The most diverse and abundant family of termites, the Termitidae, evolved in African tropical forests. They have since colonised grassy biomes such as savannas. These open environments have more extreme conditions than tropical forests, notably wider extremes of temperature and lower precipitation levels and greater temporal fluctuations (both annual and diurnal variation). These conditions are challenging for soft-bodied ectotherms, such as termites, to survive in, let alone become as ecologically dominant as termites have. Here, we quantified termite thermal limits to test the hypothesis that these physiological limits have widened in savanna termite species to facilitate their existence in savanna environments. We sampled termites directly from mound structures, across an environmental gradient in Ghana, ranging from wet tropical forest through to savanna. At each location we quantified both Critical Thermal Maximum (CTmax) and Critical Thermal Minimum (CTmin) of all the most abundant mound-building Termitidae species in the study areas. We modelled the thermal limits in two separate mixed effects models against: canopy cover at the mound, temperature and rainfall, as fixed effects, with sampling location as a random intercept. For both CTmax and CTmin savanna species had significantly more extreme thermal limits than forest species. Between and within environments, areas with higher amounts of canopy cover were significantly associated with lower CTmax values of the termite colonies. CTmin was significantly positively correlated with rainfall. Temperature was retained in both models, however it did not have a significant relationship in either.  Sampling location explained a large proportion of the residual variation, suggesting there are other environmental factors that could influence termite thermal limits. Our results suggest there has been a widening of the thermal limits in termite savanna species. These physiological differences, in conjunction with other behavioural adaptations, are likely to have enabled termites to cope with the more extreme environmental conditions found in savanna environments and facilitated their expansion into open tropical environments.

Body size is a fundamental trait that impacts many aspects of species biology and ecology. It is in turn influenced by a suite of environmental factors, and often decreases with warming. Although environmental conditions can also impact body shape, which is another functional trait that influences locomotion, resource acquisition, and potentially physiological processes, such responses are poorly understood and rarely quantified.  We experimentally tested the independent and combined effects of environmental temperature, resource level, and interspecific competition on the body shape and size responses of two model protist species. We also tested the degree to which these individual-level phenotypic responses are associated with population densities and species coexistence.  Body shape was strongly influenced by resource competition, whereas body size changes were mainly driven by environmental temperature. In both species, lower resource levels resulted in body shape elongation, suggesting that relatively more elongate individuals with potentially higher swimming speed were advantaged in the resource scarce environment. However, competition had contrasting influence on the body shape of the two species. Competition decreased the population densities of Blepharisma japonicum, which exhibited relative body shape elongation, similar to the response at low resource levels. In contrast, competition increased the population densities of Paramecium aurelia, which exhibited reduced elongation similar to body shape response at high resource levels. Hence, body shape responses could be indicative of changes in resource availability, aiding our understanding of competitive hierarchies and species interactions. Coexistence was observed in all treatment combinations, likely because body size of both species decreased similarly under warming, potentially maintaining constant per capita competitive intensity. These findings, along with recent research on phytoplankton, diverse pelagic invertebrates, and birds highlight the importance of body shape and morphology across different taxonomic groups. Hence, we call for body size and shape to be considered in concert when investigating ecological consequences of climate warming.

Variation in the degree of sexual size dimorphism (SSD) among taxa is generally considered to arise from differences in the relative intensity of male-male competition and fecundity selection. One might predict, therefore, that SSD will vary systematically with: 1) the intensity of sexual selection for increased male size, and 2) the intensity of fecundity selection for increased female size. To test these two fundamental hypotheses, we conducted a phylogenetic comparative analysis of SSD in fish. Specifically, using records of body length at first sexual maturity from FishBase, we quantified variation in the magnitude and direction of SSD in >600 diverse freshwater and marine fish species, from sticklebacks to sharks. Although female-biased SSD was common, and thought to be driven primarily by fecundity selection, variation in SSD was not dependent on either the allometric scaling of reproductive-energy output or fecundity in female fish. Instead, systematic patterns based on habitat and life history characteristics associated with varying degrees of male-male competition and paternal care, strongly suggest that adaptive variation in SSD is driven by the intensity of sexual selection for increased male size.

