A Feeding Population of Pipistrelle Bats

A

FEEDING

POPULATION

(P. PIPISTRELL

OF

PIPISTRELLE

BATS

US, LINN).

by W . V . LOVETT

Saxmundham Modern School.

INTRODUCTION

FROM 3Ist July to 29th September a team consisting of 3 boys from Saxmundham Modern School, one from Leiston Grammar School and I were studying the bats which haunt the R.D.C. dump at Westleton. Our main objective was noctules (Nyctalus noctula) but we found that we were capturing a number of pipistrelles and decided to include some work on these as far as our main work allowed. The noctules first arrived at about sundown, the pipistrelles as dusk turned to darkness at which time most of the noctules disappeared. The pipistrelles seemed to go on Aying far into the night : we usually left after they had been Aying for about an hour.

METHOD USED

A Japanese made mist net 45' X 9' was set on two poles down wind of the tip face. The tip was infested with house crickets and the wind blew these towards the net into which the pursuing bats often Aew. The bats were taken out as soon as caught and put into a coarse fisherman's keep net hung upside down with the mouth closed by an elastic band. At the end of the evening the noctules were taken out, ringed with numbered rings and released.

The pipistrelles were marked with scissors : a small patch of für was removed from across the shoulders 3Ist July—15th August, across the rump 15th August—15th September, across the middle of the back 16th September onwards. Recaptured noctules would be recognized as individuals, pipistrelles only as having been captured before. All bats were weighed before release with a Spring Balance reading to 50 gms. X l gm. The results are set out in Table 1.

Düring the same period Lord Cranbrook and Mr. H. G. Barrett were capturing bats on a tip at Great Glemham. They too caught a number of pipistrelles but did not mark them, only noting the sex as the bats were taken from the net and releasing them immediately. They may have caught the same bats two or more times on successive nights or even on the same night. Their results are set out in Table 1 Col. 5.

40

A FEEDING POPULATION OF PIPISTRELLE BATS

TABLE 1

(2) (3) (4) (5) GLEMHA NumberNumberTotal No. Weights in Numbe Week captured recaptured marked at grammes capture ending during during end of during week week week Range Average week c? ?

3 ?

«J 9 «?

9 2 5th August 5 27 — 4 5 23 5.0-8.0 6.0 12th August 8 14 — 2 13 35 6.0-7.5 6.6 19th August 7 11 - 4 20 42 5.5-8.5 6.2 — 2 26th August 1 6 1 4 20 44 6.0-8.0 7.1 7 8 2 — 1 20 45 6.0-6.5 6.2 3 13 2nd September 6 12 9th September HOLIDAY 4 13 25 52 6.0-8.5 7.1 3 2 16th September 9 20 27 62 6.0-7.5 7.0 1 2 23 rd September 2 10 1 3 30th September 1 5 — 1 28 66 6.0-7.5 6.5 5.5-7.5 6.5 4 /5th October 2 4 — 3 21 44 35 99 (1)

—

•

—

—

—

—

—

—

DlSCUSSION POPULATION

On warm evenings there was a large number of pipistrelles feeding Over the tip and this number did not seem to get any less while we were there, save from a sudden change in the weather e.g., a heavy downpour of rain. We do not of course know whether the same bats were there the whole time or whether successive waves came in, to feed for a time and be replaced. We had hope to be able to calculate the numbers feeding by capture, release, recapture data with marked bats but we found that some of the bats were moulting and the marks grew out. We do not think that für clipping can be used for marking pipistrelles in August and September. The only reliable data we have is for our first three nights as shown in Table 2.

TABLE 2 Marked bats NumberMarked bats Estimated in population capturedcapturedpopulation Date (1) X (2) (3)

3Ist July Ist August 2nd August

(1)

14 18

(2) 14 7 11

(3)

(4)

—

—

3 2

33 99

A FEEDING POPULATION OF PIPISTRELLE BATS BREEDING

41

CONDITION

Males Judging by the size of their testicles some of the males were in breeding condition when we first started on 3Ist July, though the testicles of some did not Start to swell until well into August, as the following table shows.

TABLE 3 Date lOth August

Weight 7.5 7.0 6.0

16th August

Testicles swollen slightly swollen not obvious well developed

6.5 j» >» 6.5 not obvious 6.5 ,, ,, (possibly juvenile) 5.5 We first noticed regression of the testicles and enlarged epidydimides in mid September : this was evident in all males captured on 14th September.

Barrett and Cranbrook did not note the breeding condition of males.

Females The nipples of a lactating pipistrelle or one in late pregnancy are easily identified but are difficult to find in a virgin or dry one. Of 27 females caught 3 Ist July—2nd August 20 were pregnant or lactating : 2 out of 4 caught on 24th August were the same.

Thereafter we did not see a lactating female and presume that the young must all have been weaned.

Cranbrook and Barrett took pregnant or lactating pipistrelles from 22nd June to 24th August and no lactating ones thereafter.

