Lifetime, known‐age moose reproduction in a nutritionally stressed population

Abstract

ABSTRACTQuantifying long‐term, low reproductive metrics indicative of an ungulate population's low nutritional status can help spur action to manage for moderate densities in contrast to unsustainable, high densities. We previously ranked the moose (Alces alces gigas) population described here as having the lowest nutritional status among 14 moose populations in Alaska, USA, primarily using reproductive indices (1996–2005) from moose with ages estimated by counting cementum annuli. Here, we detailed lifetime reproductive metrics from a subset of known‐age female moose initially radio‐collared at 9–10 months of age (7 cohorts; 1996–2002); we renewed radio‐collars as necessary. We radio‐tracked and circled these moose via aircraft at 24‐ or 48‐hour intervals to detect the presence of newborns during the 1998–2018 calving seasons, with meaningful annual sample sizes during 2000–2014. The number of snow‐free days in the year preceding parturition had a subtle positive effect on parturition probability, but we found no evidence for effects of the preceding February and March immobilization, cohort affiliation, or the preceding winter's snow cover on the probability of a female being parturient. The probability of twinning declined as the calving season progressed. Compared with moose production in populations with improved nutrition, our study population experienced low production primarily as a result of delayed maturation, occasional pauses in reproduction, and low twinning rates. Reproductive senescence occurred at normal advanced ages despite nutritional stress. We recorded a 28% parturition rate among 144 females 3 years of age (min. age of reproduction). Parturition rates were stable from 4 to 13 years of age ( = 77%), declined at 14 years of age, and peaked at 15 years of age. Females first twinned at 5 years of age (5%), and the twinning rate increased with age, peaking at 13 years of age (25%). Overall, 136 radio‐collared females with complete reproductive histories produced a mean of 5.3 calves/lifetime while being monitored a mean of 7.1 years at ages ≥3 years, although variability in individual production was high. Delaying or pausing reproduction increased the parturition rate at 4 and 5 years of age. However, females that delayed first reproduction produced fewer calves/lifetime on average compared with moose that first produced at 3 years of age. Virtually all moose regularly gave birth with occasional 1‐year pauses that acted to enhance production the subsequent year; the incidence of 2 consecutive nonparturient years was 2.8% (24/844). Moose experienced relatively stable, low nutritional status during 2000–2014 based upon low population‐wide twinning rates from annual aerial transect surveys (no telemetry) flown a few days after the median annual calving dates. Detailed understanding of low reproductive metrics encouraged empowered stakeholders to allow liberal harvests of female moose (2.0–4.4% of prehunt moose population numbers) and encouraged land managers to allow wildfires to burn 25% of the study area to improve moose carrying capacity. We helped manage a 31% decline in moose numbers during 2004–2012 by implementing liberal harvests of female moose. Elevated population‐wide twinning rates in 2012, 2015, 2017, and 2018, similar to elevated levels prior to 1997, were likely a positive response to lower moose densities and improved browse after wildfires. © 2018 The Wildlife Society.