Body condition and pregnancy in northern Yellowstone elk: Evidence for predation risk effects?

Abstract

Abstract. S. Creel et al. reported a negative correlation between fecal progesterone concentrations and elk : wolf ratios in greater Yellowstone elk (Cervus elaphus) herds and interpreted this correlation as evidence that pregnancy rates of elk decreased substantially in the presence of wolves (Canis lupus). Apparently, the hypothesized mechanism is that decreased forage intake reduces body condition and either results in elk failing to conceive during the autumn rut or elk losing the fetus during winter. We tested this hypothesis by comparing age-specific body condition (percentage ingesta-free body fat) and pregnancy rates for northern Yellowstone elk, one of the herds sampled by Creel et al., before (1962Creel et al., before ( -1968 and after (2000)(2001)(2002)(2003)(2004)(2005)(2006) wolf restoration using indices developed and calibrated for Rocky Mountain elk. Mean age-adjusted percentage body fat of female elk was similarly high in both periods (9.0% 6 0.9% pre-wolf; 8.9% 6 0.8% post-wolf). Estimated pregnancy rates (proportion of females that were pregnant) were 0.91 pre-wolf and 0.87 post-wolf for 4-9 year-old elk (95% CI on difference ¼ À0.15 to 0.03, P ¼ 0.46) and 0.64 pre-wolf and 0.78 post-wolf for elk .9 years old (95% CI on difference ¼ À0.01 to 0.27, P ¼ 0.06). Thus, there was little evidence in these data to support strong effects of wolf presence on elk pregnancy. We caution that multiple lines of evidence and/or strong validation should be brought to bear before relying on indirect measures of how predators affect pregnancy rates.

A survey of the effects of wolf predation risk on pregnancy rates and calf recruitment in elk

“…We concluded that ''although correlative, these results suggest that wolf predation has indirect effects on elk dynamics, driven by costs of behavioral defenses that alter reproductive physiology and demography.' ' White et al (2011) recently questioned our conclusions, using data from one herd in the Northern Range of Yellowstone National Park. First, they failed to detect changes in the mean pregnancy rate in a comparison of data from 1962-1968 (wolves absent) and [2000][2001][2002][2003][2004][2005][2006] (after wolf reintroduction).…”

“…'S CRITIQUE The results just discussed provide consistent support for the hypothesis that the pregnancy rate of elk has declined in sites with high wolf-elk ratios. Still, it remains to be explained why this pattern was not apparent in the analysis of White et al (2011). While differences among populations in the strength of risk effects are expected, several aspects of their methods would be expected to obscure any effect of predation risk on pregnancy rates.…”

“…All of the analyses in White et al (2011) compare samples collected in the 1960s with one set of methods to samples collected with different methods after wolf reintroduction. Their data on body fat from the 1960s come from measurements of kidney fat deposition in dead elk, while their data from 2000s come from ultrasound estimation of rump fat in live elk.…”

“…Second, they failed to detect a change in body fat within this herd between the two periods. Finally, they suggested that body fat was higher in Northern Range elk than in several other populations (White et al 2011). However, each of these results suffers from methodological and statistical flaws that would be expected to mask risk effects.…”

“…This decrease in the pregnancy rate was threefold to 30-fold larger than the decrease in female survival rates in the same population. It is difficult to understand why White and Garrott failed to mention these data while arguing that there is little evidence that pregnancy rates have decreased and that ''it is imperative that multiple lines of evidence and/or strong validation be brought to bear when assessing the effects of antipredator responses on elk demographic rates'' (White et al 2011).…”