LIFE HISTORY (Reproduction, Growth and Development)
Frequency:
Generally, a single pup is born each year, but in tropical areas, some species have multiple births per year.
Longevity:
1. birds of equivalent weight to mammals live longer
2. large mammals live longer than small mammals
3. EXCEPT for bats!
4. with respect to lifespan and litter size, bats are unusual mammals
The theory is that due to the low mortality rate of bats, they can afford to reproduce slower than mammals of comparable, or even larger, body size!
Reproductive patterns
Hibernation provides an incredible conservation of energy in temperate bats. This is a consequence of the decrease of energy use through a general shutdown of body physiology. This also results in a profound influence in reproductive strategy.
Consider the daily requirements of a pregnant, or lactating female as she moves around collecting her food - insects! Not only must she replace the energy required to find food, but she must also build resources to make milk, as well as store fat for her own hibernation the following fall - and in Canada, she only has a few months to accomplish this!
A reproductive effort just prior to hibernation, would be fatal for all. Spring and summer months are too short to breed, bear the young, raise them to a sufficient size and still have enough time to acquire the resources for hibernation of both the pup and the mother. Similar constraints exist for the males with respect to sperm production.
It is therefore necessary for reproduction to overlap hibernation! Hence... swarming!
Temperate bats begin sperm production, and estrous cycles in the late summer or early fall, prior to entering hibernation. After copulation, one of three things then will occur, depending on the species.
1. Delayed ovulation and fertilization, or sperm storage.
2. Delayed implantation (before implantation).
3. Embryonic diapause (post implantation).
Delayed ovulation and fertilization, or sperm storage.
This was thought to be peculiar to bats, but recent study has shown that some rabbits and some mice subscribe to this reproductive strategy. Rhinolophus, widespread usage by Myotis, Pipistrellus, Eptesicus, Nyticeius, Lasiurus, Plectous, Miniopterus, and Antrozous follow this pattern, as well as many tropical vespertilionids. This is the most specialized strategy as it requires two phenomena - delayed ovulation and overwinter storage of sperm in the female.
Copulation occurs in mid to late autumn, while the female follicle and ovum that have been developing, are vacuolated by enormous quantities of glycogen, a rich source of metabolic energy. This cycle stops and overwinters in this condition. Since the egg has not been released at this point, the sperm is stored in the uterus. Ovulation occurs in late February or March while the females are still in the hibernaculum, or shortly after they leave for their summer roosts. The young are born in May or June after a gestation period of 50 to 100 days (depending on the species). It would appear that the difference in timing is due to regional climatic variation.
This process, while seemingly simple, is more than remarkable! Consider the following: foreign cells (many thousands of sperm) survive the antigenic responses that usually follow invasion by foreign cells! In most, if not all mammals, including bats and humans, white blood cells attack and destroy unknown foreign cells. This fact, sometimes makes it very difficult for some women of our species to become pregnant, yet in the bats described, this does not happen. The study of this biological and biochemical phenomena, has provided many answers to questions pertaining to human fertility and organ transplant rejection problems. But how do the sperm manage to survive for so long a period of time? Inside the uterus, there are numerous glands that the sperm dock up against, and receive nutrition from these glands!
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Delayed implantation (before implantation)
As in the previous example, sperm production and estrous cycles occur in the fall. The females ovulate, and the copulation and fertilization take place. However, resulting embryo undergoes only the initial few stages of embryonic development, then becomes dormant. The females then enter hibernation, but the fertilized egg does not implant itself until the female emerges from hibernation in the spring. There are several vespertilionids and at least one rhinolophid that use this strategy, but none of the Canadian bats use delayed implantation, to the best of my knowledge.
It is interesting to note, that this strategy is not entirely related to hibernation. The straw coloured fruit bat (Eidolon helvum) copulates during the first annual rainy season in East Africa. Implantation is delayed for three months, until October or November, during the second rainy season. Thus, an approximate gestation period of four months is added to a three to five month period of delayed implantation, making for a total gestation period of seven to nine months, the longest for any bat!
Embryonic diapause
This strategy has only recently been discovered, and doesn’t appear in any temperate species to the best of our knowledge. It is restricted to New World leaf-nosed bats of the family Phyllostomidae such as the Jamaican fruit bat (Artibeus jamaicensis). Artibeus is poylestrous, with two or more cycles per year. Copulation and fertilization result in immediate implantation, and embryonic development with birth in July or August. This birth is followed immediately by another estrous cycle, copulation, and fertilization, but the blastocyst, after implanting, becomes dormant. After two and a half months, development resumes. This is also the case in the Californian leaf-nosed bat (Macrotus californicus), except the embryonic development does not cease. The fetus does slow down its development however.
Gestation
The length of gestation varies significantly. In small vespertilionid bats such as Myotis and Pipistrellus, gestation lasts from 40 to 60 days. In the common vampire (Desmodus rotundus), gestation lasts up to eight months. Generally, but not exclusively, the greater the body size, the longer the gestation.
Embryonic Development
At birth, all bats are helpless and depend on the mother. They are capable of climbing and clinging to the mother. All have a specialized set of milk teeth, that are needle sharp and hooked at the tips, which allows them to bite onto the mother’s nipple and hang on even during flight! They are very alert.
Baby megachiropterans are born weighing 20 - 30 percent of their mother’s weight, and unlike many other microchiropteran, they are born with their eyes open, and with lots of hair! The microchiropteran babies are not alert, naked and start to develop hair after 3 or 4 days.
Birth generally takes place during the daytime. Some bats assume a horizontal position, others remain in the normal head down position, and some in the head up position. Most births are breech (rump first) and mom may tug at the baby using her teeth to assist in the birth.
Maternal Care
There is virtually no care provided by the males after copulation, with a few exceptions in harem based relationships, where baby bats have been seen on the backs of males. It is not known if this is a case of mistaken identity or not.
There is however a strong bond between all mothers and their babies. Mother and infant establish vocal communication soon after birth, to the extent that the mother is able to identify her baby by voice, among many newborn. Baby bats will call out continuously when separated from their mother, but as they get older the communications become more refined with mother calling out to the baby and the baby responding until the mother gets close enough to recognise her baby by sight or smell. In microchiropteran species, this happens at ultra high frequencies, and it has been suggested that echolocation evolved from this procedure.
Babies that are left in a colony, huddle together in an attempt to increase thermoregulation. This is found in several species of Myotis, as well as Tadarida. There are also examples of communal care, found in Tadarida brasiliensis, Myotis thysanodes and M. lucifugus. The females left behind provide protection of the babies, and in some cases even communal suckling! In the majority of bats, however, mothers watch after their own babies.
Flight begins as early as three weeks in some species, and as late as three months in others (mainly megachiropterans). While most mammals learn to walk and run quite soon in life, they don’t generally face the challenges of the bat babies who must learn not only to fly, which is a difficult and sophisticated exercise in it’s own right, but they must also learn to navigate - in the dark! It would appear that most mother bats educate their young in various ways. This education may be in the form of providing food for the young, to having the young follow them from place to place as they learn where to hunt and how to catch food.
Most young bats do not reproduce until their second year.
