Persistence in diving American mink

Description of dive clusters and aquatic activity sessions (AAS)

Most (88.3 and 98.6%, respectively) dives occurred in clusters or in AAS (n = 3,714 total dives); only 1.4% dives (n = 53) were single isolated dives. Note that, by definition, all clusters occur within
an AAS. For individual datasets, between 60.1 and 100% of dives occurred in clusters
(mean = 84.6%), and between 68 and 100% of dives in AAS (mean = 95.4%).

Dive clusters consisted of 2–28 dives (grand median = 4, n = 633 dive clusters in total across all datasets; range of individual medians: 2–7.5,
Figure 2a); AAS of 2–80 dives (grand median = 7, n = 299; range of individual medians: 3–31, Figure 2b). As for total number of dives per day, and depth and duration of individual dives
10], both dive cluster length and AAS length were extremely variable within individuals
(coefficient of variation [CV]
_clusters
0.32–0.76; CV
_AAS
0.43 ? 1) and differed significantly among individuals (ANOVA
_cluster
: F
_{13, 619}
 = 7.3, P  0.001; ANOVA
_AAS
: F
_{13, 285}
 = 10.59, P =  0.001). The short duration of individual dives 10] meant that despite the high numbers of dives that could occur in either clusters
or AAS, both dive clusters and AAS were, on average, also short in duration: cluster
duration ranged from 9 s to 36.1 min (grand median = 2.3 min; range of individual
medians: 57 s to 12.9 min, Figure 2c) and AAS duration 11.0 s to 162.8 min (grand median = 20.3 min; range of individual
medians: 11.3–49.6 min, Figure 2d). The overall median length of pauses between dives within a cluster was 17 s, and
individual median pauses ranged between 8 and 81.1 s (data were very strongly right-skewed,
with the maximum pause between dives being 249 s; 95% of data points were 127 s,
90%  89s). Bagniewska et al. 34] described an AAS as a period of aquatic activity (diving and swimming) that may include
brief terrestrial periods and events such as jumping out onto the bank (when aquatic
prey items may be consumed) or runs to the burrow, whereas a dive cluster represents
continuous diving. However, we cannot rule out the possibility that small prey items
are taken to the riverbank and consumed during the occasional long inter-dive intervals
within a dive cluster (small prey items, e.g. aquatic beetles Dytiscidae may even be consumed during short intervals at the water’s surface). Therefore, a
dive cluster does not necessarily represent an attempt to obtain a single prey item.

Figure 2. Distributions of dive clusters and aquatic activity sessions. a, b show the duration (minutes) and c, d the length (number of dives) of dive clusters and aquatic activity sessions from
all study animals pooled.

Effect of ambient temperature, body size and light

For the number of dives in both clusters and AAS, there was a significant interaction
between ambient temperature and sex (LME
_cluster
: F
_1,609
 = 5.06, P = 0.025; LME
_AAS
: F
_1,273
 = 6.90, P = 0.009), indicating that the response of males and females to ambient
temperature differed. The main effect for ambient temperature was also significant,
and highly significant for AAS (LME
_cluster
: F
_1,609
 = 8.92, P = 0.003; LME
_AAS
: F
_1,273
 = 15.03, P  0.001). There was no significant effect of sex (LME
_cluster
: F
_1,12
 = 0.35, P = 0.568; LME
_AAS
: F
_1,12
 = 0.01, P = 0.91), but the effect of animal weight was significant (highly significant
for AAS; LME
_cluster
: F
_1,609
 = 8.79, P = 0.003; LME
_AAS
: F
_1,273
 = 18.33, P  0.001), with the number of dives decreasing with increasing weight (Figure 3). The effect of light was also significant (LME
_cluster
: F
_1,609
 = 9.11, P = 0.003; LME
_AAS
: F
_1,273
 = 8.16, P = 0.005), with the number of dives greater during the day than during the
night (Figure 4).

Figure 3. Effect of mink weight on persistence. a shows the relationship between animal weight and the median and maximum number of
dives per cluster, and b—per AAS. The dashed vertical line indicates the break in body weight between males (1 kg) and females (1 kg). Data
points are one deployment on one individual; in some cases, there were two or three
deployments on the same individual (see Additional file 1 for deployment details). Solid lines show the model fit with individual included as a random effect.

