Copulatory behaviour of cotton mice (Peromyscus gossypinus) and their reciprocal hybrids with white-footed mice (P. leucopus)

Anitn. Behav., 1979,27, 371-375

COPULATORY BEHAVIOUR OF COTTON MICE (Z’EROMYSCUS GOSSYPZAWS) A&D THEIR RECIPROCAL HYBRIDS WlTH WHITE FOOTED MICE (P. LEUCOPUS) BY DEIRDRE V. LOVECKY*, DANIEL Q. ESTEPt & DONALD A. DEWSBURYS Department of Psychology, University of Florida, Gainsville, FL. 32611

Abstxact.Cotton mice,

Peromyscus gossypinus, display a copulatory pattern with no lock, no intravaginal thrusting, multiple ejaculations, and ejaculations occasionally occurring without prior intromissions. There are relatively few ejaculations per satiety test. Mice from two distinct populations were very similar with respect to basic motor patterns, quantitative measures, and associated behavioural patterns, suggesting that these patterns may be truly ‘species-typical’. The behaviour of the reciprocal hybrids between P. gossypinus and P. kucopus was found to resemble that of the parental species.

The present research had three main objectives. One was to provide the first quantitative description of the copulatory behaviour of cotton mice, Peromyscw gossypinw. Cotton mice are small nocturnal rodents, native to the southeastern United States, that typically nest in cavities in logs and trees, and in holes in the ground in deciduous forests (Layne 1969). A second objective was to evaluate differences in behaviour among different naturally occurring populations of this species. Such studies permit consideration of the extent to which behavioural patterns are truly ‘species-typical (see Bruell 1970). Wild rats from different populations differ substantially with respect to both neophobia and poison-elicited pica (Mitchell et al. 1977). Less dramatic differences have been found in the copulatory behaviour of different populations of species of Peromyscus from other subgenera and species groups (Dewsbury 1974b; Dewsbury & Love&y 1974). If behavioural differences observed in comparative studies are to be regarded as truly ‘species’ differences, it is critical that one demonstrate greater variation among species than among populations within species. Further, this must hold generally across comparisons within a number of species. The third objective was to analyse copulatory behaviour in interspecific hybrids between cotton mice and white-footed mice, P. leucopus. Such information is relevant to questions of

both reproductive isolation and underlying genetic structure. Regarding the former, P. gossypinus and P. Ieucopus are sympatric in part of their ranges and are apparently the only species of Peromyscus for which hybrids have been reported in nature (Howell 1921; McCarley 1954). The two species interbreed in the laboratory and the hybrids are fertile (Dice 1940). One factor in maintaining reproductive isolation appears to be a preference for homospecific mating partners (McCarley 1964; Bradshaw 1968). The copulatory behaviour of interspecific hybrids has rarely been studied; its possible role in reproductive isolation is not understood. Data from hybrids relate to inferences regarding genetic structure and homology. The copulatory patterns of P. Zeucopus and P. gossypinus are quite similar. However, even patterns that are identical in two parental species can be altered or even lost in hybrids, as for example in lovebirds of the genus Agapornis (Buckley 1969). Hinde (1956) has spelled out the logic for inferences regarding homology: ‘with complex behaviour which is similar in the parent species, the whole organization of motor patterns and associated motivational conditions is likely to depend on basically similar hereditary factors in each case. If the similarity was only superficial (i.e. characters not homologous), then the complex mechanisms leading from stimulus to response would be likely to break down in the hybrid’ (page 210).

l Pre.sent address: Rhode Island Youth Guidance Center, Providence, R.I. 02906. tPresent address: Department of Psychology, University of Georgia, Athens, GA 30602. :Reprint requests to D. A. Dewsbury.

