Literature Compilation

Field/Laboratory: Data Sources: Data were collected from published sources written by experienced mammalogists. References for data sources are found in Class IV Data Structural Descriptors, Section B. These sources are numerically indexed in the references column of the data set. Reference numbers refer to the order in which these data sources were entered into the dataset. Data Collection: Each life history variable in the data set is a compilation from many data sources. The use of multiple sources reduces the influence of erroneous data points and yields an overall average for the species across environmental conditions. The original data set is in MSExcel format and each life history cell is set up with the Average function which continually recalculates the average as new data is entered into the cell. The resulting average across all sources entered is what is available in the text data file Mammal_lifehistories_v2. This method was not used for Maximum Life Span (see Life History Variables in this section) Different data sources contain different forms of data, ranging from raw data from numerous individuals, to summarized data in the forms of averages and ranges. If averages were available these were entered preferentially. Raw data were used to calculate an average which was then used in the dataset. If only ranges were available, the midpoint of the range was entered. Ranges were only used if no other data were available for that life history variable in that data source. Due to the size and complexity of the database, each source is treated as an independent estimate and sample sizes are not factored into the average. Since most data came from other compilations, it is very difficult to ensure that the same primary literature source was not used multiple times. However, data were not used if the citation for it matched a reference already in the reference list for the data set. Since many species have only a couple of citations and the number of numerically identical entries for a life history variable is small, this is unlikely to have a major influence on the results. After the entry data from reference 26, a new rule was implemented for data collection. Since time and resources are limited, continuing to collect data for species with well-characterized life history variables (i.e., 10 or more estimates for a life history variable) lead to decreased efficiency. Therefore, after any specific life history variable for a specific species reaches 10 entries, it is determined to be well characterized and data collection for that variable for that species stops. This number was chosen because after 10 entries, the average changes very little with each new entry and when there are more than 10 entries the probability of duplicating a source increases. Since this rule was not implemented until the entry of reference 26, there are still a number of species with life history variables characterized by >10 entries. Life History Variables: Nine life history variables were collected. This section contains the specific definitions used in collecting these data. Maximum life span: Maximum life span (months) is the oldest age recorded for a member (male or female) of that species. Maximum life span, unlike the other variables, represents a single datapoint not an average. If a longer maximum life span is found than the number currently in the database, then the previous datapoint is replaced with the new value. Age of First Reproduction (AFR): AFR is the age (months) at which a female individual of the species first breeds and could potentially conceive. By definition, this is only recorded for females. This variable does not include gestation. If the age of first birth is given by a data source then gestation length was subtracted from the value to yield an estimate of first breeding. Gestation length: Gestation (months) is the length of active gestating of a fetus. Gestation length does not include the period of delayed implantation. Weaning length: Weaning length (months) is the time from the birth of an offspring to the independence of that offspring from milk. Weaning mass: Weaning mass (grams) is the weight of a single offspring at the time of independence from milk. Litter size: The number of offspring birthed in a single litter. If a data source indicated that its litter size estimate was based on embryo counts or reproductive scars, it was excluded since it is relatively common for a subset of embryos in a litter to be resorbed. Litter frequency: The number of litters birthed by a female in a 12-month period. Captive studies attempting to maximize litter production were excluded as possible. Neonate mass: The weight (grams) of a single newborn offspring. If a data source indicated that the mass of the newborn was taken > 12 hours after birth it was excluded. Adult mass: The weight (grams) of a reproductive adult. If strong sexual dimorphism exists in a species, then only data sources specifying female weight were included. References 16 and 18 were only used for body masses. Furthermore, these references were only used if other body mass estimates were not available from the sources which supplied life history data. Taxonomy: Taxonomy follows Wilson, D. E., and D. M. Reeder, editors. 1993. Mammal species of the world, Second Edition. Smithsonian Institution Press, Washington, D.C., USA.