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7. Reproduction, Embryology, and Development
- University of New Mexico Press
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79 Reproduction, Embryology, and Development During the spring breeding season, a dominant male and several of his subordinates follow a female . . . This behavior, known as a “mating chase”, lasts for about 11 hours . . . After such a long day, one might guess that both sexes would be too tired to breed. —S. Zeveloff, Mammals of the Intermountain West, 1988 T his chapter is dedicated to Michael D. Rose (1972–2000). Michael earned an MS at Northern Arizona University with me. His thesis research was on the male tassel-eared squirrel reproductive cycle. Michael died in a boating accident off the coast of California in 2000. Introduction The earliest published observation about the mating of tassel-eared squirrels was by Edgar Mearns, a physician and naturalist, who wrote in 1907, “In May, which is the rutting season, whole troops of males are commonly seen chasing the females” (ref. 1). Over the past hundred years, the majority of reproductive research has been devoted to mating activities, including signs of estrus, factors affecting breeding, mating bouts, and copulation. Other reproductive research has focused on gestation , litter sizes, sex ratios, and embryology. Less studied but equally important research areas are characteristics related to reproductive activities; some examples include sperm morphology, spermatogenesis, oogenesis, and anatomical structures. 7 80 C H A P T E R S E V E N Estrus Female tassel-eared squirrels are in estrus for only eighteen hours (ref. 2, 3). During this single day, the estrous females are aggressively pursued by a group of males, both adults and juveniles. The engagement of the female in this pursuit by the males is referred to as a mating bout (ref. 4). Mating activities have been observed in every month of the year, though most mating occurs in spring and early summer (table 7.1). Female squirrels in estrus exhibit a swelling and reddening of the vulva coupled with an enlargement of the introitus (ref. 2, 4). It takes several days to reach peak estrus when the external vulvar characteristics are visible without binoculars. It is possible that a sexual pheromone is present as male squirrels have been observed investigating areas where the female has passed, such as the ground, the bole, and limbs of trees (ref. 4). Several hypotheses have been proposed to explain cues that induce estrus. One report connects onset of estrus of female tassel-eared squirrels with the development of the ponderosa pine male pollen (staminate) cones, which are produced during late May to early June and are quite ephemeral, as the pollen is quickly dispersed by the wind and the tiny cones dry and detach from the tree (ref. 3, 5, 6). However, other studies have reported estrus in January, many months before onset of staminate cone production (ref. 7). In addition, embryos and lactating females have — TABLE 7.1 — Months reported when tassel-eared squirrels were observed mating Month Subspecies Researcher, year January S. a. aberti Allred and Pogany, 1996 Mid-February–June S. a. ferreus Halloran, 1993 March, April, May S. a. aberti and Mearns, 1907; V. Bailey, 1931; S. a. ferreus Rice, 1957; Hall and Kelson, 1959; Keith, 1965; Farentinos, 1972b; Pogany and Allred, 1995 June–July S. aberti1 Leopold, 1972 (in Mexico) April–October S. a. aberti Cahalane, 1947 Early August S. a. kaibabensis Hall, 1967b Late winter/early spring S. a. ferreus Lechleitner, 1969 November, December S. a. aberti Allred, unpublished data 1 —subspecies not given [52.91.67.23] Project MUSE (2024-03-28 16:47 GMT) R e p r o d u c t i o n , E m b r y o l o g y , a n d D e v e l o p m e n t 81 been reported in March from road-killed S. a. aberti (ref. 8, 9). Another possible cue for estrus is new pine shoot growth in the late spring (ref. 10). However, since this growth occurs only in the spring it cannot explain pregnancies during the winter (ref. 7, 8). Yet another possible explanation for the onset of estrus is warming temperatures causing the melting of snow, allowing the females to obtain adequate nutrients from buried cones and false truffles (ref. 7, 11). It is possible that mild winters with little snow could similarly affect estrus onset. The onset of estrus in winter or early spring would be of no value unless males are capable of insemination , long considered possible only during the spring and early summer (ref. 3). More recent...