Home range behaviour of the monogamous Australian seahorse, Hippocampus whitei

Environmental Biology of Fishes (2005) 72: 1–12
Springer 2005
Home range behaviour of the monogamous Australian seahorse, Hippocampus
whitei
Amanda C.J. Vincenta,c, Karl L. Evansb,d & A. Dale Marsdena,e
a
Project Seahorse, Department of Biology, McGill University, Montreal, Quebec, H3A 1B1, Canada
b
Edward Grey Institute for Ornithology, Department of Zoology, University of Oxford
c
Present address: Project Seahorse, Fisheries Centre, The University of British Columbia, 2204 Main Mall,
Vancouver, B.C., V6T 1Z4, Canada (e-mail: a.vincent@fisheries.ubc.ca)
d
Present address: Biodiversity & Macroecology Group, Department of Animal and Plant Sciences, University
of Sheffield, Sheffield S10 2TN, UK
e
Present address: Fisheries Economics Research Unit, Fisheries Centre, The University of British Columbia,
2204 Main Mall, Vancouver, B.C., V6T 1Z4, Canada
Received 15 June 2003
Accepted 16 January 2004
Key words: syngnathidae, sex differences, territory, spatial behaviour, seagrass, conservation
Synopsis
We provide a quantitative account of local movements in the monogamous Australian species Hippocampus whitei, as a rare report of home range size in fishes living in seagrass habitats. Our study took place
in shallow Posidonia seagrass beds in Port Jackson (Sydney Harbour), principally during January to
March. Daily monitoring of individual seahorses during underwater observations revealed that both sexes
maintained small and apparently undefended home ranges for several breeding cycles at least. Female home
ranges were significantly larger than males, when analysed by both the minimum convex polygon and grid
cell methods. Home range size was not correlated with either body size or seahorse density. Presumably,
home ranges were small in H. whitei because camouflage (to avoid predation and to capture prey), mate
fidelity and parental brooding meant they accrued little benefit (and potentially considerable cost) from
moving more extensively. Sex differences in home range size may arise from constraints associated with
male pregnancy. These fish are among the most sedentary of vertebrates, with relatively small home ranges
equalled only by coral reef species. In terms of their conservation, relatively small protected areas may be
sufficient to support breeding populations of H. whitei although that limited movement may result in
considerable delays in the recolonisation of depleted areas.
Introduction
Many vertebrates use a particular area, or home
range, for their daily movements (Burt 1943, Sale
1978, Schoener & Schoener 1982, Mace et al.
1983). The technical delineation of the home range
varies substantially across studies (see Harris et al.
1990 for a review), but the common theme is that
an individual animal will stay within its home
range for the majority of its activities during a
certain period of the year and/or a certain portion
of its life cycle. The size of the home range can be
influenced by a variety of factors, including predation (Clarke et al. 1993), energetic requirements
and body size (Harestad & Bunnell 1979, Harvey
& Clutton-Brock 1981, Mace et al. 1983, Kelt
& Van Vuren 1999), resource distribution (Dill et
al. 1983, Grant 1997), intraspecific interactions
(Norman & Jones 1984, Grant et al. 1992), and
mating system (Hixon 1987, McCarthy &
2
Lindenmayer 1998). In some cases, individuals or
pairs of animals will defend all or a portion of their
home range, in which case it is know as a territory
(Burt 1943, Grant et al. 1992, Grant 1997).
Animals move according to their needs for survival, growth and reproduction. These needs might
include avoiding predators (survival), finding food
(survival and growth), and finding mates and
nesting or brooding sites (reproduction). Movement expends time and energy and, given limitations on both of these resources (Cuthill &
Houston 1997), we would expect any animal to
move only to the degree that the benefits of such
movements outweigh the costs. The less movement
an animal needs to fulfil its requirements for survival, growth and reproduction, the smaller its
anticipated home range.
