Territorial behaviour of the red-backed salamander: Expulsion of intruders

Anim. Behav., 1982, 30, 490-496

TERRITORIAL

BEHAVIOUR OF THE RED-BACKED EXPULSION OF INTRUDERS

SALAMANDER:

BY ROBERT G. JAEGER*, DONNA KALVARSKY~ & NAOMI SHIMIZU~ *Department of Biology, University of Southwestern Louisiana, Lafayette, Louisiana 70504 ecDepartment of Biological Sciences, State University of New York at Albany, Albany, New York 12222 Abstract. Red-backed salamanders, Plethodon cinereus, were paired in laboratory chambers where they

defended areas against each other by threat display and biting. Difference in sizes of paired individuals was not correlated with which one would enter the other's area or be more aggressive after an intrusion. We also found no sexual differences in agonistic behaviour. Both defenders and intruders assumed a threat posture for about 50 % of the invasion periods, but defenders exhibited a higher rate of biting. Defenders expelled intruders in 74 % of all encounters, while intruders expelled defenders in 18 %, with 8 % of the encounters a draw. Intruders frequently left the contested area even in the absence of biting or threat display by the defender, apparently in response to the defender's pheromones or to the high potential cost of an escalated aggressive contest (damage to the chemosensory structures). These data provide behavioural evidence that a salamander can evict intruders from a defended area and thus support the hypothesis of territoriality among terrestrial salamanders. Although territoriality is known to occur within most vertebrate groups (fish, anurans, reptiles, birds and mammals), it has not been convincingly demonstrated in salamanders. Terrestrial salamanders of the genus Plethodon, and perhaps other genera of Plethodontidae, appear to be territorial, based on behavioural studies in laboratory chambers. However, no thorough studies have been performed in natural habitats of salamanders. The red-backed salamander, Plethodon cinereus, in particular has been extensively studied (Jaeger 1972, 1980; Gergits 1981). It occupies the forest-floor habitat of eastern North America, foraging during rainy periods in the leaf-litter where invertebrate prey are abundant and readily available. As the leaflitter dries between rains, the salamanders move to patches of moisture under rocks and logs, where prey are scarce. Jaeger et al. (1981) suggested that individuals establish territories under and around these cover objects in defence of a limited but reliable supply of food. Gergits (1981) stated that four conditions must be satisfied to conclude that territoriality exists. First, individuals must exhibit site tenacity. This has been shown to occur, in homing studies, for both the red-cheeked salamander, Plethodon jordani (Madison 1969), and P. cinereus (Samuelsen 1977), and for the latter species, in a mark-recapture study (Gergits 1981). Second, defence of the area must occur. Cupp (1980) showed that males of the green salamander, Aneides aeneus, will aggressively attack in-

truding conspecifics and force them to retreat within chambers. Thurow (1976) observed intraand interspecific aggression and, apparently, defence of small areas by 14 species of Plethodon. Gergits (1981) and Wrobel et al. (1980) suggested that P. cinereus is aggressively superior to the Shenandoah salamander, P. shenandoah, in laboratory chambers, which reflects the competitive dominance of the former species over the latter in natural habitats in Virginia (Jaeger 1972). Individuals of P. cinereus compete for sites by visual threat and submissive postures that occasionally escalate to biting (Jaeger 1981). However, 'dear enemy recognition' (Wilson 1975) reduces the probability of an escalated contest between salamanders that are familiar with each other's pheromones, as would be expected of territorial neighbours (Jaeger 1981). Third, the individual's presence in the area must be advertised. Observations of behaviour and morphology indicate that pheromones are commonly used by salamanders in sexual communication and that the nasolabial grooves of plethodontid salamanders function as chemosensory structures (Arnold 1976). The nasolabial grooves transmit chemical cues from the substrate to the nasal area when a salamander taps its snout to the substrate. Both intra- and interspecific communication via pheromones was demonstrated for P. cinereus and P. shenandoah by Jaeger & Gergits (1979), and within P. jordani by Madison (1975). These pheromones confer individual recognition among conspecitics, at least within P. einereus (Jaeger 1981; 490

