studies in managed loblolly pine forests in Mississippi. Prior studies of bats in eastern Texas forests have focused on roosting ecology of species of conservation concern, such as Rafinesque's big-eared bat Corynorhinus rafinesquii and southeastern myotis Myotis austroriparius, or have reported results of broad faunal surveys (Packard 1966; Michael et al. 1970; Schmidly et al. 1977; Schmidly 1983, 1991; Thies 1994; Walker et al. 1996; Yancey and Jones 1996; Higginbotham and Jones 2001; Mirowsky et al. 2004).

We are unaware of any previous studies focusing on the bat community of this ecosystem in Texas. Such a survey is essential and timely because the U.S. Forest Service lacks detailed data about the bat assemblages in southeastern Texas (D. Jauregui, Sam Houston National Forest, personal communication), and National Wildlife Refuges located in Texas lack well-developed plans appropriate for bat conservation (Dixon et al. 2013). Bats in North America face many challenges due to habitat loss, white nose syndrome, and wind turbine collisions (Dixon et al. 2013); therefore, it is critical that such data are collected to enable development of conservation plans. In addition to compiling inventories, effective conservation plans require knowledge of how bats are using any given area. This demands identification of roost sites, including tree roosts for migrating bats, and maternity colonies and hibernacula (places, usually caves, where bats hibernate over winter), as well as understanding of reproductive patterns and success. Knowledge of timing of bat parturition and lactation is essential because many parts of the Texas pine forest are managed with prescribed burns, and direct smoke and heat exposure might be detrimental for bats (Rodrigue et al. 2001). Thus, the purpose of this study was to characterize the assemblage and investigate reproductive patterns of the summer bat community in the pine forest of eastern Texas.

We conducted the study in Sam Houston National Forest, which is located in southeastern Texas and encompasses an area of 65,217 ha (Thomlinson 1995). This forest belongs to the Coastal Plains region of the southern forests, and is composed mostly of coniferous trees <80 y of age (87%), with only 1% of the stands containing trees >100 y of age (Azevedo et al. 2000). Approximately 35% of the forest is occupied either by pine plantations <20 y of age or by recent clear-cuts. The dominant pine is loblolly pine Pinus taeda, while shortleaf pine Pinus echinata is present in most of the older stands and dominates some drier sites. Longleaf pine Pinus palustris occurs in mixed stands on the eastern side of the forest. A variable mixture of American sweetgum Liquidambar styraciflua, southern red oak Quercus falcata, post oak Quercus stellata, white oak Quercus alba, water oak Quercus nigra, bluejack oak Quercus incana, American beech Fagus grandifolia, and magnolia Magnolia sp. constitutes the canopy, with a well-developed mid-story of hardwoods in unmanaged areas. Mixed hardwood forest occurs close to most drainages. At least eight forest-dwelling bat species are expected to summer in these forests (Schmidly 2004; Ammerman et al. 2012): Rafinesque's big-eared bat, big brown Eptesicus fuscus, eastern red bat Lasiurus borealis, northern yellow bat Lasiurus intermedius, Seminole bat Lasiurus seminolus, southeastern myotis, evening bat Nycticeius humeralis, and tri-colored bat Perimyotis subflavus.

Suitable mist-netting sites were scarce because most ponds were ephemeral, many drying out by mid-June. We conducted our sampling at two sites where ponds remained water-filled throughout the summer (June–August): Kelly Pond and Henry Lake Creek. Kelly Pond, in Montgomery County (30°30′37″N, 95°39′42″W), was approximately 36 m in diameter, and was surrounded by pine forest canopy except on one side that opened to a clearing. Henry Lake Creek was in San Jacinto County (30°32′31″N, 95°7′29″W), located approximately 55 km east of Kelly Pond. An access road crossed one of the branches of Henry Lake Creek at this site. The road interrupted water flow, forming a slow-moving pool of water similar in size to Kelly Pond. This road section had an open canopy, thus allowing bats to approach the pond, but otherwise was surrounded by dense hardwood canopy.

We collected field data during the summers of 2009–2011. At Kelly Pond we sampled with either three stacked mist nets (each 2.6 m high and 12 m wide; Avinet Inc., Dryden, NY) or a single large net (7.8 m high and 12 m wide) attached to a single triple-high net system (BatNets.com, Austin, Texas). Both net arrangements presented equivalent surface areas. Sampling effort at Kelly Pond was six nights (40 netting-hours) in 2009, six nights (24 netting-h) in 2010, and four nights (23.5 netting-h) in 2011. We netted at Henry Lake Creek, using two triple-high net systems for nine nights (82.5 netting-h) in 2010, and a single triple-high net for four nights (24 netting-h) in 2011. During 2011, both sites were surveyed simultaneously.

