Biparental Beetle Research Articles

Differential movement and activity patterns of sexes in a biparental beetle during the reproductive season

1. Biparental care is stabilised if parents perform different tasks during care. Specialised parental roles may require different time and energy budgets that in turn are expected to influence the activity and space use of sexes.2. Here we investigate movement patterns of the biparental Lethrus apterus beetle using a grid of pitfall traps in their natural habitat.3. Sexes of the burrow building L. apterus perform different roles during caregiving, as females collect most of the leaves, which serve as food for the offspring while paired males stay mostly in the burrow. We hypothesised that sex differences in mate search and parental activities are reflected in movement patterns.4. We found that females frequently travelled short distances, whereas males were detected less often but when detected, they travelled significantly longer distances than females.5. Our results are consistent with the notion that efficient parental food provisioning requires more localised movement and activity patterns. Furthermore, the long distance movements of some males may indicate active mate searching behaviour.

Seasonal changes in immune response and reproductive investment in a biparental beetle

Immunity and reproduction are physiologically demanding processes, therefore trade-offs are expected between these life history traits. Furthermore, investments in these traits are also known to be affected by factors such as sex, body size, individual condition, seasonal changes and parasite infection. The relationship between immunity and reproduction and the effect of other factors on this relationship were investigated in many species, but there are a small number of studies on these patterns in biparental invertebrates. Lethrus apterus is an iteroparous biparental beetle with predominant female care in respect of collecting and processing food for larvae. Males guard the nest built underground and also their mate. Here we investigate how sex, body size, time within the reproductive season and parasite load may influence the relationship between immunocompetence and reproductive investment in this species. In beetles from a natural population we quantified immune response by measuring the encapsulation response, antimicrobial activity of hemolymph, the investment into reproductive tissues by measuring the size of testis follicles in males and total egg size in females, and parasite load by counting the number of mites on the beetles. We found that the encapsulation response is condition-dependent, as large individuals showed significantly higher encapsulation ability than small ones. Antimicrobial capacity was significantly higher in females than in males. In case of antimicrobial activity there was also a seasonal change in the relationship between immunity and reproductive investment, but only under heavy mite load. Reproductive investment was influenced by the interaction between body size and season (in females) and by body size and season (in males). Furthermore in females the interaction between antimicrobial activity and season indicated that reproductive investment increased with antimicrobial activity early in the reproductive season. By investigating the relationship between immunity and reproductive investment in a natural population of a biparental beetle species, we can conclude that investments into these important life history traits are governed by complex interactions between physiological and environmental factors. Our results are discussed in the context of life history evolution, highlighting the role of the assessed factors in shaping trade-offs themselves (in invertebrates).

Willingness to initiate a fight but not contest behaviour depends on intruder size in Lethrus apterus (Geotrupidae)

In resident-intruder contests, residents are expected to win more often than intruders unless the intruder has significantly higher competitive ability that is often determined by its size. Therefore, small intruders are expected to be less motivated to engage in contests than large ones and intruder size is predicted to have a positive relationship with the duration and escalation of the encounters. In a field experiment we tested these hypotheses in Lethrus apterus, a biparental beetle breeding in underground tunnels, by placing either a small or a large male in front of a tunnel occupied by a resident male. In agreement with our predictions residents won most of the encounters. Small intruders were less willing to engage in a contest and were less successful in their takeover attempts than large intruders. Contrary to many studies however, the duration and escalation (measured by the occurrence and latency of the different contest stages) of the fight in front of the burrow did not differ between the two intruder size categories. These findings suggest that in this species, small and large intruders adjust their prior decisions to their competitive abilities but once a fight started, they behave similarly during the contest.

The effects of adult sex ratio and density on parental care in Lethrus apterus (Coleoptera, Geotrupidae)

Theoretical models suggest that adult sex ratio (ASR) and population density are expected to influence parental roles by reducing the mating opportunities of the commoner sex and by changing the intensity of sperm competition, although experimental evidence for these predictions is sparse. In biparental species with a high risk of extrapair paternity and consecutive egg laying over the breeding period, males are expected to reduce their parental investment and to spend more time on mate guarding if male density is high, to maximize their fitness. We conducted a field experiment to test this hypothesis in Lethrus apterus, a flightless biparental beetle species from the Geotrupidae family. Using seminatural enclosures, we assigned individuals to nine treatment groups differing in adult sex ratio (three levels) and individual density (three levels) using a full factorial experimental design. Nest attendance and parental provisioning (i.e. collecting and transporting leaves to the nest) were recorded as well as the number, size and sex ratio of the offspring. We found that as the level of male–male competition increased, generated either by the increased density of individuals or by the male-biased sex ratio, pairs showed higher nest attendance and collected fewer leaves. Male-biased groups also produced fewer offspring under high and low densities indicating a possible conflict of interest between the sexes over paternity and brood size. These results support the increased paternity assurance hypothesis under a high level of intrasexual competition.

Transgenerational effects of parent and grandparent gender on offspring development in a biparental beetle species

Parental effects on offspring life-history traits are common and increasingly well-studied. However, the extent to which these effects persist into offspring in subsequent generations has received less attention. In this experiment, maternal and paternal effects on offspring and grand-offspring were investigated in the biparental burying beetle Nicrophorus vespilloides, using a split-family design. This allowed the separation of prenatal and postnatal transgenerational effects. Grandparent and parent gender were found to have a cumulative effect on offspring development and may provide a selection pressure on the division of parental investment in biparental species.