to colony fission under conditions of stress, such as those caused by drought. ... in this species of CheIaner, with more detailed observations on colony fission.
Insectes Sociaux, Paris
9
Masson, Paris, 1983
1983, V o l u m e 30, n* 3, pp. 308-316
DIFFERENT MODES OF REPRODUCTIVE BEHAVIOUR
(INCLUDING A DESCRIPTION OF COLONY FISSION) IN A SPECIES OF CHELANER (HYMENOPTERA : FORMICIDAE) D.T. BRIESE * Department .of Environmental Biology, Australian National University, P.O. Box 475, Canberra 2601, Australia Re~u le 17 juillet 1982.
Accept6 le t8 novernbre 1982.
SUMMARY In successive years, two different modes of reproductive behaviour were observed in two nests of the myrrnieine ant Chelaner sp. ; emission of ,winged sexuals follo~ved by nlaustral colony foundation, and colony fission. A detailed description is given of the first six days of colony fission. After this period 16 incipient daughter colonies had separated from the maternal nest, and the process ~vas still continuing. While it is possible that this species is polymorphic with respect to reproductive behaviour, evidence is presented vghich suggests an alternative possibility, namely that claustral foundation occurs in this species ~vhen environmental conditions are favourable, but that it switches to colony fission under conditions of stress, such as those caused by drought.
RESUME
Dffferent$ modes du comportement reproducteur (y compris la description du bouturage d'une soci6t~) chez une esp~ce de Chelaner L'observation pendant deux armies successives du comportement reproducteur de deux soci6t~s de la Myrmicine Chelaner sp. a permis de mettre en ~vidence 2 modes de reproduction: ~rnission d'individus ail~s aboutissant ~ une fondation par fernelle isol6e et bouturage de la socidt6. On prdsente une description d6tailMe des six premiers jours qui suivent le bot~turage de la soci~t& A l'issue de cette p~riode, t6 Colonies filles s'6taient s6par~es du hid m~re, le bouturage se poursuivant encore. II est done possible que cette esp6ce pr~sente un polymorphisrne reproducteur: Iorsque ins conditions de l'euvironnement sont favorables, il y a fondation ind~pendante; des conditions moins favorables caus~es par exemple par la s~cheresse entra$nent Ia r6alisation des bouturages.
* Present address : Division of Entomology, C.S.I.R.O, Canberra 2601, Australia.
MODES OF REPRODUCTIVE BEHAVIOUR OF C H E L A N E R
309
INTRODUCTION Colonies of s o d a l insects r e p r o d u c e in two m a i n ~ a y s . They m a y either emit sexuals which mate and initiate new colonies by ctaustral foundation, o r m a y g r o w a n d b r e a k u p i n t o p a r t s (BRIARS, 1965). I n t h e h i g h e r a n t s t h e d o m i n a n t m e t h o d is t h e f o r m e r o n e , i n w h i c h t h e q u e e n e s t a b l i s h e s a n e s t cell a f t e r a n u p t i a l flight a n d r e a r s t h e f i r s t b r o o d o n t h e f o o d r e s e r v e s o f h e r o w n b o d y (I-I6LLDOBLER a n d WILSON, 1977). H o w e v e r , i n s o m e s p e c i e s , m o d i fications involving colony fission or hesmosis are found, w h e n q u e e n s depart w i t h a f o r c e of s u p p o r t i n g w o r k e r s . D u r i n g a s t u d y o n a n a n t c o m m u n i t y i n s e m i a r i d A u s t r a l i a (BRIESE a n d MACAULEY, 1977), tWO n e s t s s i t e s of a m y r m l c i n e a n t , Chelaner sp. [ g r o u p of rothsteini (FOREL] w e r e m a r k e d i n a p e r m a n e n t s t u d y .quadrat. I n F e b r u a r y 1972, w i n g e d m a l e s a n d f e m a l e s w e r e o b s e r v e d e m e r g i n g f r o m o n e c o l o n y , w h i l e i n F e b r t i a r y 1973 t h e s e c o n d c o l o n y w a s o b s e r v e d i n t h e p r o c e s s of c o l o n y fission. T h e f o l l o w i n g is a d e s c r i p t i o n of t h e taro m o d e s of r e p r o d u c t i v e b e h a v i o u r i n t h i s s p e c i e s of CheIaner, w i t h m o r e d e t a i l e d o b s e r v a t i o n s o n c o l o n y f i s s i o n . T h e a d a p t i v e a d v a n t a g e s o f p o s s e s s i n g a l t e r n a t i v e m o d e s of r e p r o d u c t i o n a r e d i s c u s s e d f o r this species. METHODS The observed colonies Here in a semi-arid shrub steppe habitat, dominated by the saltbush Atriplex vesicaHa Hew. ex Benth., ~hich Has located 51 km north of Deniliquin in southwestern New South Wales, Australia. The location has been described in detail by BRIESZ and MaCaULEV (1977). The two nest sites were 4.8 m apart. Specimens of male, female and ~eorker ants Here collected from both colonies at the times of observation. Robert W. TAYLOR(C.S.I.R.O. Division of Entomology) examined these and could not differentiate either between males or between workers of the two colonies. Hffvgever, the queens taken from the colony which reproduced by claustral foundation had dark brc~xn gasters