Mandibular Layering in Delphinapterus leucas and Age Determination
LAWS1 investigated mandibular periosteal layers as an aid to ageing the Sperm whale Physeter catodon. Continued studies of P. catodon indicate that mandibular layers and tooth layers occur in equal numbers2, and it has been postulated that two tooth layers are deposited annually3. Two tooth and one mandibular layer in Delphinus delphis are annual4.
Validation of Growth Layer Group (GLG) depositional rate using daily incremental growth lines in the dentin of beluga (Delphinapterus leucas (Pallas, 1776)) teeth
Counts of Growth Layer Groups (GLGs) in the dentin of marine mammal teeth are widely used as indicators of age. In most marine mammals, observations document that GLGs are deposited yearly, but in beluga whales, some studies have supported the view that two GLGs are deposited each year. Our understanding of beluga life-history differs substantially depending on assumptions regarding the timing of GLG deposition; therefore, resolving this issue has important considerations for population assessments. In this study, we used incremental lines that represent daily pulses of dentin mineralization to test the hypothesis that GLGs in beluga dentin are deposited on a yearly basis. Our estimate of the number of daily growth lines within one GLG is remarkably close to 365 days within error, supporting the hypothesis that GLGs are deposited annually in beluga. We show that measurement of daily growth increments can be used to validate the time represented by GLGs in beluga. Furthermore, we believe this methodology may have broader applications to age estimation in other taxa.
The Natural History of a Summer Aggregation of the Big Brown Bat, Eptesicus fuscus fuscus
There is a surprising paucity of detailed information concerning Eptesicus fuscus in the eastern United States, although a few colonies, have been studied in some detail in the West Coast. In contrast Myotis has been studied in great detail by a number of authors. The reproductive cycle of Eptesicus closely parallels that of Myotis with a few important exceptions, and will be discussed below. MATERIAL AND METHODS Dr. J. K. Doutt kindly provided the field data for 146 Eptesicus fuscus collected by the Pennsylvania Mammal Survey between 1946 and 1950. This material was used primarily in plotting the growth curve, but the available reproductive data were also used. Fifty-two female and 26 male Eptesicus collected during the warmer months of 1951, 1952 and 1953 in Washington Grove, Montgomery County, Maryland, were the source of all histologic, dental, and most life history data. The Maryland bats were killed with chloroform, weighed and measured. After skinning and removing the skull, the carcass was placed in 10 percent neutral formalin with all viscera well exposed. Gross reproductive condition was noted and the testes measured prior to f6xation. Adrenal weights were obtained after fixation. The gonads and adrenal glands of 69 bats were serially sectioned at 7 micra, and stained routinely with haematoxylin and eosin. The other viscera were sectioned in 14 animals. Mr. Herbert G. Ward provided assistance in preparing the histologic material. After cleaning, anterior and lateral photographs of the skulls were taken by Dr. Merrill Wheatcroft of the Dental Division, Naval Medical Research Institute. I also owe Dr. Wheatcroft my thanks for advice and assistance on the problem of tooth wear in these bats. OBSERVATIONS The colony.-Washington Grove, Maryland, is in an old oak woods at an elevation of 625 feet. The tree canopy is dense, but the understory is a cleared park. The houses are frame structures, usually with wooden slatted shutters. These shutters, primarily on the second story, are the roosting places for summer aggregates of Eptesicus fuscus. Althoutgh there is a fair amount of movement from house to house following disturbances, certain shutters seem to serve as semipermanent annual roosts for individual colonies. In early summer these are nursery colonies composed of females and young. Males begin to