Experiment I Experiment I was conducted in order to collect normative data regarding the copulatory behaviour of P. gossypinus. 371

372

ANIMAL

BEHAVIOUR,

27,

2

Preliminary observations indicated that it would be appropriate to adapt standard measures used with laboratory rats. Co$ulatory behaviour is composed of several organized groups or series of mounts without vaginal penetration (mounts), mounts with penetration but no sperm transfer (intromissions), and mounts with penetration and sperm transfer (ejaculations), with each series terminated by an ejaculation. Measures taken were defined by Dewsbury (1975b).

Methods The subjects were laboratory-born offspring of either wild-trapped orfit-generation, laboratory-reared parents from two populations af P. gossypinus. Original stock were trapped near either Gainesville, Alachua County, Florida (Gainesville population), or Palmdale, Glades County, Florida (Palmdale population). All mice were at least 90 days of age and no animals in male-female pairs were litter mates. The numbers of males and females from each population that either mated or failed to mate are presented in Table I. In addition a total of 13 males and 14 females mated but produced unusable data because of inconsistent mating, malfunction of the event recorder, or death. Animals were housed in clear plastic cages; those of the males measured 48 x 27 x 13 cm, whereas those of the females were 29 x 19 x 13 cm. The colony room was maintained on a reversed 14 : 10 cycle of fluorescent white light with dim red light shining at all times. Tests for individual animals were spaced at least two weeks apart, with two to five tests per pair. Females were brought into oestrus with injections of 0.06 mg oestradiol benzoate three days before testing and O-6 mg progesterone at least 6 h before testing. Tests were initiated approximately 6 h after the lights went off. Females were introduced into the males’ home cages, and the pair was permitted 90 min to initiate copulation. If copulation occurred, the test was continued until attainment of a satiety criterion of 30 min with no intromissions.

ReSIlltS The occurrence of copulation was somewhat inconsistent, with mating on just 39 % and 51% of the tests for the Gainesville and Palmdale mice respectively (Table I). The basic motor patterns involved in copulation were essentially identical to those described for P. leucopus (Dewsbury 1975b). Means and standard errors for all measures were calculated with data from all pairs equally weighted. The mean mount latencies for the Gainesville and Pahndale populations were 1100 s (SE= 188) and 1062 s (SE = 243) respectively ; intromission latencies were 1138 s (SE= 193) and 1085 s (SE = 243) respectively. For the Gainesville population a mean of 2.4 (SE= 0.2) ejaculatory series preceded attainment of the satiety criterion. The mean number of ejaculations (complete series) in the Palmdale population was I.9 (SE= 0.2). Data for measures that characterize individual series are summarized in Table II. The general similarity of the data from the two populations

Table I. Summary of Positive and Negative Animals and Tests* No. of

mice in

Percentageof tests positive (all tests>

Sex

Data Set

No. of mice not mating

(Gainesville)

F”

:66

:22

P. gossypinus (Palmdale)

F”

15 15

2 1

F”

14 14

2

27(107/288) -

F”

::

15 15

19(42/225) -

F”

I

10 10

38(76/201) -

Taxon P. gossypinus

P. leucopus LxG GxL *From Dewsbury (1975b).

39(86/218) 51(92/181) -

LOVECKY

ET AL.:

COPULATION

is readily apparent. The first ejaculatory series occupied a mean of approximately 3 min, and included three intromissions. There were few mounts. Ejaculation without prior intromissions (IF = 0) occurred on a few tests (38%, Gainesville population; 3.2 %, Palmdale population). Three-way analyses of variance with repeated measures were used to assess the statistical significance of effects of population, series, and repeated tests on the five measures that characterize each series. Individual comparisons were made using the Newman-Keuls procedure. Because pairs completed different numbers of tests and series per test, the maximum that could be used in these analyses was three tests, two series, 12 Gainesville pairs, and 9 Palmdale pairs. The only significant effect due to population was on PEI (post-ejaculatory intervals) (F(1, 19) = 5.56, P < 0.05); with those of the Palmdale animals longer than those from Gainesville. Significant effects of series were found for EL (ejaculation latency) (F (1, 19) = 864, P < O*Ol), MI11 (mean inter-intromission interval) (F (1, 19) = 7.26, P < O-05), and PEI (F (1, 19) = 16.08, P < OWl). Both EL and MI11 showed significant reductions from the first to the second series. By contrast, PEI showed a significant increase from the first to the second series. Effects of sequential tests were found for MF (mount frequency) (F (2, 38) = 5.01, P < 0.05) and PEI (F (2, 38) = 6.17, P < 0.01). Mount frequency showed a significant increase from test 1 to test 2. The mean PEI in the first test was longer than that for either the second or third tests. In 12.7 % and 16.7 % of the test for the Gainesville and Palmdale populations respectively, the 30-min satiety criterion followed immediately upon an ejaculation. For all other tests the TableII.

