Re-copulation and post-copulatory mate guarding increase immediate female reproductive output in the dragonfly Nannophya pygmaea. Rambur. Received: 30 ...
Behav Ecol Sociobiol (1994) 35:219 225
© Springer-Verlag 1994
Yoshitaka Tsubaki • Michael T. Siva-Jothy Tomohiro Ono
Re-copulation and post-copulatory mate guarding increase immediate female reproductive output in the dragonfly Nannophyapygmaea Rambur
Received: 30 May 1993 / Accepted after revision: 2 July 1994 Abstract After copulation, male Nannophya pygmaea dragonflies mate guard by hovering over ovipositing females and repelling conspecific males. Copulation is not always a prerequisite for oviposition in the females of this species because females can store the sperm received during previous visits/copulations. An oviposition "episode" consists of several "bouts" of oviposition separated by periods of perching. We conducted two types of male-removal experiments to examine the effects of mating and post-copulatory mate guarding on the oviposition behaviour of females. In the first experiment, we removed all males from the habitat to eliminate the effect of re-copulation, mate-guarding and harassment by males. In the second experiment, we removed males immediately after copulation to eliminate the effects of guarding and other post-copulatory malefemale interactions. We compared these experimental data with data obtained under natural conditions. The dipping rate in an oviposition bout was not influenced by copulation or guarding. However, guarded females made more dips per episode than did solitary females. The proportion of time actually spent ovipositing (total bout duration/oviposition episode duration) of guarded females was higher than that of solitary females. Solitary females often oviposited in more than one territorial site, while guarded females usually oviposited within a single territorial site during an oviposition episode. Because males tend to hold territories at sites where egg survival is high, guarded females (and the male guardian) benefit from guarding in terms of egg hatcha-
Y. Tsubaki ( ~ ) Laboratory of Wildlife Conservation, National Institute for Environmental Studies, Onogawa, Tsukuba, 305 Japan M.T. Siva-Jothy Department of Animal and Plant Sciences, University of Sheffield,Sheffield,S10 2UQ, U.K. T. Ono Laboratory of Biology, Kinjo Gakuin University, Moriyama, Nagoya 463, Japan
bility. The possible benefits for solitary females are discussed. Key words Dragonfly • Re-copulation Mate guarding - Oviposition. Site selection
Introduction The males of many dragonfly species guard their mates after copulation (e.g. Sakagami et al. 1974; Waage 1984; Higashi et al. 1987). Post-copulatory guarding in this order usually consists of males perching or hovering near ovipositing females and repelling other males from the immediate area: this behaviour is often referred to as non-contact mate guarding. In all odonate species so far examined, the sperm of the male that copulates with a female immediately before oviposition will fertilise most of her eggs (Waage 1979a,b; Fincke 1984; McVey and Smittle 1984; Siva-Jothy and Tsubaki 1989; Wolf et al. 1989). Post-copulatory mate guarding is therefore adaptive because it prevents sperm displacement (Parker 1970) and thereby loss of sperm precedence. Observations on Pachydiplax longipennis(Sherman 1983) and Nannophya pygmaea (Tsubaki and Ono 1985) showed that males usually hovered above and behind ovipositing females unless the female approached the territory boundary, when the male would move into a position that prevented his mate from moving out of the territory. Guarding may therefore also function to keep the female within territory boundaries, as well as repelling intruders. Both of these functions prevent other males from copulating with a recently mated female and therefore benefit the guarding male. Although several authors have discussed the benefit of guarded oviposition to females (Waage 1978, Higashi et al. 1987), the effect of mate guarding on female oviposition site selection (and/or mate choice) has been neglected. If the function of guarding is solely to repel intruders, females may be able to deposit eggs more safely and efficiently with a guard. However, if guarding
220 also functions to keep females within a territory d u r i n g oviposition, a n o t h e r q u e s t i o n arises: is it better (in fitness terms) for a female to c o p u l a t e with a male a n d be c o n s t r a i n e d by t h a t male to oviposit within his territory, or to c h o o s e oviposition sites w i t h o u t c o p u l a t i o n a n d therefore g u a r d i n g ? In o t h e r words, d o u n g u a r d e d females have access to oviposition sites suitable for egg a n d / o r larval survival t h a t g u a r d e d females c a n n o t utilise because of their g u a r d i a n ? This p a p e r presents results a i m e d at addressing these questions.