Seahorses might be expected to be among the
most sedentary of vertebrates. In terms of survival,
seahorses are generally highly cryptic fishes, and
remain immobile rather than swimming away from
a predator (A. Vincent pers. obs.). They can temporarily attach themselves to a substrate using
their prehensile tail, so remaining in one place
should be easier for them than for most other
fishes, which must swim against currents and thus
expend energy. In terms of growth, seahorses are
ambush predators that attack their small invertebrate prey as they pass the seahorse’s holdfast
(Vincent 1990, James & Heck 1994). In terms of
reproduction, many seahorse species will not need
to move about in search of additional mates
because they maintain monogamous pair bonds at
least for the duration of a single breeding season
(Vincent 1990, Perante et al. 2002, Foster & Vincent 2004). As well, they need not search for
brooding sites because the male guards the eggs
within a specialised tail pouch.
This paper presents an extensive field study of
the home range behaviour of an Australian seahorse species, Hippocampus whitei (Bleeker 1855),
in Sydney Harbour. We set out to assess the spatial
fidelity of this species, and to evaluate whether
males (encumbered by pregnancy) might have
smaller home ranges than females. As no overt
spatial defence had been noted in this species
(Vincent & Sadler 1995) and their home ranges
overlapped considerably (Vincent et al. 2004), we
inferred that, unlike many monogamous fishes
(Barlow 1984, Roberts & Ormon 1992, Grant
1997), H. whitei may not actively defend their
space as territories. Such lack of territoriality
might reduce the probability of a tight correlation
between home range size and body size.
Methods
Study site and species
Our study site comprised the entire seagrass meadow in a small protected bay in Port Jackson, New
South Wales, Australia (also known as Sydney
Harbour: 3351¢S, 15117¢E). The primary locations of our observations were two tracts of seagrass (called the North and South seagrass beds),
located 20 m apart, that were dominated by Posidonia australis but also contained Zostera capricorni and were surrounded by Halophila ovalis
(Figure 1). The area of the North bed was 196 m2
and that of the South bed was 353 m2. Water
temperatures during the study ranged from 16 to
Figure 1. Map of the study site, showing seagrass types in Port
Jackson, New South Wales, Australia.
3
21C and horizontal visibility was typically 1–5 m,
except during brief periods following very heavy
rainfall. Water depth at the site varied from 0.4 to
4.3 m, according to location and tide. Tides were
semi-diurnal, with about 1 m vertical range.
Hippocampus whitei live along the south-eastern
coast of Australia (Munro 1958, Lourie et al. 1999,
Kuiter 2001) and is a medium sized seahorse (in
this study X ± SD standard length was
12.9 ± 1.4 cm
and
X ± SD
mass
was
6.4 ± 2.5 g, n ¼ 105). It is most abundant in P.
australis seagrass meadows (Middleton et al. 1984)
but also occupies other habitats that offer holdfasts, including artificial structures such as shark
nets. Like other seahorses, H. whitei generally
appear to rely on camouflage and crypsis to avoid
predation, move slowly, and are ambush predators
(A. Vincent, pers. obs).
This population forms monogamous pair bonds
that endure throughout the reproductive season
(Vincent & Sadler 1995). Such pairing is reinforced
by morning greetings: the female moves to her
male, then dances with him for about 6 min before
they part for the rest of the day (Vincent & Sadler
1995). Paired seahorses do not respond to nonpartners even when they are the recipients of social
displays from unpaired seahorses (Vincent & Sadler 1995). Moreover, they remain faithful even if
one partner is unable to reproduce for a time, and
delay finding a replacement mate if their partner
disappears (Vincent & Sadler 1995). Courtship is
prolonged and active and includes colour changing
(Vincent & Sadler 1995).
Field methods
The field study ran from 9 November 1991 to 22
April 1992, through the southern summer, when
H. whitei breed (R. Kuiter pers. comm.). We
conducted our analyses, except where otherwise
noted, on observations made during the period of
13 January to 15 March 1992, which we call the
focal study period. This is when we had the
greatest concentration of observations and the
most stable population; whereas few seahorses either arrived on or left the study site between 13
January and 15 March, we found new seahorses
throughout November and December, and virtually all had left the site by late April (Figure 2). We
included all sightings for any given animal, what-
Figure 2. Number of seahorses on the North and South seagrass beds for the duration of the study. The vertical dotted
lines show the beginning and end of the focal study period over
which we have calculated home range sizes.
ever its partner. However, four of the 38 males and
two of the 42 females were known to have found
new partners (after losing their original mates)
between 13 January and 15 March. The possible
impacts of such pair changes on home range estimates are considered in the discussion.
