Anim. Behav., 1993,45,47-53

Nest placement by loggerhead turtles, Caretta caretta

GRAEME C. HA YS & JOHN R. SPEAKMANDepartment of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen AB9 2TN, U.K.

(Received 12 September 1991; initial acceptance 13 November 1991;final acceptance 15 May 1992; MS. number: 3931)

Abstract. Nest placement by loggerhead turtles nesting on the island of Cephalonia, Greece, wasexamined. The spatial distribution of nests was not uniform (N=27 nests): they tended to be laid awayfrom the sea, but not in vegetation that backed the beach. Hatchling success (recorded for eight nests)increased significantly for nests laid further from the sea. Hence most nests were laid in positions wherehatchling success was high. Six individuals were observed to nest more than once. Each of these multiplenesters showed little consistency in the distance that they nested from the sea (i.e. within-individualvariation was high), and hence no significant differences in nest positioning between individuals weredetected ..Digging attempts in the vegetation behind the beach were frequently aborted (71 % of attempts),but attempts in the open sand between the vegetation and the sea were aborted less often (11 % ofattempts). Sand temperatures at the depths where eggs incubated were higher further from the sea. Theimplications of nest placement for hatchling sex ratio are discussed.

For animals that lay their eggs in a nest, the selec-tion of a nest site may strongly influence offspringsurvival and therefore have important conse-quences for the reproductive fitness of the adult(Martin 1988). For sea turtles the survival of theoffspring may be strongly related to the distance thatthe nest is laid from the sea and from supra-littoralvegetation behind the beach (Mrosovsky 1983).Inundation of nests by sea water leads to egg mor-tality from suffocation (Whitmore & Dutton 1985)and/or chloride toxicity (Bustard & Greenham1968). The survival of eggs to hatching may there-fore be higher in nests laid further from the seawhere inundation is less likely. Tides in the easternMediterranean are negligible (Couper 1983) andhence inundation of nests may be less ora threatthere than in other parts of the world. However,nests may potentially still be indundated as a resultof storm surges. Counterbalancing this selectivepressure favouring nests laid further from the sea ithas been suggested that hatchlings emerging fromnests sited further inland may take longer to reachthe sea or be unable to find the sea at all (Mrosovsky1983), and may therefore have a higher risk of dyingof desiccation, heat stress or predation on the beach.In addition, nests excavated amongst supra-littoralvegetation may suffer high egg mortality as a resultof roots invading the egg chamber (Witherington1986).

0003-3472/93/010047 +07 $08.00/0

Nest placement may also have consequences forthe sex of sea turtle hatchlings, since their sex isdetermined by the incubation temperature (reviewsin Raynaud & Pieau 1985; Janzen & Paukstis 1991)which in turn varies with nest location on a beach(Morreale et al. 1982; Mrosovsky & Provancha1989). There is concern therefore that predictedrises in global temperatures may induce highlyskewed sex ratios if turtles continue to nest in theirpresent locations (Davenport 1989).

From both a life history and a conservation stand-point there is therefore considerable interest in nestplacement by sea turtles. In this paper we examinethis process for loggerhead turtles, Caretta caretta,on the Mediterranean island of Cephalonia, Greece(38°09'N,20033'E).

METHODS

Every night between 1 June and 31 August 1990,Potamakia beach, Cephalonia, was patrolled onfoot to locate nesting loggerhead turtles. Wedefined the date during the season as the days since1June. Potamakia beach is backed by supra-littoralvegetation consisting mostly of thorny bushes,Eryngium maritimum, approximately O·5 m inheight. We termed the border between these bushesand the clear sand the 'vegetation line'.

© 1993 The Association for the Study of Animal Behaviour

47

48 Animal Behaviour, 45, J

When a turtle had finished laying, her curve cara-pace length was measured from the anterior of theprecentral scute at the carapace mid-line to theposterior of the post-centrals, using a l-m flexibletape measure. She was also tagged on each rearflipper with a numbered plastic livestock tag('Jumbo' tag, Dalton Supplies Ltd.), or identified ifpreviously marked. Once the turtle had returned tothe sea we measured the distance of the nest fromthe water's edge (N =27 nests) and also the distancefrom the vegetation line (N = 22 of these 27 nests)using a 30-m flexible tape measure. We were unableto discern any tidal variation in sea height which isconsistent with the very limited tidal fluctuationsreported for the eastern Mediterranean (Couper1983). Phase of the moon was recorded each nighton a scale from 0 to I, where O=new moon, andI = full moon.