Materials and methods Study site The study was conducted on a moor near the campus of Kinjo Gakuin University, Omori, Nagoya, Japan in 1986. Within the moor, we chose a site for our behavioural observations that contained several small pools. The study site and marking procedure have been described in more detail by Tsubaki and Ono (1986). In the observation area there were seven territorial sites where at least 1 male dragonfly held a territory on at least one day during the observation period. The observation area was covered with 32 square grids (numbered 1-32) of 50 x 50 cm so that sites where oviposition occurred could be identified. Each grid corner was marked with a small vinyl tag. Dragonflies were individually marked between 13 June and 20 August 1986. Behavioural observations Observations of oviposition and related behaviours were made on 12 sunny days from 1 to 20 August 1986. The observation days were separated into three periods. During the first period (5 fine days between 1 and 6 August), we observed oviposition without manipulation. Unguarded oviposition without copulation was observed during the 4 fine days between 11 and 16 August when we removed all males from the observation area. Finally, prior to the third period (3 fine days between 18 and 20 August), we introduced new males caught at other pools from the same moor. During this period, we observed the oviposition behaviour of females after copulation: mated males were removed soon after copulation. After copulation was completed, both male and female perched briefly, making it possible to remove the male without disturbing the female. We therefore recorded the oviposition behaviour of females under three different conditions: (1) guarded oviposition after mating, (2) unguarded oviposition without mating and (3) unguarded oviposition after mating. By comparing conditions 1 and 3, we could examine the effects of guarding, and by comparing conditions 2 and 3 we could examine the effects of copulation on female oviposition behaviour. We refer to those females ovipositing under conditions 1, 3 and 2 as guarded, copulated/solitary, and solitary females respectively. When a female arrived at the observation area, we recorded the sequence of reproductive behaviours (e.g. copulation, oviposition, mate guarding) with the aid of a tape recorder, and terminated the recording when the female left the area. The location (grid number) and the number of dips made by females during oviposition was also recorded. As most females not ovipositing or mating are usually perching on nearby vegetation (within 10 m) and since there are no other suitable pools for oviposition, we are confident that we observed all the reproductive events of our marked females during the observation periods. Sometimes a female's oviposition status changed during an episode of oviposition. Guarded ovipositing females (i.e. type 1) occasionally left the reproductive area (due to disturbance by intruding males) but later came back to the same area to oviposit as
"type 3" females. Two such females were categorised as "type 1" because they copulated with, and laid eggs whilst being guarded by, new mates. The duration of an oviposition episode was defined as the length of time between the moment a female initiated the first oviposition bout and when she terminated the final oviposition bout on that day. The duration of an oviposition bout was defined as the length of time between the moment when a female left her perching site to oviposit to the moment when she perched again. Egg survival To assess territory quality, we estimated the proportion of eggs that survived to hatching at each territorial site. We obtained eggs for this procedure by catching a receptive female and dipping her abdomen into water in a glass tube: 100-200 eggs per female could be collected by this method. Eggs were collected from 8-10 females and then mixed (to randomise any maternal effect on egg survival). Between 10 and 50 eggs were placed on a 0.2-mm mesh attached to a metal ring (13 mm in diameter) that was placed at the bottom of a pool in the observation site. The surface of an egg is covered with jelly-like material that becomes hard after 4 h: this helped eggs attach to the mesh. We placed five to seven rings with eggs at each territorial site on 26 July and collected them on 6 August 1986 and repeated this procedure from 13 to 21 August 1986. We combined the data from both replicates because the combined data represent survival (and therefore territory quality) over the period we made our behavioural observations. Eggs were examined with a stereo-microscope to distinguish hatched from unhatched eggs: a hatched egg has a large split along its length (the hatching line). Measurement of water temperature and dissolved oxygen Water temperature and dissolved oxygen concentration in the vicinity of eggs was measured with a self-calibrating digital thermometer (Takara D211) and a dissolved oxygen meter (YSI model 57) at around noon (1130-1230 hours) on 6 days throughout the reproductive season (7, 12, 22 July, 1, 3, 9 August). Statistics Means are presented _+ SDs unless otherwise stated.
