Reproductive biology

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In their life histories, bats are long-lived with low reproductive output. In the wild, individually marked bats (little brown bats and greater horseshoe bats [Rhinolophus ferrume-quinum]) have survived more than 30 years, and females have the capacity to produce one young per year. In Britain, greater horseshoe females appear to have young only every second or third year. Furthermore, 70% of these bats born in any year do not survive their first winter. The litter size in bats is typically one, though a few species bear twins at least some of the time, and another few, notably red bats, may have litters of even three or four.

During birth, female bats turn heads-up to allow gravity to assist with the birth process. The ligaments holding the two halves of the pelvic girdle together are capable of great flexibility to allow birth. Young are born back-end first. Newborn bats are huge compared to their mothers: single young

White bats (Ectophylla alba) rest under the sloping roof of a heliconia leaf that they shaped in the tropical rainforest of La Selva Biological Station, Costa Rica. (Photo by Gregory G. Dimijian/Photo Researchers, Inc. Reproduced by permission.)

are 25-30% of their mother's postpartum mass. Young consume their own weight in milk every day and grow quickly. In some bats, for example, little brown bats (0.3 oz [8 g]) from North America, young reach adult size (forearm length) by about age 18 days. By then, their milk teeth have been replaced by adult dentition, they have started to fly, and insects first appear in their diets. Big brown bats (0.5 oz [15 g]) take about 28 days to reach this stage, and young vampire bats continue to nurse until they are six months old.

Young bats have huge appetites for milk, which is expensive to produce. Female bats roosting in nurseries with hundreds or even thousands of others use a combination of spatial memory, voice, and odor to recognize their own young. This ensures that her young receives enough milk and maximizes its chances of survival. The challenge of recognizing her young depends upon the female's situation. A red bat roosting only with her own young, has a different task than the Daubenton's bat roosting with tens of other Daubenton's bats. Female Brazilian free-tailed bats with nurseries numbering in the millions have a huge challenge in this regard—one they regularly meet and overcome.

Gestation periods in bats range from 60-100 days, and in most bats, fertilization follows copulation. Most species of bats are monestrus, with females having one reproductive event per year. Some tropical species are diestrus, have two reproductive events per year, and females in a few species (e.g., lesser-crested mastiff bats, Chaerephon pumila) may bear up to five young per year (one per estrous cycle).

Some species of bats extend the time between mating (typically polygynous) and birth. Sometimes fertilization follows copulation, but development or implantation of the fertilized egg is delayed, extending the gestation period. This occurs in some New World leaf-nosed bats (e.g., California leaf-nosed bats, Macrotus californicus) and plain-nosed bats (some populations of Schreiber's long-fingered bats). The other approach, known from plain-nosed bats and horseshoe bats, is to delay fertilization. In this case, females store sperm in the uterus after copulation. Storage can last from less than 20 days in some tropical species, to almost 200 days for north temperate forms. Delayed fertilization does not extend the gestation period. Extension of the time between mating and birth ensures that young are born at the most productive (in terms of food) time of the year.

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Pregnancy And Childbirth

Pregnancy And Childbirth

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