The two seagrass beds were permanently
marked into a 2 m by 2 m grid and we recorded
seahorse locations within 1 m by 1 m sections of
this grid square. We tagged seahorses with uniquely numbered green PVC discs (5.5 mm ·
3 mm) hung around their neck by a cotton thread;
we loosened these every 3–4 weeks to allow for
growth. The stress of tagging appeared minimal
since seahorses resumed usual social interactions
within minutes of being returned to the water.
We measured seahorses with Vernier calipers.
Standard length was the sum of the head length
(from snout tip to mid-point on the opercular
ridge/cleithral ring) and body length (from midpoint on the opercular ridge to the tip of the
straightened tail: Lourie et al. 1999). Seahorse
mass varies greatly with reproductive state, and so
we did not use that measure in our analyses.
We conducted survey work while using SCUBA.
We made observations for 490 h between 13 January and 15 March. Most dives (80%) started
before 07:00 h, while 17% began between 07:05
and 11:00 h, and the remainder started at 15:00 h,
17:00 or 19:00 h. Dives lasted a mean (±SD) of
168 ± 71 min. The bias toward morning observations resulted from our concurrent research on
4
social interactions, which occurred mostly in the
early morning (Vincent & Sadler 1995). At the
beginning of a dive, we quickly located as many
seahorses as possible and recorded their reproductive state. We paused in our survey when there
was the prospect of social interaction between
seahorses and observed the outcome before
resuming our census. We conducted observations
by floating 1 m away. This did not appear to affect
the seahorses’ behaviour; for example, they often
mated in our presence. Most seahorses could
generally be located with prolonged searching. The
number of fixes per seahorse varied, however, because we first tried to find a defined subset of the
seahorses, only moving on to a larger group of
animals if time and social activity levels permitted.
Analysis
Home range size
We calculated home range sizes in Wildtrak 1.2, a
programme designed for non-parametric analysis
of radio tracking data (I.A. Todd, University of
Hertfordshire, U.K.), employing two methods as
recommended by Harris et al. (1990). We used the
minimum convex polygon (MCP) method that
calculates the home range as the area of the
polygon that connects the outermost fixes for each
animal. It has the advantage of robustness when
the number of fixes is low and is directly comparable among studies (Harris et al. 1990). However,
it provides no indication of use within the home
range and is strongly influenced by peripheral
fixes. We decided to include all fixes in the calculation of home ranges because: (1) we had no
biological justification for discarding fixes for most
animals, and (2) we did not wish to discard data
for the many animals with relatively few fixes. We
controlled for the effect of differing numbers of
fixes by including that variable in our analysis.
We also used the grid cell (GC) method, which
provides an easily interpretable representation of
habitat usage (Harris et al. 1990). This method
overlays a grid of squares on the animal’s range
and counts the number of squares in which at least
one fix was obtained. We used a grid of 1 m by
1 m, as this matched the resolution at which fixes
were recorded in the field. We treated all fixes as
independent. Temporal autocorrelation of fixes is
not a serious concern when using non-parametric
home range estimates (Harris et al. 1990). Moreover, there was little potential for autocorrelation
of fixes in our study because: (1) dives were always
at least one hour apart, which is sufficient for a
seahorse to traverse the entire seagrass bed, and (2)
60% of all fixes were the sole fix for that individual
on that day.
To examine temporal change in home range size
during the breeding season we compared home
range size during two arbitrary 21-day periods, the
length of gestation for individuals that did not
change partners between the two periods. The
periods were: (1) 13 January to 2 February, and (2)
24 February to 15 March. We also compared
home range sizes between two 21-day pregnancy
cycles that included: (1) 15 January, and (2) 1
March, the exact dates depending on the individuals’ pregnancy cycles. We examined the shift in
home range location from one 21-day period to the
next by: (1) calculating the distance between the
home range centres (the average of the x- and ycoordinates) for the two periods, and (2) calculating the percentage of the home range during the
first period that overlapped with the home range
during the second.
Seahorse standard length
Because seahorses grow continuously, we had to
standardise our length measurements, taken at
various times throughout the study period, to a
single date to make them comparable. We standardised the measurements to 15 January (the
mid-point of the entire field study) by fitting our
measurement data for the entire season to a von
Bertalanffy growth function, using Munro’s (1982)
method.
