Behav Ecol Sociobiol (1990) 27:159-168 Behavioral Ecology and Sociobiology O Springer-Verlag 1990 Provisioning behavior and the estimation of investment ratios in a solitary bee, Calh'opsis (Hypomacrotera) persimilis (Cockerell) (Hymenoptera: Andrenidae) Bryan N. Danforth Snow Entomological Museum, Department of Entomology, University of Kansas, Lawrence, KS 66045-2106, USA Summary, One aspect of behavioral ecology that has male offspring production were calculated. Estimates of received considerable attention, especially by students the cost ratio based on the amount of time spent forag- of social insects, is the relative amount of energy invested ing, adult dry body weight, and pollen ball dry weight by parents in the rearing of male versus female offspring. all give similar values. Female offspring receive an ener- Sexual selection theory makes predictions about how getic investment of from 1.3 to 1.5 times that of males. individuals should allocate their total investment in the These results support the use of adult dry body weight sexes. To test these predictions we must accurately quan- ratios in the estimation of cost ratios. tify the relative "cost" incurred by a parent in the pro- duction of an average individual of either sex. Body weight ratios are the most common estimate of cost ra- tio, but the correspondence between offspring body Introduction weight and energetic investment on the part of the parent has rarely been determined. Calliopsis (Hypomaerotera) The relative amount of resources invested by parents persimilis is a solitary, ground-nesting bee whose natural in the total output of male versus female offspring is history makes it particularly convenient for studies of termed investment ratio. In the classic model of sex ratio investment patterns and foraging behavior. Each day evolution, R.A. Fisher (1958) predicted that population- females construct and provision from 1 to 6 cells in lin- wide investiment in male and female offspring should ear, closely-spaced series. Each cell is provisioned with approach equality, in panmictic populations. Any devia- pollen from Physalis Wrightii flowers, which is collected tion from a 50:50 investment ratio within a population on two or three foraging trips. However, the temporal should be corrected by selection favoring females that sequence in which two- and three-trip foraging bouts produce the under-represented sex. I will follow Booms- occur is not random. Females invariably begin each day ma (1989) in considering investment ratio (IR) as the provisioning cells with three trips worth of pollen and total investment in female production divided by the usually switch to provisioning the latter cells of the day total investment in male production. The investment ra- with just two trips worth of pollen. The sex of the off- tio can be calculated by multiplying the sex ratio (SR, spring within the same co-linear series of cells also varies no. females/no, males) by some measure of the relative non-randomly - female offspring predominate in the cost to the parent of an average female divided by the first cells of each series and male offspring in the latter relative cost of an average male ( CR) : IR = CR x SR. cells. The correspondence between the number of forag- The question of how to measure cost is fundamental ing trips to provision a cell, the total time spent foraging, to studies of investment ratios. Trivers (1972) pointed and offspring sex was determined for 36 cells. The data out that the true cost of an offspring to the parent should indicate a close, though not absolute, relationship be- be measured in terms of the diminution of total resources tween the number of foraging trips and the sex of the available for investment in other offspring. However, this offspring: males usually received two trips of pollen, can rarely be measured and approximations are usually though some received three, whereas female offspring used. The most commonly used estimate of offspring invariably received three trips worth of pollen. A number cost is average wet or dry body weight (Trivers and Hare of potential estimates of the relative cost of female and 1976; Cowan 1981; Frohlich and Tepedino 1986; Boomsma 1989). The accuracy of this estimate has been Present address: Department of Entomology, National Museum questioned because of metabolic differences between the of Natural History, Smithsonian Institution, Washington, DC sexes (males require more food per unit of adult body 20560, USA mass than females) and variation in water content 160 (Trivers and Hare 1976; Boomsma 1989). More funda- for at least 3 days prior to weighing. In the results presented below mentally, the degree to which offspring body weight cor- means are given with their standard errors. Voucher specimens responds to the energetic input on the part of the parent are deposited in the Snow Entomological Museum. is poorly understood. The