Res. Popul. Ecol. (1963) V, 139--146

ON T H E P R O B L E M OF T R A P - R E S P O N S E T Y P E S

O F S M A L L M A M M A L P O P U L A T I O N S

Ryo TANAKA

Zoological Laboratory, Kocbi Women ' s U n i v e r s i t y , Kocbi

The mark-and-release method is basically useful in the study of population

dynamics for small mammals , especially required in the research of long-term suc-

cessive fluctuations in population parameters ; by this means we can obtain data

for the estimation of N, B, P (in the terminology of LESLIE et al, 1952, 1953) and

home range sizes at regular intervals during a study period, through which the

population under study is expected to keep its natural status, suffering hardly so

serious disturbance as would be caused by the removal method.

However , some diff icul t ies a re inhe ren t in the m a r k i n g method , and the

heterogenei ty in t rappabil i ty between marked and unmarked individuals will be most

important. In te rms of my definition (TANAKA, 1956), the alloresponsive type Seems

to occur far more usually than the isoresponsive for all local populations of every

species of ra ts and mice.

The earliest paper that adduced a proof for increased t rappabil i ty of animals

after once marked seems to be CHITTY and KEMPSON (1949), and later not a few

authors discussed the same problem. In 1956, I proposed three types of t rap response

in the manner of differentiation of t rappabil i ty between the marked and the unmarked

on the basis of est imated probabilities of capture n (marked) and p (unmarked) .

The proposal has hereto been supported by but few ecologists, but I am becoming

still more certain of its t ruth by having examined further my data and related

results of other workers. Then the further consideration of this problem is here

described mainly about its relation to speciation and subspeciation and its effects on

the estimation method of population parameters in small mammals .

GROUPING POPULATIONS BY THE RESPONSE TYPE

Any local populations of small mammals , excepting insectivores for the present,

can be grouped under three response types by means of the marking method. A

d iagrammat ic a r rangement of the type for ra t and mouse populations in te rms of

interrelation between re- and p-values was exhibited in the previous paper (TANAKA,

1956), and a similar figure, including the further added data (Table 1) which w e r e

secured af ter or had been done before 1956, are also presented here (Fig. 1) ; all the

values of n and p were calculated by myself, and a group of open or solid circles

belonging to the same species or subspecies is circumscribed with a solid or broken line.

140

Table 1. Characters of the added populations in Fig. 1.

Habitat Months Response Authors Species No. (Locality) (Year) lr P type

Animal shed July Rattus 2 A (Tokyo) (1956) 0. 17" 0. 42* II OSADA et al. ('57)

Animal shed May to norvegicus 3 B (Tokyo) June (?) 0. 05 0. 18 II OSADA et al. ('59)

Mus molossinus

Mus musculus

Apodemus speciosus

Apodemus argenteus

Microtus montebelli Microtus

pennsylvanicus pennsylvanicus

Clethrionomys rufocanus bedfordiae

Eothenomys smith•

Coastal grassland April O, 61 •

(Fukuoka) (1957) 0. 062

Grass and Sept. to brush land Oct. O�9 44•

(Guam) : (1945) .046

Woodland ! (Mt. Ishizuchi)

Woodland 1 (Hokkaido)

Bamboo-grass 2 land

(Mr Tsurugi)

Sept. (1951) 0 .48

Sept. (1957)

July (1960)

Woodland I Nov. 3 (Nagano Pref. ) (1956)

(East Lansing, I July Michigan) (1942)

8

Grassland 9 (Hokkaido)

10

3 Bamboo-grass land

4 (Mt. Tsurugi)

Sept. (1958)

Oct. (1955)

0. 41

0. 61

0. 20+ 0. 070

0. 18

O. 73=t= O, 032 O, 73• O. 041 O, 625 • O, 040

0, 46• 0, 044 0, 511 0, 056

. 27• �9 067

O. 28• �9 087

0. 31

O. 23

O. 43

0. 064• 0. 0072

0. 25

0. 34=k 0. 059 0. 21• 0. 076 0. 31• 0. 068

0. 20• 0. 044 0. 16• 0. 033

TANAKA • HA~tAJI~A (unpublished)

BAKER ('46, Table 2)

TANAKA (unpublished)

UDAGAWA et al. I ('56), TANAKA ('59)

HAYNE II?*~ ('40, Table 2)

TANAKA ('61)

TANAKA ( '56)

* recalculated from his trapping record, but he gave n=0. 196 and p=0. 357. ** special comment is needed and given in the text.