We monitored mist nets continuously during all surveys. For each captured bat, we recorded species, age, mass, standard external measurements (lengths of body, forearm, tail, ear, tragus, and hind foot), and reproductive status. We identified each bat as adult or subadult by examining the epiphyseal–diaphyseal fusion of the fourth metacarpal–phalangeal joint (Anthony 1988). Individuals with opened joints were classified as subadults; individuals with closed joints were classified as adults. Furthermore, we determined reproductive status as pregnant, lactating, or non-reproductive for females, and as reproductive (scrotal) or non-reproductive for males (Racey 1988). Bats were released within 1 h of capture. We followed the Guidelines of the American Society of Mammalogists for the use of wild mammals (Gannon et al. 2007) and worked under protocol 10-05 approved by Baylor University Institutional Animal Care and Use Committee and Texas Parks and Wildlife scientific permit (SPR-0706-704).

Reported results represent the combined data sets for both sites for all sampling years (Tables S1 and S2). We computed the proportion of the overall bat community comprised by each species. Bats of each species were grouped according to gender, age, and reproductive condition: pregnant females, lactating females, adult non-reproductive females, adult males, and subadults. We compared the reproductive phenology of our most-frequently-captured bat species in eastern Texas forests with reproductive timing in other locations where these species occur.

We captured 382 bats representing eight species (Seminole, evening, big brown, eastern red, southeastern myotis, tri-colored, hoary Lasiurus cinereus, and Mexican free-tailed bats Tadarida brasiliensis) throughout our study (Table 1). During 2009, captures totaled 122 bats of all eight species. During 2010 we captured 136 bats comprising six species, all but hoary and Mexican free-tailed bats. The same six species were also captured during 2011 (124 bats; Table 1). Seminole bats dominated the assemblage at both sites in all sampling years, present in nearly twice the number of the next most common species, evening bats (Table 1). Together, Seminole and evening bats comprised 65.2% of the bats captured in our surveys. Big brown bats were third most common (13%), with the remaining five species accounting for the final 20% of captures. For Seminole and southeastern myotis, females represented the majority of the bats captured (62% and 67%, respectively); whereas, males were in the majority for eastern red, evening, big brown, tri-colored, and Mexican free-tailed bats (67%, 62%, 60%, 58%, and 75%, respectively).

We captured 163 Seminole bats: 20 pregnant females, 34 lactating females, 20 non-reproductive females, 31 adult males, and 58 subadults (Table 1). Pregnant bats first appeared in samples during the last week of June and were present continuously through the end of July (Table 2). Lactating females, however, were captured nearly a month earlier than the first pregnant bats; lactating bats were present every week from the first sample in late May through the third week in July, a span of 7 wk. The earliest appearance of subadults in our samples was during the last week of June. Though not found in our samples for the first week of July, subadults comprised from half to nearly two-thirds of the samples through the first week of August, when our field seasons ended. Male and female subadults were captured in similar numbers (31 and 27, respectively). Adult males were uncommon in May and early June (only two captures), but their proportions increased from late June through early August, representing up to a quarter of the captures in subsequent weeks. Non-reproductive females were also uncommon, reaching their greatest proportion (26%) during the last week of sampling.

Of the 86 evening bats captured during the survey, adult males represented 60% of all adult evening bats captured, and were present in every weekly sample except for the first week of June (Table 1). Most of the males were captured in the last 2 wk of June. Pregnant females (n  =  22) were captured throughout June and July but were most numerous during 15–30 June (Table 2). Lactating females (n  =  5) were captured only during a 3-wk interval in June (Table 2). Non-reproductive females (n  =  4) were occasional captures during June, July, and August. Subadults (n  =  8, all but one male) were captured from mid-June through late July, with 50% of subadults captured during the third week of June.

Males represented almost 58% of big brown bat captures (n  =  57; Table 1). They were present in samples throughout the summer except for the first week of June. Pregnant females (n  =  18) were captured May through July, and they represented the second most numerous group among big brown bats. The highest capture numbers of pregnant females were on 31 May and during the second week of June (Table 2). Lactating females (n  =  4) were only captured in the third week of June (Table 2). Two male and two female subadults (mid-June) and one non-reproductive female (early August) were captured.

The remaining five species were captured in low numbers during this survey (Table 1). We captured 31 eastern red bats, with number diminishing over the three summers: 20 captures in 2009, 8 in 2010, and only 3 in 2011 (Table 1). Reproductive females (pregnant and lactating) were captured only during June when they dominated the sample. No non-reproductive adult females were captured. Adult males were captured occasionally throughout the summer. Subadults (all male) represented nearly one-third of captures, and almost all were captured in July of 2009.