Series

Mean

Scores

Number of tests G* Pt

:

4:

ii

: 5 Inc.$

27 : 55

15 : 50

on Measures

of Copulatory

Ejaculation Glatency 9 142.5 78.1 107.7 39.3 15.7 2076.7

IN PEROM

313

YSCUS

criterion was reached within an incomplete ‘series’. All such incomplete ‘series’, except one for each population, included long intromissions (see Dewsbury 1975b); 44% and 17% respectively for the Gainesville and Palmdale animals included both regular and long intromissions. In the remaining tests, only long intromissions intervened between the last ejaculation and attainment of the satiety criterion. Experiment II Experiment II was conducted in order to collect normative data on the copulatory behaviour of the reciprocal hybrids between P. gossypinus and P. leucopus. Methods The subjects were 30 males and 30 females of a cross of P. leucopus females with P. gossypinus males (L x G cross), and 28 males and 28 females of the reciprocal cross (G x L cross). The P. gomypinus parents were from the Gainesville population; information regarding the P. leucopus population is provided by Dewsbury (1975b). All animals were at least 90 days of age at the time of testing. Mice were maintained on a 16 : 8 photoperiod. An attempt was made to complete at least four positive tests for each malefemale pair. All remaining procedures were identical to those of experiment I. ReSUltS

Only 19% (42/225) of tests with the L x G mice and 38 % (76/201) of the tests with the G x L mice resulted in copulation (see Table I). The disproportionate success of the G x L cross also was reflected in the breeding data from these animals. Basic motor patterns displayed by mice of both crosses were identical to those

Bebaviour

for Two

Intromission frequency G P

Populations

Mount

of

Peromyscus gossypinus

Mqn i$er-introPost-ejaculatory miss. mtervt (s) Ginterva1 (?

GfrequenY

205.8 92-9

3-l

;:;

;::

8:;

25.9 48.6

31.0 81.6

326.3 305-s

456.8 406.8

104.2 83.0

;:i jt4 14-7

;:;

8:; 0.0 l-3

8:;

26.1 11-5

18.0 12.9

G

17Z.7

426.3 459.2 655.3 -

507.4 499.5 -

2076.7

li%

224-8

*G indicates Gainesville population. indicates Palmdale population. :Refers to incomplete ‘series’. Data are calculated for all tests ending within such a series using the same measures as for complete series. tP

374

ANIMAL

BEHAVIOUR,

of the parental species. Data resulting from quantitative analyses of copulatory behaviour are presented in Table III. One-way analyses of variance were used to assess the statistical significance of differences among the four groups. Because of differing numbers of series per test and tests per pair, mean scores for each pair for each series were used in these analyses. Significant analyses of variance were followed by individual comparisons using the NewmanKeuls procedure (see Table III). Comparison of the two reciprocal hybrids revealed that the G x L animals generally were the faster copulators. For EL, MIII, and PEI, the means of the hybrids generally fell within the range of the parental species, with L x G and G x L mice scoring most like the animals of their maternal species. On most measures there was no consistent pattern to the scores of the hybrids relative to those of the parents. However, the hybrids did show consistently lower numbers of intromissions preceding ejaculation (IF) than the parental species. Ejaculation on a single insertion occurred on 16 of 100 ejaculations for the L x G hybrids and 26 of 292 for G x L hybrids. Both hybrids more closely resembled P. leucopus than P. gossypinus with respect to their most frequent mode of attaining the satiety criterion. The percentages of tests ending between series (i.e. with no copulations after the last ejaculation) were 64.3% for L x G hybrids, 50.0% for the G x L hybrids, 53.8 % for P. leucopus, and 12.7% for P. gossypinus. Table

III.

