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Feit, Elizabeth C.
Cambios en el comportamiento femenino en las avispas eusociales Myschocyttarus mastigophorus (Hymenoptera: Vespidae) debido a la eliminacin de los machos del nido
Changes in female behavior in eusocial wasp Mischocyttarus mastigophorus (Hymenoptera: Vespidae) due to the removal of males from the nest
Males of independent founding hymenoptera contribute little to the nest by not foraging for food or feeding larvae. Males are a potential cost to the nest because they consume resources and are not used for
reproduction, but males of Myschocyttarus mastigophorus can be found on the nest year. To investigate if the behavior of females changes when the males are not present, baseline behaviors of both males and females were recorded for four nests M. mastigophorus, then the males were removed and observations continued. Baseline behaviors indicate that females departed (t = 2.33) and arrived (t = 2.81). Departures
increased after the males were removed (t = 2.33, all p < 0.10) and there was a trend for females to arrive more frequently (t = 1.67, p = 0.14). Females possibly increased these behaviors to forage while the males
are not present so the males cannot take food away from the females. It is possible that the males are present year round purely for reproduction with non-sibling females.
Los machos de Hymenoptera contribuyen muy poco a la colonia ya que no cumplen ninguna funcin de forrajeo o alimentacin de las larvas. Los machos son un posible costo para el nido debido a que consumen recursos y no son utilizados para la reproduccin, pero los machos de Myschocyttarus mastigophorus pueden ser encontrados en los nidos aun un ao despus. Para investigar si el comportamiento de las hembras cambia cuando los machos no estn presentes, datos basales del comportamiento tanto de machos como de hembras fueron tomados para cuatro nidos, luego se removieron a los machos y continuaron las observaciones.
Text in English.
Tropical Ecology 2009
Ecologa Tropical 2009
t Monteverde Institute : Tropical Ecology
Feit 1 Changes in female behavior in eusocial wasp Mischocyttarus mastigophorus (Hymenoptera: Vespidae) due to the removal of males from the nest Elizabeth C. Feit Department of Psychology, University of Wisconsin Madison ABSTRACT Males of independent foun ding hymenoptera contribute little to the nest by not foraging for food or feeding larvae. Males are a potential cost to the nest because they consume resources and are not used for reproduction, but males of Myschocyttarus mastigophorus can be found on t he nest year. To investigate if the behavior of females changes when the males are not present, baseline behaviors of both males and females were recorded for four nests M. mastigophorus then the males were removed and observations continued. Baseline b ehaviors indicate that females departed ( t = 2.33) and arrived ( t = 2.81). Departures increased after the males were removed ( t = 2.33, all p < 0.10) and there was a trend for females to arrive more frequently ( t = 1.67, p = 0.14) Females possibly incre ased these behaviors to forage while the males are not present so the males cannot take food away from the females. It is possible that the males are present year round purely for reproduction with non sibling females. RESUMEN Machos de hymenoptera c ontribuyen muy poco a la colonia ya que no cumplen ninguna funcin de forrajeo o alimentacin de larvas. Los machos son un posible costo para el nido debido a que consumen recursos y no son utilizados para la reproduccin, pero los machos de Myschocyttaru s mastigophorus pueden ser encontrados en los nidos an un ao despus. Para investigar si el comportamiento de las hembras cambia cuando los machos no estn presentes, datos basales del comportamiento tanto de machos como de hembras fueron tomados para c uatro nidos, luego los machos se removieron y se continuaron las observaciones. Estos datos indican que las hembras se marchan (t= 2.33) y arriban (t= 2.81). Las partidas aumentan despus de que los machos se removieron ( t = 2.33, all p < 0.10) y hay un a tendencia a arribar ms frecuentemente ( t = 1.67, p = 0.14). Las hembras posiblmente aumentan el comportamiento de forrajeo cuando los machos no estn presentes, as los machos no pueden tomar alimento del que transportan las hembras. Es posible que lo s machos que estn presentes todo el ao para reproducirse con hembras no relacionadas genticamente INTRODUCTION Independent founding eusocial wasps exhibit a dominance hierarchy. This linear hierarchy is used not only for the foundress to prevent low er females from reproducing, but also to make them do nest maintenance activities, such as foraging (de Souza 2008). Females of Mischocyttarus mastigophorus perform the majority of the tasks associated with colony maintenance, including foraging, nest bui lding, and feeding larvae. Males usually do not participate in these tasks, or participate in very low rates (O'Donnell 1995). Giannotti (2004) found the behavior "remaining immobile on the nest" comprised more than 82 percent of all observed behaviors i n males of Polistes fuscatus another independent founding Vespidae. Males performing behaviors relating to colony
