Effects of prolonged immunocontraception on the breeding behavior of American bison

Abstract

In late 2009, the Catalina Island Conservancy began using fertility control to replace periodic removals to manage an introduced population of American bison (Bison bison) on the island. Through the application of the immunocontraceptive vaccine porcine zona pellucida (PZP), population growth was slowed within 1 year, and halted over time. In response to lingering questions about the use of PZP to manage large, free-ranging wildlife populations, we sought to determine the reversibility of PZP by ceasing the annual application to a subset of 15 bison cows and monitoring for subsequent calf arrival, and to document changes in the timing and length of the breeding season in response to PZP by monitoring breeding behavior and assessing fecal progesterone (FP) levels for all 60 resident cows over a 13-month period. As of June 2017, no new calves had been observed on the island, suggesting that, following repeated annual treatment with PZP (3 or 4 years), bison do not resume normal reproduction for at least 4 or 5 years, and that fewer treatments would be advisable if a faster return to fertility is desired. Based on observations of bull and cow behavior, and FP levels, cows displayed estrous cycles consistently throughout the study period, indicating that bison may ovulate year-round when conception and its consequences, e.g., lactation and presence of calves, are blocked. Because there is little evidence that an extended breeding season would negatively impact the health of bulls or result in large numbers of out-of-season births on Catalina, PZP appears to be a highly effective tool for managing the population of introduced bison on the island. However, the extended period of contraception and breeding activity of both cows and bulls may make PZP less suitable in high-latitude, predator-rich environments where bison conservation remains a top priority.

Keywords: Bison bison; Catalina Island; PZP; immunocontraception; non-lethal population control; reproductive behavior.

Fig. 1.

Map of Santa Catalina Island, California, illustrating locations of 3 cross-island fences that partition the island into 4 zones. Zone 1 is free of bison (Bison bison), whereas Zones 2, 3, and portions of 4 are occupied by bison. Avalon and Two Harbors are the largest human communities on the island. Inset shows the location of Catalina in relation to mainland southern California.

Fig. 2.

Behavioral and FP profiles of representative PZP-treated and Reversal bison (Bison bison) cows on Catalina Island. (top) PZP-treated cow 9011, for which 4 cycles were detected based on behavior, 7 cycles were detected based on FP levels (black stars), with 2 occasions in which behavior and FP overlapped. (bottom) Reversal cow 9009, for which 7 cycles were detected based on behavior, 8 cycles were detected based on FP levels (black stars), with 6 occasions in which behavior and FP overlapped. FP = fecal progesterone; PZP = porcine zona pellucida.

Fig. 3.

(A) Observations of light and heavy tending and copulations by bison (Bison bison) bulls, and (B) all bull breeding behaviors (all tending and copulations) per day of observation, tallied by month, in response to PZP treatment in cows on Catalina Island from 1 June 2014 to 30 June 2015. PZP = porcine zona pellucida.

Fig. 4.

(A) Number and (B) percentage of bison (Bison bison) cows displaying estrous cycles, tallied by month, on Catalina Island from 1 June 2014 to 30 June 2015 in response to PZP treatment. Percentage was calculated as a fraction of the number of cows sampled in a given month. Prior to PZP treatment, the typical calving season on Catalina was from March to mid-May, indicating breeding from June to mid-August. PZP = porcine zona pellucida.