Should females prefer older males as mates? Male survival to old age might indicate the presence of fitness-enhancing genes that increase offspring fitness. However, many correlational studies show that mating with older males can lower female fecundity and even reduce offspring fitness due to epigenetic or germline mutation effects. One problem in quantifying female choice based on male age is that age is usually confounded with mating history. This begs a question: Do females choose males based on their age or their mating history? The answer requires an experimental approach, but few such studies exist. Here, we test if experimentally induced variation in the mating history of old and young males (12-week difference in postmaturity age) affects female choice in the eastern mosquitofish (Gambusia holbrooki). To vary mating history, adult males were either allowed to freely mate with females for 3 weeks or they only had visual contact with females. Immediately thereafter, we ran four-choice mating trials, using association time, to test the effects of male age and mating history (2 × 2 design) on male attractiveness. Females did not show a clear preference for males based on either characteristic. This was not due to a lack of female choice: females spent significantly more time with larger males. In addition, female choice was significantly repeatable across four trials: twice as a virgin and twice as a nonvirgin. Finally, female mating status (virgin or nonvirgin) did not affect her choice of mate, although virgin females spent significantly more time associating with test males.

An experimental test to separate the effects of male age and mating history on female mate choice

Behavioural traits are considered animal personality traits when individuals differ consistently in their expression across time and across context. Here, we test this idea on three metrics derived from social interaction networks (strength, betweenness and closeness). Using experimental data from house sparrows in captive populations, and observational data from house sparrows in a wild population, we show that all three metrics consistently exhibit repeatability across both study populations and two methods of recording interactions. The highest repeatability values were estimated in male-only captive groups, whereas repeatabilities estimated in single-sex networks subsetted from mixed-sex groups showed no sex specificity. We also show that changes in social group composition led to a decrease in repeatability for up to six months. This work provides substantial and generalizable support for the notion that social network node-based metrics can be considered animal personalities. Our work suggests that social network traits may be heritable and thus could be selected for.

Repeatable social network node‐based metrics across populations and contexts in a passerine

An enormous amount of work has been done on aging in Drosophila melanogaster, a classical genetic and molecular model system, but also in numerous other insects. However, these two extensive bodies of work remain poorly integrated to date. Studies in Drosophila often explore genetic, developmental, physiological, and nutrition-related aspects of aging in the lab, while studies in other insects often explore ecological, social, and somatic aspects of aging in both lab and natural populations. Alongside exciting genomic and molecular research advances in aging in Drosophila, many new studies have also been published on aging in various other insects, including studies on aging in natural populations of diverse species. However, no broad synthesis of these largely separate bodies of work has been attempted. In this review, we endeavor to synthesize these two semi-independent literatures to facilitate collaboration and foster the exchange of ideas and research tools. While lab studies of Drosophila have illuminated many fundamental aspects of senescence, the stunning diversity of aging patterns among insects, especially in the context of their rich ecology, remains vastlyunderstudied. Coupled with field studies and novel, more easily applicable molecular methods, this represents a major opportunity for deepening our understanding of the biology of aging in insects and beyond. 

The Biology of Aging in Insects: From<i>Drosophila</i>to Other Insects and Back

In theory, a male should change the allocation of fighting and mating efforts in relation to his age. Thus, the consequences of sexual selection may be complicated by changes in the male resource allocation due to aging. However, previous studies have focused on changes in female mate choice and male-male competition with aging separately, and the impact of aging on the relationship between mate choice and male competition is unknown. Here, we examined how male competitiveness and attractiveness and their relationship changes over male lifespan in Gnatocerus cornutus. In this species, males perform courtship displays and fight rival males for mates. Older males are more competitive in male fighting and aggressive toward females than younger males. The aggression is also directed toward females. Almost all older males who attacked a female failed to copulate, while younger males never attacked the females. As a result, copulation success decreased with age. When an older male did not mistake a female for a male, he exhibited more frequent courtship. However, mating with older males imposed direct costs on females in terms of both fecundity and lifespan, with no offsetting indirect benefits for her offspring. The courtship behavior of older males does not supply females with cues for mate-choice benefits but are used to coerce females into mating. Our results suggest that male–male competition constrains female preference for older males. Thus, female choice and male–male competition may not be reinforcing in older G. cornutus males. When males get older, they often pay more attention to females in a variety of ways, such as courting and fighting for access. This is because older males have not long to live. In Gnatocerus cornutus as well, older males attend to fighting and courting. The older males seem to be competitive in male fights and attractive to females. However, they are not only aggressive toward rival males but also sometimes use violence on females. As a result, these older males are rejected by the assaulted females and cannot mate. Additionally, the violence of older males can reduce female fecundity and lifespan. Thus, mating with older males is costly to females in G. cornutus, and a female preference for older males is not expected to evolve.

Katherine Woodcock

Papers

Older males attract more females but get fewer matings in a wild field cricket