Managing Time and Energy

鄭先祐 (Ayo)國立台南大學 環境與生態學院

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Managing time and energy

1. Introduction2. The function approach3. The benefits of energy reserves4. The costs of energy reserves5. Relating short-term ( 短期 ) behaviour

to lifetime reproductive success6. Final considerations

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Introduction

All behaviour takes time and consumes energy. ( 所有的行為都會耗時間和耗能量 )

Feeding strategies?

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The functional approach

Cost-benefit analysis ( 本利分析 ) In practice it is often difficult to assess f

uture reproductive success, and so attention has often been focused on simple currencies which have the following properties: They are easy to measure Maximizing them will maximize fitness

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5.3 The benefits of energy reserves

The value of feeding ( 覓食的價值 ) is to provide the raw material for self-maintenance, growth and reproduction. 生活於高緯度的動物，冬季黑夜加長，覓食

時間縮短，食物減少。 冬眠是個化解的辦法，但在冬眠前必要累積

脂肪 ( 能量 ) 。

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實驗證明，許多種鳥類於低溫時，體內累積的脂肪較多。 (Ekman & Hake, 1990)

倘若晚上的溫度變化較大， great tits 的體重會較大。 (Bednekoff et al., 1994)

Photoperiod is a good predictor of body mass in birds (reviewed by Blem, 1990)

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Fig. 5.1 (a) Great tits were switched from a 9-h feeding day (long days) to a 6-h feeding day (short days) while holding photoperiod constant.

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Fig. 5.1 (b) starlings were exposed to fixed length interruptions in their feeding day.

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Several authors have found differences in mass and fat correlated with factors expected to affect the predictability ( 可預期性 ) of food intake.

The effects of social dominance on fat-storage.

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5.4 The costs of energy reserves

Animals with naturally high levels of fat, or high mass are considered to be in ‘good condition’ or of ‘high quality’.

However, fat storage must have costs.

1. Acquisition costs2. Storage costs3. The cost of being fat

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5.4.1 Acquisition costs ( 獲取的代價 )

There are two types of cost associated with feeding

1. Acquisition of the food2. Maintenance of the energy reserve once

secured. 獲取脂肪的累積，覓食的投入加大，增

加被掠食的風險。

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5.4.2 Storage costs

獲取食物後，可以貯存，或是立即食用，轉變成脂肪，貯存於體內。

許多種動物有貯存食物的行為。 貯存食物，必要記憶貯存的地點。

什麼情況下，動物會貯存較多的食物？ 食物供應變動性高的情況

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Fig. 5.2 Hurly (1992) measured seed hoarding ( 貯藏 ) and body mass changes in captive marsh tits. When an automatic feeder delivered seeds at highly unpredictable times, the birds hoarded more seeds per day, but did not increase body reserves.

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5.4.3 The costs of being fat

The most obvious costs of fat storage in human life are the various pathological conditions associated with obesity( 肥胖 ): heart disease, gall and kidney stones, diabetes and various forms of cancer and arthritis.

However, there are almost no data on pathological effects of fat storage in wild animals. If migrating birds have heart attacks, how do we fi

nd out?

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身體脂肪的多寡，或是體重，可能會影響其身體的活動能力，因此而影響其逃避敵害的能力。

Mass increases associated with being gravid have been shown to impair mobility and reduce escape speeds in several species of lizard and snake (Madsen, 1987; Siegel et al., 1987; Cooper et al., 1990)

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Fig. 5.3 (a) adding artificial masses to starlings resulted in a shallower angle of ascent ( 起飛的角度 ) when escaping from a simulated predator

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Fig. 5.3 (b) reduction of natural body mass by means of short-term food deprivation, also showed that heavier birds had lower take-off angles.

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Fig. 5.3 ( c) The natural daily variation in body mass of zebra finches is correlated with the time they take to ascend a given vertical height.

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Fig. 5.4 (a) In years when sparrowhawks were absent, the resident great tits maintained higher body mass than when the predators were present.

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Fig. 5.4 (b) Starlings were assigned randomly to experimental aviaries differing in the amount of protective cover they contained.

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5.5 Relating short-term behaviour to lifetime reproductive success

The costs and benefits of energy acquisition and storage 生殖成功是最重要的利益。

個體有其基本的貯存脂肪的需要。 低於基本貯存量，生殖將會失敗。 這個基本貯存需求量，會隨時間改變。

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Fig. 5.5 The optimal policy for an animal over a season, where the decision whether to feed or rest depends on both energetic state and time. If reserves are less than some critical level Xc, the future reproductive success is 0.

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Stochastic dynamic programming (SDP)

SDP is a numerical technique for finding the optimal behavioural decision as a function of state and time ( 狀態和時間 ).

The animal is characterized by one or more state variables and a set of possible behavioural options. For technical details see Mangel and Clark (1988)

Dynamic Modelling in Behavioural Ecology. Princeton University Press.

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Fig. 5.6 When food availability is low, birds forage at a high level for most of the day (open circles). When food availability is higher, the overall level of foraging is lower.

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5.6 Final considerations

Is the dynamic programme really necessary?

1. Dynamic programming is a particular technique for finding optimal solutions. But it is not always necessary to use this technique just because a simple rate-maximizing approach is inadequate.

2. Stochasticity can be analyzed without using dynamic programming.

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