We counted the number of eggs in each nest bycarefully excavating the nest as soon as the turtle hadreturned to the sea, and we then carefully reburiedthe eggs in the original eggchamber. This procedurewas unlikely to affect the survival of the eggsadversely since in previous studies careful exca-vation and handling of sea turtle eggs shortly (lessthan 3 h) after they had been laid did not induce eggmortality (pannenter 1980;Harry & Limpus 1989),presumably because extra-embryonic membranesthat are critical for successful development begin toenclose the embryo only after this time (Blanck &Sawyer 1981). Furthermore no nests were subse-quently excavated by predators and the meanhatchling success (the number of hatchlings thatemerged divided by the number of eggs laid for eachnest, multiplied by 100; Dodd 1988) that werecorded (N = 8 nests, mean = 71'6%) was withinthe previously reported range for loggerhead turtles(53'1-83'8%; Dodd 1988).

The minimum incubation tim'e previouslyreported forloggerheads is49days(Dodd 1988).Wetherefore began to monitor hatchling emergence 45days after each nest was laid. We recorded howmany hatchlings emerged at night either by catch-ing hatchlings under large (approximately I mdiameter) inverted plastic bowls placed over the nestsites, or by using wire mesh to funnel the hatchlingsasthey crawled towards the sea so that they fell into,and were trapped in, a bowl sunk in the sand. Bowlswere checked at regular (maximum 3-h) intervalsthroughout the night. When hatchlings were foundunder the bowls, they were counted and thenreleased together at the nest site. Prior to release all

lights were switched off so that the hatchlings werenot artificially disorientated (Mrosovsky 1978). Bydirect observation we then recorded the number ofhatchlings that successfully reached the sea. (Inpractice all the hatchings that emerged at nightsuccessfully reached the sea.)

Fewer than 20 hatchlings per- nest emergedduring the day. We monitored how many did so bycounting the tracks leading from the nests. Trackswere counted at approximately 1200 and 2100hours. The number of day-time emerging hatch-lings that died on the beach was recorded by count-ing the number of tracks that did not reach the sea.In all such cases (N = 3 hatchlings), we found adesiccated dead hatchling at the end of the track.We attempted to monitor hatchling success for 11nests ranging from 9 to 30 m from the sea, i.e. for asample that spanned most of the total range of nest-ing distances from the sea. However, for three ofthese nests we failed to record hatchling successeither because hatchling traps were stolen duringthe night or because hatchlings emerged outside thetraps.

Using thermistor probes linked to a data logger(Grant Instruments) we recorded the sand tempera-ture at 30, 40 and 50 cm (the approximate depth atwhich incubating eggs are located) adjacent (< I m)to the 11 nests for which we attempted to monitorhatchling success.We recorded sand temperature onthree occasions: between 1950and 2130 hours on 4August; between 1440and 1700hours on 5August;and between 0125 and 0315 hours on 6 August. Tomake the temperature measurements we dug a pitdown to the specified depth and inserted fourthermistor probes about 6 cm horizontally intoundisturbed sand on the sidewall ofthe pit. We thenimmediately recorded the sand temperature, so thatthe readings would not be influenced by the ambientair temperature in the pit. We never re-dug the samearea of sand twice. Sand temperature was indepen-dent of the order in which the 11 sites were sampled(F,.9<0·94, P>0·05). We therefore grouped thedata from the three sampling occasions to give amean temperature for each depth adjacent to eachnest.

RESULTS

The turtles nested on average 22·6 m from thewater's edge (N=27, range=9-34m, sE=I·24).The distribution of nests from the sea (Fig. 1) dif-fered significantly (G6= 16,8, P<O·OI) from that

Hays & Speakman: Nest placement by turtles 49

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Figure 1. The observed distribution of nests at differentdistances from the sea. Distances are the mid-points foreach 5-m-wide class interval.

expected for a uniform distribution between 0 and34 m (the maximum distance that a nest was laidfrom the sea). There were significantly (G, = 11·6,P <0·0 I) more nests laid between 20 and 34m fromthe sea than there were between 0 and 14m. Thedistance that a nest was laid from the sea was notsignificantly related to the curve carapace length ofthe female, the number of eggs she laid, the dateduring the season or the phase of the moon on thatnight (F,,2S <:;; 0·9, P>0'05), or any combination ofthese factors (stepwise regression P> 0'05) butincreased significantly as the distance from the seato the vegetation line increased (Fig. 2). However,the distance that a nest was laid from the vegetationline was independent of how far the vegetation linewas from the sea (F,,20=2'9, P>0·05).