JAEGER ET AL.: SALAMANDER TERRITORIALITY McGavin 1978), and are partially responsible for inter-individual spacing within and between P. cinereus and P. shenandoah (Jaeger & Gergits 1979). Despite the wealth of data on agonistic behaviour among salamanders, there has been no demonstration of Gergits' (1981) fourth condition of territoriality: that one individual can exclude a potential competitor from the defended site. Such data are a necessary test of the hypothesis of territoriality in Plethodon. Agonistic behaviour per se among individuals may be merely a manifestation of inter-individual spacing (mutual avoidance) while 'a defended area' is a general requisite of territoriality (Brown & Orians 1970). In the present experiment we test the fourth condition of territoriality: exclusion of competitors from a defended site. Individuals of P. cinereus were allowed to mark areas with pheromones in separate chambers for one month. Paired chambers were then placed contiguously and the two salamanders were allowed to interact across the interface. We tested several predictions derived from the hypothesis of territoriality: (1) salamanders would show less threat and submissive behaviour when alone than when together in a chamber; (2) the defending animal would perform more aggressive and less submissive behaviour than the intruder; and (3) the defender would exclude the intruder in most interactions. Methods

Plethodon cinereus were collected during autumn 1979 at Hawksbill Mountain, Shenandoah National Park, Virginia. Adults (N = 68, range = 35-41 mm snout-vent length) were randomly partitioned into pairs and each member of a pair was placed in a 3.8-1itre clear glass jar (25 cm long, 16 cm diameter) in late March, 1980. Each jar rested on its side and contained 3 cm of damp soil, which produced a substrate that was level with the mouth of the jar. Paired jars were placed mouth to mouth, but for one month a partition isolated the two salamanders so that they exchanged no physical, visual or olfactory communication. One month provided ample time for the salamanders to mark areas with pheromones (Jaeger & Gergits 1979; Jaeger 1981). The salamanders were kept at 20 C with a light cycle of 12L : 12D and illuminance of ___4 lx. They were fed three times weekly with the flies Drosophila melanogaster and D. virilis.

491

During April and May, 1980, 34 pairs of salamanders were tested for territorial behaviour by removing the partition between paired jars for 1 h. Interacting individuals were identified by unique dorsal pigmentation. Post mortem inspection of the gonads showed that there were 13 pairs of males, 9 pairs of females and 12 pairs of male-female. None of the females was in reproductive condition, which reduced the possibility of courtship in the behavioural parameters observed. We defined three behavioural states of a salamander. Resident: a salamander that was on its own marked area while the other salamander was likewise on its own marked area (which served as a control condition, since no physical interactions could occur). Intruder: a salamander that entered the area of the other animal. Defender: a salamander whose area was invaded. Within a 1-h test period, a salamander could alternate among the three states. Using a stopwatch, we measured the amount of time that a salamander spent in each behavioural state. We monitored, for each state, the following parameters: amount of time spent in a resting, threat or submissive posture, rate of nose-tapping, rate of biting, and the area of the opponent's body that was bitten. The postures were defined by Jaeger (1981). Generally, a salamander in a resting posture (also normally used while moving and feeding) has its front legs extended downward and the hind legs extended to the side, so that only the head and anterior aspect of the trunk are raised off the substrate. In a threat posture, all legs are extended downward so that all parts of the body, except perhaps the terminal half of the tail, are raised. In a submissive posture, the head, trunk and tail lie flat on the substrate with all legs extended outward. This posture is used primarily when escape from an attacking salamander is impossible (Jaeger 1981), and it effectively prevents foraging (Jaeger et al. 1981). Nose-tapping (Arnold 1976; Tristram 1977) occurs in plethodontid salamanders when the nasolabial cirri are touched to the substrate. Arnold (1976) suggested that tapping is the primary means of chemoreeeption in plethodontid salamanders and may be used in detecting pheromones. Non-parametric statistics were used to compare independent data sets (Mann-Whitney Utest, Kruskal-Wallis one-way analysis of variance by ranks test, Chi-square, binomial test, Spearman rank correlation) and related data sets (Wilcoxon matched-pairs signed-ranks test)