and fully developed wings, while queens from the colony undergoing fission had bright orange gasters and only poorly developed ~wings. They ~were otherwise indistinguishable. The gaster of a brachypterous queen, taken with workers and immatures back to the laboratory, changed from orange to dark brown after three weeks. While the possibility cannot be completely discounted that the two forms are sibling species, it is more likely that both the typical queen and the brachypterous queen are eonspecific. The occurrence of winged and brachypterous forms of female in the same colony has been observed in closely related species by WHEELER (1917). Observations on both nests were made from February 20 to 25, 1972, during Hhich time the emergence of winged reproductives from one of the nests was noted. Observations on colony fission in the other nest commenced at 1 030 on February 5, 1973, when the process had just commenced. During the subsequent period temperatures were quite high (daily maximum over 30~ C), and Chetaner sp. showed a bimodal diurnal activity pattern, remaining inactive during the hotter part of the day (BRIESz and MaCaVLEV, 1980). Observations on the progress of colony fission were made twice during the morning period of activity and t~ice during the evening period. The positions of emigrating columns of workers ,were mapped, and the lengths of time taken to travel
310
D.T. B R J E S E
bet-aveen locations were recorded. By counting the numbers of ~orkers, queens and immatures passing a point in given time intervals, and recording the length of time over ~vhich a column passed this point, rough estimates of the populations of emigrating columns ~vere made. Unfortunately, because of other experimental commitments, close observations could only be made over a period of six days. However, the site was revisited six months later to examine the consequences of this colony fission. Voucher specimens of CheIaner sp. have been placed in the Australian National Insect Collection, C.S.I.R.0, Division of Entomology, Canberra.
RESULTS
CIaustral colony foundation Heavy rain (40 mm) had fallen o n F e b r u a r y 16 anti 17, 1972, a n d rainfall had been average prior to that (fig. 1). During the first two days of observations (February 20 and 21) all colonies of Chetaner sp. w e r e engaged solely in nest w o r k and the collection of pebbles to decorate nest m o u n d s . No foraging activity was observed. A f u r t h e r 12 m m of rain fell on F e b r u a r y 21 and 22, and in the late a f t e r n o o n of F e b r u a r y 22, winged males and females e m e r g e d f r o m one nest of Chelaner sp. in the saltbush s t u d y plot. These exhibited the typical r e p r o d u c t i v e pattern, s w a r m i n g in the air, m a t i n g a n d falling to the ground. The females subsequently shed their wings a n d began to search for nest sites. This p a t t e r n was repeated on F e b r u a r y 23, b u t on F e b r u a r y 24 no m o r e Mates were released and the w o r k e r ants h a d r e c o m m e n c e d foraging for food. During this p e r i o d the second colony in the saltbush s h o w e d n o signs of a b n o r m a l activity. I t was not k n o w n w h e t h e r o t h e r colonies f r o m outside the s t u d y plot m a y have c o n t r i b u t e d to the m a t i n g swarm. S u b s e q u e n t searches did n o t reveal any incipient colonies in the 500 m 2 study plot, though these m a y have been present elsewhere.
Colony fission After a long dry spell (fig. 1), heavy rain (30 ram) fell on the night of F e b r u a r y 4-5, followed by a very hot day (38~ max.). Attention was initially d r a w n to the second colony on the m o r n i n g of F e b r u a r y 5 b y the presence of male and female ants m a t i n g on a n d a r o u n d the nest. The males were black a n d fully alate, whereas the females h a d orange gasters a n d only p o o r l y developed wings. This type of female has been described by WttEELER (1917) as a mesonotai, s u b a p t e r o u s f o r m , a n d is morphologically i n t e r m e d i a t e between the typical fully-winged queen a n d the apterous w o r k e r ant. Sixty three of these b r a c h y p t e r o u s queens w e r e observed emerging f r o m the m a t e r n a l nest and m a t i n g with waiting males which had flown in from elsewhere. The source of these males w a s not found. As m a t i n g was already in progress w h e n observations began this figure represents a m i n i m u m c o u n t
MODES OF REPRODUCTIVE BEHAVIOUR OF CHELANER
311
Observations Commenced i Emission of Alates COLONY . . . . . . . . . . 1 v~
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Colony Fission
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120
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80 >. .,..I
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