T h e figure man i f e s t s tha t commensa l ra t s in res ident ia l hab i t a t s are p la in ly of

type II ( n < p ) , the same in the previous and added data , agreeable to m y proposal

even if only an opinion in favour of this type has so far been afforded by another

au thor and his coworkers (OsADA el al., 1957), while the fieral mice of Apodemus,

Peromyscus, Microtus, Clethrionomys and Eothenomys (the last is regarded as a sub-

genus of Clethrionomys by HooPER and HART, 1962) are of type I ( n > p ) or III ( n =

p). As explained before, it is r emarkab l e tha t the local popula t ions of Cl. rufocanus

bedfordiae m a y occasional ly be near or just isoresponsive.

T h e record of Microtus pennsylvanicus pennsylvanicus, given by HAYN~ (1949) ,

worked out to tend to type II unexpectedly. Since his record was obta ined by checking

t raps twice a day ( A M and PM), no cons tan t probabi l i ty of capture could surely be

141

0.7 Peromyscus leueopus noveboracensls

0.6

o.5

O.k

~h

o.3

0.2

0.11

0 0

Fig. 1

Table 2.

Ra t tus

r a t t u s ,2] c:~th~io.o~y,// 7@ / r u t i l u s / / / mikado / / / " l/ /Apod,~u~ /,

montebelll I "\ ... .... \ /'~101

\ \

N k

2~g \

.:..@:o I OMua molossinu o2J I

C):2::2n:" . . . 1 b e d f o r d i a e

I t Q J , ~ I I I I I I 0.I 0.2 0.3 O.t~ 0.5 0.6 0.7 0.8

Showing trap-response types by interrelation between x and p for rat and mouse populations of several species at different times and in different

habitats or localities ; the bisector is the isoresponsive line.

Supposed trap-response types afforded by other workers for mouse and vole populations.

Species or Subspecies Response type Authors Remarks

Mus musculus II SaALANDnR et al. ('58) in a residence

Mus musculus III wild, hand-caught

Peromyscus maniculatus abietorum

CROWCROFT ~JEFFRRS ('61) populations in pens

I MORRIS ('55)

Peromyscus leucopus III GETz ('61) through 5 weeks noveboracenszs

Microtus agrestis I CHITTy ~Z KEMPSON ('49)

1948 to 1950, Microtus agrestis I LESLIE et al. (%3) 2 day prebaiting

two populations in

G~Tz ('61) Microtus pennsylvanicus

I MORRIS ('55)

LESLIE et al. ('53)

Clethrionomys gapperi ochraceous

Clethrionomys glareolus III

different habitats, through 13 months; one day p r e b a i t i n g

1948 to 1950, 2 day prebaiting

142

expected from the set of catches as it was, so I used the AM catches alone to estimate

N, and then p and ~ were computed as usual. GETz (1961), however, demonstrated

that two populations of the same species (perhaps the same subspecies) in different

habitats kept type I on the average from the records, collected after one day practice

of a sort of a prebaiting, over as long as 13 months ; I feel his method may give

a stricter result than ours, inasmuch as he counted the rate of capture from known

populations, instead of estimated ones. One exception, he spoke of, that the capture

rate for the unmarked was slightly greater than that for the marked in April with

the marsh sample, appears to suggest an occasional occurrence of type III in this

subspecies, too. Thus we may well consider the ttAYNE'S data never to be significantly

deviated from type III. In another view, the trend to type II might have been caused

by his sample restricted to female adults.

As for Peromyscus leucopus (subspecies noveboracensis according to BURT, 1940),

GETZ (1961) inferred from the Bu•w's records based on 5 trapping periods at weekly

intervals that the species was isoresponsive in harmony with my earlier inference

from the ~- and p-values counted from one period record of BuRT. But all other races

of this species are not yet assured to be of the same type ; it is too early to declare

that this species is in general isoresponsive.

The response type supposed from evidence given by various methods different

from ours in literature is summarised as in Table 2. We can see from this table

together with Table 1 that some populations of Peromyscus and Clethrionomys may

be isoresponsive, whereas none of Microtus has ever been so except for one record

of HAYNE and an exceptional one of GETZ with pennsylvanicus.

