DISCUSSION
We found that of four carnivore species, only two appeared to actively
search for elk and none searched for mule deer neonates. The fact that
no carnivores encountered parturient mule deer more often than expected
by chance was consistent with our predictions that the young of less
abundant species would not be targeted (Figure 1b); mule deer are at
least 5 times less abundant than elk in this ecosystem (Oregon
Department of Fish and Wildlife, unpublished data). This result suggests
that mule deer fawn mortalities in this study area (Jackson et
al. 2021) are likely the result of fortuitous encounters (from the
perspective of the predator) and that risk to mule deer from incidental
predation may thus depend on the amount of overlap between deer and elk
parturition habitat for predators that are primarily searching for elk
calves. Our finding that two of the four carnivore species actively
searched for elk (the more abundant species) partially aligned with our
expectations that a predatory response would be greater toward the more
abundant prey species. We also expected the more generalist carnivores
(bears and coyotes, which are highly omnivorous) would exhibit a more
fluid response to an ephemeral prey source. This hypothesis was
supported for bears but not coyotes. The lack of response of bobcats and
coyotes to both elk and deer may be explained for several reasons.
First, elk calves are large prey items for mesopredators such as coyotes
and bobcats particularly when subject to maternal defense from elk.
While mule deer young are much smaller, their rarity on the landscape
may not have warranted search behavior. Nonetheless, we can speculate
that had mule deer been the more abundant species in this area, we may
have observed some level of search behavior by coyotes and/or bobcats.
Second, bobcats are efficient predators of small prey while coyotes in
this system gain a substantial amount of protein via scavenging
(Ruprecht et al. 2021b), both of which potentially reduce the need to
pursue neonates. Finally, our sample size of collared bobcats was small
(N = 6) so results may have been subject to Type II error for
that species.
Our analyses to evaluate whether
carnivores’ use of elk parturition habitat tracked the phenology of the
birth pulse revealed further contrasting behaviors among the predators
in our study. Use of elk parturition habitat by male bears was best
explained by a model with a quadratic effect of Julian week, with its
maximum aligning almost perfectly with the peak of the birth pulse
(Figure 6b). This result suggests that male bears exhibited a spatial
shift in habitat consistent with an effort to maximize encounters with
elk neonates immediately following parturition, which is logical given
that bears are limited by a short window in which they can efficiently
hunt neonates. This idea is further supported in that the number of
encounters between GPS-collared bears and parturient elk in the 30 days
post-parturition peaked between 7-10 days after an elk gave birth. By
contrast, the quadratic effect was not supported for cougars (Figure
6d). Although previous research has shown that juvenile elk constitute a
large fraction of the diets of cougars (Clark et al. 2014),
several aspects of cougar predatory behavior may explain why cougars did
not exhibit a quadratic effect indicative of searching areas used by
neonates. First, because elk become solitary to give birth before
rejoining the herd several weeks later (Paquet & Brook 2004), it may be
inefficient for cougars to target solitary mother-young pairs when they
would have access to more individuals by pursuing larger herds of mixed
age classes. Such “nursery herds” (Paquet & Brook 2004) would present
naïve prey such as yearling elk that may have recently lost maternal
guidance, or vulnerable young of the year after the mother-young pair
rejoined the herd after parturition. The number of encounters between
GPS-collared cougars and parturient elk in the 30 days post birth was
greatest around 20 days after an elk gave birth which further supports
the idea that cougars pursued calves only after they had matured for
several weeks. We were initially concerned that the apparent use of
parturition habitat by bears coinciding with the birth pulse could be
driven by selection for other dynamic resources such as green vegetation
that was correlated with elk parturition habitat, causing a spurious
result. However, only male bears exhibited this pattern, and we would
expect females to exhibit the same response to bottom-up resources.