492

ANIMAL

BEHAVIOUR,

(Siegel 1956). We set a = 0.05 divided by the number of times that a data set was used in statistical tests. Results Intrusions occurred between six of the 13 pairs of males, three of the nine pairs of females and five of the 12 pairs of male-female. Within several pairs, multiple intrusions occurred either with one individual intruding several times or with mutual consecutive intrusions where individuals apparently 'chased' each other across the boundary between areas. S i z e and Agonistie Behaviour There were no significant correlations (Table I, A - D ) between difference in snout-vent length and percentage of time that salamanders spent intruding, regardless of sexual pairings. Once an intrusion had occurred, there also were no significant correlations between difference in size and percentage of intrusion time devoted to threat or submissive postures (Table IE, IF) or in rate of biting (Table IG). These data imply that larger salamanders were not prone to be more aggressive than smaller ones and that smaller individuals were not more submissive than larger ones. S e x and Agonistie Behaviour Male were not significantly different in levels of aggression and submission toward other males than they were toward non-gravid females

30,

2

(Table IIA). Also, non-gravid females did not behave differently toward other females than they did toward males (Table liB). When comparing male-male interactions with non-gravid female-female interactions, there still were no significant differences (Table IIC). We conclude that under our experimental conditions the salamanders did not alter their agonistic behaviour on the basis of sex. Behavioural State and Agonistic Behaviour Since body size and sex did not affect the amount of agonistic behaviour between individuals, we now pool all of the data and ask what behavioural differences occurred among and between residents, defenders and intruders. There was a significant difference in the percentage of time allocated to the resting posture among these three behavioural states (Table IliA). Using pair-wise Mano-Whitney U-tests to identify differences between states (Table IliA), defenders and intruders did not differ significantly, but both spent significantly less time than residents in a resting posture. The reason for this change becomes clear from an analysis of aggressive behaviour. The percentage of time devoted to the threat posture also differed significantly a m o n g states (Table IIIB). Again defenders and intruders did not differ significantly, but both used the threat posture significantly more than residents (Table IIIB). However, the percentage of time spent in the submissive posture did not significantly differ

Table I. Behavioural Interactions of Defender and Intruder Correlated with Difference in Body Size

Prediction* A. Larger3 intruded upon smaller d~ B. Larger~ intruded upon smaller ~ or larger ~ intruded upon smaller C. Larger ~ intruded upon smaller ~ D. Larger salamander intruded more E. Larger salamander threatened more F. Smaller salamander was more submissive G. Largersalamander bit more

N

r~

t

P

13 12

0.111 0.080

0.370 0.254

> 0.20 > 0.20

9 34 14

0.104 0.154 0.193

-0.882 0.681

> 0.10 > 0.20 > 0.20

14 14

0.123 0.181

0.429 0.638

> 0.20 > 0.20

*Rows A-F predict that the total percentages of time spent intruding, threatening or submitting were correlated with difference in size. Row G predicts that rate of biting (bites/min) was correlated with difference in size. N : Number of pairs. r s : Spearman rank correlation coefficient. t: Value of Student's t, used when N > 10. P: Two-sided probability, a = 0.05.

JAEGER ET AL.: SALAMANDER TERRITORIALITY

493

Table II. Behavioural Interactions of Defender and Intruder as a Function of Sex

Prediction*

N

A. 3 - 3 interactions were greater than c~-~ interactions ~ time intruding time threatening 700time submitting rate of biting

3-3

B.

~-~

~-$ interactions were greater than ? - 3 interactions ~ time intruding time threatening time submitting rate of biting

C. ~-~ interactions were greater than ?-~ interactions ~ time intruding ~o time threatening time submitting rate of biting

26 12 12 12

18 6 6 6

X(-4-1 sE)

X(zk 1 sE)

U

P

~-~ 11.3 55.2 8.6 0.16

(zk 0.6) (-4- 12.5) ( • 6.9) ( • 0.08)

12 5 5 5

12.3 i(4- 7.7) 150.0 20.0 ( + 12.3) 15.0 29.6 ( • 18.4) 12.0 0.02 (-4- 0.01) 25.5

