Energetic constraints on mating performance in the sand goby

Behavioral Ecology Vol. 9 No. 3: 297-300 Energetic constraints on mating performance in the sand goby Kai Lindstrom Department of Ecology and Systematics, Zoological Laboratory, PO Box 17, University of Helsinki, Helsinki FIN-00014, Finland I tested the effect of food addition on reproductive success in male sand gobies, Pomatoschistus minuius, by comparing foodsupplemented males with unfed, control males. The sand goby is a small marine fish with paternal egg care. The males were breeding in artificial nest sites in otherwise natural conditions in the field. I quantified energy reserves by extracting nonpolar lipids. The food supplement unproved the fat reserves of the fed males as compared to unfed males. Fed males spent more time at the nest, whereas unfed males spent a much smaller proportion of their time at the nest. As a consequence, fed males mated sooner than unfed males and tended to get more eggs. In the unfed group, mating speed was correlated to body length so that bigger males mated sooner. The results suggest that the reproductive success of breeding sand goby males is constrained by the availability of energy but that this constraint is most severe for small males and less severe for bigger males. Energy availability through its effect on condition will affect the investment in reproductive effort. Key words: condition dependence, fat reserves, mating success, Pomatoschistus minutus, sand goby, sexual selection. [Behav Ecol 9:297—300 (1998)] A nimals are generally constrained in their resource allocation patterns due to limited availability of resources (Roff, 1992). These constraints will lead to various trade-offs in reproductive decision making. This is an important assumption of indicator mechanisms of sexual selection (Andersson, 1994). A general constraint on animal activity is food availability and the need to forage. Reproducing individuals have to divide their time between feeding, mating, and parental care, activities that often are mutually exclusive. How much time an individual can devote to mating effort versus feeding depends on its present energetic condition. Individuals in good condition may, due to their higher energy reserves, be able to allocate more time to mating than individuals in poor condition. In studies where individuals have been given supplemental food, mating performance has improved. In the bicolor damselfish, Stegastes partitus, fed males maintained their courtship rate throughout the breeding season, whereas unfed males showed a decline in courtship rate (Knapp, 1995). In a wolf spider, males with high drumming rates are preferred by females, but a high drumming rate is also costly and lowers survival (Kotiaho et al., 1996; Mappes et al., 1996). Drumming rate of the males decreases over the season except in food-supplemented males, who can maintain a constant drumming rate (Mappes et aL, 1996). The purpose of this study was to test if the reproductive success of naturally breeding male sand gobies, Pomatoschistus minutus, is constrained by food availability. I compared the reproductive success of food-supplemented and non-foodsupplemented ("unfed") males in the field. The sand goby is a small marine fish species that breeds on shallow, soft bottoms. It lives only 1 year (Healey, 1971) but has a prolonged breeding season beginning in May and lasting until July—August during which males undertake repeated brood cycles and females spawn repeatedly (Lindstrom K, unpublished data). The male builds a nest under a suitable mussel shell or stone, which he then guards. The eggs of one or more females are laid in a monolayer in the ceiling of the nest. Being confined Received 22 May 1997; first revuion 2 September 1997; second revision 16 October 1997; accepted 14 November 1997. O 1998 International Society for Behavioral Ecology to the nest will limit the male's opportunities to forage, and, because parental care is energetically costly (Lindstom 1998; Lindstrom and Hellstrdm, 1993), breeding males are likely to be severely food constrained. Because of the prolonged breeding season, sand gobies should experience a trade-off between investing in present reproduction and future reproduction. More intensively courting males are preferred by female sand gobies (Forsgren, 1997), and courtship has often been found to be energetically expensive (see, e.g., Andersson 1994), so it is possible that sand goby males must trade off mating for foraging. Several fish species store energy as nonpolar lipids and use these fat reserves during periods of exceptionally high energy demand or low food availability (Love 1970; Reznick and Braun, 1987). I therefore used the fat reserves of the males to quantify the energy reserves. MATERIALS AND METHODS This study was done during May 1995 at the Klubban Biological Station on the Swedish west coast (58°15' N, 11°28' E). The study site was a shallow bay situated close to the station. The bottom consists of fine sand with a water depth between 50 cm and 1 m. Natural nest sites are readily available, but sand gobies in the area also accept halved flower pots as nest sites. I put out 64 halved clay flower pots (diam 6 cm) at relatively constant water depths (maximum difference about 20 cm). The distance between flower pots was 1.5 m, and they were arranged in four squares of 4X4 pots on areas of clean sand with no vegetation or other objects. Once a nest had been colonized by a male, die male was randomly assigned to be either fed or not during the experimental period. The males in the fed group were offered small, 2 x 2 X 1 mm, pieces of mussel, Mytitus edulis, meat as food. Individual pieces of the food were delivered to the males using a clear plastic tube 35 cm in length. I only gave a male as much food as he would eat but no more than four pieces, and I took great care that no uneaten pieces of food were left in the vicinity of the nest that could attract other fish or egg predators to the nest I checked each nest every day by snorkeling and making notes on waterproof paper. When approaching a male's nest, I recorded whether he was away from the nest, at the nest, or in the nest. If he was in the vicinity of the nest, I recorded his Behavioral Ecology VoL 9 No. 3 298 a) Table 1 A. comparison of ffco sod unfco m**** Variable Fed males Total length (mm) 60.0 ± 6.1 Colonization time (days) 3.1 ± 2 . 8 • Unfed mala 2.5- df 59.4 ± 7.2 0.200 18.6 .843 4.4 ± 4.0 0.880 18.9 388 Meant i SD for different variables and the results of t tests comparing the two groups are presented. Colonization time is the time until a male occupied a nest site. For both variables, the sample sizes are 9 and 12 for the fed and unfed males, respectively. RESULTS I followed the nesting cycle of 21 males. Out of these, 9 males were assigned to the fed group and the remaining 12 to the unfed group. The first males occupied a nest site on the first day after the nest sites were put out in the field. The last (...truncated)