Timing and Synchrony of Births of Bighorn Sheep

Implications for Reintroduction and Conservation

Populations of bighorn sheep, which once numbered in the thousands, have declined markedly since the latter part of the nineteenth century, and these ungulates face a precarious future. Since the 1920s, considerable money has been spent to restore populations of bighorn sheep into historic ranges. Despite those efforts, from 1923 to 1997, only 41% of translocated populations of bighorn sheep were considered successful. Many factors, including diseases and predation, may impact reestablished populations of bighorns. Little information exists, however, regarding how the behavior of released bighorns influences the success of reintroductions, especially timing and synchrony of births in new environments.

Bighorn sheep occupy areas from Canada to Mexico. In general, bighorns occurring at northern latitudes and higher elevations give birth late in spring, and the birthing period is constricted, which allows females to exploit the shortened growing season. Bighorn sheep that occupy southern latitudes give birth during most months, likely because growing seasons are much less predictable. Parturition in Rocky Mountain bighorns usually occurs when favorable temperatures and onset of nutritious forage are most conducive to support lactation and thus survival of lambs.

Young that are born late, however, suffer decreased survival to their first winter, reduced survival to yearling age, and decreased future reproductive potential.

We studied timing and synchrony of births from 2005 to 2007 in four populations of reintroduced Rocky Mountain bighorn sheep occupying the Central Basin and Range and Wasatch and Uinta Mountain ecoregions in Utah to investigate whether bighorns would adjust timing and synchrony of births to climate and green-up of vegetation in the two ecoregions (Fig. 1). We also compared timing and synchrony of births for bighorns in their source herd (Antelope Island) with bighorns in an ecologically similar release site (Stansbury Mountains) that was 36 km (22 miles) away during 2006 and 2007 (Fig. 1).

Figure 1. Study areas and ecoregions in Utah, where we quantified timing and synchrony of births in populations of reintroduced bighorn sheep from 2005 to 2007.

We determined 274 birthdates, and although only separated by 57 km (35 miles), reintroduced populations of bighorn sheep occupying the Central Basin and Range Mountains gave birth an average of 29 days earlier than those on the Wasatch and Uinta Mountains, which corresponded with the initiation of vegetation green-up. Additionally, bighorn sheep on the Stansbury Mountains (ecologically similar release site) gave birth at similar times as bighorns on Antelope Island (source area). Populations of bighorn sheep that were reintroduced into adjacent ecoregions adjusted timing of births to environments and green-up of vegetation in restoration areas. Timing and synchrony of births for reintroduced bighorn sheep in an ecologically similar release site were the same as those of their source area.

So what does this mean? 

• Young that are born late suffer decreased survival to their first winter, reduced survival to yearling age, and decreased future reproductive potential. This is especially critical for the small number of individuals that are reintroduced in bighorn populations. Therefore, biologists should consider this adjustment of timing and synchrony of births when reintroducing bighorn sheep, especially when animals are released into different ecoregions.
• When possible, managers should select release sites that are ecologically similar to source areas, thereby reducing the potential effects of animals adjusting the timing and synchrony of births to environmental conditions of restoration areas. A failure to do so may cause increased mortality of late-born young born.

Acknowledgments
These studies were supported by the Utah Division of Wildlife Resources, the Utah Chapter of the Foundation for North American Wild Sheep, U.S. Department of Defense, and Utah State Parks. We thank T. Smith, L. Chase, J. Oyster, and K. Sproat for their assistance with these studies.

This edition of Trophy Points was based on information from the following studies:

• Whiting, J. C., D. D. Olson, J. M. Shannon, R. T. Bowyer, R. W. Klaver, and J. T. Flinders. 2012. Timing and synchrony of births in bighorn sheep: implications for reintroduction and conservation. Wildlife Research 39: 565-572.
• Whiting, J. C., R. T. Bowyer, J. T. Flinders, and D. L. Eggett. 2011. Reintroduced bighorn sheep: fitness consequences of adjusting parturition to local environments. Journal of Mammalogy 92:213-220.
• Whiting, J. C., K. M. Stewart, R. T. Bowyer, and J. T. Flinders. 2010. Reintroduced bighorn sheep: do females adjust maternal care to compensate for late-born young? European Journal of Wildlife Research 56:349-357.

Trophy Points: Big Game Research On Line is complied and edited by David G. Hewitt, a Professional Member of the Boone and Crockett Club and the Stuart W. Stedman Chair for White-tailed Deer Research at the Caesar Kleberg Wildlife Research Institute.