0.22 :> 0.10 > 0.05 > 0.10

14.8 60.2 21,8 0.06

(zk 7.2) ( • 22.0) (:k 17.9) (:k 0.06)

95.5 12.5 15.0 11.0

0.50 > 0.66 > 0.50 0.54

9.4 38.8 28.2 0.14

(• (zk (• (~z

4.6) 219.0 19.6) 32.0 18.3) 29.0 0.07) 34.5

0.79 > 0.10 > 0.10 > 0.10

$-~ 9.4 38.8 28.2 0.14

(zk 4.6) (=tz 19.6) (-4- 18.3) (:k 0.07)

3'-~ 26 12 12 12

N

12 5 5 5 ~-~

11.3 ( + 0.6) 55.2 ( ~ 12.5) 8.6 (-4- 6.9) 0.16 ( • 0.08)

18 6 6 6

*Predictions are that (A) males were more aggressive toward males than toward females, 03) females were more aggressive toward females than toward males, (C) males were more aggressive toward males than females were toward females. N: Sample size of pairs being compared. X: Mean (:k 1 standard error) percentage of time or rate (per min) for each pair. U: Value of Mann-Whitney U-test. P: Two-sided probability, c~ = 0.025. a m o n g the three states (Table I I I C ) . T h e rate o f biting was significantly faster for defenders t h a n for intruders (Table I I I D ) , a n d the rate o f noset a p p i n g differed significantly a m o n g the three states ( T a b l e I I I E ) . This was caused b y b o t h defenders a n d intruders t a p p i n g faster t h a n residents, b u t they did n o t differ significantly f r o m each o t h e r (Table IIIE). Exclusion of Intruders W e n o w c o n s i d e r each i n t r u s i o n to be a separate event, a n d find t h a t some s a l a m a n d e r s never i n t r u d e d , some d i d so once a n d others int r u d e d several times within the 1-h o b s e r v a t i o n period. T h e r e were a t o t a l o f 50 intrusions summ e d over all 68 salamanders. T a b l e IV shows which s a l a m a n d e r (defender, intruder, b o t h o r neither) b i t d u r i n g an invasion a n d which (defender, intruder, b o t h o r neither) t h e n left the contested area. W h e n the defender bit the intruder, the latter left the a r e a signific a n t l y m o r e often t h a n d i d the f o r m e r ( b i n o m i a l test, two-sided, P = 0.022). W h e n the i n t r u d e r bit the defender, there was n o significant difference in w h i c h one left ( P = 0.69); however, s a m p l e sizes were small for this c o n d i t i o n due to the i n f r e q u e n c y o f biting b y the intruder. W h e n n e i t h e r s a l a m a n d e r bit, the i n t r u d e r still left

significantly m o r e frequently t h a n the defender ( P < 0.00006): 11 o f the 24 i n t r u d e r s left directly after a threat d i s p l a y by the defender, a n d 13 i n t r u d e r s left even w i t h o u t such a display. T h e r e was little tendency f o r b o t h d e f e n d e r a n d i n t r u d e r to r e m a i n in one a r e a o r to leave simult a n e o u s l y (Table IV). Areas o f Body Bitten S a l a m a n d e r s did n o t bite an o p p o n e n t rand o m l y relative to the length o f its t r u n k (X~ = 48.37oo o f t o t a l length), tail ( X = 46.77o) a n d s n o u t ( X = 5 . 0 ~ ) : Z ~ : - 679.5, P < 0.001. Tails were bitten significantly m o r e frequently t h a n t r u n k s (Z~ ~ 6.6, P = 0.01) a n d snouts were bitten m o r e often t h a n t r u n k s (Z 2 = 456.8, P < 0.001). Significantly m o r e bites were directed a t the snout t h a n at the tail (Z 2 = 350.7, P < 0.001). Discussion Defenders a n d intruders spent little t i m e in a resting p o s t u r e (relative to residents), b u t b o t h exhibited the t h r e a t d i s p l a y f o r a b o u t h a l f o f the t i m e together. I n terms o f displays, then, defenders were n o t m o r e aggressive t h a n intruders. T h e submissive d i s p l a y was used infrequently, because the i n t r u d e r t e n d e d to leave after being