Meao

Measure lELF EL-l EL-2 EL-3 IF-l IF-2 IF-3 MF-1 MF-2 MF-3

*From

27,

differed significantly from the other three groups with respect to the frequencies of positive and negative tests (~2 = 33.88; df = 1, P < O-01).

P. gossypinus

Diaeussion P. gossypinus display a copulatory pattern with

no lock (i.e. mechanical tie between penis and vagina), no intravaginal thrusting, multiple ejaculations, and occasional ejaculations without previous intromissions (Pattern 15 of Dewsbury 1972). P. gossypinus share this copulatory pattern with species such as western harvest mice, Reithrodontomys megalotis; Florida mice, P. floridanus; desert wood rats, Neotoma iepida; and plateau mice, P. melanophrys (see Dewsbury 1975a). The occurrence of such a copulatory pattern in P. gossypinus is consistent with predictions based on the penile morphology and accessory gland complement of this species (see Linzey & Layne 1969; Hooper 1958; Dewsbury 1974a, 1975a). Copulatory patterns with ejaculation possible on a single brief insertion may represent adaptations to habitats with minimal cover and consequent exposure to predation. Both western harvest mice and Florida mice inhabit regions with little cover (Dewsbury 1975a). P. gossypinus resemble these species in that they often nest in cavities in logs or trees, or under objects on the ground (Layne 1969). Only relatively small quantitative differences were found between the two different populations of P. gossypinus. A similar pattern emerged

Scores and Results of Statistical Analysis of Quantitative Measures of Copulatory Bebaviour in Peromjwcus bcopus, P. goseypinne, and Their Rerripoenl Hybrids P. leucopd

P. gossypinus

LXG

GXL 851.1 3.7 113.4 71.6 93.0 2.2

1301.0 20:.: 317.4 311.9

1137.9 142.: 78.1 107.7

1161.2 2lZ 132.8 236.6

::; 5.6 1.2

::: ;:;

;:; 2.9

::;

f :: g :;

MIII-2 MIS1 MIII-3 PEI-2 PEI-1

63.0 448.6 551.5

48.6 25-9 26.1 362.3 305.5

7:.: 62.6 63.0 569.2 520.1

PEE3

637.0

426.3

648.1

Dewsbury

2

49.6 86.4

(1975b).

Significant GL>L,G,LG L>GL

GL Ls;

kGb$, 3

NS

::: 1.1

4. ;

8:;

i?>G LG > G,GL

Z.S 32.0 %:: 397.1

diis.

zLGtbLG ,

i%

> G, GL

pL;

