Reproductive biology

Rock-dwelling pikas produce few young each year, whereas burrowing forms produce many large litters each season.

Most pikas originate from first litters as there is strong selection pressure for young to become active as early in the short summer season as possible. Juvenile pikas must grow up fast, claim a vacant territory (if one exists), and construct a haypile before the next winter's snow descends on the talus. Young normally remain close to their site of birth, and dispersal to a territory far away on their home talus, or movement to another talus patch, is rare.

Young become independent approximately three weeks after parturition, and they remain on their family burrow system territory throughout the summer and the following winter. Thus, as litter after litter appear on the meadow, the number of animals in each family grows and the overall population density skyrockets. Some populations reach densities of more than 120 animals per acre (300 per ha). Mortality is high during the harsh Tibetan winters and most animals perish; it is rare for any pika to live into its second year. The breeding season begins in early spring, and this is also the time that the composition of families is determined. There is a modest reshuffling of pikas before mating occurs. Nearly all pikas born the preceding summer remain on their home territory, but some, in particular males, disperse short distances and join neighboring families. The most common movements are to an immediately adjoining neighbor, but some pikas move as far as five territories away. The result of most of these movements is an equalizing of density across the meadow. Males do not move to families with more females. Instead, they move to families with more males.

The mating system expressed by plateau pikas results from the combination of the random nature of over-winter mortality and the few dispersal movements of animals alive just before the mating season. Most family burrow systems are occupied by a single adult male and female, which results in monogamy. However, when the number of surviving males and females is higher than can be accommodated on the restricted number of territories by only pairs, more complex situations arise. Some families are composed of one male and many females (polygyny), others with multiple males and one or more females (polyandry). These three different mating systems can occur in neighboring family burrow systems, a

An American pika (Ochotona princeps) in Colorado, USA. (Photo by John Shaw. Bruce Coleman, Inc. Reproduced by permission.)
An American pika (Ochotona princeps) gathering vegetation. (Photo by Goerge D. Lepp/Photo Researchers, Inc. Reproduced by permission.)

situation unique in mammals. In addition, the formation of these mating systems does not appear to result from variance in habitat quality on the meadow; there is no correlation between the type of mating system on a burrow system in one year with what will appear there the following year.

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