Statistical methods
We calculated seahorse density in each of the
seagrass beds as the number of animals found in
the seagrass bed divided by the area of the bed. We
then calculated the mean density for the focal
study period as the mean daily density.
We used a general linear modelling (GLM) approach to investigate the influence of: (1) sex, (2)
seagrass bed (North or South), (3) number of fixes,
(4) seahorse standard length, and (5) time period
on home range parameters. We included all seahorses in the analysis unless otherwise noted. We
5
treated the number of fixes as a covariate, in order
to compare home ranges across sexes and beds
while controlling for variation in the number of
fixes. The response variables and continuous predictor variables were normally distributed unless
stated otherwise, in which case the data were BoxCox transformed (Sokal & Rohlf 1981). Home
range parameters were then modelled in a GLM in
Systat (SPSS Inc., Chicago, U.S.A.).
We constructed a full model that included all
two-way interaction terms and then simplified it
using a backward stepping procedure as follows.
We removed the non-significant interaction with
the highest p-value, and then re-ran the model. This
procedure was repeated, with one interaction removed for each run, until all remaining interactions
were significant at a ¼ 0.05. We then removed nonsignificant main effects in the same way until all
terms remaining in the model were significant. We
report all main effects but interactions are only
reported if they are significant. All results are presented as X ± SE, unless otherwise noted.
Seahorses generally remained faithful to the site
throughout the 63-day focal study period before
moving offshore at the end of the southern summer. The mean (±SD) tenancy during this period
was 51 ± 18 days (n ¼ 80) and 62% of seahorses
were present for at least the entire study period;
most were also present before the study started
and many stayed after it ended (Figure 2; see
Vincent & Sadler 1995). From early March, previously resident seahorses were no longer re-sighted, probably because migration from the study
site started at this time. The date on which a seahorse was last seen was independent of sex (GLM:
p ¼ 0.506),
seagrass
bed
F1,95 ¼ 0.48,
(F1,96 ¼ 1.30, p ¼ 0.258) and seahorse standard
length (F1,97 ¼ 2.01, p ¼ 0.159). Within pairs,
neither sex showed a tendency to leave the site first
(15 male vs. 11 female first departures respectively,
with both members of two pairs leaving simultaneously; paired t-test on departure day:
t27 ¼ )0.53, p ¼ 0.604).
Home ranges
Results
Population description
Hippocampus whitei seahorses clearly occupied
areas of P. australis and Z. capricorni rather than
shorter seagrasses or bare areas, so much so that
we were rapidly able to define our focal seahorse
study area by the extent of P. australis. We continued to search for seahorses in the sparse
Z. capricorni and H. ovalis throughout the study,
with little success.
The population density on the North seagrass
bed was more than double that on South during
the entire focal study period. When the area
available was calculated as the whole seagrass bed,
mean seahorse densities during the focal study
period were 0.215 m)2 on North and 0.080 m)2 on
South. When the area available was calculated as
that delimited by seahorse occupancy at any point
during the study, mean densities were 0.215 m)2
on North and 0.088 m)2 on South (see Figure 2).
Given that only one tagged seahorse was ever
sighted on both seagrass beds, we treated the two
beds as independent and compared home ranges
on the two seagrass beds.
Our analysis revealed that each male or female
seahorse maintained a small home range in which
it remained for many months during the reproductive season (Figure 3, Table 1). Our calculations provide an index of the size and location of
these home ranges.
We used linear regression to examine the correlation of home range size between partners. We
excluded seahorses that switched partners from
this analysis. We found no relationship (regression
on Box-Cox transformed MCP: F1,30 ¼ 1.53,
p ¼ 0.226, r2 ¼ 0.048) and have thus treated the
sizes of female and male home ranges as independent in all further analyses.
Home range sizes and locations were relatively
constant over time within the focal study period,
from 13 January to 15 March. Neither GC nor
MCP home range sizes differed between the first
and second 21-day assessment periods (GLM
(GC): F1,111 ¼ 0.045, p ¼ 0.832; GLM with BoxCox transformation (MCP): F1,111 ¼ 0.064,
p ¼ 0.800). The home range locations shifted
minimally between the two 21-day periods. The
average position of a seahorse moved a mean
(±SD) of 2.2 ± 1.5 m between the two periods
(n ¼ 59). A mean (±SD) of 49 ± 24% of a
6
Figure 3. Maps of the MCP home ranges of seahorses. Different hatching patterns serve to distinguish the different home ranges and
have no other meaning. The three labelled male home ranges on North are anomalous home ranges (see Figure 4).
