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Field biologists almost invariably report relative humidity as a measure of moisture in the air and assume that relative humidity somehow predicts evaporative water loss from an animal. In this paper, I use the vapor pressure gradient to show that, under conditions of constant relative humidity, evaporative water loss from the body surface of a hibernating bat can vary by more than 100%, depending on ambient temperature. Potential evaporative water loss at constant relative humidity is an increasing curvilinear function of ambient temperature for a torpid bat that has a surface temperature equal to surrounding air temperature, but a decreasing curvilinear function of air temperature for an aroused bat in the hibernaculum. Under some circumstances, evaporative loss actually can be greater in a hibernaculum with higher relative humidity than in one with lower relative humidity. When examining potential differences in evaporative water loss between sites, habitats, or treatments, biologists should consider the absolute (not relative) level of ambient moisture, as well as the surface temperature of the animal, which greatly affects the tendency of water molecules to evaporate.
Dietary differentiation can be a key mechanism for the coexistence of syntopic species with similar niches. On the Caribbean island of Puerto Rico, three species of bat from the family Mormoopidae — the Antillean ghost-faced bat (Mormoops blainvillei), sooty mustached bat (Pteronotus quadridens), and Parnell's mustached bat (Pteronotus parnellii portoricensis) — are aerial insectivores that roost in the same caves. To investigate the possibility of dietary differentiation, we estimated the percent volume and percent frequency of occurrence of the orders of arthropods consumed by these three species of bat, using standard fecal analysis. We also compared dietary diversity among species, as well as the amount of dietary overlap, with respect to season and habitat. Lastly, this study used canonical correspondence analysis (CCA), a method of ordination, to assess the effects of species, sex, age, reproductive condition, season, and habitat on intraspecific differences in the diet of the Puerto Rican Mormoopidae. Eight orders of arthropods were found in the diet of these mormoopids, with Lepidoptera, Hymenoptera, and Coleoptera being major staples. The CCA revealed differences in diet among the three species, suggesting that dietary differentiation is at least one mechanism for coexistence. In addition, the variables habitat and season correlated significantly with the diet of M. blainvillei and P. quadridens, whereas habitat and sex correlated with the diet of P. p. portoricensis. Thus, our study shows dietary differences among the three species of Mormoopidae living in the same caves on Puerto Rico, as well as intraspecific differences within the diet of each species.
We investigated the occurrence of abbreviated pinnae (squared ears) in little brown bats (Myotis lucifugus) and northern bats (Myotis septentrionalis) in Michigan, USA. Although the trait was not observed in 308 M. septentrionalis, squared ears occurred in 56 (0.1%) of 5,863 M. lucifugus that were examined. Squared ears were equally common in males and females. Both ears typically were affected to the same degree and most often reduced by 50% of their height, although the amount of pinna that was missing varied from 5 to 50%. Previous authors speculated that the trait was an ontogenetic malformation, but microscopic examination of squared ears from M. lucifugus in Michigan indicated that the trait consistently was associated with trauma, most likely frostbite.
Evening bats (Nycticeius humeralis) are ubiquitous in the southeastern United States, but only one maternity colony occurs in the Great Lakes region, in Michigan, and it is the northernmost colony known in North America. We hypothesized that diet would vary throughout the season and between years and that diet of evening bats in northern areas would differ from diet of those in more southern parts of the continent. Almost 600 fecal pellets of evening bats were collected and analyzed from May through August 2006–2007. Thirteen orders of insects and two orders of arachnids were found, but Coleoptera, Diptera, Hymenoptera, and Hemiptera composed 87% of dietary volume. Lepidoptera, which forms a substantial portion of the diet for most vespertilionids in eastern North America, contributed only 4.7% to the volume. Diet was broadly similar to evening bats in the core of their range, although evening bats in Michigan incorporated substantially more Diptera into their diet. Dietary composition and diversity differed little between years but showed variation among weeks within years. The lack of dietary specialization and similarity in major components in different parts of the range suggest that availability of prey or dietary competition with other species at the ordinal level are not factors limiting the northern distribution of the evening bat.
We examined changes in the assemblage of bats in southern Lower Michigan, USA, using results of paired netting surveys conducted with similar techniques but separated by 12–26 years. Species diversity declined by 18–35% and evenness decreased by 0–35% throughout the region and in two specific areas. Changes in diversity and evenness were attributed primarily to decreases of 44% or more in relative abundance of the foliage-roosting eastern red bat (Lasiurus borealis). Number of L. borealis captured per net-night decreased 52–85%. The decline in relative abundance of L. borealis suggested by mist netting was supported by a 10-fold decrease over 38 years in the proportion of L. borealis that were tested for rabies by the state health laboratory. The apparent decline in L. borealis is especially alarming in light of the recent upsurge in use of wind power and the large number of L. borealis that are killed at such developments. We recommend that other previously completed surveys in eastern North America be duplicated, as one way of helping to confirm or refute the trend that we have identified.
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