Figure 4. Effects of light on persistence. a shows the median length (number of dives) of clusters made in darkness and daylight
by males and females. b shows the proportion of the total dives made in darkness of the total for males and
females.

To compare effects on average and maximal persistence, the analyses were repeated
on summary statistics for each individual, using median and maximum values for the
number of dives in a cluster, and the number of dives in an AAS (Table 1). The effect of animal weight was significant for both median and maximum values
of cluster and AAS length (Figure 3). The main effect for ambient temperature was not statistically significant for either
median or maximum persistence; however, the interaction between temperature and sex
was (for maximum cluster length, and maximum AAS length, but not for median cluster
length or AAS length). Plotting maximum persistence against ambient temperature for
both sexes separately revealed that for females the maximum number of dives increased
with decreasing temperature, but for males there was no evidence of a similar relationship
(Figure 5). No other effects were statistically significant.

Table 1. LME model summary

Figure 5. Effect of ambient temperature on persistence. The figure depicts the relationship
between temperature and the number of dives per a cluster and b aquatic activity session for females (red) and for males (black). The slopes for significant relationships for females are included in the figures
(clusters: persistence [no. of dives] = ?0.53(ambient temperature)[
^o
C] + 21.93; F
_1,8
 = 8.89, R
²
[adjusted] = 0.47, P = 0.0175. AAS: persistence [no. of dives] = ?1.76 (ambient temperature)[°C] + 58.92;
F
_1,8
 = 6.19 R
²
[adjusted] = 0.37, P = 0.038).

Contrary to our predictions based on allometry and thermoregulation, small mink were
more persistent divers than large mink (Figure 3), and (maximum) persistence was greater when it was colder; but the latter was only
true for females (Figure 5). Although the effect of body size was apparent for both median and maximum persistence,
it was greatest for maximum cluster length and maximum AAS length, with the result
that, on average, maximum cluster length for females (1 kg body weight) was approximately
twice that of males (1 kg body weight). Maximal female cluster lengths ranged between
11 and 28 dives (grand median = 17), whereas maximal male cluster lengths ranged between
4 and 18 dives in a cluster (grand median = 9), although the apparent difference between
sexes was due to differences in overall body size, rather than in sex per se.

That the smallest females, with the highest rates of heat loss, showed the greatest
persistence, and that this observation occurred during the coldest ambient temperatures
experienced during the study, demonstrate that persistence, or ‘time spent diving’
by mink, was not limited by body size or cold temperatures at least within the seasonal
temperature range of southern England (mean daily ambient temperature did not drop
below 0°C during our study). It is possible that, in colder climates, there is a lower
critical temperature beyond which foraging activities are temporarily prohibited.
It is also possible that temperature affects diving behaviour differently on different
temporal scales (e.g. within a day, or within a period of days).

Several authors have referred to the possibility of inter-sexual niche partitioning
in mink with females taking relatively more aquatic prey (fish and amphibians) and
males more large terrestrial prey (e.g. lagomorphs in the UK 35]; see also 36], 37]). These data provide some support for that suggestion in that the smallest individuals
(the females) exhibited the longest duration aquatic activity, and thus could be inferred
to spend the most time pursuing aquatic prey, whereas the largest individuals (the
males) performed fewer dives per day 10], the shortest dive clusters, and the shortest aquatic activity sessions, which suggests
more time spent in terrestrial habitats, and, accordingly, might reflect greater use
of terrestrial prey. However, short monitoring periods and small sample size mean
that we cannot dismiss short-term exploitation of a patchy resource (as opposed to
consistent, long-term differences), or the possibility of individual specialisation
(as opposed to inter-sexual differences; discussed in 10], 14], see also 38]). Further, because we were unable to monitor diving males during the coldest winter
temperatures (only 1 male was tagged in winter when ambient temperatures were below
7°C, but this individual dived fewer than five times during the 5- to 6-day monitoring
period, and thus was excluded from analysis) we could not determine whether males,
the larger individuals, also sometimes performed long dive clusters (or AAS), only
that females, the smallest individuals, could.