Table 1. Descriptive statistics of seahorse home ranges during the focal study period (13 January to 15 March) regardless of pair
status. Only animals for which we had more than 25 fixes are included.
Sex
MCP home range
Female
Male
GC home range
Female
Male
a
Seagrass bed
X ± SE (m2)
Mediana (m2)
Range (m2)
Number of seahorses
Both sites
North
South
Both sites
North
South
22.0
13.4
43.3
7.8
9.4
5.2
±
±
±
±
±
±
6.4
2.4
13.5
1.6
2.3
1.5
13.8
10.5
33.0
6.0
6.5
3.8
3.5–37.5
6.5–37.5
3.5–124
1–39.5
2.5–39.5
1–16
22
13
9
26
16
10
Both sites
North
South
Both sites
North
South
14.4
14.0
14.9
9.0
10.1
7.2
±
±
±
±
±
±
0.8
1.0
1.5
0.6
0.7
1.0
13.5
13.0
16.0
9.0
9.5
6.5
6–21
9–21
6–20
4–17
7–17
4–15
22
13
9
26
16
10
The median is a better indicator of central tendency than the mean in these untransformed data.
seahorse’s home range during the second period
overlapped with the home range during the first
period (n ¼ 56). Neither GC nor MCP home range
sizes differed between the two pregnancy periods
we analysed (GLM (GC): F1,55 ¼ 0.02, p ¼ 0.900;
GLM (MCP): F1,56 = 0.009, p ¼ 0.925).
We examined MCP home range size as a function of: (1) number of fixes, (2) sex, (3) seagrass
bed, and (4) seahorse length for all animals for
which we had at least five fixes. MCP home range
sizes were uncorrelated with number of fixes
(F1,75 ¼ 0.87, p ¼ 0.880), so we removed this co-
variate from the analysis. Females’ MCP home
ranges were larger than males’ (GLM with BoxCox transformation: F1,78 ¼ 12.7, p ¼ 0.001,
r2 ¼ 0.140), but MCP home range sizes did not
differ between seagrass beds (F1,77 ¼ 0.252,
p ¼ 0.617). MCP home range sizes were uncorrelated with seahorse length (F1,74 ¼ 0.13,
p ¼ 0.724).
We examined GC home range size as a function
of: (1) number of fixes, (2) sex, (3) seagrass bed,
and (4) seahorse length for all animals for which
we had at least 25 fixes (Table 1). GC home range
7
Figure 4. Maps of three anomalous MCP home ranges. Male 11 had two home ranges, each with a different partner. Male 57 ranged
widely early in the study before settling into a home range. Male 61 kept a small home range, but went on a single foray 10 m away on
February 7. The full home range is shown in Figure 3.
sizes increased with the number of fixes in all cases
(GLM with Box-Cox transformation: F1,74 ¼ 34.3,
p < 0.001, r2 ¼ 0.480), but this increase was
greater for females than for males (F1,74 ¼ 4.96,
p ¼ 0.029), confounding the interpretation of sex
and site differences. We therefore removed number
of fixes from the model and used a two-factor
ANOVA. This revealed that female GC home
ranges were larger than those of males (ANOVA
with Box-Cox transformation: F1,78 ¼ 12.7,
p ¼ 0.001, r2 ¼ 0.14), with no difference between
the two seagrass beds (F1,77 ¼ 1.91, p ¼ 0.171).
This difference between sexes is probably underestimated in our results, because the means we
report include a number of animals with few fixes,
and because we had more fixes for males than for
females (t-test with Box-Cox transformation:
t78 ¼ 1.98, p ¼ 0.051). An additional analysis,
conducted only on the 50 animals for which we
had 25 or more fixes, revealed yet larger female
GC home ranges than in the full analysis (Table 1), but similar male GC home ranges. GC
home range size was uncorrelated with seahorse
length (GLM: F1,74 ¼ 0.01, p ¼ 0.938).