Southeastern myotis (n  =  21) were captured intermittently throughout the summer (Table 1). All adult females (n  =  10) were reproductive. Only four adult males were captured. Subadults (four females and three males) were captured in mid-June. Tri-colored bats (n  =  19; Table 1) were captured during every month of sampling (May–August). Pregnant females (n  =  6) were only captured during the third week of June. No lactating females were captured. Almost 58% of the tri-colored bats captured were adult males. Both subadults were females. Mexican free-tailed (n  =  4) and hoary (n  =  1) bats were captured only during summer 2009 (Table 1). We captured one Mexican free-tailed bat (male) on 24 July, three Mexican free-tailed bats (all females) on 7 August, and a single female hoary bat on 12 June.

Our two pond sites appeared representative of the expected bat community assemblage in Sam Houston National Forest, because we captured all six bat species expected to be present during summer in this type of habitat (big brown, eastern red, Seminole, southeastern myotis, evening, and tri-colored bat). The few other bat species occurring in eastern Texas have habitat requirements not widely present in Sam Houston National Forest. Rafinesque's big-eared bats are known in easternmost Texas (Schmidly 2004) but are rare (Mirowsky et al. 2004; Ammerman et al. 2012); they prefer to roost in hollow water tupelo Nyssa aquatica trees (Gooding and Langford 2004) in bottomland hardwood forests (Mirowsky et al. 2004). Northern yellow bats prefer roosting either in Spanish moss Tillandsia usenoides hanging from oaks or in palm groves (Webster et al. 1980). Areas broadly surrounding our netting sites lacked tupelo, Spanish moss, and palm groves; thus, we did not expect to find these two bat species in our samples.

Cumulatively, at least nine species of bats have been documented in southeastern forests, with a maximum of eight species found at any particular site (Table 3). At least two of these species (evening and tri-colored bats) occur in every sampled community. Their abundance in communities varied substantially, ranging from as high as 28% for both species to as low as 2.8% for evening bats and 1.4% for tri-colored bats, respectively. Eastern red bats, or eastern red–Seminole calls, were also documented in all of the studies. Abundance of eastern red bats in communities also varied widely. Big brown, southeastern myotis, and hoary bats were documented in five of the seven studies even though both big brown and hoary bats have distributions across the most of the United States (Wilson and Ruff 1999). Hoary bats have been previously reported in southeastern Texas (Schmidly 1994; Ammerman et al. 2012), but they usually only winter in Texas and migrate north for summer. Even though hoary bats were reported in five of seven studies (Table 3), this species was always in low abundance and never constituted >4.3% of the community. Southeastern myotis was not found in the North Carolina study (Morris et al. 2010), which is not surprising because its known distribution extends only into the southern corner of this state (Wilson and Ruff 1999).

Unexpectedly, presence of Seminole bats was confirmed in only three studies other than ours—those conducted in Mississippi (Miller 2003; Welch 2003) and Alabama (Kilgore 2008). In these studies, the Seminole bat was usually the most numerous member of the community with captures ranging from 29% to 42%. The geographic range of Seminole bats extends into North Carolina and Arkansas, but the studies conducted there did not report this species' presence.

Mexican free-tailed bats were captured in only two studies—ours (Texas) and Morris et al. (2010; North Carolina), though their range extends into all of the states reported in Table 3. The absence of Mexican free-tailed bats in most studies is understandable because these are not considered to be tree-roosting bats; rather, they roost primarily in caves and man-made structures (Schmidly 2004; Ammerman et al. 2012). Some man-made structures (bridges and sheds) are available as potential roosting sites for Mexican free-tailed bats in the vicinity of Kelly Pond site, but we found no occupied roosts. The nearest caves known to serve as roosts for Mexican free-tailed bats are in the Hill Country of Central Texas (e.g., Bracken Cave, approximately 320 km from Sam Houston National Forest).

Our surveys yielded samples of adequate size and temporal coverage to enable consideration of reproductive patterns for several of the study species. Our findings offer new insights for patterning in three species—Seminole, evening, and big brown bats. Annual patterns and synchrony of reproduction in bats relate to annual and seasonal climate cycles (Racey and Entwistle 2000). Availability of food resources is one of the main factors affecting timing of parturition (Arlettaz et al. 2001). Insectivorous bats living in temperate zones tend to compress their reproductive activities within summer and autumn, corresponding to seasons when insect availability and temperatures are high (Racey and Entwistle 2000). Reproductive cycles of these bats progress as follows: during autumn, gametogenesis and mating occur; during winter, development is suspended as most temperate bat species hibernate; during spring, gestation is stimulated by arousal from hibernation; during mid-summer, parturition occurs; and during mid-and late summer, lactation occurs (Racey and Entwistle 2000). Because of climatic constraints (mainly temperature and rainfall) that directly impact food availability, most temperate bats are synchronously monoestrous, having one litter per year born during a constrained time period in summer. By comparison, tropical bat species show a wider range of reproductive patterns, including seasonal monoestry, seasonal polyestry, and aseasonal polyestry (Racey and Entwistle 2000).