biology of mass-provisioning bees and wasps provides an economically and energetically simple sys- Results tem with which to investigate questions of parental in- vestment. Several correlates of offspring cost can poten- tially be quantified: the time spent by females for cell Phenology construction and provisioning, the total weight of the provisions consumed by larvae, and the weight of the When nests were first located on 7 August, it appeared adult offspring produced by those provisions. Because that the nesting season had just begun because all nests of some unique features of the biology of Calliopsis (Hy- excavated contained only a few cells, and the brood con- pomacrotera) persimilis, it was possible to determine the sisted exclusively of eggs and small larvae. Nests contain- time spent by females in provisioning cells destined to ing pupae were first detected on 16 August, and thereaf- contain male offspring versus those destined to contain ter a range of nests, from newly initiated nests containing female offspring. Average time spent foraging for off- predominantly eggs to nests containing predominantly spring of either sex should provide a realistic estimate pupae, could be found. By 7 September the number of of offspring cost, especially for oligolectic bees, in which active nests had declined, and all nests excavated had resource availability is temporally limited. Interestingly, pupae or callow adults emerging from cells. The last this estimate of cost corresponds very closely to esti- instar larva is most likely the overwintering stage, as mates of energetic cost based on adult dry body weight in other panurgines (Rozen 1989). and the dry weight of the pollen and nectar provisions. These results have implications for the estimation of in- vestment ratios in other insects. Nest occupancy and structure Although Cockerell (1899) considered C. persimilis (previously Hypomacrotera; see Ruz, 1988) a subspecies All of the nests excavated (n= 59) contained a single of C. callops, there are several morphological characters female. How long a female occupied a nest and whether that allow one to unambiguously separate them into two females occupied more than one nest during their lives species. These features include those listed by Cockerell was not determined. Nests were located on horizontal and two not mentioned by him (Danforth, in prepara- ground and most entrances were surrounded by a circu- tion). The two species have overlapping ranges in south- lar tumulus 3-4 cm in diameter. The entrance remained ern Arizona and New Mexico. Rozen (1970) studied C. open while bees foraged but was closed 4-5 min after callops near Douglas, Arizona, and found no evidence the last trip of each foraging bout. of serial cell construction. The main tunnel entered the ground at a steep angle (between 45 ° and 90 ° to the horizontal) and thereafter followed a straight path downward (Fig. 1). The first cells were encountered 5 to 7 cm below the surface and Materials and methods the deepest cells were found at 12 cm. The maximum s number of completed cells per nest was 22. Cells oc- This study was carried out at a locality 1 km north of Animas, Hidalgo County, New Mexico, USA. Approximately 90 nests were curred singly or, more commonly, arranged in co-linear, found between 7 August and 7 September 1988 along a dirt road closely-spaced series of from two to six cells (Fig. 1 d, running parallel to state road 338. This area is a mixed grassland e). The distance between the ends of cells in series was habitat with scattered Sphaeralcea (Malvaceae), Sida (Malvaceae), 3-5 mm. Single cells, containing the oldest and siginifi- Charnaesaracha (Solanaceae) and a dense patch of Physalis Wright- cantly male-biased brood, were the shallowest. As in ii (Solanaceae), the sole source of pollen for C. persimilis, adjacent some other panurgine bees (Rozen 1967), the cells of to a cotton field. The times of departures and returns of individual female bees C. persimilis were lined with a waterproof material. were recorded by placing clear plastic cups over the nest entrances. When a bee emerged from the nest into the plastic cup, it was immediately released, and when a bee was seen flying around a Parasitism and predation cup, the cup was removed and as the bee entered her nest the presence or absence of pollen loads was noted. It was essential A total of 320 cells were located while excavating mature to allow bees to fly normal orientation flights on the first departure of the day, otherwise they could not find their nest entrances when nests (nests containing primarily last instar larvae and they returned. Using plastic cups thereafter did not seem to affect pupae). Some of these cells (6.9%) contained larvae or the behavior of the bees and allowed me to follow up to six nexts provisions infected with fungi, and some (2.2%) con- at a time. tained bombyliid fly larvae.