Variation in populational size and composition within two summer colonies of endangered gray bats (Myotis grisescens)
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For >30 years, the endangered gray bat (Myotis grisescens) has been a priority in conservation efforts in the southeastern United States. As part of these conservation efforts, the Alabama Department of Conservation and Natural Resources, conducts population counts of summer colonies of M. grisescens annually. During annual population counts by personnel of the agency’s State Lands Division at Blowing Spring Cave, Lauderdale Co., Alabama, strong fluctuations in numbers of M. grisescens were noticed among counting sessions. This prompted me to question why these fluctuations occur, and whether sex and age vary accordingly throughout the year. Research was conducted at each full moon and new moon March-October, 2004 and 2005. Population counts were made on the first night. On the second night, bats were captured, examined, and released. For comparison, the same protocol was followed at Anderson Cave, Shelby Co., Alabama, which also is home to a summer colony of M. grisescens. Both colonies were identified as maternity colonies. Fluctuations were observed at both caves, and sex and age ratios varied significantly throughout both years. At Blowing Spring Cave, size of populations increased in March and April, decreased from April to mid-June, strongly increased in July, decreased again in August, and increased again before bats abandoned the cave in mid-October of 2004. A similar pattern was observed in 2005, but no decrease in size of population was observed in August. Similar fluctuations were observed at Anderson Cave in 2004; however, size of population began to decrease in August, and no increase in size of population was observed from August to mid-October 2004. This led to the conclusion that Blowing Spring Cave serves as a migrational stopover site for gray bats on their way to winter hibernacula, and that Anderson Cave does not serve as such. Fluctuation in size of populations can be explained by migrational movements among caves. Populational fluctuations in maternity colonies can be explained further in such colonies by activities associated with gestation, parturition, lactation, volant young, and early migration and hibernation of adult females. In June 2005, gray bats were not present at Anderson Cave due to unknown reasons. Emergence patterns varied among sampling sessions, and length of emergence time increased when bats were pregnant or when young were present. Sex and age ratios varied in both caves throughout the 2004 and 2005 annual activity seasons. Ectoparasites collected from patagia were identified as mites of the species Spinturnix americanus (Family Spinturnicidae, Subclass Mesostigmata). Abundance of S. americanus on patagia decreased before hibernation in both years. Weight of young-of-the-year increased with time, weight of young females was greater than in young males until October 2004, when weights were equal. Sexual dimorphism in weight also was observed in adults, with females heavier than males. Both Blowing Spring Cave and Anderson Cave are accessible to cavers and the public. Gray bats are extremely sensitive to disturbances by humans, especially when young are present. To continue successful management of M. grisescens and to better protect the species, access to both caves should be restricted to prevent disturbance by humans.