ANIMAL

494

9 V V A

BEHAVIOUR,

dde V V A

30,

2

idddd V A c~

~

~1~

V

V

A

+1

qq

~Hq-H

,,5

ooo

o~

oo oo oo

,5

o
-H

~Hq +1

V

,/=

oo

q

o m

,,m

9

~

0

O0

JAEGER ET AL. : SALAMANDER

Table IV. The Number of Cases ia Which a Defender or Intruder Left a Contested Area FolJowing a Bite by One, Both or Neither

Animal biting

A n i m a l leaving

Defender Intruder

Both

Neither

Defender Intruder Both

1 4 2

9 2 2

0 0 0

1 0 0

Neither

2

24

0

3

bitten (18% of all contests), after a threat display by the defender (22 %), or after no apparent provocation from the defender (26 %). The last case is particularly interesting because Jaeger & Gergits (1979) found that salamanders tended to avoid areas marked with pheromones by another salamander even in the absence of the marking animal. Our data show that after an intrusion occurred, both defenders and intruders increased their rates of nose tapping, probably gathering information (via pheromones) on each other's identity. Only 8 of the intruders took the option of remaining submissive in a defended area rather than leaving or attacking the defender. A salamander, then, apparently had three ways of expelling an intruder from its area (listed in order of increasing aggression): pheromonal advertisement of the area, threat display, and biting. The rate of biting was twice as fast for defenders as for intruders, indicating that the defenders were more willing to escalate an aggressive contest beyond the stage of threat display. An escalated aggressive contest can have a high cost to the loser (i. e. the one that is bitten). There were more bites directed at the tail than at the trunk and more at the snout than at the tail. Jaeger (198l) noted that biting an opponent's trunk can cause little damage, while bites to the tail can cause autotomy of that organ, with consequent loss of fat reserves. Bites to the snout can damage the nasolabial grooves and thus impair the chemoreceptive abilities of the bitten animal. The cost of being bitten, then, is a potential loss of fitness, either through loss of fat used in reproduction (Maiorana 1977) or through impairment of chemoreceptive abilities needed to find prey (Jaeger 1981). In our experiment, intruders seldom initiated biting attacks (12% of all invasions) and frequently left the contested area without being bitten (52%). Thus salamanders were more likely to risk the cost of an escalated

TERRITORIALITY

495

contest when defending an area than when entering another individual's area. Difference in sizes between paired salamanders was not a predictor of which one would intrude or be more aggressive. The defender bit more often than the intruder regardless of whether it was larger or smaller. The rule for settling spatial disputes by red-backed salamanders may be similar to that of speckled wood butterflies (Pararge aegeria) studied by Davies (1978): 'resident wins, intruder retreats'. Davies felt that such a rule would be effective when escalated contests are costly, as they can be for P. einereus. Unlike Davies's butterflies, however, the defending salamanders did not always win. Defenders won (i.e. the intruder left) in 74 % of the encounters while intruders won in 18 %, with 8 ~ of the encounters a draw (neither left). This suggests that the decisions (sensu Krebs 1978) involved in settling spatial disputes may be complicated for salamanders. Both males and non-gravid females defended areas, and none of our measures of agonistic behaviour revealed a difference among malemale, male-female or female-female interactions. This suggests that males and females may hold mutually exclusive territories in forest habitats, at least when females are not in breeding condition; adult female red-backed salamanders breed only once every two years while males breed yearly (Sayler 1966). Since food is limited in availability during rainless periods (Jaeger 1972, 1980), we hypothesize that for both sexes territories periodically serve to protect a scarce supply of prey under retreat sites (rocks and logs). However, the 'feeding territory' hypothesis does not explain all of the available data. Jaeger et al. (1981) found that males established territories much faster than females, as estimated by the rate of marking behaviour. Jaeger & Gergits (1979) showed that both sexes of P. cinereus avoided male-marked substrates (in the absence o f the marker), but did not avoid female-marked substrates. Gergits (1981) and Wrobel et al. (1980) observed males of P. cinereus and P. shenandoah to bite conspecific males more frequently than females. Consequently the data on the function of territoriality for male and female salamanders are ambiguous. We can only guess at this point that males and females may hold mutually exclusive feeding territories in forest habitats during the noncourtship period (summer, when prey are often unavailable to salamanders due to frequent periods of dryness; Jaeger 1980), but they may