WEIGIITS OF INDIVIDUAL BATS

Barrett Hamilton & Hinton (1910-21) record the weights of pipistrelles weighed by various workers as varying from 4.9 to 6.3 grammes. Our results are set out in Table 1 Col. 4 and Table 4. The variations we found, 5.5 to 7.5 gms. i n ^ r j and 5.5 to 8.5 in are wider, and both maximum and minimum higher. We were, though, dealing with feeding bats with a greater or lesser amount of food in the stomach. Bats in captivity will eat up to J or more of their body weight and the weight of a wild caught bat varies according to the amount of food in its stomach. We took two noctules weighing 33 and 32 grammes respectively on 17th August and kept them without food overnight. When released on 18th August both weighed 26 gms. Cranbrook and Barrett found in 1960 that a noctule caught on different days

42

A FEEDING POPULATION OF PIPISTRELLE BATS

could vary by up to 15% and assumed that that depended upon whether it was caught at the beginning or end of the feeding period.

If the workers quoted by Barrett Hamilton & Hinton had weighed only emptv bats—some at least were winter caught—it would only need one or two grammes of food in the stomach to bring their weights up to ours. Really accurate weights could only be obtained by keeping bats overnight and weighing them empty before release the next evening. We couldn't visit the tip every night so that was out of the question.

The 2? were consistently heavier on the average and in maxima than the cJcJ. This was not due to pregnancy : we took dry 2$ eat of 8.0 and 8.5 grammes. It does not seem likely that more than or that more 22 than were captured at the end of the feeding period : the difference therefore is probably sexual.

The slight increase in weights in September is probably due to the growth in size of the juveniles. Though some $ $ with fully developed testicles and some lactating 22 weighed as little as 5.5 grammes we think that most of these light weight bats were juveniles and none of that weight was taken in September. One September marked $ caught in October weighed 5.5 gms. This bat must have weighed at least 6 gms. in September.

TABLE 4 Analysis by sex and weights of all bats caught.

Week ending 5th August 12th August 19th August 26th August 2nd September 9th September 16th September 23rd September 4/5th October (visits)

Max. gms.

6.5 7.5 6.5 6.0

Males Min. gms.

5.5 5.5 5.5 6.0

7.0 7.5 6.5

6.0 7.0 6.0

Average gms.

6.0(5) 6.5(8) 6.0(7) 6.0(2)

Max. gms.

8.0 7.5 8.0 8.0 — 6.5 HOLIDAY 8.5 6.4(13) 7.3(2) 7.5 7.5 6.3(2)

Females Min. gms.

5.5 6.0 5.5 6.0 6.0

Average gms.

6.0(31) 7.0(16) 6.8(15) 7.3(10) 6.3(3)

6.5 6.0 5.5

7.5(33) 7.1(10) 7.0(7)

APPEARANCE OF YOUNG OF THE YEAR

The only way in which we could distinguish young of the year was by weight combined with breeding condition. A pregnant or lactating $ is obviously adult but it is difficult to distinguish a virgin juvenile from a non-breeding adult since the teats of a dry but adult bat are very small. Similarly adult in breeding condition with enlarged testicles are easily recognized but it is

Photograph

Dr. K. St.

Joseph.

A FEEDING POPULATION OF PIPISTRELLE BATS

43

not easy to distinguish between juvenile and adults at other times. Weight alone is not diagnostic. The weights of five lactating $$ caught on Ist August were four at 6.5, one at 7.5 gms., nine on 2nd August, 6.0, six at 6.5, 7.5 and 8.0 gms. A virgin $ caught Ist August at 5.5 gms. may have been a juvenile but we are more confident about another, 5.5 gms. taken on 17th August in Company with three&J, the testicles of which had not developed, weighing 5.5, 5.5 and 6.0 gms. By that date the testicles of sexually mature were starting to develop, indeed we think that most had. Since neither we nor Cranbrook and Barrett took any lactating $$ after 24th August we think that most juveniles must have been weaned and Aying by that date.

It is often said that young bats are carried about by their mothers for up to a fortnight after birth. There are many records of breeding colonies of e.g., noctules moving, mothers and young together, so obviously the feat is possible, but whether it is a general practice does not seem to be known. Starting as we did on 3Ist July young bats, weaned by 24th August the latest date on which we took lactating would have been half grown at least. Cranbrook and Barrett started on 15th June but neither they nor we found bats in the net with young attached, nor young lying on the ground below the net.

MOULT

We think that the adults were moulting from the beginning of August onwards. Some bats had longer and finer für than others, e.g., two out of five lactating $$ on Ist August. On 14th September four $$ weighing 6.5, 8.0, 8.5 and 8.5 grammes respectively had this longer für on the shoulders, shorter and sparser für on the rump so we assume that this long silky für is the winter coat and that the moult starts over the shoulders finishing at the tail.

Certainly für clipping marks on the shoulders grew out pretty quickly in August, though one $ was recaptured on 29th September with the August mark still obvious.

ACKNOWLEDGEMENTS

This bat research was only made possible by grants from the Carnegie United Kingdom Trust through the Council for Nature and a Morley Bursary from the Suffolk Naturalists' Society. I am grateful to Lord Cranbrook for help in the preparation of this paper.

REFERENCES

Barrett Hamilton, G. & Hinton, M., 1910-21. A History of British Mammals. London. Gurney & Jackson p. 110.

Cranbrook, Earl of, 1959. Netting Bats. Trans. Suff. Nat. Soc. 11.3.271.