Our estimates of home range exaggerated the
routine daily movements of at least three males
(Figures 3 and 4), all of which we retained in the
analysis. For two of them, a change in partners (as
a result of the female’s disappearance) greatly affected the total area in which they were found
during the focal period (13 January to 15 March).
In both cases, our analysis treated the home range
as the entire area the males occupied during the 63
focal days, ignoring the shifts of location. The third
male had a very small home range, but he made one
foray to a location 10 m away, thus greatly
increasing his apparent home range; he also
rarely went to this spot before and after the focal
period.
Discussion
As expected, H. whitei of both sexes and in both
seagrass beds maintained very small home ranges
throughout the 63-day focal study period. Observations before and after this period (Vincent &
Sadler 1995) confirmed that many individuals
maintained these home ranges for at least 150 days
of the reproductive season. All home ranges were
very close to one another, minimising any ecological differences. Similar patterns were found in
a brief follow-up study in 1993 (A.C.J. Vincent
unpublished data). At the end of the breeding
season, however, it appears that much of the
population migrated offshore.
Hippocampus whitei presumably maintain small
home ranges because the costs of movement are
greater than the benefits in terms of survival,
growth and reproduction. Certainly, the costs of
movement over a home range may be greater in
H. whitei than in many other fish species because
H. whitei lack a stream-lined body design. In
addition, movement might compromise site specific camouflage – H. whitei often change colour to
match their immediate surroundings – and draw
the attention of both predators and potential prey
(for a review, see Foster & Vincent 2004). We did
not observe any predation events on the urban
habitat of the study site, although (among the
few possible predators of seahorses) flatheads
were common and the small number crabs, rays,
and anglerfishes might also have posed a risk
(A.C.J. Vincent pers. obs.). In turn, individual
8
H. whitei were seen to use only ambush capture to
prey on mysid swarms, juvenile fishes, and a wide
variety of other benthic, epibenthic and planktonic
fauna on the site. Given the aforementioned costs
of movement, these faithfully monogamous seahorses may accrue yet further benefits from
remaining near the pair’s daily greeting location,
especially as they are not involved in mate
searching once paired.
The small home ranges in H. whitei initially are
arguably more similar to those of monogamous
syngnathids than to their polygamous relatives.
Other monogamous syngnathid species in the
genera Hippocampus (Dauwe 1992, Jones & Avise
2001, Perante et al. 2002: for a review, see Foster &
Vincent 2004) and Corythoichthys (Gronell 1984,
Paulus 1991, Matsumoto & Yanagisawa 2001)
certainly hold small home ranges. The same is
true, however, for species in Australian macroalgal
clumps, Hippocampus breviceps, and in Portuguese
seagrasses, Hippocampus guttulatus, that are not
clearly monogamous (Moreau & Vincent 2004,
J. Curtis unpublished data). In contrast, five
polygamous species (in the genera Entelurus,
Nerophis, and Syngnathus) exhibit no apparent site
fidelity in Swedish seagrass beds (Vincent et al.
1995). Similarly, a seahorse species that is at least
socially polygamous, Hippocampus abdominalis,
ranges hundreds of metres in the course of a day
(K. Martin-Smith & A.C.J. Vincent unpublished
data). Unfortunately, the data on syngnathids are
still too few to allow analytical comparisons of
home range behaviour across species of different
body sizes, habitats, and mating patterns (Foster
& Vincent 2004), but this seems a promising area
of research.
Using published data we compared H. whitei
home ranges with those of a wide range of fish
species from different habitats, whilst controlling
for body size (Figure 5). We caution that
the regressions presented (Figure 5) do not control for phylogeny. However, it is clear that H.
whitei are among the most sedentary of vertebrates. Their home ranges are much smaller than
those of all other similar sized vertebrates, except
coral reef fish, which have similar sized home
ranges.
In our analysis, we were struck by the dearth of
quantitative reports on home ranges in seagrasses,
especially for species where adults occupy sea-
Figure 5. Home ranges of H. whitei compared to those of
vertebrates in other taxa. Points with error bars show X ± SE
H. whitei home ranges from this study; females are the upper
point, males the lower. All lines (except that for coral reef
fishes) are regressions reported by authors; we calculated the
regression for coral reef fishes ourselves. None of the regressions control for phylogeny. There was no significant difference
between home ranges (after controlling for body weight) in our
two sources for coral reef fishes (ANCOVA: F1,31 ¼ 2.93,
p ¼ 0.097), so we present one regression for the combined data
set. Sources: birds ¼ Schoener (1968); mammals (M) ¼ McNab
(1963);
lizards ¼ Turner
et
al.