Six turtles were observed nesting on the beachmore than once. Each multiple nester showed littleconsistency in the distance that she nested from thesea for each of her nests (i.e. within-individual vari-ation was high; Fig. 3), and hence no significantdifference in nest positioning between individualscould be detected (ANOVA, FS,'2=2'3, P> 0'05).

Hatchling success averaged 71·6% (N = 8, SE=4,75, range=43'7-89'9%), and was significantlyhigher for nests laid further from the sea (Fig. 4).Thus for a nest laid 15m from the sea, mean hatch-ling success was 53·0% (95% CI=44·7-61·3%),compared with 87·6% (95% CI=80'1-95'1 %) at30m. However, no nests were ever inundated by thesea.

On average, 99·5% of hatchlings emerging froma nest reached the sea (N=8, range = 98'4-100%,

32

SE=0·2); the percentage was not related to the dis-tance of the nest from the sea (F',6 = 0'2, P> 0,05).No nests monitored for hatchling emergence werelocated beyond the vegetation line.

Sand temperature between 30 and 50 cm wasgreater further from the sea (Fig. 5).For example, at50 cm the mean temperature difference between 10and 30m from the sea was + 1'2°C (26·1-27·3°C) .

Digging attempts beyond the vegetation linewereaborted significantly (G, = 9,1, P< 0·01)more oftenthan attempts in the open sand in front of the veg-etation line (Fig. 6). Of all digging attempts 71%were aborted in the vegetation zone, but only I1%were aborted in the open sand.

DISCUSSION

Bacon (1970) and Eckert (1987) have previouslyreported for leatherback turtles, Dermochelyscoriacea, that the distance the nest was laid from thesea was not influenced by the size of the female. Wefound that this was also the case for loggerheads,and additionally that the number of eggs laid, thedate and the phases of the moon did not affect howfar the nest was laid from the sea. However, nestswere not deposited uniformly. Instead they tendedto be laid away from the sea (Fig. 1), and close to,but not beyond, the vegetation line (Fig. 2). Theincrease in the distance that turtles laid from the seawhen the vegetation line was further from the water(Fig. 2) suggests that the vegetation may have con-strained the length of the turtle's inland crawl.Although the mechanism of nest site selection isvery poorly understood for loggerheads (Dodd1988),it has been tentatively suggested that thermalcues in the sand may initiate digging (Stoneburner& Richardson 1981). A possible role of vegetationhas not been considered before.

The lower hatchling success for nests nearer thesea (Fig. 4) was not caused by sea-water inundation,since no nests were inundated. Hatchling success isinfluenced by several abiotic factors including tem-perature (Mrosovsky et al. 1984), oxygen levels(Ackerman 1980), chloride levels (Bustard &Greenham 1968) and moisture content (McGehee1990; Mortimer 1990) in the nest. Sand tempera-ture, the only one of these parameters measured,varied with distance from the sea (Fig. 5). Clearly,however, temperature alone may not have beenfully responsible for the observed variation inhatch ling success, since it may have eovaried with

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Figure 2. The relationship between the distance that nests were laid from the sea and the distance from the sea to thevegetation line. Distance of nest from the sea (m)=0·66 (distance from vegetation line to the sea)+4·28 (r' =O·35, F,lO =11'0, P < 0·0 I). The line representing distance from nest to sea=distance from vegetation to sea (i.e. the line representingnests laid at the vegetation line) is shown.

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Figure 3. The distance that nests were laid from the sea (m) by individuals observed laying more than once.

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Figure 4. Hatchling success at different distances from the sea. Hatchling success (%) = liS log (distance from the sea(m»-82'1 (1"=0'88, F, .s=42·3, P<O·OI).

