Mating strategies of woodland caribou: Rangifer tarandus caribou

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Mating behavior of woodland caribou, Rangifer tarandus caribou, was studied in populations from contrasting environments that were expected to influence reproductive tactics. A small population of caribou on Brunette Island (Newfoundland) lived at high density in mostly open habitat, without predators. A larger population in Spatsizi Wilderness Park (British Columbia) lived at low density in mostly open habitat, with heavy predation pressure. A high density population, on the Slate Islands (Ontario), lived in closed habitat without predators. The major objectives of this study were to examine the functional aspects of rutting behavior of males and to examine the relationships between the environment and the mating behavior of males and females. Sparring behavior did not appear to have strong consequences for dominance ranks. Fighting was limited to the period immediately prior to and during breeding. Fight duration tended to be longer for males from rutting groups (one dominant male plus females) than for males interacting in mating herds (many large males and females). Hock-rubbing, a behavior pattern of dominant large males, may stimulate estrus in females, and intimidate males. Bush-thrashing, by dominants and subordinates, occurred most frequently in the absence of male conflict, or intense courtship activities. Ground-hitting appeared to be associated with conflict between males. Bush-gazing varied in frequency between populations and appeared to have little social significance. Males may low-stretch to females to test their estrous status. Males chased females in both herding and non-herding contexts. Chase and low-stretch may also stimulate reproductive physiology and behavior in females. Ratios of chase to low-stretch acts were lower for males from mating herds than from rutting groups. Chasing was also infrequent in male-female associations in closed canopy. The generally accepted role of lip-curl (flehmen) and "mouth-open", postures frequently performed by rutting males, is questioned. The main reproductive strategy of mature males is to acquire and keep females by aggressive conflicts with other males. Other factors also contributed to reproductive success of mature males, such as efficiency at finding, herding and courting females, and timing of reproductive effort. Young males bred females opportunistically, responding quickly to the absence or diversion of dominant males. Measures of female association with individual males (in rutting groups) were positively correlated with male chase rates and male dominance ranks. Males displayed little preference for individual females. Tending behavior during estrus varied greatly. Males in large herds tried to breed prereceptive females by "surprise" mounts and by persistent mounting attempts while careful tending appeared to be more common in rutting groups. The evolution of male antlers may have been influenced by the social environment at breeding; size of groups may alter the costbenefit ratios that determine the tactics of male fighting and display (as well as tending and herding options). Sexual dimorphism in body size was not a function of the degree of polygyny. Rather, sexual dimorphism varied as a function of conditions for body growth. Males adopt rutting tactics that are appropriate to female grouping patterns, which themselves are a function of habitat, predation and demography. In essence, the environment determines the social requirements of females, which in turn, determines the mating strategies of males.
Bibliography: p. 536-565.
Butler, H. E. (1986). Mating strategies of woodland caribou: Rangifer tarandus caribou (Unpublished doctoral thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/15292