Although we are unaware of previous research on search behavior of
cougars toward ungulate neonates, our work both aligns and contrasts
with patterns described for black bears and coyotes elsewhere. The
density of elk neonates in our study area was closer in magnitude to the
number of caribou calves in the study by Rayl et al. (2018) that
determined bears actively searched for neonates than it was to the study
by Bastille-Rousseau et al. (2011), in which bears
opportunistically encountered neonates. Our finding that male bears were
much more likely to encounter neonates than were females also aligns
with Rayl et al. (2015) who found that male bears were more
likely to visit caribou calving grounds than were females. Further,
several other studies have documented higher predation or meat
consumption by male black bears than females (Boertje et al.1988; Jacoby et al. 1999), which should be expected given that
previous research has shown that larger, male bears require more
animal-borne protein to gain weight than do smaller, female bears (Rode,
Robbins & Shipley 2001). Our results did not suggest that coyotes
actively searched for elk calves which accords with a cause-specific
mortality study of elk calves in this region that found coyote predation
to be minor (Johnson et al. 2019). In other ecosystems, however,
coyotes have been implicated as nontrivial sources of mortality for elk
calves (Barber-Meyer, Mech & White 2008) although there is mounting
evidence that coyote predation on mule deer neonates occurs largely only
when small mammal populations are low (Hamlin et al. 1984; Hurleyet al. 2011). We unfortunately did not have sufficient data on
occurrence and abundance of alternative prey to assess whether this
occurred in our study.
It is important to view the distinction between active search and
incidental encounter in light of the effects that predator population
dynamics may have on prey. If predators employ active search behavior,
then a reduction in predator density may not yield increased neonate
survival because neonates spared by that individual become targets for
the remaining pool of searching predators (unless prey density is such
that predation is limited by handling time or satiation). But if neonate
predation occurs because of incidental encounters, then a reduction in
predator density benefits a focal prey species by reducing encounter
rates both because each prey is less likely to be incidentally
encountered per unit time and because predators spend time handling
other species. Although population growth rates of ungulates are most
sensitive to survival of adult females, this demographic rate is
consistently high and stable (Gaillard, Festa-Bianchet & Yoccoz 1998)
and many ungulate populations are instead limited by insufficient
recruitment due to low neonate survival (Raithel, Kauffman & Pletscher
2007). Thus, knowledge of how different species of carnivores search or
encounter different species of prey will be needed to determine the
extent to which predator control would be an effective strategy for
managing ungulate populations.
A necessary assumption in our study was that predator and adult female
ungulate locations within a 200-meter proximity at the time of
simultaneous GPS recordings constituted an encounter with an ungulate
neonate in the weeks after birth. This assumption was required because
we did not have GPS transmitters on neonates and thus assumed the
location of the adult female was a reasonable proxy for the location of
the neonate. This assumption certainly introduced some amount of data
contamination that may have obscured a stronger signal than what we
observed. Although our dataset was unique in that it included
contemporaneous GPS recordings on four species of carnivores and two
species of ungulates, with fix intervals up to three hours on predator
GPS collars, we are certain to have missed additional encounters.
Another limitation in our study was that we were required to use
inference methods to predict some of the parturition events for deer and
all parturition events for elk which are subject to error in the exact
timing of births. Further, some number of neonates likely died in the
days following birth which our analysis could not consider. These
factors should only have acted to dilute any potential signals in the
data and not cause spurious correlations. Nonetheless, we caution that
our results are conservative and should be interpreted with the
possibility that Type II error may be present. These issues will be
minimized as GPS transmitters become sufficiently lightweight to be
placed on neonates and as battery life improves such that more frequent
GPS readings can be obtained on both predators and prey.
An emerging frontier in animal ecology is understanding if and how
behavior influences population dynamics. Consequently, elucidating how
predatory tactics (e.g. active search vs. incidental encounters) affect
prey populations should feature prominently in future research. Our
results suggest there was a behavioral response by two of the four
carnivores toward elk, but no response by any of the carnivores toward
mule deer. This result combined with previous research suggests that the
foraging tactics by predators in response to a pulsed resource differ by
both predator and prey species, are likely ecosystem-specific, and can
change dynamically through time as the availability of vulnerable
neonates fluctuates.