(1969);
mammals
(H&B) ¼ Harestad & Bunnell (1979); lake fishes and river
fishes ¼ Minns (1995); coral reef fishes ¼ Sale (1978), Kramer
& Chapman (1999). Kramer & Chapman (1999) reported only
lengths for fishes in their literature survey, so weights were
calculated from length–weight relationships for each species (or
the median of such relationships for congenerics) as reported in
FishBase (Froese & Pauly 2002).
grasses as a primary habitat; we sought such data
with extensive queries through Aquatic Sciences
And Fisheries Abstracts,1 BIOSIS,2 and FishBase.3 As more information for other such species
becomes available, comparative studies investigating the association of home range size with
body form, swimming speed, predator avoidance
mechanisms, feeding mechanisms, and mating
pattern would be useful.
1
2
3
www.fao.org/fi/asfa/asfa.asp.
www.biosis.org.
www.fishbase.org.
9
Sex differences in home ranges
Lack of territorial defence
As predicted, female H. whitei had larger home
ranges than males. Our data almost certainly
underestimated the magnitude of the sex difference in home range sizes for two reasons. First,
we estimated home ranges over an arbitrary
period of 63 days rather than over the duration
of a pairing. This created the potential for exaggerating male home ranges in the few cases where
they formed new pairs, because widowed males
moved in search of mates, whereas widowed females did not (Vincent & Sadler 1995). Second,
we primarily recorded locations early in the
morning when females were approaching males at
the greeting location (Vincent & Sadler 1995),
thus potentially estimating female home ranges to
be smaller than if we had tracked them
throughout their daily movements. Frequent
observations throughout the day, while we were
engaged in other activities and thus not recording
locations, lead us to infer that females’ home
range would have been considerably larger if
locations of all sightings had been recorded, while
the same is not true for males (A.C.J. Vincent
pers. obs.).
The smaller home range of male H. whitei
matches the pattern of sex differences found in
many other seahorses and pipefishes (Gronell
1984, Dauwe 1992, Vincent et al. 1994, 1995,
Matsumoto & Yanagisawa 2001, Moreau and
Vincent, 2004). One probable explanation is that
energetic and physical constraints associated with
brooding embryos in a large pouch – with consequent increased body mass and drag – favour reduced male movement. This conclusion is
supported by the observation that male H. whitei
that lost their partner waited until they gave birth
before moving to locate a new mate (Vincent &
Sadler 1995). Although the reasons are uncertain,
male seahorses have elevated metabolic rates during pregnancy (Masonjones 2001). It should,
however, be noted that H. guttulatus and H. hippocampus showed no apparent sex differences in
home ranges in seagrass habitats in Portugal
(J. Curtis unpublished data). We were unable to
examine empirically the hypothesis that pregnancy
limits male home range size, because high rates of
male pregnancy meant we had very few sightings
of non-pregnant males in our study.
The results of this study, combined with previous
work, indicate that H. whitei do not defend their
home ranges as territories. First, the substantial
overlap that we found in home ranges – and in the
smaller areas of core use (unpublished data) – argues a lack of territoriality, as territorial animals
are usually spatially segregated (Grant 1997).
Second, previous research on H. whitei revealed no
overt defence of home ranges, through display or
aggression, even though visual displays are clearly
used in social communication by this species
(Vincent & Sadler 1995). Third, the lack of a
relationship between home range size and seahorse
size in this study suggests that home ranges may
not be defended, since size often confers competitive advantage in territoriality (e.g., Neat et al.
1998). Fourth, the similarity in the home range size
of H. whitei on our two sites (despite a more than
twofold difference in seahorse density) argues
against territoriality, because territory size is usually related to the density of competitors in the
surrounding areas (Sale 1975, Nursall 1977, Larson 1980, Norman & Jones 1984, Tricas 1989).