Extended seasonal monoestry, known in tropical African bat species (Racey and Entwistle 2000), is a strategy with births of single litters spanning a particular season. In such cases, births might extend over a longer period of time than in typical seasonal monoestry as seen for most temperate bat species. It could be beneficial for temperate species to adopt this reproductive pattern in regions where winters are not severe and warm temperatures dominate most of the year. None of the species present in Texas have been reported previously to produce multiple litters (Ammerman et al. 2012), and are not expected to have them because multiple litters are usually present only in areas with multiple rainfall seasons (Racey and Entwistle 2000). However, asynchrony in reproduction might still be beneficial because it might reduce competition for resources (roosts and food) among reproductive females (both pregnant and lactating) and fledglings.

Shifts in the timing of parturition and lactation in most vespertilionid (family Vespertilionidae) bat species have been reported in relation to latitude (Cockrum 1955). Cockrum (1955) also noted that, in vespertilionid bats from temperate climates, it is possible to induce earlier ovulation (and earlier parturition as a consequence) by moving female bats from hibernation into warm laboratories. The reproductive timing of three vespertilionids in our study (i.e., Seminole, evening, and big brown bats) all differed from that previously reported in the literature. We documented an extended season of parturition and lactation lasting ≥3 mo, approximately 1 mo longer than previously observed elsewhere. Seminole bats have been considered synchronous breeders, with pregnancy occurring during May and June and lactation occurring mostly from late May until late June (Cockrum 1955; Jennings 1958; Wilkins 1987; Schmidly 2004). However, we captured lactating females 31 May–21 July, and pregnant females as late as 31 July. Among past studies concerning the reproductive patterns for Seminole bats, only Miller (2003) reported pregnancy extending into July (mid-July in Mississippi). Given this, our results suggest extended intervals of parturition and lactation that are not as synchronous as previously understood. However, because of lack of data from April and early May, we are uncertain whether parturition in Seminole bats might occur continuously throughout the summer, or if it is completely asynchronous and occurring in multiple waves (for example, one in early May, next in July, etc.).

According to Schmidly (2004), evening bats in Texas give birth in late May and early June. However, in Presidio County, Texas, Dowler et al. (1999) captured a juvenile female on 20 April, suggesting an earlier parturition date (late March or early April). In contrast, pregnant females were captured in Ohio from late April until the end of June (Cockrum 1955) and, in Arkansas, a single pregnant bat was captured on 12 May (Perry et al. 2010). Watkins (1969) reported capture of pregnant evening bats in Missouri on 8 July. We captured pregnant evening bats as early as 1 June and as late as 21 July, and lactating females throughout June, suggesting pregnancy may be more prolonged than reported by Schmidly (2004).

We captured pregnant big brown bats 31 May through 19 July, and lactating females throughout June. Ammerman et al. (2012) reported that parturition in big brown bats in Texas occurs May–June, and lactation continues through July. Thus, our study suggests that Seminole, evening, and big brown bats in southeastern Texas might have a longer reproductive season than documented for these species elsewhere.

Acoustic surveys conducted in our study area during the November–March period confirm year-around bat activity, although activity levels were lower during winter compared with summer (Pettit 2011). In southeastern Texas, average temperatures fall below 16°C (60°F) only from mid-November to mid-March. During this 4-mo period, nighttime temperatures are often above 4°C (45°F) and daytime temperatures tend to approach or exceed 16°C (60°F; National Weather Service Forecast Office 2012). This thermal regime might allow bats to enter torpor rather than hibernation (the latter strategy generally being followed by bats in colder winters of higher latitudes in the United States). The pattern of warm temperatures and high humidity during summer, enabled by geographic proximity of the Gulf of Mexico, could allow insect populations to prosper from early spring through late autumn, which would likely provide bats with food resource over a longer period than for these species at higher latitudes. As a consequence, parturition in females need not be restricted to a brief interval in mid-summer, but could potentially extend asynchronously through much of the summer. Furthermore, Wear and Greis (2012, 2013) indicated that all predictive models for southern U.S. forests predict temperature increase. It is possible that earlier parturition might be driven in the future, if not already, by global warming.

