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). 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