In Fig. 1, the group of Microtus montebelli is apart from that of CI. rufocanus

encircled with a broken line, within which some other fieral species and house mice

are involved. The fact might indicate lower values of ~ and p on account of its

population shyness stronger than others', but the indication is as yet uncertain until

further data are available, because both values may vary under the influences of

trap densities and weather conditions on study areas.

Now I shall discuss the response type of Mus musculus, which can be either a

commensal stock or a field one. The Japanese mammalogists have not yet come

to agreement in the view about whether the Japanese house mouse belongs to Mus

musculus or another species molossinus, but some populations inhabit a grassland

far away from residential areas and others get along well in residences and warehouses,

corresponding with the habit of European house mice. Anyhow, we can merely say

from the examples in the tables that a field population of the house mouse may be

of type I (M. molossinus in Fukuoka, M. musculus on Guam in Table 1) or of type

III (M. musculus, hand-caught from corn-ricks, in pens in Table 2), while a residential

population might be of type II such as with commensal rats by reference to slight

evidence of S~ALANDER et al. (1958). A further supply of similar proofs is required

to ascertain the inference.

143

How about is the response type of field populations of commensal rats which

are completely isolated f rom residential ones? The fieral types (I and III) might be

imagined as such is the case with the house mice, but I am terribly sor ry to be

unable to comment on that at all by reason of no available data up to the present.

The field populations of Norway rats, I have studied in the south-western coastal

region of Shikoku, appears to interflow usually with the residential ones which always

tend to become more dominant in winter months.

RESPONSE TYPE AND SPECIATION

It is very interesting f rom the view-point of not only ecology and pest control

but also speciation theories, but it is as yet little elucidated to what degree the

differentiation of the response type is correlated with speciation or subspeciation in

small mammals . A subspecific population of Peromyscus leucopus in a woodlot has

been demonstrated to suggest that the subspecies would be innately isoresponsive,

and yet we cannot affirm that the same type occurs always to any populations in

different habitats of the subspecies, needless to say of other races of the species,

for it seems likely in view of the above evidence on fieral mice that the type m a y

va ry f rom I to III depending on habitats and intra-population circumstances among

local populations of the same taxonomic group. Thus we should hesitate in saying

something characterist ic of the race, Clethrionomys rutilus mikado, by its one record.

Nevertheless, there m a y probably be a s tronger tendency to type III in some groups,

for instance the subspecies noveboracensis, than in other groups. There still remains

much room for research f rom this angle.

I suppose that type II is associated with the highest level of population shyness,

because the capture ra te is al together lower in an initial capture and turns still less

af ter the first confinement in a trap, and that although the distinction in this respect

between types I and III is not clear, the lat ter is ra ther more akin to the lowest

level of shyness, because a typically isoresponsive population is apt to have higher

values in both n and p.

The genetic and distributional s tudy of geographical races or subspecies appears

to have most progressed for North American species, in particular with Peromyscus (SUMNER, 1932; DICE, 1940; HOOPER, 1944), SO that their populations could be dealt

with in the order of subspecies here. As for most of Japanese species in the main

islands, a profound study of their subspeciation have scarcely been made insomuch

as the local populations can be strictly treated as subspecies.

DICE and CLARK (1962) experimental ly verified a significant difference in several,

psychological and physiological, behaviors among three subspecies closely related to

each other ; these behaviors are well assumed to be in some degree associated with

the population shyness. DICE (1940) emphasized sexual isolation as an essential first

step in speciation of Peromyscus, and the isolation mechanism is supposed to be highly

144

correlated with the t rap-wary response in the psychological aspect. Then, if a

difference in some characters reflected in t rap-response types is developed among

local populations, it may possibly induce sexual isolation to provoke speciation of

the populations.