An alternative explanation of short dive clusters in males might be that males are
more efficient at catching aquatic prey (i.e. perhaps they can catch fish quicker,
can catch larger fish, and therefore need to spend less time foraging in the water).
However, because we cannot identify successful dives, and cannot assume that a single
dive cluster relates to an attempt to catch a single fish (or other aquatic prey)
it is not possible, given the current data, to draw conclusions regarding either efficiency
or success. Further, the number of dive clusters recorded over the 5- to 6-day monitoring
period was hugely variable for both males (5–82) and females (8–96), which is inconsistent
with clear inter-sexual differences in foraging strategy but perhaps consistent with
individual differences.

That the largest effect sizes were seen for maximal persistence rather than median
persistence, shows that, on average, there was little difference in persistence between
female and male mink (or small and large mink), and no difference in ‘average’ behaviour
between winter and summer (i.e. even the small ‘high diving’ females did not perform
very long clusters or AAS all the time). Nevertheless, there was clearly no reduction
in persistence in winter. Eurasian otters also maintain consistent activity time in
the water, regardless of water temperature 39], despite the energetic costs of swimming in 2°C water (approximate minimum British
river temperature 40]) being 2.7 times higher than in 20°C water (approximate maximum British river temperature
40]) 41]. The increased thermoregulatory costs of swimming in winter relative to swimming
in summer are likely to be even higher for mink due to their smaller body size, relatively
greater surface area, and thus greater heat loss rate, compared with otters 15]. Given the energetic costs involved, the extreme persistence observed in winter suggests
that the costs of occasional prolonged diving in cold water are outweighed by the
energetic gains. Harrington et al. 10] suggest that the relatively high diving rates observed in winter probably reflect
the behaviour of some mink capitalising on the increased susceptibility to capture
of ectothermic fish in cold water 8], 13]. However, our prediction that these high winter diving rates would be achieved in
shorter clusters of dives to minimise prolonged periods of heat loss was not upheld:
cluster length (for females) was longer in winter. Unfortunately, the data do not
allow us to determine whether long clusters reflect an attempt to catch a single fish
(if the chance of catching a large fish in torpor is high, it might be worth pursuing
the opportunity for longer) or reflect an extended period of catching several smaller
fish (that were consumed in the brief inter-dive intervals). Nor can we distinguish
between successful and unsuccessful dives.

From a thermoregulation perspective, Williams 13] estimates that mink, swimming in water at 20°C, are only able to maintain their body
temperature for ca. 5 min. Although we did not measure body temperature, and we cannot
quantify total time spent swimming, our results show that mink were able to remain
active (diving) in the water for much longer than 5 min although they did not often
do so. Mink were recorded continuously diving for 36 min with brief inter-dive intervals
of only a few minutes at most, at ambient temperatures of between 2.5 and 5.5°C (water
temperature recorded at our study site in winter, 27]).

Both males and females performed longer dive clusters and AAS during the day than
during the night (Figure 4a), although, in general, male mink were more nocturnal in their diving than females
(Figure 4b). Three possible explanations for increased persistence during the daylight are:
(1) hunting underwater is more profitable in daylight for an animal with relatively
poor underwater vision in daylight, and thus worth greater time investment; (2) hunting
is more risky at night due to the presence of larger, nocturnal competitors (e.g.
otters 26], and thus restricted to shorter periods; (3) day and night dives have different functions
(e.g. hunting and travelling), that might be characterised by different temporal structures.
As surface swimming may be interrupted by periods of underwater swimming to reduce
drag, or as an evasive manoeuvre 42], clusters of dives might also represent periods of travelling in the water. Presumably,
underwater visual acuity is not as crucial for travelling as for hunting, and so travelling
dives might be more likely to take place at night; however, it is not obvious why
clusters of travelling dives would be shorter than clusters of hunting dives. Further
study of the diving behaviour of mink, in the absence of otters, is required to distinguish
between the first two possibilities. Harrington et al. 26] suggest that male mink may be less affected by competition with otters, which may
explain the apparent diurnal difference between the sexes in diving behaviour, if
mink dive during the day to avoid otters. Alternatively, males may spend more time
travelling than females, partly because they have larger home ranges 14], 43], and so may dive more during the night, if mink dive during the night predominantly
for travel.