Feit 2 maintenance was scarce, including giving alarm 4.8% of the time and feeding larvae 0.8% of the time (Giancotti 2004). Males of both Poliste s fuscatus and Mischocyttarus cerberus also do not participate in colony defense against predators (Judd 1999; Togni & Giannotti 2006). Males of most social Hymenoptera species depart their natals nests to mate with unrelated females and do not return (O' Donnell & Beshers 2004; Strassman 2001). Males of M. mastigophrous, however, return to the natal colony after mating and remain on the nest for up to several weeks (Jeanne 1972; O'Donnell 1999; West Eberhard 1969). Unlike other Vespidae, males are aggress ive to the females on their natal nests, and the females respond to male aggression with submissive behaviors such as crouching, fleeing and departing the nest (O'Donnell 1999). These aggressive, seemingly dominance behaviors may be related to the observa tion that males do not forage, and rely on female foragers for food. Since the males consume resources, but do not contribute to nest maintenance activities such as foraging and feed larvae, it is possible that they are significant cost to the nest (O'Do nnell 1999). This is especially true since they remain on their natal nests longer than males of other social hymenoptera. As the ratio of males to working females increases, workers cannot bring enough food to support the entire colony, which can lead t o colony decline (Jeanne 1980). Males may be an even greater cost during the dry season, when the colony already has reduced colony growth due to low prey abundance (O'Donnell 2000). This sex biased workload and consumption of resources provides interesti ng questions as to the function of males on the nest. They do no reproduce with females on their natal nests, and can be found on the nests all times of the year and during all stages of colony development (O'Donnell 1999). This constant male presence su ggests that they may have some specific role in the nest other than reproduction, even though they appear to contribute little. There is an overall lack of knowledge of male behavior and function in paper wasps, especially tropical species (O'Donnell 199 9). The purpose of this study is to investigate if the females change their behavior after the males are removed from the nest. The results of this study will give more insight into the importance, if any, of producing males that remain on their natal ne st. If males have a role in the nest other than reproduction, females may adjust their actions to compensate for the sudden male absence. METHODS Procedure Observations took place at the Monteverde Cloud Forest Reserve (MVCFR) in Monteverde, Costa Ri ca. Four nests of Mischocyattarus mastigophorus were located on the rafters of buildings within the MVCFR complex (n = 5 to 8 individuals per nest) from 20 April 2009 to 3 May 2009. All observations took place between 0930 and 1200 for morning observation s, and 1230 and 1500 for afternoon observations, with one colony observed at a time. The following methodology was used for each colony sampled. One day of baseline behavioral observations was taken with two hours of observation in the
Feit 3 morning and two ho urs in the afternoon. At sunset (approximately 1800 local time) the day of baseline behavioral observation, males were removed from the nest after observations were completed. Males were distinguished from females by their long, filamentous antennae and w hite faces (O'Donnell 1999). After males were removed, behavioral observations were continued the next day, with two hours of observations in the morning and two hours in the afternoon. Behavioral Observations The researcher stood on a ladder directly in front of the nest, within 0.5 meters. A hand held tape recorder was used to record all observations for each two hour block of time. Observed behaviors included nest maintenance tasks, social interactions, flying departures and arrivals, transfers of f ood and building materials, chasing, biting, and displacing (O'Donnell 1999). Recordings were transcribed and quantified at a later date for statistical analysis. Statistical Analysis A sum of all behaviors from each nest was transcribed from the tape recordings and then transformed into rate per individual per hour for each behavior. The average rate per individual per hour for each behavior was used in independent sample t tests for comparing differences in male and female behaviors and paired t test s for comparing female behavior after the males were removed to baseline behavior. RESULTS Primary Findings During baseline observations, females departed the nest more than males ( t = 2.33, p = 0.058)(see Fig. 1). Females also arrived on the nest more than males, ( t = 2.81, p = 0.03; Fig. 1). There was also a trend for females to arrive on the nest with food more frequently than males ( t = 1.67, p = 0.14; Fig. 1). Females departed the nest more after the males were removed than when the males were pres ent ( t = 2.33, p = 0.10; Fig. 1). Similarly, there was a trend for females to arrive on the nest at a higher rate when the males were absent compared to when the males were present ( t = 2.35, p = 0.13; Fig. 1). While the rate of females arriving on the ne st with food decreased after the males were removed, this decrease was insignificant ( t = 0.84, p = 0.57; Fig. 1).