Results General behaviour T h e r e p r o d u c t i v e b e h a v i o u r of male a n d female N . pygmaea has been described by N a k a m u t a et al. (1983) a n d T s u b a k i a n d O n o (1985, 1986). Here, we briefly summarise oviposition, a n d related behaviour. Males defend oviposition sites, such as pools or shallow rivulets. T h e size of a territory is a b o u t 1 1.5 m in diameter. Sexually receptive females arrive at these sites to oviposit a n d are usually intercepted by territorial males. C o p u l a t i o n lasts a b o u t 30 s a n d last-male s p e r m precedence is 0.98 i m m e d i a t e l y after c o p u l a t i o n (Siva-Jothy a n d T s u b a k i 1994). After m a t i n g the male a n d female perch less t h a n a b o u t 20 c m a p a r t on e m e r g e n t grass for 3 0 - 6 0 s. Females then c o m m e n c e oviposition within their m a t e ' s territory, repeatedly depositing eggs into the water at that site. A c o m p l e t e r e p r o d u c t i v e event
221 Table 1 Duration of oviposition bouts and number of dippings in a bout for (1) guarded, (2) copulated/solitary and (3) solitary females
Ovipositon type
Bout duration (sec) -+ SD
No. dippings _+SD
Dipping rate _+SD
Sample size n
(1) Guarded (2) Copulated/solitary (3) Solitary Probability (Kruskal-Wallis test)
8.54_+5.11 7.96 -+ 5.50 7.54+4.62 0.187
8.84_+7.19 10.02-+ 9.67 7.92_+6.66 0.882
0.974-+0.449 1.086-+ 0.629 0.986-+0.505 0.450
192 50 128
600 7 ~
500 6
,~ •~.
24
g
400
5
g ]..,-~_-4
t
.~ 300 g
4
27
o
P, 200
~,
3
Z
100
I
I
Guarded female
Copulated/ solitary female
I
Solitary female
I
Guarded female
I
I
Copulated/ solitary female
Solitary female
Fig. 1 Duration of an oviposition episode (s), shown with 95% confidence intervals, in guarded, copulated/solitary and solitary females. Numbers are sample size
Fig. 2 Number of oviposition bouts, shown with 95% confidence intervals, in guarded, copulated/solitary and solitary females. Numbers are sample size
(from female arrival to female departure) takes about 5 min if the pair is not disturbed. An oviposition episode consists of several bouts of oviposition, which are separated by periods of inactivity (perching), generally lasting 10-60 s. Females strike the water surface ("dipping") usually 5-20 times in an oviposition bout of 5-20 s. In each dip the female lays 2.95_+0.42 SE (n -- 31) eggs. Whilst the female oviposits, the male usually guards her by hovering above her.
multiple pairwise comparison of means (Sokal and Rohlf 1981), P0.1). As a result, there were fewer dippings in an oviposition episode for solitary females (31.1 ___20.2, n = 27) than for guarded females (63.4 -+ 39.9, n = 24) or copulated/solitary females (61.4_+ 28.9, n = 9) (Kruskal-Wallis test, P < 0.01 ; GT2, P < 0.01 and P < 0.05, respectively). The proportion of actual oviposition (total bout time) in an oviposition episode was greater in guarded females (0.208_+0.095, n = 24) than in copulated/solitary (0.144_+0.094, n = 9) or solitary females (0.144_+0.103, n = 27) (Kruskal-Wallis test, P ,~
10
._
5 O
.~_
~
9
.15
1
27
2
~
~
Copulated/solitary ales
--~
~
A0
g R m05
I
i
i
1
Guarded Copulated/ Solitary female solitary female female Fig. 3 Proportion of total bout time against the duration of oviposition episode in guarded, copulated/solitary and solitary females. Vertical bars indicate 95% confidence intervals. Numbers are sample size
(mean = 1.15_+0.36). Those females that oviposited in two different territorial sites did so because they were captured by neighbouring territorial or non-territorial males, and were carried to other territories or vacant sites. In contrast, a substantial number of copulated/ solitary and solitary females visited two or more territories without remating (copulated/solitary = 2.0 + 0.82; solitary = 1.79_+0.72). The average number of territories visited by females was significantly smaller in guarded females than copulated/solitary or solitary females (Kruskal-:Wallis test, P