496

ANIMAL

BEHAVIOUR,

share areas as the females a p p r o a c h reproductive c o n d i t i o n (spring a n d a u t u m n ) . I n conclusion, our data indicate that the redbacked s a l a m a n d e r is territorial, with the ability to expel intruders from advertised a n d defended areas. Jaeger (1979) a n d Wells (1980) f o u n d under-dispersion of Plethodon cinereus a n d P. glutinosus, respectively, in n a t u r a l habitats. This spacing can n o w be interpreted as a response to intraspecific territoriality. T h e tendency of displaced individuals o f P. cinereus (Samuelsen 1977) a n d P. jordani ( M a d i s o n 1969) to ' h o m e ' m a y be a consequence o f terrestrial salamanders r e m a i n i n g near a n d defending territories. It also seems likely, based on o u r data a n d those o n interspecific c o m m u n i c a t i o n (Jaeger & Gergits 1979) a n d aggression (Gergits 1981; T h u r o w 1976), that parapatric distrib u t i o n s between a n u m b e r of species of plethod o n t i d salamanders (Highton 1972; H a i r s t o n 1980) are the consequence o f interspecific territoriality.

Acknowledgments We t h a n k D e b r a Barnard, William Gergits a n d Kiisa Nishikawa for critical c o m m e n t s o n this manuscript. W. Gergits' master's thesis o n interspecific territoriality o f salamanders provided the stimulus for m u c h of our research. The research was supported by G r a n t 1523 from The A m e r i c a n Philosophical Society a n d N S F G r a n t DEB-80t7447. We t h a n k D e n n i s Carter of the N a t i o n a l Park Service, S h e n a n d o a h N a t i o n a l Park, for permission to conduct research in the park.

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Krebs, J. R. 1978. Optimal foraging: decision rules for predators. In: Behavioural Ecology: An Evolutionary Approach (Ed. by J. R. Krebs & N. B. Davies), pp. 23-63. Sunderland, Mass. : Sinauer Associates. Madison, D. M. 1969. Homing behaviour of the redcheeked salamander, Plethodon jordani. Anirn. Behav., 17, 25-39. Madison, D. M. 1975. Intraspecific odor preferences between salamanders of the same sex: dependence on season and proximity of residence. Can. J. Zool., 53, 1356-t361. Maiorana, V. C. 1977. Tail autotomy, functional conflicts and their resolution by a salamander. Nature, Lond., 265, 533-535. McGavin, M. 1978. Recognition of conspecific odors by the salamander Plethodon einereus. Copeia, 1978, 356-358. Samuelsen, P. 1977. The home range and homing behavior of the red-backed salamander, Plethodon cinereus. Unpublished M.S. thesis, University of Rhode Island. Sayler, A. 1966. The reproductive ecology of the redbacked salamander, Plethodon einereus, in Maryland. Copeia, 1966, 183-193. Siegel, S. 1956. Nonparametrie Statistics for the Behavioral Sciences. New York: McGraw-Hill. Thurow, G. 1976. Aggression and competition in eastern Plethodon (Amphibia, Urodela, PIethodontidae). J. HoTetoL, 10, 277-291. Tristram, D. A. 1977. Intraspecific olfactory communication in the terrestrial salamander Pletkodon cinereus, Copeia, 1977, 597-600. Wells, K. D. 1980. Spatial associations among individuals in a population of slimy salamanders (Plethodon glutinosus). Herpetologica, 36, 271-275. Wilson, E. O. 1975. Sociobiology: The New Synthesis. Cambridge, Mass.: Harvard University Press. Wrobel, D. J., Gergits, W. F. & Jaeger, R. G. 1980. An experimental study of interference competition among terrestrial salamanders. Ecology, 61, 10341039. (Received 26 May 1981; revised 25 September 1981; MS. number: a2659)