Feit 4 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Arrival Departure Bring food to the nest Female, Baseline Male, Baseline Female, Post Removal ______________________________________________________________________ FIGURE 1. Comparison of female behavio r, male behaviors, and females, and female behavior after the males are removed from the nest. Females arrived and departed the nest significantly more than males. Female departures increased after the males were removed. Females also put their head in a cell significantly more than males, but this data was excluded from the analysis_________________________________________ Other Observations The most commonly observed behavior was when a female put her head in a cell. Females performed this behavior at a significantly higher rate than males ( t = 3.47, p = 0.01; Fig. 2). The rate of this behavior decreased in females after the males were removed, but this change was insignificant ( t = 0.43, p = 0.69). However, this behavior was excluded from the ana lysis because it is not clear what task the wasps were performing. When an individual sticks its head in a cell, the individual could be feeding larvae, or the larvae could be feeding the individual (Y. Molina, pers. comm.). Since it was important to det ermine exactly what task the male and female wasps were performing to determine if behavior was changing post male removal, this behavior was excluded from the analysis. Another interesting observation was the number of individuals on each nest. Of the four nests included in this analysis, there was a range of five to eight total individuals per nest, which was much lower than found by previous studies, some of which occurred at the same location (Table 1). However, the percent composition of males on the nest was very similar. This stud found males comprised an average of 46% of individuals on the nest, while O'Donnell (1999) found males comprised 41%.
Feit 5 0 2 4 6 8 10 12 14 16 Female, Baseline Male, Baseline Female, Post Removal Head in cell _______________________________________________________________________ _F IGURE 2. Comparison of the rates of individuals putting heads in cell for female baseline, male baseline, and female after the males were removed.___________________ _______________________________________________________________________ TABLE 1. Compar ison of the number and composition individuals on the nest with previous studies. While the total number of individuals has declined in comparison to previous years, the percent composition of males on the nest is similar. _______________ Study Location i n Costa Rica Species Wasps per nest Avg. number of wasps per nest Avg. % composition of males % Range of males per nest O'Donnell, 1999 Monteverde M. mastigophorus 2 128 23.9 41 25 58 O'Donnell, et al, 2007 Monteverde M. mastigophorus 7 13 9.9 -------------------Feit, 2009 Monteverde M. mastigophorus 5 8 6.5 46 20 63 DISCUSSION
Feit 6 Females participated in behaviors at a higher rate than males for departures, arrivals, and bringing food to the nest. This sex biased division of labor suppor ts results found in other studies that females perform colony maintenance behaviors at a higher rate the males (Judd 1999; Giannotti 2004; O'Donnell 1995; Togni & Giannotti 2006). After the males were removed from the nest female behavior increased in arri vals and departures, but bringing food to the nest decreased after the males were removed compared to when the males were present. Perhaps the females increased departures and arrivals to forage. If there are no males on the nest to take the food, then th ere is more for the other females and the larvae. If the females are taking advantage of the absence of males to increase foraging, this is perhaps a clue that the males are a cost to the nest. However, females brought less food after the males were rem oved than when the males were present. This does not mean they are foraging less, but rather that with the males on the nest they do not need to bring as much food back because the male will not take any of it away. Another explanation for the changes i n behavior seen after males were removed is that these changes are normal for females while the males are gone, since males usually leave during the day to search for mates (Jeanne 1980). It is possible that females always increase foraging while the male s are gone so the males cannot steal food away from them when the females arrive on the nest with food. With the males being removed permanently in this study, they were gone for an extended period of time compared to the baseline observations. The removal allowed the females to continue increasing the arrivals and departures throughout observations. While explanations are varied as to reasons why females may change behavior when the males are removed, they clearly do change their behavior, indicating some underlying mechanism of male presence on the nest. A second question of this study is why are there males on the nest year round if they contribute little to the nest? Related to this question is the observation that the male composition of males on the nest in this study was very similar to male nest composition in previous studies. It is possible that ~41% may be the optimal percentage of males to maximize fitness. Since males are haploid, they are more related to the foundress (their mother, 100% rel ated) than diploid sisters are to the foundress (average of 75% related). Therefore the foundress increases her fitness, via genetics, when her sons reproduce when compared to producing sisters. Perhaps M mastigophorus does not have a single breeding se ason, so females produce males year round to maximize their fitness. However, it is not beneficial to produce as many males as possible because males only consume resources from the nest and contribute little, which is a possible cost to the nest. It is possible that the male composition observed in this study and in previous studies on the nest is stable and maximizes the fitness of the foundress without sacrificing the cost of maintaining the nest via consumption of resources. Despite the hypotheses b eing presented, the function of having males on the nest year round is still unknown. If the males do not perform activities that benefit the nest (such as foraging for food and feeding larvae) and if females increase foraging while the males are absent s o they do not steal the food, then it would seemly costly to the nest to have them present year round. This study was conducted at the end of the dry season when colonies should be experiencing a decline in growth (O'Donnell 2000). However, there were ma les present on each nest found despite there being a small amount of