Hippocampus whitei, like some other syngnathids
studied to date (Gronell 1984, Dauwe 1992, Nijhoff
1993, Vincent et al. 1995, Matsumoto & Yanagisawa 2001), appear to provide an exception to the
general pattern that monogamous and egg-brooding fishes are territorial (Barlow 1984, Thresher
1984). The most common motivations for territoriality – the need to guard food, mates, spawning
sites, or offspring (Davies & Houston 1984, Carpenter 1987, Grant 1997) – may not apply to
H. whitei. It is unlikely to be economically viable
for seahorses to defend their widely dispersed and
unpredictable prey (plankton, benthic organisms
and small fish) (Thom et al. 1995, Walsh & Mitchell
1998). Mate guarding is probably unnecessary because: (1) the potential reproductive rates of the
two sexes may be very nearly equal, as in Hippocampus fuscus (Vincent 1994), (2) seahorses can be
certain of maternity and paternity (Vincent & Sadler 1995, Jones & Avise 2001), and (3) extra-pair
matings in H. whitei are most unlikely (Vincent &
Sadler 1995). A finding that Hippocampus subelongatus males sometimes switched mates between
pregnancies (Kvarnemo et al. 2000) did not establish whether both partners were still available.
10
Male pregnancy means that seahorses do not need
to defend their oviposition sites or the embryos
they are brooding, other than by ensuring their
own survival. Other obligate slow-swimming fishes,
such as butterflyfishes (family Chaetodontidae)
and trunkfishes (family Ostraciidae), also function
without territoriality (Itzkowitz 1974).
use. The results were inconclusive on the small
spatial scale under consideration (a total of around
550 m2, including both beds) and with the available data, but a more thorough investigation of
spacing behaviour in seahorses would be valuable.
Acknowledgements
Conservation implications
This study of H. whitei can assist in developing
conservation action for depleted populations, by
providing a baseline against which other species
can be evaluated. Many seahorse species, although
not H. whitei, are threatened by overexploitation
in target fisheries and indiscriminate capture in
non-selective fishing gear, especially trawls (Vincent 1996). In heavily fished species it can be very
difficult to find unexploited populations in which
to study basic biological parameters, so we are
forced to rely on studies of close relatives.
The spatial behaviour H. whitei, and of other
Hippocampus species (for review, see Foster &
Vincent 2004), suggests that seahorses could be
very vulnerable to overfishing. Their small home
ranges and lack of movement between seagrass
beds implies that recolonisation of over fished
areas by adults could be slow. Moreover, the lack
of territoriality suggests that all seahorses will be
resident on the seagrass bed (with no reservoir of
transient animals excluded from the site), and thus
vulnerable to fishing in this habitat. On the other
hand, the small and overlapping home ranges
suggest that quite small no-take areas might suffice
to protect a viable population of seahorses (Kramer & Chapman 1999). In addition, the seahorses’
small home ranges and lack of aggression among
neighbours suggests that these fish might survive
and breed in captivity. Investigations of dispersal
by young and by adult seahorses (perhaps in seasonal migrations) would greatly assist conservation decision-making.
Given the importance of habitat loss in managing and conserving wild populations, it clearly
behoves us to learn more about the environmental
variables that might influence spatial movement in
H. whitei and other seahorses. We did originally
examine the relationship between home range size
and (1) position in the seagrass bed (relative to
edges, and to the seaward side), and (2) habitat
This is a contribution from Project Seahorse. We
particularly thank Laila Sadler, Alison Phillips and
Cathy King for their wonderful assistance with the
fieldwork. For advice and suggestions on statistical
analysis and content of the manuscript, we are
most grateful to Jonathan Anticamara, Shaun
Goho, Don Kramer, Keith Martin-Smith, Jessica
Meeuwig, Laila Sadler, Melita Samoilys, and the
helpful anonymous referees. Our great thanks to
the Sydney Harbour pilots and pilot cutter crews.
This work was carried out while AV was a Visiting Scholar in the Department of Zoology, University of Sydney. Financial support to AV was
provided by The Royal Society (Overseas Research
Grant and the John Murray Travelling Studentship) and National Geographic magazine. The
1989 pilot study was financed by the Johnstone and
Florence Stoney Studentship from the British
Federation of University Women and a grant from
the Lerner-Grey Fund, American Museum of
Natural History (both to AV). AV was supported
by the Ernest Cook Research Fellowship, Somerville College, Oxford during the field study and
DM was supported by a generous gift from Guylian Chocolates Belgium during the analysis.
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