Reproductive biology

Most of our general knowledge of reproduction in lago-morphs comes from investigations on the European rabbit (Oryctolagus cuniculus). Rabbits are known to be reflex, or induced, ovulators. Females may come into heat and be attractive to males, but the act of copulation is needed to stimulate ovulation, which occurs about 12 hours after mating. It is assumed that this pattern is followed in other lago-morphs. Another interesting twist in lagomorph reproductive biology is the phenomena of "superfetation." In European hares (Lepus europaeus), a female in late pregnancy can go into heat, mate, ovulate, and be impregnated and thus carry two litters of different age in her uterus at one time. This process naturally can lead to a rapid increase in the local population in a short time. Most pikas exhibit a post-partum estrus, thus initiating sequential litters back-to-back, with the same effect of truncating reproduction into as short an interval as possible—a particularly effective strategy given the short alpine summers experienced by most pika species.

Another interesting aspect of reproduction in lagomorphs is their apparent lack of parental care—in particular, the in-frequency of nursing bouts initiated by mothers. Termed "absentee parental care" by Eisenberg, most mothers visit their young to nurse only about once every 24 hours. Fortunately lagomorph milk is extremely rich and the young are able to survive these long intervals between meals. This strategy may have arisen for mothers to avoid bringing their young, exposed in a shallow form, to the attention of predators.

Pikas do not express elaborate courtship or mating behavior. For both talus-dwelling (Ochotona princeps) and burrowing (O. curzoniae) pikas, males generally approach females, sniff the genital area briefly, and mount. Copulation lasts only a few seconds after which the pair separate. Rock-dwelling pikas tend to be monogamous; Asian forms live in pairs, and males and females in the American species generally occupy neighboring territories and show some familiarity with each other. Burrowing pikas live in family groups that also define their mating system. However, over-winter mortality is high and capricious, thus the availability of males and females within family territories can vary across the landscape. Most families begin the breeding season with one male and one female, thus demonstrating true monogamy. Some families have more adult females than males and are polygynous, whereas the reverse, a family with more adult males than females, yields a polyandrous mating system. All three types of mat ing systems can be found in adjoining family territories. Young pikas are born beneath tons of rock or deep in burrow systems, making observations of parental care difficult. Some captive studies and anecdotal field observations indicate that while pika mothers visit their young infrequently, they nurse more often than in the "absentee" system of most leporids. When young rock-dwelling pikas are weaned and appear on the talus, they are treated aggressively by their parents. Normally young avoid confrontations by choosing to be active at times when their parents are inactive and confining their movements to areas near the borders of adult territories. The opposite is true of newly weaned burrowing pikas. Fathers, and to a lesser degree mothers, are highly attentive toward their offspring and engage in numerous affiliative behaviors with them.

Reproduction is highly seasonal in all pikas beginning at the end of winter and extending through early summer. Timing and length of breeding season depends on altitude, latitude, that year's climate, and whether the species is a rock or burrowing form. Rock-dwelling pikas initiate two litters each year, although normally only one is successful. Litter size is small, generally three young (range of one to five), and not all of these are successfully weaned. Animals first breed as yearlings; gestation is 30 days. In contrast, burrowing pikas initiate many (up to five) sequential large litters (range of one to 12), and weaning success appears to be high. In some burrowing forms young mature quickly and breed initially in their season of birth. Gestation is approximately 20-21days. Thus, reproductive performance is greater in all dimensions in burrowing versus rock-dwelling pikas.

The most elaborate courtship pattern in lagomorphs has been described in the European hare. First, large mating groups aggregate from which animals seemingly pair off. Apparently these partners are faithful and live together for about one month. The extended courtship involves repeated chases and approaches, tail flagging, and contact. Copulation finally ensues, although lasting for less than 10 seconds. The pair separate with a giant leap by the female used to throw off the male. Vigorous pursuit and repeated copulations may follow. Most other hares and rabbits express a muted version of this behavior, and in some instances, copulation occurs without any preliminary courtship. In European rabbits mating is based on the dominance hierarchy within a warren; the mating system in most other species is either unknown or assumed to be based on either a dominance hierarchy or random encounters.

Hares are precocial (being born fully furred and with their eyes open) at birth. This condition follows a relatively long gestation period of about 40 days. Coupled with the lack of attention given young during the lactation period, there appears to be little in the way of parental care expressed toward young hares. Rabbits have a shorter gestation period of about 30 days, and their young are altricial (born without any fur and with their eyes closed) at birth. Mothers construct a nest, sometimes in a protected burrow dug for this purpose, lined with hair from their own belly and plant material to secure their young. The European rabbit enters a social world in the tightly knit warren, thus benefits from behaviors of parents and others. Most other rabbits, however, do not receive substantial parental care and independently enter the world following weaning.

Leporids are legendary for their reproductive potential. Indeed, many species have many large litters each year, and young may reach sexual maturity at a relatively early age. However, lagomorphs live in a wide variety of ecosystems, and many of these are stressful and energy limited. In these conditions some species may not be as fecund. In the far north, where the summer breeding season is short, native hares (Le-pus othus, L. arcticus) normally conceive only one large litter per year. In desert situations where some hares (L. californi-cus, L. alleni) may face a shortage of resources over an extended breeding season, litter sizes may consist of only one to three young, although they may produce four to seven litters each year. Latitude, and thus its effect on season length, may significantly influence litter size. For example, the Eastern cottontail (Sylvilagus floridanus) produces larger litters in the northern part of its range (average is 5.6), with a steady decline in litter size to the southern limit of the species (average is 3.1). Thus, seasonality and productivity of the habitat appear to be major influences on the fecundity of leporids.

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