Courtship display by great frigatebirds, Fregata minor: an energetically costly handicap signal?

A framework to unlock marine bird energetics

Energetics can provide novel insights into the roles of animals, but employing an energetics approach has traditionally required extensive empirical physiological data on the focal species, something that can be challenging for those that inhabit marine environments. There is therefore a demand for a framework through which to estimate energy expenditure from readily available data. We present the energetic costs associated with important time- and energy-intensive behaviours across nine families of marine bird (including seabirds, ducks, divers and grebes) and nine ecological guilds. We demonstrate a worked example, calculating the year-round energetic expenditure of the great auk, Pinguinus impennis, under three migration scenarios, thereby illustrating the capacity of this approach to make predictions for data-deficient species. We provide a comprehensive framework through which to model marine bird energetics and demonstrate the power of this approach to provide novel, quantitative insights into the influence of marine birds within their ecosystems.

Selection for male stamina can help explain costly displays with cost-minimizing female choice

Abstract

In many species, male lifespan is shorter than that of females, often attributed to sexual selection favouring costly expression of traits preferred by females. Coevolutionary models of female preferences and male traits predict that males can be selected to have such life histories; however, this typically requires that females also pay some costs to express their preferences. Here we show that this problem diminishes when we link coevolutionary models of costly mate choice with the idea of stamina. In our model, the most successful males are those who can combine high attendance time on a lek — or, more generally, tenacious effort in their display time budgets — with high viability such that they are not too strongly compromised in terms of lifespan. We find that an opportunistic female strategy, that minimizes its costs by mating with highly visible (displaying) males, often beats other alternatives. It typically resists invasion attempts of genotypes that mate randomly in the population genetic sense, as well as invasion of stricter ways of being choosy (which are potentially costly if choice requires e.g. active rejection of all males who do not presently display, or risky travel to lekking sites). Our model can produce a wide range of male time budgets (display vs. self-maintenance). This includes cases of alternative mating tactics where males in good condition spend much time displaying, while those in poor condition never display yet, importantly, gain some mating success due to females not engaging in rejection behaviours should these be very costly to express.

Significance statement

In many species, males spend much time and energy on displaying to attract females, but it is not always clear what females gain from paying attention to male displays. The tradition in mathematical models attempting to understand the situation is to assume that random mating is the least costly option for females. However, random mating in the population genetic sense requires females to behave in a manner that equalizes mating success between displaying and non-displaying males, and here we point out that this is biologically unlikely. Opportunistically mating females can cause males to spend much of their time budgets displaying and will shorten male lifespans in a quality-dependent manner.

Songs versus colours versus horns: what explains the diversity of sexually selected traits?

Papers on sexual selection often highlight the incredible diversity of sexually selected traits across animals. Yet, few studies have tried to explain why this diversity evolved. Animals use many different types of traits to attract mates and outcompete rivals, including colours, songs, and horns, but it remains unclear why, for example, some taxa have songs, others have colours, and others horns. Here, we first conduct a systematic survey of the basic diversity and distribution of different types of sexually selected signals and weapons across the animal Tree of Life. Based on this survey, we describe seven major patterns in trait diversity and distributions. We then discuss 10 unanswered questions raised by these patterns, and how they might be addressed. One major pattern is that most types of sexually selected signals and weapons are apparently absent from most animal phyla (88%), in contrast to the conventional wisdom that a diversity of sexually selected traits is present across animals. Furthermore, most trait diversity is clustered in Arthropoda and Chordata, but only within certain clades. Within these clades, many different types of traits have evolved, and many types appear to have evolved repeatedly. By contrast, other major arthropod and chordate clades appear to lack all or most trait types, and similar patterns are repeated at smaller phylogenetic scales (e.g. within insects). Although most research on sexual selection focuses on female choice, we find similar numbers of traits (among sampled species) are involved in male contests (44%) and female choice (55%). Overall, these patterns are largely unexplained and unexplored, as are many other fundamental questions about the evolution of these traits. We suggest that understanding the diversity of sexually selected traits may require a shift towards macroevolutionary studies at relatively deep timescales (e.g. tens to hundreds of millions of years ago).

Frigate birds track atmospheric conditions over months-long transoceanic flights

Understanding how animals respond to atmospheric conditions across space is critical for understanding the evolution of flight strategies and long-distance migrations. We studied the three-dimensional movements and energetics of great frigate birds (Fregata minor) and showed that they can stay aloft for months during transoceanic flights. To do this, birds track the edge of the doldrums to take advantage of favorable winds and strong convection. Locally, they use a roller-coaster flight, relying on thermals and wind to soar within a 50- to 600-meter altitude band under cumulus clouds and then glide over kilometers at low energy costs. To deal with the local scarcity of clouds and gain longer gliding distances, birds regularly soar inside cumulus clouds to use their strong updraft, and they can reach altitudes of 4000 meters, where freezing conditions occur.

Metabolic efficiency in courtship favors males with intermediate mass in the Australian redback spider, Latrodectus hasselti

Recent studies have suggested that metabolic efficiency may be an important factor in male mating success when females require vigorous and/or prolonged courtship. In capital breeding animals in which a male's resource pool is fixed at adulthood the relationship between energy expenditure and courtship performance may be especially important, as males are expected to utilize their finite resources efficiently when soliciting mates. Males may benefit from being efficient, i.e., achieving a sufficiently high level of courtship signaling at low energetic cost, if it enables them to acquire mates before their limited energy reserves are depleted. We investigated the relationship between metabolic efficiency and courtship vibrational signaling in the Australian redback spider, Latrodectus hasselti, a semelparous capital breeder where males invest heavily in courtship to secure a mating. We assessed metabolic rate in a sample of males and measured two courtship components (duty cycle and amplitude) that reflected the energy content of web-borne vibrations. We then calculated two indices of metabolic efficiency for these courtship properties. There was a quadratic relationship between mass and duty cycle such that the highest duty cycle signals were performed by males having intermediate mass. Furthermore, intermediate-mass males were also the most metabolically efficient. Prolonged courtship is necessary in L. hasselti for successful mating, and the results of this study suggest that intermediate-mass males are superior courters because they utilize their finite resource pool most efficiently to produce high energy vibrational signals.

Duration of courtship displays corresponds to social status in male greater sac-winged bats (Saccopteryx bilineata)

In species with complex courtship displays, male courtship performance is often used by females to assess male quality. The greater sac-winged bat ( Saccopteryx bilineata (Temminck, 1838)) lives in a harem-based resource-defence polygyny. Courting males perform complex hover displays in front of roosting females. Males differ in their social status by having females permanently or sporadically in their day-roost territory (harem males vs. nonharem males). We compared the frequency and duration of hover displays from harem and nonharem males in free-living colonies. Male social status was correlated with male age and the number of females being courted; thus, these two effects were removed to compare the frequency and duration of hover displays for harem and nonharem males. The frequency of hover displays per hour did not differ between harem and nonharem males, whereas the mean duration of hover displays was linked to male social status, with harem males exhibiting significantly longer hover displays than nonharem males. When analysing each social status separately, the hover display duration of both harem and nonharem males was neither influenced by the number of competing males nor by the number of females being courted. Male age did not influence the hover display duration of nonharem males; however, it had a significant effect on the hover display duration of harem males, with older harem males hovering significantly longer than younger harem males. Because females are free to choose in which male territory to roost, they might use the duration of hover displays to evaluate the quality of courting males.