Feit 7 females. This observation supports the hypothesis that males increase fitness of the foundress because of their haplodiploidy. Future studies could repeat this one, increasing the num ber of nests, since this study only included four. This study could also be extended throughout the year since many studies conduct observations between May and September. This could confirm the changes behaviors, as well as possibly show other changes in behavior. This study could also be repeated with marking individuals and seeing if the rates of individual females increases or decreases differently than other individuals. To test if the hypothesis that females increase behaviors normally while males are not on the nest, a study could compare the behavior of females while males are present versus absent, except without manipulation of removing the males. This would show if females do try to prevent males from taking food by foraging when they are abse nt. Another important area of investigation is continuing to study the role of the males on the nest, since their role other than reproduction is still unknown. Overall, there were marked changes in female behavior after the males were removed from the nest. The rate of females arriving and departing increased when the males were not present. This possibly indicates that females are trying to compensate for males using resources by increasing foraging while the males are absent so the males cannot take food that could be used for the females and the larvae. On the other hand, it may also be beneficial to have males on the nest year round because their haplodiploidy increases the fitness of the foundress when her sons mate with females from other nests. Despite these hypotheses, the role of the male on his natal colony is still unknown. ACKNOWLEDGEMENTS A big thank you to Anjali Kumar for all her help and support in the planning execution of this project, as well as all her help with data analysis an d interpretation. I also thank Yamile Molina from the University of Washington for her tremendous assistance with methodology and the statistical analysis. Thank you to the Monteverde Cloud Forest Reserve (MVCFR) for allowing me access to reserve buildin gs to conduct my observations. Thank you to the CIEE Tropical Biology program for providing funding for supplies and giving me the opportunity to complete this project. LITERATURE CITED DE S OUZA A.R., I.L. R ODRIGUES J.V.A R OCHA W.A.A R EIS J.F.S L OP ES AND F. P REZOTO Foraging behavior and dominance hierarchy in colonies of the neotropical social wasp Polistes ferreri (Hymenoptera, Vespidae) in different stages of development. Sociobiology. 52: 293 303. G IANNOTTI E 2004. Male behavior in colonie s of the social wasp Polistes lanio (Hymenoptera, Vespidae). Sociobiology. 43: 551 555. H ANSON P.E. 1995. The biology of hymenoptera. Pages 70 73 in P.E. Hanson and I.D. Gauld. The Hymenoptera of Costa Rica. Oxford University Press, New York, NY. J EANNE R.L. 1972. Social biology of the neotropical wasp Mischocyttarus drewseni Bulletin of the Museum of Comparative Zoology Harvard University. 144: 63 150.
Feit 8 J EANNE R.L. 1980. Evolution of social behavior in the Vespidae. Annual Reviews in Entomol ogy: 25, 371 396. J UDD T.M. 2000. Division of labour in colony defence against vertebrate predators by the social wasp Polistes fuscatus Animal Behaviour. 60: 55 61. M OLINA Y., AND S. O'D ONNELL 2008. Worker reproductive competition affects divisio n of labor in a primitively social paperwasp ( Polistes instabilis) Insectes Sociaux. 56: 14 20. O'D ONNELL S. 1995. Division of labor in post emergence colonies of the primitively eusocial wasp Polistes instabilis de Saussure (Hymenopter: Vespidae). Insectes Sociaux. 42: 17 29. O'D ONNELL S. 1999. The function of male dominance in the eusocial wasp, Mischocyttarus mastigophrous (Hymenoptera: Vespidae). Ethology. 105:273 282. O'D ONNELL S. 2000. Eusocial wasps: (Vespidae: Polistinae). In N.M. N adkarni and N.T. Wheelwright. (Eds.). Monteverde: ecology and conservation of a tropical cloud forest, pp 129 131. Oxford University Press, New York, NY. O'D ONNELL S., AND S. N. B ESHERS 2004. The role of male disease susceptibility in the evolution of haplodiploid insect societies. Proceedings of the Royal Society of London B: 979 983. O'Donnell, S., N. Donlan, & T. Jones. 2007. Developmental and dominance associated differences in mushroom body structure in the paper wasp Mischocyttarus mastigophoru s Journal of Neurobiology. 67: 39 46. S TRASSMAN J 2001. The rarity of multiple mating by females in the social Hymenoptera. Insectes Sociaux: 48, 1 13. T OGNI O. C. AND E. G IANNOTTI 2006. Male behavior of Mischocyttarus cerberus (Hymenoptera, Vespi dae) during attacks on the nest by ants. Sociobiology. 48: 553 558. W EST E BERHARD M. J 1969. The social biology of Polistine wasps. Miscellaneous Publications of the Museum of Zoology University of Michigan. 140: 1 101. W EST E BERHARD M. J. 1983. Polistes. In D.H. Janzen. Costa Rican Natural History, pp 758 760 University of Chicago Press, Chicago, IL.