Hays & Speakman: Nest placement by turtles 51

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Figure 5. The mean sand temperature at 50 cm (T ,"om) adjacent to nests, plotted against the distance that the nest was laidfrom the sea (dmetres). TSOom=0·058d+25·5 (F1.9=26·0, ",=0·71, P<O·OI). At 30cm depth: T30om=0'044d+26'3(F1.,= 16'0, ,.'=0'60, P<O·OI). At 40 cm depth: T4l1om=0'055 d+25'8 (F1.9 = 23'1, ",=0·69, P<O·OI).

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Figure 6. The number of aborted (.) and successful (0) attempts at nest excavation in the vegetation zone and in opensand in front of the vegetation zone.

other factors such as oxygen levels, chloride levelsor water levels.

Maximizing hatchling survival (Fig. 4) clearlyprovided a selective advantage for females to layaway from the sea. There may, however, have'beencounter-balancing selective pressures that limitedthe inland crawl of the turtles (Figs 1 and 2).Loggerhead nests laid amongst vegetation maysuffer poor hatchling success (Witherington 1986)and in addition hatchlings emerging in vegetationmay be less likely to reach the sea (Mrosovsky1983). However, we did not monitor hatchlingemergence from any nests in the vegetation zone,and so were unable to test these hypotheses. All thehatchlings that wemonitored emerged in open sandoutside the vegetation zone, and those emergingfurther inland were not less likely to reach the sea.

However, one disadvantage that was identified forfemales crawling beyond the vegetation line wasthat their digging attempts tended to be aborted(Fig. 6), presumably because roots or sand textureimpeded successful digging.

For leatherback turtles nesting in the Guianas andin the V.S. Virgin Islands, it has been suggested thatthere may be no pattern to nest placement becausethere is no predictable pattern to hatchling survivalat different distances from the water (Mrosovsky1983; Eckert 1987). In contrast, Horrocks & Scott(1991) found that for nests of the hawksbill turtle,Eretmochelys imbricata, on Barbados, West Indies,hatchling survival varied with the elevation of thenest above sea level and that nests were clumped inthe region where hatchling survival was highest.Thus on beaches where there is a predictable

52 Animal Behaviour, 45, 1

pattern to hatchling survival in different nestlocations, turtles tend to nest in the area that maxi-mizes hatchlingsurvival and thus their reproductivefitness (this study; Horrocks & Scott 1991).

For loggerhead turtles, cooler incubation tem-peratures produce male hatchlings, an intermedi-ate ('pivotal') temperature produces a mixed sexratio and warmer temperatures produce females(Mrosovsky 1988). The pivotal temperature forloggerheads in the Mediterranean has not, how-ever, been investigated. We therefore do not feeljustified in attempting to calculate hatchling sexratios from our sand temperature data. Further-more, the sand temperatures we recorded (Fig. 5)do not equate to incubation temperatures, firstbecause the eggs themselves may cause localizedheating of the sand (Morreale et at. 1982), andsecond because there may be a seasonal change insand temperature (Mrosovsky & Provancha 1989).However, the temperatures we recorded will stillgive an indication of the relative difference in tem-perature that will occur between nests laid at differ-ent distances from the sea. Nests closer to the seawould tend to be cooler, and therefore produce agreater proportion of males. Manipulating beachconditions might therefore provide a means of miti-gating global temperature increases. For example,if the vegetation line was altered so that it wasnearer the sea, this might cause individuals to laycloser to the water (Fig. 2) and thus in cooler sand(Fig. 5), although this might also lower hatchlingsurvival (Fig. 4). If individuals were consistent inthe distance that they nested from the sea, thenthis might also provide a mechanism for mitigatingfuture environmental temperature rises. Forexample, a response to global warming might be theselection for individuals that nested closer to thewater and thus in cooler sand. However, we foundthat there was no detectable consistency in the dis-tance that each individual nested from the sea(Fig. 3) as has been found previously in leather backturtles (Eckert 1987).

ACKNOWLEDGMENTS

We thank all the volunteers who helped with fieldwork in Greece, particularly Simon Wakefield. Wealso thank Care For The Wild for co-ordinating theproject. GCH was supported by Care for the Wild,and an Aberdeen University studentship. We thankJack Hayes for statistical advice, Scott Eckert and

Nicholas Mrosovsky for stimulating discussionsconcerning this work, and John Davenport and ananonymous referee for commenting on an earlierdraft of the paper.

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