METHODOLOGICAL DIFFICULTIES

It is already incontrovertible that we have to rule out the LINCOLN index method

for es t imat ing parameters with alloresponsive populations, but we have not to do

so with a limited number of isoresponsive ones. In regard to the former, a procedure

of prebait ing for one or a few days is often a t tempted to protect a population f rom

falling into the heterogeneous trappabili ty. However, I incline to doubt great utility

of the prebait ing with this aim. Really the heterogenei ty may occur even after the

practice of prebaiting, as exemplified by GETZ (1961) with Microtus pennsylvanicus

and by LESLIE et al. (1953) with M. agreslis. If we want to make an exhaustive

prebait ing through a population, quite a long t ime is needed for such populations as

to show lower p-values; too short a prebait ing would give, on the contrary, a new

disturbance to them, for an unknown number of trap-experienced animals being a

part of a population are led to coexist with the rest which have never entered a trap,

with the result that uncontrolable heterogeneity is yielded.

LESLIE et al. (1952) put forward good but laborious formulae for the est imation

of parameters changing with t ime f rom a very long chain of sample. In the actual

application to their small m a m m a l data obtained af ter the two day prebait ing

(LESLIE et al. 1953), they found the Clethrionomys population to be isoresponsive

but that of Microtus to be alloresponsive by a statistical test.

By this method, they could utilize only a half as much information as the Microtus

sample could afford, since they had to discard the information to be drawn f rom

unmarked individuals. Though this is excellent in that the calculation is based on

variances between total catches in each t rapping period, they set at naught any

variances between daily catches in each period. Thei r short-cut method, three-point

sampling, seems only limitedly available for small m a m m a l s because, aside f rom

the problem of response types, entry values just outside a t r iangle will be seldom

near zero in view of the likely mean life-span as long as a year in rats and mice.

So far as small m am m al s are concerned, our method (TANAKA, 1954) founded

on the principle of the routine census method (regression method af ter ZIPPIN 1956)

is more pertinent only f rom the view-point of the demerits of their me thod ; we

est imate population sizes at a t ime by t reat ing the marked and the unmarked sample

separately f rom each other, hence this method being valid for every populations whether

iso-or allo-responsive. If we can thus est imate marked populations M1, M~, .--, Mt

and unmarked ones N1, N2, ..,, Nt at regular intervals f rom t ime 1 to t, successive

values of 2 are computed f rom the N series and those of P are given f rom the M

145

series, and B is known assuming P for the marked to be equal to that for the

unmarked, as 2 = P + B according to LESLm'S definition.

Another problem in censusing from mixed populations of marked and unmarked

animals was discussed before in connection with effects of multiple collisions with

a single-catch trap (TANAKA, 1963). It may be really conceived that marked ones

tend to behave toward traps in advance of unmarked ones in populations of type I,

while the reverse trend occurs in those of type II, but neither will take place in

isoresponsive ones.

These phenomena make us to think of the more difficulty of random sampling

from the mixed populations with alloresponsive types, and when the routine census

formula is applied to mark-and-release sample, we should most care about the effect

of trap numbers for field populations liable to be type I under a study plan with

single-catch traps. This formula is employed on the assumption that the marked and

the unmarked group behave independently of each other, but the assumption can

not be exactly sound because of the heterogeneous behavior toward traps between

both groups.

CONCLUmONS

( 1 ) So far as the population data thus far amassed go, commensal rats in residential

habitats are of type II; house mice may be of type I or III in fields but might show type

II in residences, and fieral species or subspecies may range from type I to III. Altogether

alloresponsive populations seem to be much commoner than the isoresponsive.

( 2 ) It is as yet little disclosed to what degree the differentiation of the type is

correlated with speciation or subspeciation, even if only a proof that a subspecific

population would hold type III as its fixed feature is given.

The response types are supposed to be associated with the levels of population

shyness. Hence some characters reflected in those may possibly be connected with

sexual isolation provoking speciation.

( 3 ) Some awkward respects due to the heterogeneous trappability, depending upon

the trap-response type, among a mixed population of marked and unmarked animals

have been confirmed regarding the methodology for estimating parameters.

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146

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1956~}~J-~ , 5 ~ : ~ N ~ & o ) ~ (~, p) r ~0, ~ d ~ J ~ _ I 3 ~C, ~ N ( ~ I ~ 3 ~ ) ~ , J ; ~ , ~

~-�9 , . ~ g ~ � 9 v ~" ~. x ~, ~ v ~. ~ ~ ~ $ ~ I ~ Or<p) , , , "~ .~ ~. 7: t ~ - c ~ : ~ K ~ > P )