Our study was conducted near the westernmost extent of the partially managed pine-forest ecosystem that characterizes vast expanses of the southeastern United States. Though we sampled only two sites, this investigation provides the initial report of the bat community assemblage in southeastern Texas. Further, this study provides data revealing reproductive patterns not previously reported for several species found in Sam Houston National Forest. Findings provide limited evidence suggesting that three bat species deviate from synchronous monoestry (the reproductive pattern typical of insectivorous bat species in the temperate zone) in favor of extended seasonal monoestry more often seen in tropical bat species. We anticipate that these reproductive adaptations might pertain to these species, and perhaps others, widely across the coastal plain of the southeastern United States. Study of historical natural-history records associated with museum specimens and additional studies conducted in similar forests in other states where the influence of the Gulf of Mexico can be felt will be key to evaluating these predictions. Additional studies of this sort will help in obtaining the data needed to formulate comprehensive conservation plans for tree-roosting bats in southeastern Texas and other parts of the austroriparian province.

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Table S1. Bat capture data collected in Sam Houston National Forest, Texas, at Kelly Pond site over three summers (2009–2011). Table includes bat identification numbers, species, gender (M—male, F—female), reproductive state (R—reproductive, NR—non-reproductive, L—lactating, and P—pregnant), and age (A—adult, or SA—subadult). Species were assigned four letter designations as follows: LABO (Lasiurus borealis—eastern red bat), LASE (L. seminolus—Seminole bat), LACI (L. cinereus—hoary bat), EPFU (Eptesicus fuscus—big brown bat), NYHU (Nycticeius humeralis—evening bat), MYAU (Myotis austroriparius—southeastern myotis), PMSU (Perimyotis subflavus—tri-colored bat), TABR (Tadarida brasiliensis—Mexican free-tailed bat).

Found at DOI: http://dx.doi.org/10.3996/022014-JFWM-014.S1 (53 KB XLSX).

Table S2. Bat capture data collected in Sam Houston National Forest, Texas, at Henry Lake Creek site over two summers (2010–2011). Table includes bat identification numbers, species, gender (M—male, F—female), reproductive state (R—reproductive, NR—non-reproductive, L—lactating, and P—pregnant), and age (A—adult, or SA—subadult). Species were assigned four letter designations as follows: LABO (Lasiurus borealis—eastern red bat), LASE (L. seminolus—Seminole bat), LACI (L. cinereus—hoary bat), EPFU (Eptesicus fuscus—big brown bat), NYHU (Nycticeius humeralis—evening bat), MYAU (Myotis austroriparius—southeastern myotis), PMSU (Perimyotis subflavus—tri-colored bat), TABR (Tadarida brasiliensis—Mexican free-tailed bat).

Found at DOI: http://dx.doi.org/10.3996/022014-JFWM-014.S2 (52 KB XLSX).

Reference S1. Wear DN, Greis JG, editors. 2012. The Southern Forest Futures Project: summary report. Gen. Tech. Rep. SRS-GTR-168. Asheville, North Carolina: U.S. Department of Agriculture-Forest Service, Southern Research Station.

Found at DOI: http://dx.doi.org/10.3996/022014-JFWM-014.S3; also available at http://www.treesearch.fs.fed.us/pubs/42526 (4.8 MB PDF).

Reference S2. Wear DN, Greis JG, editors. 2013. The Southern Forest Futures Project: technical report. Gen. Tech. Rep. SRS-GTR-178. Asheville, North Carolina: U.S. Department of Agriculture-Forest Service, Southern Research Station.

Found at DOI: http://dx.doi.org/10.3996/022014-JFWM-014.S4; also available at http://www.treesearch.fs.fed.us/pubs/44183 (1.9 MB PDF).

We thank T. Pettit, N. Green, H. Li, R. Rotondi, C. Skrovanek, M. Weber, C. Hanks, and numerous undergraduate research assistants for their efforts in the field. D. Jauregui (Wildlife Program Manager, Sam Houston National Forest) generously provided us with access to Sam Houston National Forest and U.S. Forest Service facilities. Funding for this project was provided by Texas Academy of Science, Southwestern Association of Naturalists, and Baylor University (Biology Department Folmar Research Award and Travel Award; Graduate School Travel Award; Undergraduate Research and Scholarly Achievement program; Glasscock Award). Dr. R. Doyle and the Baylor Biology Department provided field vehicles and funded rabies prophylaxis. We thank the Subject Editor and anonymous reviewers for insightful comments and suggestions on how to improve the manuscript.

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