Size at maturity in ectotherms commonly declines with warming. This near-universal phenomenon, formalised as the temperature-size rule, has been observed in over 80% of tested species, from bacteria to fish. The proximate cause has been attributed to the greater temperature dependence of development rate than growth rate, causing individuals to develop earlier but mature smaller in the warm. However, few studies have examined the ontogenetic progression of the temperature-size response at high resolution. Using marine planktonic copepods, we experimentally determined the progression of the temperature-size response over ontogeny. Temperature-size responses were not generated gradually from egg to adult, contrary to the predictions of a naïve model in which development rate was assumed to be more temperature-dependent than growth rate, and the difference in the temperature dependence of these two rates remained constant over ontogeny. Instead, the ontogenetic progression of the temperature-size response in experimental animals was highly episodic, indicating rapid changes in the extent to which growth and development rates are thermally decoupled. The strongest temperature-size responses occurred temporally mid-way through ontogeny, corresponding with the point at which individuals reached between ~5- 25% of their adult mass. Using the copepod Oithona nana, we show that the temperature-dependence of growth rate varied substantially throughout ontogeny, whereas the temperature dependence of development rate remained constant. The temperature-dependence of growth rate even exceeded that of development rate in some life stages, leading to a weakening of the temperature-size response. Our analyses of arthropod temperature-size responses from the literature, including crustaceans and insects, support these conclusions more broadly. Overall, our findings provide a better understanding of how the temperature-size rule is produced over ontogeny. Whereas we find support for the generality of developmental rate isomorphy in arthropods (shared temperature dependence of development rate across life stages), this concept should not apply to growth rates.

1. Body size affects rates of most biological and ecological processes, from individual performance to ecosystem function, and is fundamentally linked to organism fitness. Within species, size at maturity can vary systematically with environmental temperature in the laboratory and across seasons, as well as over latitudinal gradients. Recent meta-analyses have revealed a close match in the magnitude and direction of these size gradients in various arthropod orders, suggesting that these size responses share common drivers. 2. As with increasing latitude, temperature also decreases with increasing altitude. Although the general direction of body size clines along altitudinal gradients has been examined previously, to our knowledge altitude-body size (A-S) clines have never been synthesised quantitatively, nor compared with temperature-size (T-S) responses measured under controlled laboratory conditions. 3. Here we quantitatively examine variation in intraspecific A-S clines among 121 insect species from 50 different global locations, representing 12 taxonomic orders. While some taxa were better represented in the literature than others, our analysis reveals extensive variation in the magnitude and direction of A-S clines. Following the assumption that temperature on average declines by 1°C per 150m increase in altitude, order-specific A-S clines in the field appear to deviate from laboratory T-S responses. Specifically, the magnitude of A-S clines and T-S responses are more closely matched in some taxonomic orders (e.g. Diptera) than others (e.g. Orthoptera). These findings contrast with the strong co-variation observed between latitude-size clines and T-S responses, and between laboratory and seasonal T-S responses. 4. The lack of clear size relationships with elevation, and hence temperature, is likely due to the counteracting effects of other major drivers with altitude, including season length and oxygen partial pressure. Switches in voltinism within species across altitude, and the dispersal of individuals across different elevations, may also obscure trends.

Changes in the timing of life history events within the year alter the degree to which the activity patterns of different species coincide, making the dynamics of interspecific interactions sensitive to the phenology of the interacting parties. For parasites, the availability of suitable hosts to infect represents a crucial determinant of dynamics, and changes in the host (and parasite) phenology may thus alter disease epidemiology and the conditions for disease maintenance. We tested the hypothesis that the incidence of a sexually transmitted mite infection, Coccipolipus hippodamiae, in Adalia bipunctata ladybird beetles in Sweden was determined by host phenology, namely presence/absence of sexual contact between cohorts of the host. We observed that the pattern of mite presence/absence across Swedish A. bipunctata populations was highly reproducible between years, implying a persistent biological/ecological basis underlying the incidence. Further, ladybirds from populations where the mite was absent were able to acquire mites during copulation, develop a mite infection, and transmit infection onward, indicating an ecological (rather than biological) driver of mite incidence. Observations of ladybird phenology in natural populations provided evidence of sexual contact between overwintered and new cohort adults in populations where the mite was present. In contrast, new cohort ladybirds in the two northern Swedish populations where the mite was not present had not had sexual contact with the overwintered generation, creating a ‘hard stop’ to mite transmission. We conclude that variation in host phenology may be an important driver of the incidence of sexually transmitted infections (STIs) by determining the presence/absence of sexual contact between generations. More generally, we hypothesize that sensitivity to variation in host phenology will be highest for parasites like STIs that infect one host species, one host life stage and are directly transmitted on contact between host individuals.