FLhG

Lij>G,L>G

LOVECKY

ET AL. : COPULATION

in data categorizing behaviours associated with copulation (not reported, but available from the authors). These data, together with those from other species (Dewsbury 1974b; Dewsbury & Lovecky 1974), suggest that in contrast to some other behavioural patterns in other taxa, Peromyscus copulatory behaviour varies little among different natural populations. Although the hybrids of P. gossypinus with P. Ieuwpus can display complete, integrated copulatory behaviour, copulation occurred in just 28 “/o of the tests. Hybrids may be at a slight competitive disadvantage relative to parental species with respect to copulation in the natural habitat. However, the difference is relatively small. The low probabilities of mating in the L x G mice may represent a maternal effect (i.e. due to pre- or post-natal maternal environment), as P. leucopus were less likely to mate than were P. gossypinus. The close resemblance between the basic copulatory patterns of the hybrids and parental species, even though very different patterns are found within the genus, suggests a homology (see Hinde 1956). There was an overall low probability of mating in the test situation and of producing litters in breeding cages. This appears characteristic of these mice, as it contrasts with other species (e.g. P. maniculatus) under identical circumstances and has been reported by other investigators (e.g. Rood 1966). Acknowledgments This research was supported by Grant BMS7508658 from the National Science Foundation. Experiment I is based upon part of a doctoral dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree at the University of Florida. Hormones were provided through the courtesy of the Schering Corporation, Bloomfield, New Jersey. D. V. L. was supported on a traineeship from Training Grant NIH-MH-10320 to the Center for Neurobiological Sciences, University of Florida. REFERENCES Brad&w, W. N. 1968. Progeny from experimental mating tests with mice of the Peromyscus leucopus group. J. Mammal., 49,475-480.

IN PEROMYSCUS

315

Bruell, J. H. 1970. Behavioural population genetics and wild MIAS musculus. In: Contributions to BehaviorGenetic Analysis: The Mouse as a Prototype (Ed. by G. Lindzey & D. D. Thiessen), pp. 261291. New York: Appleton-Century-Crofts. Buckley, P. A. 1969. Disruption of species-typical behaviour patterns in Ft hybrid Agapornis parrots. Z. Tierpsychol., 26, 137-143. Dewsbury, D. A. 1972. Patterns of copulatory behavior in male mammals. Q. Rev. Biol., 47, l-33. Dewsbury, D. A. 1974a. Copulatory behaviour of whitethroated wood rats (Neotoma albigula) and golden mice (Ochrotomys nuttalli). Anim. Behav., 22, 601-610. Dewsbury, D. A. 1974b. Copulatory behavior of wildtrapped and laboratory-reared cactus mice (Peromyscus eremicus). Behav. Biol., 11, 315-326. Dewsbury, D. A. 1975a. Diversity and adaptation in y4deie:copulatory behavior. Science, N. Y., 190, Dewsbury, D. A. 1975b. Copulatory behavior of whitek60t;tl;; (Peromyscus leucopus). J. Mammal., . Dewsbury, D. A. & Lovecky, D. V. 1974. Copulatory behavior of old-field nwe (Peromyscus polionotus) from different natural populations. Behav. Genet., 4, 347-35s. Dice, L. R. 1940. Relations between the wood-mouse and the cotton-mouse in eastern Virginia. J. Mammal., 21, 14-23. Hinde, R. A. 1956. The behaviour of certain cardueline Ft inter-species hybrids. Behaviour, 9, 202-213. Hooper, E. T. 1958. The male phallus in mice of the genus Peromyscus. Misc. Publ. Mus. Zool. Univ. Michigan, 105, l-24. Howell, A. H. 1921. A biological survey of Alabama. I. Physiography and life zones. II. The Mammals. North Amerkan Fauna, 45, 1-88. Layne, J. N. 1969. Nest building in three species of deermice, Peromyscus. Behaviour, 35, 288-303. Linzey, A. V. & Layne, J. N. 1969. Comparative morphology of the male reproductive tract in the rodent genus Peromyscus (Muridae). Am. Mus. Novitates, 2355, l-47. McCarley, W. H. 1954. Natural hybridization in the Peromyscus leucopus species group of mice. Evolution, 8, 314-323. McCarley, .H. 1964. Ethological isolation in the con~~~~ Peromyscus leucopus. Evolution, 18, Mitchell, D., Bkatty, E. T. & Cox, P. K. 1977. Behavioral ddferences between two populations of wild rats: implications for domestication research. Behav. Biol., 19,206-216. Rood, J. P. 1966. Observations on the reproduction of Peromyscus in captivity. Am. Midl. Nat., 76, 496-503. (Received 7 April 1977; revised 26 May 1978; MS. number: ~2042)